# High Resolution Gamma-Ray Spectroscopy at the RIBF

Europe/Berlin
April 10: Institut für Kernphysik April 11-12: Lichtenberghaus, Darmstadt
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Description

We are pleased to announce a workshop on high resolution gamma-ray spectroscopy that will be held from April 10-12, 2019 at the Technische Universität (TU) Darmstadt.

We are currently developing a germanium-based gamma-ray spectrometer composed of MINIBALL and several Ge tracking detectors from Japan, Europe, and the USA for experimental fast beam campaigns at the RIBF in 2020. The objectives for this workshop therefore include, but are not limited to:

i) Discussion of physics case,
ii) Planning, schedule, and organizational matters towards the campaign in 2020,
iii) Plans beyond 2020.

It is foreseen that individual proposals arise from the physics discussion to be submitted to the next RIBF NP-PAC, held in November/December 2019. A second workshop will be held in Asia later this year. In order to avoid overlaps and ensure feasibility, we expect that all proposals are discussed at either one of the workshops. Furthermore, we will request Letters of Intent from all proponents following the first workshop. The Letter of Intent should summarize the physics case, and include realistic estimates of count rates (LISE++ and GEANT simulations), and specify the configuration of the array. The LOI should not exceed 2 pages. More details on the expected performance and possible configurations will be provided at the workshop.

Participants
• Alexandre Obertelli
• Andrea Gottardo
• Andrea Jungclaus
• Andres Illana Sison
• Andrey Blazhev
• ANNA CORSI
• Asahi Kohda
• Bui Dui Linh
• Byul Moon
• Christian Sürder
• Christoph Fransen
• Claus Müller-Gatermann
• Corinna Henrich
• daisuke suzuki
• Dmitry Testov
• Eiji Ideguchi
• Francesco Recchia
• Frank Browne
• Freddy Flavigny
• Heather Crawford
• Hongna Liu
• Janne Pakarinen
• Jeongsu Ha
• Jesus Pereira Lopez
• Joa Ljungvall
• Jonas Refsgaard
• Joochun Park
• Joonas Konki
• Jose Javier Valiente Dobon
• Juan Saiz
• Juergen Gerl
• Kathrin Wimmer
• Liam Gaffney
• Magda Gorska
• Marcus Scheck
• Marina Petri
• Martha Liliana Cortes
• Michael Bentley
• Nigel Warr
• Nobu Imai
• Norbert Pietralla
• Nori Aoi
• Oliver Wieland
• Paul Fallon
• Pavlos Koseoglou
• Peter Reiter
• Pieter Doornenbal
• Rosa-Belle Gerst
• Ryo Taniuchi
• serge franchoo
• Silvia Lenzi
• Takuma Koiwai
• Thorsten Kröll
• Volker Werner
• Yasutaka Yamamoto
• Zsolt Podolyak
Contact
• Wednesday, 10 April
• 14:00 15:30
Technical: Details I
Convener: Kathrin Wimmer (The University of Tokyo)
• 14:00
Welcome 10m
• 14:10
Technical considerations for in-beam gamma spectroscopy experiments 20m

Technical considerations for in-beam gamma spectroscopy experiments at RIBF will be discussed.

Speaker: Pieter Doornenbal (RIKEN)
• 14:30
Opportunities with MINOS+MINIBALL at RIBF 20m

The status of the MINOS device will be presented. Possible coupling to MINIBALL will be discussed in terms of feasibility and physics interest.

Speaker: Dr Anna Corsi (CEA Saclay)
• 14:50
The GRETINA DAQ system 20m

The GRETINA DAQ system

Speaker: Heather Crawford (Lawrence Berkeley National Laboratory)
• 15:10
Development of the gamma-ray tracking detector system at RCNP 20m

Development of the gamma-ray tracking detector system at RCNP

Speaker: yasutaka Yamamoto
• 15:30 16:00
Coffee Break 30m
• 16:00 18:00
Technical: Details II
Convener: Pieter Doornenbal (RIKEN)
• 16:00
CNS Plunger device and life time measurement at RIBF 20m

Information on the CNS plunger device and the experiment plan using the device will be presented.

Speaker: Eiji Ideguchi (RCNP, Osaka University)
• 16:20
Simulations for the High Resolution array 20m

The simulation framework and the analysis of simulated data will be explained.

Speaker: Kathrin Wimmer (The University of Tokyo)
• 16:40
Status of Miniball 20m

Status of Miniball

Speaker: Peter Reiter
• 17:00
Coulex at intermediate beam energies 20m

Coulex at intermediate beam energies

Speaker: Andrea Jungclaus (Instituto de Estructura de la Materia - CSIC)
• 17:20
Development of a gamma-ray tracking detector and its performance test 20m

The gamma-ray tracking detector is a germanium detector realizing both high efficiency and Compton background suppression by reconstructing the scattering process of the incident gamma-rays from the positions and energy deposits of the gamma-rays at each interaction points in the detector. Its high position resolution is also beneficial for accurate Doppler correction. In the tracking detector, the interaction positions are determined three-dimensionally with high position resolution by analyzing the signal waveform from the segmented electrodes. We have measured waveforms for different interaction points of gamma-rays using a GRETINA Quad Detector. The experiment was performed using a gamma-ray beam from the GACKO beam line at the NewSUBARU electron storage ring facility. The three dimensional position of the interaction in points are selected first by collimating the incident gamma-rays and then by measuring the gamma-rays scatted at 90 degree in the detector by using a narrow slit. Obtained waveforms were compared with the simulated waveform.

Speaker: Asahi Kohda (RCNP, Osaka University)
• 17:40
Symbiotic mass measurement with ZD-MRTOF 20m

We will place the third prototype setup of SLOWRI which consists of a gas catcher and a multi-reflection time of flight mass spectrograph (MRTOF-MS) at the beam dump of the Zero-degree forward spectrometer of RIKEN RI-beam factory. The gas catcher thermalizes energetic RI-beam from BigRIPS and extracts low-energy bunched ions from the catcher using RF-carpet ion guide technique. The bunched ions are stored for several milliseconds in the MRTOF device for mass measurement with a relative precision of 0.1 ppm level. Thanks to the spectrographic feature of MRTOF-MS, many different nuclides can be measured at once without any scans. This feature is essential to perform comprehensive mass measurements of all available nuclides at RIBF.
With this setup (ZD-MRTOF), we will be able to run mass measurements of short-lived nuclides simultaneously with other BigRIPS experiments. We discuss possible collaboration with the in-beam gamma team for such “symbiotic” mass measurement.

Speaker: Prof. Michiharu Wada (WNSC, IPNS, KEK)
• Thursday, 11 April
• 09:00 10:30
Proposals: Physics I
Convener: Heather Crawford (Lawrence Berkeley National Laboratory)
• 09:00
Shape coexistance in the N=Z nucleus $^{80}$Zr 15m

State-of-the-art beyond mean field methods with the Gogny D1S interaction has predicted for the N=Z $^{80}$Zr nucleus five 0$^+$ states corresponding to different nuclear shapes within 2.25 MeV, where several rotational and γ-bands are built upon those five 0+ states [1]. We propose to study the rich low-lying energy spectrum of $^{80}$Zr, by using a 1n and 2n knock-out reaction from $^{81,82}$Zr, respectively. The $^{81,82}$Zr fragments will be produced from the fragmentation of a primary $^{124}$Xe beam at 345 MeV.A on a $^{9}$Be target. The reaction fragments will be separated and identified by he BigRIPS separator. The fragments of interest will impinge on a $^{9}$Be target surrounded by a high-purity germanium array (MINIBALL). The final reaction products will be identified by the ZeroDegree spectrometer. Currently T.R. Rodriguez together with J. Tostevin are calculating the spectroscopic factors for the various excited states in order to quantify the population to the various low-lying excited states in $^{80}$Zr. Gamma-gamma coincidences, together with angular gamma distributions will help us to build up the level scheme of $^{80}$Zr at low-energies.

[1] T. R. Rodríguez and J. Luis Egido Physics Letters B 705 (2011) 255–259

Speaker: Jose Javier Valiente Dobon (LNL-INFN)
• 09:15
Study of multiple shape coexistence in 80Zr and isospin symmetry breaking of its vicinity 15m

Nuclear physicists have kept their attention to the nuclei that the proton number (Z) and the neutron number (N) are same, i.e. N=Z, because they are interesting for several reasons. First, they provide a test ground of exchange symmetry between protons and neutrons. Approximately, the nuclear force acting between nucleons does not distinguish the kind of nucleons. However, each proton has a charge, thus Coulomb force affects the configuration of nuclei. It implies that the nuclei that consist of (Z, N+1) and (Z+1, N) show a different behavior due to the charge difference even if their number of nucleons are same.

Second, some theoretical predictions suggest the emergence of multiple shape coexistence in N=Z nuclei [1]. The phenomenon has been observed for 16O and 40Ca. 80Zr can pose five different shapes, spherical, prolate, and three different triaxial according to Ref. [2]. Moreover, 80Zr has a potential of being a dodecahedron shape, one of the platonic solids. A recent research found that the alpha clustering nuclei form a balloon-shaped configuration like a fullerene [3].

In this study, the goal of research will focus on revealing the properties of 80Zr and its vicinity in the nuclear chart. The two main objectives of this study are as follows:

(1) A multiple shape coexistence of 80Zr: this study is aimed at scrutinizing the properties of the low-lying states of 80Zr. A theoretical prediction using beyond mean field approach indicates that the multiple shape coexistence can exist in 80Zr. The experiment will concentrate on the discovery of the hypothetical low-lying 0+ states.

(2) Isospin symmetry breaking around N=40, Z=40: as described above, the isospin symmetry breaking takes place due to Coulomb force. The extent of deviation from the charge-symmetry and charge-independence are presented with the term Mirror Energy Difference (MED) and Triplet Energy Difference (TED), respectively. Mass region A=80-100 are little known due to the difficulty of ion production. However, the development of measurement techniques and ion production mechanism realize the exploration of the proton-rich nuclei. We expect that the nearly symmetric low-lying states can be observed in pairs of 79Zr – 79Y, 85Mo – 85Tc, and so on. The study of charge-independence and charge-symmetry breaking towards more heavier nuclei will reveal the role of protons in fp shell and its neighboring orbitals in the symmetry breaking.

References
[1] K. Heyde and J. L. Wood, Rev. Mod. Phys. 83, 1467 (2011)
[2] T. R. Rodriguez and J. L. Egido, Phys. Lett. B 705, 255 (2011)
[3] A. Tohsaki and N. Itagaki, Phys. Rev. C 97, 011301(R) (2018)

Speaker: Jeongsu Ha (Seoul National University)
• 09:30
T=0 pairing along the N=Z line in the g9/2 region 15m

Enhanced neutron-proton (np) pairing correlations can arise when both particle types occupy the same orbitals. In addition to the T = 1 np pairing phase, the opportunity for isoscalar (T = 0) correlations is also present, especially on the N=Z line. Competition between these np-pairing mechanisms is of much interest. Recent work on $^{92}$Pd [1] has indicated the possibility for the existence of a new type of spin-aligned isoscalar np pair. The observation of the β-decaying 16$^+$ isomer in $^{96}$Cd has also revealed evidence for the importance of the T = 0 np interaction at higher spins [2]. Very recently, the gamma rays in the ground state sequence of $^{96}$Cd have been observed [3], following decay of the spin-trap isomer, although the ordering of the transitions could not be confirmed. Calculations suggest that spin-aligned pair approximation should contribute significantly to the structure of the states in the sequence, but varying with spins, and may reduce significantly for the 8$^+$ state [e.g. 4].

To gain further insight into the np-pairing effects, we propose here to use the knockout methodology to populate states in the N=Z systems $^{96}$Cd and $^{92}$Pd up to 8$^+$ through (probably 1-neutron) knockout. The aim will be to firmly establish the ordering of the transitions in both nuclei and also to investigate the extent to which the knockout cross-sections might be affected by changes in the underlying structure along the yrast line.

[1] B. Cederwall, F. G. Moradi, T. Bäck, A. Johnson, J. Blomqvist, E. Clement, G. de France, R. Wadsworth, K. Andgren, K. Lagergren et al., Nature 469, 68 (2011).
[2] P. J. Davies, H. Grawe, K. Moschner, A. Blazhev, R. Wadsworth, P. Boutachkov, F. Ameil, A. Yagi, H. Baba, T. Bäck et al., Phys. Lett. B 767, 474 (2017).
[3] P. J. Davies et al. Phys. Rev C 99, 021302(R) (2019)
[4] G. J. Fu, J. J. Shen, Y. M. Zhao and A. Arima. Phys. Rev C 87, 044312 (2013)
[5] Z. X. Xu, C. Qi, J. Blomqvist, R. J. Liotta and R. Wyss. Nucl. Phys. A 877 (2012) 51

Speaker: Michael Bentley (University of York)
• 09:45
MED and TED in the rotational A=78, 82 T=1 triplets 15m

D. Bazzacco, A. Goasduff, S.M. Lenzi, S. Lunardi, R. Menegazzo, D. Mengoni, A. Montaner-Pizá, G. Pascualato, F. Recchia, D. Testov
T. Bayram, A. Illana, G. de Angelis, D. Brugnara, M.L. Cortés, F. Galtarossa, A. Gottardo, E. Gregor, T. Marchi, D.R. Napoli, J.J. Valiente-Dobón, I. Zanon
INFN Laboratori Nazionali di Legnaro, Legnaro, Italy
P. Doornenbal, K Wimmer
RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom

Isobaric multiplets along N=Z line have been object of constant interest due to the fact that this region of the nuclide chart is the only place where it is possible to find answers to some fundamental problems in nuclear physics, such as the isospin symmetry. Isospin symmetry is violated by the electromagnetic interaction and by nuclear forces. One of the consequences of this symmetry is that the level schemes of mirror nuclei (obtained interchanging neutrons and protons) should be identical in the absence of symmetry-breaking interactions.
Signatures of the isospin symmetry breaking in mirror nuclei are, therefore, the differences between the excitation energy of analogue states, called mirror energy differences (MED). Although the Coulomb interaction is the main responsible for the isospin symmetry breaking (ISB), sizable contributions may arise from the residual nuclear interaction [1,2]. This has been systematically studied in the f7/2 shell. However, the particularity of the structure of nuclei of the f7/2 shell is that this orbital largely dominates the wave functions. This may mask the fact that the ISB VB term could not be an exception of this orbital, but, more generally, could appear in other orbitals. The extension of these investigations to other mass regions is therefore very important to check the limits of validity of the isospin symmetry for different masses, to look for possible ISB contributions, such as those identified in the f7/2 shell, and to search for other isospin non-conserving effects. Preliminary calculations on this line suggest that the additional isovector VB term is necessary to reproduce the MED in the sd shell.
While MED are very sensitive to the nuclear structures properties (and in particular in monopole effects), the difference of excitation energy of analogue states in triple T=1nuclei (TED), depend only on multipole effects [3,4].

In this LoI we propose to measure the MED and TED in the triplet T=1 A=82 and A=78, by measuring for the first time excited states in 82Mo and 78Zr. These nuclei will be populated by two-neutron knock out from 84Mo and 80Zr. With present day primary beam of 124Xe on a Be target. In the same experiment it be also possible to measure the MED in the T=1/2 mirrors A=83 and A=79 for the first time in one-proton and one-neutron knock out. These studies will allow to extend the systematics to nuclei in which the wavefunctions are characterized by configurations where the role of the g9/2 orbit is dominant and therefore determine the eventual ISB arising from this orbital in both the TED and the MED.
[1] A.P. Zuker, S.M. Lenzi, G. Martinez-Pinedo and A. Poves, Phys. Rev. Lett. 89, 142502 (2002).
[2] M.A. Bentley and S.M. Lenzi, Prog. Part. Nucl. Phys. 59, 497-561 (2007)
[3] S. M. Lenzi, M. A. Bentley, R. Lau, and C. Aa. Diget, Phys. C 98, 054322 (2018).
[4] K. Kaneko, Y. Sun, T. Mizusaki, and S. Tazaki, Phys. Rev. C 89, 031302(R) (2014).

Speaker: Silvia M. Lenzi (University of Padova and INFN)
• 10:00
Test of nuclear collectivity above 100 Sn 15m

Collectivity in nuclei in the vicinity of the N = Z line may be enhanced by neutron-proton
interactions occupying similar orbits near the Fermi level. Therefore, information on single-particle
energies and residual interactions with respect to the 100 Sn core are extremely important. The region
of 100 Sn has been extensively investigated due to unusual B(E2;0 + →2 + ) values observed for light Sn
isotopes. But the direct study of the properties of 2 + state neutron-deficient Sn-isotopes is
challenging due to presence of long-lived isomers up to the range of nanoseconds.
In the proposed experiment we propose to probe the collectivity in the neighbouring light Sb
nuclei. The dominant feature of low-energy levels of the odd-mass Sb nuclides with 54<n<82 (6-467)="" (a="" 1="" 111="" 133="" 2="" 2+="" 2+)="B(E2;2+→0+)," 2+→="" 5="" 7="" 852="" 9="" 963="" [1].="" [2]:="" [3]="" [4],="" a="" above="" adjacent="" and="" another="" approach="" are="" arises="" as="" b(e2;9="" below="" bohr-mottelson.="" by="" calculations="" cd-bonn="" certain="" chong="" confirm="" consequence="" core.="" coupled="" coupling="" d="" diminishes="" dramatic="" effect="" energies="" eq.="" even-even="" follow="" for="" from="" full="" g="" g7="" heavier="" how="" however="" ii="" in="" increases.="" indeed,="" interaction="" interpreted="" is="" isotopes="" it="" kev="" level="" levels="" monopole="" moves="" n="" neutrons="" occupy="" odd-a="" of="" one="" or="" orbits="" particle-vibration="" peculiarity="" performed="" position="" postulated="" presence="" proton="" protons="" pure="" qi="" ration="" ref.="" result="" sb="" see="" shell-model="" shift="" shifted="" simple="" single-particle="" sn="" spherical="" spin-orbit="" state="" states.="" that="" the="" this="" to="" trend.="" two="" using="" volume="" which="" z="">113) due to the monopole shift the proton is moved to g7/2 and as a
result, the mixing between d5/2 and g7/2 orbitals which leads to the significant drop in the
calculated B(E2; 9/2+→g.s.) transition strength. The energy of 9/2+ 1 is relatively insensitive to the
neutron number and remains in close proximity to 2 + level in the underlying Sn core. The second
9/2+ 2 might be due the promotion of a g 9/2 proton into a higher orbital, which gives
2p1hconfiguration. Both 9/2+ states, which have different intrinsic configuration, are closely spaced
and, therefore, may be mixed with each other. The situation is similar to one observed near Z = 28
shell [5].
We propose to study lifetime of the low-lying states in 105,107,109 Sb, by using a 1n and 2n
knock-out reaction from 106,108,110 Sb, respectively. The 106,108,110 Sb fragments will be produced from
the fragmentation of a primary 124 Xe beam at 345 MeV/A on a Be target. The reaction fragments
will be separated and identified by he BigRIPS separator. The fragments of interest will impinge on
a 9 Be target surrounded by a high-purity germanium array (MINIBALL). The final reaction
products will be identified by the ZeroDegree spectrometer.
[1] J.Shergur, D.J.Dean, D.Seweryniak Phys. Rev. C. 71 064323 (2005)
[2] J. P. Schiffer et al., Phys. Rev. Lett. 92 (2004)
[3] C. Qi, Z.X. Xu Phys. Rev. C 86, 044323 (2012)
[4] Qi Chong, private comm.
[5] I. Stefanescu et al. Phys. Rev. Lett. 100, 112502 (2008)</n<82>

Speaker: Jose Javier Valiente Dobon (LNL-INFN)
• 10:15
Spectroscopy of 100In with neutron knockout reactions 15m

The robustness of the proton and neutron shells for the doubly magic nucleus $^{100}$Sn has been studied in $\beta$-decay experiments, resulting in the smallest log $ft$ value for the decay of the $^{100}$Sn ground state to the $(1^+)$ state in $^{100}$In. A decay spectroscopy experiment at the RIBF has improved the statistical uncertainties on the corresponding Gamow-Teller decay strength $B_{GT}$ by a factor of ~3, due to a tenfold increase in statistics. At the same time, a sizable reduction in $B_{GT}$ compared to the previous results was observed.

However, the extraction of the $B_{GT}$ value requires an accurate knowledge of the level scheme of the daughter nucleus $^{100}$In. In comparison with large-scale shell model calculations, multiple arrangements of $\gamma$ rays in $^{100}$In are possible due to unobserved weak $\gamma$-ray branches and a limited set of $\gamma\gamma$ coincidences. Furthermore, $\beta$-decay branches to higher-lying $(1^+)$ states in $^{100}$In have not been measured. The resulting systematic uncertainty on the $B_{GT}$ value is now comparable to the statistical uncertainty.

In order to ascertain and expand on the level scheme of $^{100}$In for tests of SM and improvements in the precision on the Gamow-Teller decay properties of $^{100}$Sn, a neutron knockout experiment on $^{101,102}$In is proposed. Doppler-corrected $\gamma$-ray energies separated by as little as 40 keV at $E_\gamma \sim 100$ keV should be resolved with the HPGe array.

Speaker: Joochun (Jason) Park (Lund University)
• 10:30 11:00
Coffee 30m
• 11:00 12:30
Proposals: Physics II
Convener: Nori AOI (RCNP, Osaka Univ.)
• 11:00
Gamma spectroscopy near 78Ni 15m

We propose the $\gamma\gamma$ spectroscopy of $^{77,79}$Cu, which is of paramount interest for tracing the evolution of proton single-particle levels near $^{78}$Ni. Despite the limited resolution of the Dali-2 scintillators during the Seastar campaign, a level scheme could be constructed for $^{79}$Cu. The $\pi f_{7/2}$ strength turned out to be fairly fragmented, resulting in a level population and a decay pattern that was richer than anticipated. Spin assignments were suggested only from comparison with MCSM calculations. A more precise determination of the level feedings would enable for exclusive cross sections to be obtained, with together with a refined level scheme would constrain the possible spin values. To this purpose the improved resolution of a germanium array of 1%, against 9% for Dali-2, is particularly significant.

Since Seastar the intensity of the primary $^{238}$U beam has increased from 12 to 40 pnA, which compensates the lower $\gamma$ efficiency of 9% instead of 27% (after addback). We would retain the Minos liquid hydrogen target with its TPC for identifying proton knock-out on an incoming zinc beam. We expect we would need the same amount of beam time as was used for Seastar, that is 5.5 days.

The study of particle-hole states in $^{80}$Zn, such as the $g_{9/2}^{-1} d_{5/2}$ neutron multiplet that breaks the $N=50$ core, would inform us on the size of the eponymous shell gap. They would be accessed through neutron knock-out from a $^{81}$Zn beam, for which a different but nearby setting of the spectrometer should be chosen. The beam-time estimate for this measurement equals 3 days.

Speaker: S Franchoo (IPN Orsay)
• 11:15
Competition between core excited and single-particle proton and neutron states out of 78Ni studied via lifetime measurements. 15m

D. Bazzacco, S. Lenzi, S. Lunardi, D. Mengoni, A. Montaner-Piza’,
A. Goasduff, R. Menegazzo, G. Pasqualato, F. Recchia, D. Testov
Dipartimento di Fisica, Universita’ di Padova and INFN (Italy)

P. Doornenbal, K. Wimmer
RIKEN Nishina Center (Japan)

T. Bayram, D. Brugnara, L. Cortes, G. de Angelis,
A. Gottardo, E. Gregor, A. Illana, D.R. Napoli, I. Zanon, J.J. Valiente Dobon
INFN Laboratori Nazionali di Legnaro (Italy)

In the vicinity of $^{78}$Ni the quadrupole collectivity is expected to be large because the orbits close to the Fermi level allow the realization of a quasi-SU(3) symmetry, having orbits with Δj =2 Δl = 2 quasi-degenerated. As a consequence the quadrupole interaction produces a shape transition in which highly correlated many-particles -holes configurations gain binding energy and become as bound as the spherical states. These intruder deformed bands often appear at low excitation energy in the magic nuclei and also $^{78}$Ni is expected to show such features.
Potential energy surface calculated for $^{78}$Ni [1] shows a spherical minimum which is very flat. This interesting pattern is predicted also for the first 2$^+$ state, reflecting a particular fluctuation. This fluctuation is much narrower in the lighter Nickel isotopes such as $^{68}$Ni, where the E2 excitation from the ground state goes to very high $2^+$ states. The overlap probabilities with the closed shell are predicted to be 60%, 53%, and 75% for $^{56,68,78}$Ni, respectively in [1].
On the other hand, recent large scale shell model calculations [2] predict a ground state of doubly magic 65% character, but the first $2^+$ excited state at 2.88 MeV belongs to the (prolate) deformed band based in the intruder second 0$^+$ state at 2.65 MeV. The B(E2) for the decay to the g.s. is B(E2: $2_2^+$--> $0^+$) = 32 e2 fm4 These calculations give a second 2$^+$ of 1p-1h nature at 3.15 MeV, connected to the ground state with B(E2) = 110 e$^2$ fm$^4$.

The neighboring odd-even nuclei are expected to present a relatively simple structure, at least for the lowest lying states, and as consequence such nuclei constitute a unique test bench for the effective interactions and the nuclear degrees of freedom. However, as a consequence of the presence of intruder configurations in even-even nuclei, it is of fundamental importance to characterize the states of the odd-even nuclei in terms of their single particle character versus a core-excited character.

The nucleus $^{79}$Cu can be exploited to probe the proton excitations outside $^{78}$Ni, provided that the character of the states is understood. Calculations reported in [3] show that indeed the lowest lying states are predicted to have very different microscopic structure. The 5/2$^-$ g.s and the 3/2$^-$ first excited correspond mainly to a proton in the $f_{5/2}$ and $p_{3/2}$, respectively, while the first 1/2$^-$ state has a more mixed character, with an occupancy near 50% for both $p_{1/2}$ (single particle configuration) and $f_{5/2}$ (core coupled configuration). We propose here to study the structure of the excited states via lifetime measurement. Most of the states of interest are predicted to have a lifetime in the ps range that is accessible with plunger techniques.

It would be very interesting to study the nucleus $^{79}$Ni to probe the neutron excitations along the N=51, however this nucleus is not at reach. Instead $^{81}$Zn will be the most exotic N=51 isotope that will be accessible for in-beam experiments. This nucleus [4] is expected to present different states corresponding to a 1/2$^+$ states of dominating single particle $s_{1/2}$ character, two 5/2$^+$ states, one corresponds to a core-coupled configuration and the other having a neutron $d_{5/2}$ single-particle character. Lifetime measurement will be able to disentangle the character of such states.

[1] Y. Tsunoda et al. PRC 89, 031301(R) (2014)
[2] F. Nowacki et al. PRL 117, 272501 (2016)
[3] L. Olivier et al. PRL 119, 192501 (2017)
[4] C.M. Shand et al. PLB 773 492–497 (2017)

Speaker: Francesco Recchia (INFN LNL)
• 11:30
Detailed spectroscopy of 78Ni 15m

Detailed spectroscopy of 78Ni

Speaker: Pieter Doornenbal (RIKEN)
• 11:45
Lifetimes of the 4+ states in 72 and 74Ni 15m

The seniority scheme was introduced by Racah, initially to identify multiparticle configurations in the atomic spectrum, latter was extended for the atomic nucleus, where is useful to classify the jn states in the jj-coupling. The concept of seniority in bound to the pairing. The seniority is generally conserved up to a large extend and it is well known the conditions that need to satisfy the interaction to preserve it [1].
In the mid 90´s isomeric states in the nuclei with N = 50, 40 < Z < 50 were investigated [2]. The isomerism in this region is due to the occupation of the proton g9/2 orbital, forming seniority isomers in stretched gn9/2 configurations. This discovery was shortly followed by the discovery of another island of isomerism in the corresponding Z=28 valence mirror nuclei 69Ni and 70Ni, originating from neutron gn9/2 configurations and that allowed to deduce the (g9/2)2 effective interaction for the neutron-rich nickel isotopes [3].
Already these early results suggested that the (g9/2)2 effective interaction might not preserve the seniority with significant implications for the valence-mirror symmetry between the Z = 28 isotopes with A = 70 to 76 and N = 50 isotones with A = 92 to 98 [4]. This suggestion was reinforced by the experimental results on the -decay of Co isotopes towards the middle of the g9/2 shell (mass 72 and 74), where it has been suggested that the 8+ seniority isomer is not present [5] [6]. Nevertheless, the -decay of Co isotopes high spin isomer are expected to have I = 5-,6- and 7-, being the population of the 8+ isomeric state in the Ni isotopes an indirect process. Moreover, A.I.Morales in Ref. [6], points out the inconsistence of the level-scheme de-excitation branching ratios with an interaction inverting the seniority scheme (see also Ref. [7]).
In this LoI we propose the measurement of lifetimes of the 4+ states (possibly also the 6+ states in some case) in 70Ni, 72Ni and 74Ni, that will help to shed light on the seniority scheme validity in the (g9/2)n configurations.
The present results on the 72Ni 2+ lifetime corresponds to half the B(E2) compared with the neighbouring 70Ni and 74Ni and, therefore, a short measurement could help to shed light on the systematics of the 2+ B(E2) values in the isotopic chain [8]. The expected lifetimes are in the range of tens of ps, thus, the lifetime measurement can be perform with lineshape analysis or with plunger measurement (see for example Ref.[8]).
The production of the secondary beam will be done by the fission of a 238U primary beam at 345 MeV/u and with an intensity of about 40 pnA. The intensities of the corresponding secondary beams will be about 106 for 71Cu, 2x105 for 73Cu and 3x104 for 75Cu. The one proton knock-out reactions are expected to have cross sections of the order of 6 mb, and the population of the 4+ state in the final Ni isotopes is expected to be of the order of 35%. In our most unfavourable case (74Ni) we expect about 120 gamma-Ion coincidences in 1 hour.

[1] P. Van Isacker, Phys. Rev. Lett. 100 (2008) 052501
[2] R. Grzywacz et al., Phys. Lett. B 355 (1995) 439
[3] H. Grawe et al., Prog. Part. Nucl. Phys. 38 (1997) 15.
[4] A.F. Lisetskiy et al., Phys. Rev. C 70 (2004) 044314; Eur. Phys. J. A 25 (2005) s01.
[5] C. Mazzocchi et al., Physics Letters B 622 (2005) 45.
[6] A.I.Morales et al., Physics Letters B 781 (2018) 706.
[7] C. Qi, Physics Letters B 773 (2017) 616.
[8] K. Kolos et al., Phys. Rev. Lett. 116 (2016) 122502

Speaker: Jose Javier Valiente Dobon (LNL-INFN)
• 12:00
Coulomb excitation in neutron-rich isotopes around Ni 15m

A main focus of nuclear physics nowadays is the study of the evolution of shell structure in exotic isotopes. An interesting region of the nuclear chart to test shell evolution is around the Ni isotopic chain. For this chain, $B(E2;~0^+_{\mathrm{g.s.}}\rightarrow2^+_1)$ measurements have shown a behavior that differs, not only from the seniority scheme, but also from different theoretical models available, and represents a puzzling situation. In the same region, the study of transition probabilities of low-lying states on odd-$A$ Cu isotopes has produced interesting results on the structure of these states and its collective or single-particle nature. For the odd-mass Co isotopes, which corresponds to the hole-core configuration, very little information on the energy or transition probabilities of the excited states is known. For the Fe isotopes, $B(E2;~0^+_{\mathrm{g.s.}}\rightarrow2^+_1)$ values have been measured up to $^{68}$Fe, and an effective lifetime measurement of the $2^{+}_1$ state of $^{70}$Fe was recently reported, showing the deformation of these isotopes and outlining the evolution of collectivity between $N=40$ and $N=50$.

A proposal to measure the $B(E2;~0^+_{\mathrm{g.s.}}\rightarrow2^+_1)$ values of $^{70,72,74,76}$Ni was presented at the last NP-PAC of the RIBF, which graded it as deferred. We aim to re-propose the cases of $^{70,72,74}$Ni using the HR-Ge array at RIBF, while the measurement of $^{74,76}$Ni are to be performed using DALI2$^+$. The experiment considers a $^{238}$U beam at 345 MeV/u impinging on a Be target to produce the isotopes of interest, which are then separated using the BigRIPS spectrometer. Three magnetic settings are considered to populate the isotopes of interest. A secondary Au target is used to induce Coulomb excitation and the HR-array of RIBF will be used to detect the $\gamma$-rays emitted after the reaction. Outgoing particles are identified with the ZeroDegree spectrometer. Thanks to the HR-Ge array, within the same experimental settings, further information can be extracted from the analysis of the odd$-A$ Cu and Co isotopes, as well as on odd-even and even-even Fe isotopes.

Speaker: Martha Liliana Cortes (RIKEN Nishina Center)
• 12:15
Coulomb Excitation of $^{70}$Fe 15m

One of the most compelling regions of the nuclear chart within reach of current experimental facilities is that of the N=40 isotopes near $^{64}$Cr. Due to the strong effect of the pn interaction between the protons and neutrons at the Fermi surface, the removal of $f_{7/2}$ protons below Ni effectively alters the $f_{5/2}$ neutron spin-orbit partner energy relative to the $g_{9/2}$ and $d_{5/2}$ and narrows the $N$=40 gap, resulting in deformation as strong quadrupole and pairing correlations favor promotion of neutron pairs across the gap. However, while the mechanism driving deformation is well understood, it is experimentally critical to map the deformation in this region as one moves to and across $N$=40 toward $N$=50. Coulomb excitation allows a direct measure of deformation and collectivity, and has been performed in $^{66,68}$Fe and $^{64}$Cr – the high-resolution array at RIBF will allow extension of these measurements to $^{70}$Fe, adding a critical point to the systematics beyond N=40.

Speaker: Heather Crawford (Lawrence Berkeley National Laboratory)
• 12:30 14:00
Lunch 1h 30m
• 14:00 15:45
Proposals: Physics III
• 14:00
Neutron single-particle structure above N=50 towards 78Ni 15m

We propose to perform the spectroscopy of N=51 isotones of 83Ge and 81Zn. The goal is to use the selectivity of neutron knockout from 84Ge and 82Zn to characterize the evolution of the ν(s1/2-d5/2) energy splitting and to identify for the first time (2p-1h) intruder state (ν(g9/2)-1(sd)+2 ) possible signature of shape coexistence above N=50 close to 78Ni. Such a study is uniquely possible at the RIBF due to the exoticity of the beams involved and high-resolution gamma spectroscopy is crucial due to the proximity in energy of the populated states in odd-even products

Speaker: Freddy Flavigny (IPN Orsay)
• 14:15
E2 Coulex measurements in $^{79,81}$Zn to understand nuclear intruder states and shell evolution 15m

E2 Coulex measurements in $^{79,81}$Zn to understand nuclear intruder states and shell evolution

Introduction
Recent work [Delafosse] has pointed out that the physics around the N=50 shell closure close to $^{78}$Ni may be driven by the effects of ρ-meson exchange potential. On the one hand, this is causing a reduction of the N=50 gap going towards Z=32, inducing a sudden increase of across-shell quadrupole coherence in $^{84}$Ge. On the other hand, this short-range interaction could be the responsible of the observed lowering in energy of the $\nu s_{1/2}$ shell above N=50, which is separated from the ground-state $\nu d_{5/2}$ by only 200 keV in $^{83}$Ge. Indeed, by looking at the $\nu s_{1/2}$ energy trend in N=51 isotones, one may be tempted to hypothesize that the $\nu s_{1/2}$ shell becomes ground state already in $^{81}$Zn. A $^{81}$Zn beta-decay measurement found a very small feeding to the $5/2^-$ $^{81}$Ga ground state, further hinting at a possible $1/2^+$ spin-parity assignment for $^{81}$Zn ground state [Paziy]. Recent theoretical calculations made for $^{78}$Ni do predict degenerate $\nu d_{5/2}$ and $\nu s_{1/2}$ shells at Z=38 [Taniuchi].

Coulex measurement to detect the $^{81}$Zn ground state
If the $5/2^+$ states and $1/2^+$ states are very close in energy in $^{81}$Zn (<100 keV), it may be very difficult to identify which is the ground state, even with particle transfer gated by $\gamma$ rays. Here we propose to use Coulomb excitation at relativistic energies with high-resolution $\gamma$-ray spectroscopy to investigate the matter. If 81Zn has a “normal” $5/2^+$ ground state, one would expect to observe a multiplet of five Coulomb-excited states, from $1/2^+$ to $9/2^+$, which will be the coupling of the $^{80}$Zn $2^+$ state to the unpaired neutron in the $\nu d_{5/2}$ shell. On the contrary, if $^{81}$Zn has a $1/2^+$ ground state, the multiplet will be composed of only two states. The energies of all these states will be within few hundred keV of the $2^+$ of $^{80}$Zn (1456 keV), and they will share the B(E2) of $^{80}$Zn, in a weak-coupling approximation. However, the measured Coulex cross sections will be enhanced for the case of $1/2^+$ ground state, due to the spin factor. Shell-model calculations with the JUN45 interactions give the B(E2) distribution among the states of $^{81}$Zn reported in Tab. 1, and the corresponding Coulex cross sections have also been calculated with Dweiko.

In conclusion, the proposed measurement will allow one to probe the spin of the $^{81}$Zn ground state by measuring the multiplet populated by the Coulex reaction.

Experimental considerations:
We consider the use of a $^{81}$Zn beam of 200 pps, and a $^{208}$Pb target of 1 g/cm$^2$. We consider a $\gamma$-ray detection efficiency of 5% (no addback and high $\gamma$-ray energies). Using the cross section quoted above, this would imply the number of counts reported it Tab. 1 in each peak in a 5-day measurement. A 15% error on the measured B(E2) is foreseen on the biggest peaks, while 20-25% error for the smallest ones.

Coulex of isomeric intruder states in $^{79}$Zn
Shape coexistence in the $^{78}$Ni region has been reported in Refs. [Gottardo, Yang] in $^{80}$Ge and $^{79}$Zn. The shape-coexisting $1/2^+$ state in $^{79}$Zn has also the peculiarity of being a long-lived isomer (seconds). The wave function of this isomeric level has been investigated through g-factor measurement with LASER spectroscopy. It is an N=50 intruder level with a $(\nu g_{9/2}){-1}-(\nu s_{1/2})^1$ configuration. Contamination of the wave function from the other known intruder $(\nu g_{9/2})^{-1}-(\nu d_{5/2})^1$ state is possible, though limited [Yang]. The same state was previously observed in a ${78}$Zn(d,p)$^{79}$Zn transfer measurement at ISOLDE [Orlandi], and indeed the transferred angular momentum was compatible with an s wave. The measurement in Ref. [Yang] not only determines the intruder $(\nu g_{9/2}){-1}-(\nu s_{1/2})^1$ nature of the state, but also shows a large isomer shift of the $1/2^+$ isomer with respect to the $9/2^+$ ground state. In their paper, Yang et al. suggest this is due to a large quadrupole deformation with $\beta$=0.22, against the $\beta$≈0.14 value of the ground state [Yang]. This result was obtained by assuming an axial deformation and that the mean radius of the wave function of this intruder state follows the standard 1.2·A$^{1/3}$ fm rule (5.1 fm for $^{79}$Zn). However, since the g factor clearly points out a major $\nu s_{1/2}$ component in the wave function, the <$r^2$> of the $1/2^+$ isomer may actually be 10-15% larger [Bonnard2], justifying the measured isomeric shift even with an almost spherical shape.
In [Bonnard1, Bonnard2] it is shown how the s-wave function radius may be even larger than what was previously thought, having implication in shell formation. Indeed, the rapid decrease of the $\nu s_{1/2}$ energy in N=51 isotones leading to a possible $1/2^+$ ground state in $^{79}$Ni, has been attributed to the coupling to the continuum of the $\nu s_{1/2}$ shell and to an increase in nuclear diffusivity [Hagen, Nowacki, Delafosse].
The understanding of how the $\nu s_{1/2}$ determine shape coexistence can thus help to infer important information on the neutron shell structure at N=50. More specifically, $^{79}$Zn may represent a very particular case of coexistence between the slightly deformed ground state shape and a similarly spherical intruder shape but with a large radius.
The proposed Coulomb excitation measurement will help to understand the nature of the shape coexistence of the intruder $1/2^+$ isomer: a (almost) spherical larger $\nu s_{1/2}$ intruder state as suggested by recent calculations on s shell in this region, or a more deformed configuration (likely involving at least some $\nu d_{5/2}$ components) as suggested in Ref. [Yang]. The measured energies and transitional quadrupole moments will help, also in comparison with theoretical calculations, to verify if the large measured isomer shift is due to a large radius or to a large deformation. The levels populated by the Coulex reaction will already be indicative of the wave function we want to probe. Another important observable we will get from coulomb excitation are B(E2) values from $3/2^+$ and $5/2^+$ states coming from the coupling of the intruder states with the $2^+$ of the $^{78}$Zn core. The B(E2) value of the $2^+$ of the $^{78}$Zn core is 8 Wu↓. In the case of an almost spherical, s-wave state, we will observe slightly larger (5-10%) reduced E2 matrix elements in the band built on the intruder-state, due to the isomer shift of the $1/2^+$ state (since B(E2) ∝⟨$r^2$ ⟩$^2$). On the contrary, if the deformation is the large one claimed in Ref. [Yang], $\beta$=0.22, B(E2) values will be much larger, several times the 8 Wu↓ value of the $^{78}$Zn core. The combination of the energy and B(E2) measurements with the coulex reaction will thus provide an unambiguous characterization of the shape coexistence in this region, also helping to understand the role played by the $\nu s_{1/2}$ in the N=50 gap stability and size going towards $^{78}$Ni.

Experimental details for Coulex of $^{79}$Zn
We consider the use of a $^{79}$Zn beam of 5*10$^3$ pps, and a $^{208}$Pb target of 1 g/cm$^2$. We consider a $\gamma$-ray detection efficiency of 5% (no addback and possibly high $\gamma$-ray energies). The isomeric ratio for the $1/2^+$ isomer is known from a previous decay study at EURICA [Delattre]. Using an isomeric ratio of 10% and the cross section deduced from the B(E2) values quoted above, 100 mbarn to be conservative, this would imply 1200 counts in $\gamma$-ray peaks in a 2-day measurement. A 15% error on the measured B(E2) is foreseen.

Bibliography
[Bonnard1] J. Bonnard, A. Zucker, S. Lenzi, Phys. Rev. Lett. 116, 212501 (2016)
[Bonnard2] J. Bonnard and A. Zucker, arXiv:1606.03345 (2016)
[Gottardo] A. Gottardo et al., Phys. Rev. Lett. 116, 182501 (2016)
[Yang] X. F. Yang et al., Phys. Rev. Lett. 116, 182502 (2016)
[Hagen] G. Hagen et al.,Phys. Rev. Lett. 117, 172501 (2016)
[Delafosse] C. Delafosse et al., Phys. Rev. Lett. 121, 192502 (2018)
[Delattre] M.C. Delattre, PhD thesis IPN Orsay (2016)
[Nowacki] F. Nowacki et al., Phys. Rev. Lett. 117, 272501 (2016)
[Taniuchi] Taniuchi et al., to be published in Nature (2019)

Speaker: Andrea Gottardo (LNL - INFN)
• 14:30
Secondary knockout reactions and lifetime measurement with the plunger technique 15m

Secondary knockout reactions and lifetime measurement with the plunger technique

Recent work [Delafosse] has pointed out that the physics around the N=50 shell closure close to $^{78}$Ni may be driven by the effects of $\rho$-meson exchange potential. This potential causes a reduction of the N=50 gap going towards Z=32, inducing a sudden increase of across-shell quadrupole coherence in $^{84}$Ge. Indeed, collectivity in N=52 isotones has been shown to point out a shape change from $^{86}$Se to $^{84}$Ge, from soft-triaxial to frankly prolate [Delafosse]. The large error bars on lifetime measurements available in $^{84}$Ge, and the absence of measurements in $^{82}$Zn, prevent to draw firm conclusions, in particular concerning the occurrence of intruder configurations already in the ground state of $^{84}$Ge, anticipating the occurrence [Delafosse].

Experimental considerations:
We consider the use of a 510$^4$ pps $^{85}$As beam, and a $^9$Be target of 1 g/cm$^2$. We consider a gamma-ray detection efficiency of 10%. Using a cross section of 1 mbarn for the states of interest in ${84}$Ge, one would have 120 counts per hours. Considering the need measuring five distances, with about 100 counts per peak, one would need about 12 hours of measurement. For the same measurement in $^{82}$Zn, the $^{83}$Ga secondary beam is only 210$^3$ pps. As a result, one would need 12 days of measurement. Restricting to only three distances, one could measure the lifetime of the $2^+$, $4^+$ states in 5-6 days.

Bibliography
[Delafosse] C. Delafosse et al., Phys. Rev. Lett. 121, 192502 (2018)

Speaker: Andrea Gottardo (LNL - INFN)
• 14:45
Onset of collectivity beyond N=50 studied in zinc isotopes 15m

Neutron-rich zinc isotopes will be populated in (p,2p) on the MINOS liquid hydrogen target.

Recently the 2+ and 4+ levels of N=52,54 82,4Zn nuclei were measured at RIKEN within the SEASTAR programme [1]. The energies of the newly observed levels suggest the onset of deformation towards heavier Zn isotopes, which has been incorporated by taking into account the upper sdg orbitals in the Ni78-II and the PFSDG-U models. The measured E(4+)/E(2+) ratio is close to 2, characteristic for vibrational nuclei (see figure 1). Here we propose to quantify the collectivity in zinc nuclei beyond the N=50 shell closure, and to obtain direct information on the relevant single neutron orbitals.

The study of the odd-mass 85,83Zn is expected to provide information on the neutron orbitals above N=50. Most importantly it will determine the relative position of the d5/2, d3/2 and s1/2 orbitals. Prediction of two different shell model calculations [2] are shown for 83Zn (N=53) in figure 2. (MCSM has only d5/2 above N=50, so no low lying ½). Systematics of N=51 nuclei suggest that the ½+ state would be very close to the 5/2+, and might become even ground-state. The neutron orbitals beyond N=50 could be studied more directly in 79Ni, populated from 80Cu(p,2p). However, the single-neutron states will be not populated directly, but via higher-lying states [3].

The lifetimes of excited states in 82,84Zn will be determined by line-shape analysis [4]. The expected lifetimes of the 2+ states are around 50 ps, while those of 4+ around 15 ps, therefore, these can be measured from the gamma-ray lineshapes, and the collectivity quantified. We will attempt to get information on excited states also on 86Zn. This has 6 neutrons above N-50, which would fill the d5/2 orbitals. Excited states need neutron excited to the higher lying d3/2 or s1/2 orbitals. The measured level scheme will provide information on this higher lying orbital.

The proposed experiment is feasible, as the 81,82,83,84Zn studied were previously populated with the SEASTAR campaign [1,2]. For example, 84Zn was populated by 85Ga(p,2p). The rate of 85Ga was 7 ion/s (with 30 pnA 238U primary beam on a 3mm Be target). In 24 hours, about 40 counts in the 2+->0+ transition of 84Zn were detected using DALI2. The better energy resolution of MINIBALL++ will compensate for its lower efficiency,and will allow lifetime determination.

Fig.1. Systematics of E(2+), E(4+) (top) and R4/2=E(4+)/E(2+) (bottom) for the Zn isotopic chain, compared with theoretical values. Taken form [1].

Fig.2. Two different shell model calculations compared with the tentative experimental level scheme for 83Zn [2].

[1] C.M. Shand et al., Phys. Lett. B 773, 492 (2017).
[2] C.M. Shand, PhD thesis, University of Surrey, UK (2017)
[4] P. Doornenbal et al., Nuclear Instruments and Methods in Physics Research A 613, 218 (2010)
[3] L. Olivier et al., Phys. Rev. let. 119, 192501 (2017)

Speaker: Magdalena Gorska (GSI)
• 15:00
Structural investigation at the boundary of gamma-ray spectroscopy: investigation of extremely neutron rich $^{84,86,88}$Ge 15m

Neutron rich Ge isotopes represent a puzzle so far as the few existing data
yield a contradicting picture. A recently determined large $B(E2;2^+ \to 0^+)$
value with a large uncertainty in $^{84}$Ge hints for a completely unexpected
shape transition from soft triaxiality in neutron rich Se isotopes with
Z=34 to prolate deformation for neighboring Ge with Z=32 [1]. Such an
"island of inversion" is not expected. The shell model only predicts
such for much lighter systems (Z<28) [1]. On the other hand, recent
investigations of the level schemes of $^{84,86,88}$Ge hint for a new region of
rigid triaxiality similar to $^{76}$Ge [2]. However, these interpretations
suffer heavily from the lack of knowledge on transition strengths between
the lowest states. Therefore, we aim to determine transition strengths
between the lowest states from level lifetimes in the picosecond range
in extremely neutron-rich $^{84,86,88}$Ge. These observables will be determined
at the RIBF facility at RIKEN with the recoil distance Doppler-shift method
after proton knockout using fast (v/c~60%) secondary beams of $^{85,87,89}$As,
a dedicated Cologne plunger device and a segmented highly efficient gamma-ray
spectrometer coupled to the zero degree spectrometer for recoil
identification. It should be stressed that $^{88}$Ge is the most neutron
rich Ge isotope where excited states are known so far at all.

[1] C. Delafosse et al., Phys. Rev. Lett. 121, 192502 (2018)
[2] M. Lettmann et al., Phys. Rev. C 96, 011301(R) (2017)

Speaker: Christoph Fransen (Institute for Nuclear Physics, University of Cologne, Germany)
• 15:15
Intermediate-energy Coulomb-excitation of neutron-rich isotopes at Z<38, N>50 15m

The evolution of collectivity, mirrored in B(E2) excitation strength, along the N=52 isotonic line is of special interest toward the Z=28 shell closure. The degree of collectivity depends on the possibility of cross-shell excitations, and on the evolution of shell structure in general. The energies of 21+ states in the N=52 isotones minimize at Z=32 (Ge), only four protons above the Z=28 magic proton shell. This has previously been attributed to a continuous weakening of the N=50 closure when moving to lower-Z isotones [1], where also the effect of the down-sloping 3s1/2 neutron orbital was highlighted, opening the possibility of a N=58 sub-shell at low Z. B(E2) values are available and show a rise down to 84Ge (Z=32), however, the recent value for 84Ge [2] has large uncertainties, not allowing for a conclusive structural interpretation. Shell model predictions are supportive of enhanced collectivity in the first 2+ state of 84Ge, but the calculated B(E2) value would not be significantly larger than the known value in 86Se. The continuation of the trend toward doubly-magic 78Ni, i.e., in 82Zn, is lacking to date. In order to shed light on the onset of collectivity in this region, in which also triaxial features (e.g., in 84Ge and 86Ge) have been suggested [3,4], B(E2) measurements in intermediate-energy Coulomb-excitation are proposed. In two settings of the BigRIPS separator, the important B(E2) values of 84Ge and 82Zn could be obtained simultaneously, as well as B(E2) values of 86Ge and 88Se, probing the evolution toward N=56. With the availability of a high-resolution γ array, not only the B(E2) strength of the first-excited, but also those of the second-excited 2+ states could be obtained. In addition, from the recent proton-knockout work [4,7] candidates for octupole-excited states are known in the Ge isotopes, which would be accessible in through Coulomb excitation. This would result in another key observable for the γ-degree of freedom, besides the energy ratio R2/2=E(22+)/E(21+), namely the ratio B2/2 = B(E2;22+→01+) /B(E2;21+→01+) [5,6]. The evolution of B(E2) strengths could even be followed up to N=60 in the Se isotopes, cutting deeper into the shell and covering the region of transition to deformed nuclei.

[1] J.A. Winger et al., Phys. Rev. C 81, 044303 (2010).
[2] C. Delafosse et al., Phys. Rev. Lett. 121, 192502 (2018).
[3] M. Lebois et al., Phys. Rev. C 80, 044308 (2009).
[4] M. Lettmann et al., Phys. Rev. C 96, 011301 (2017).
[5] A.S. Davydov and A.A. Chaban, Nucl. Phys. 20, 499 (1960).
[6] T.R. Saito et al., Phys. Lett. B 669, 19 (2008).
[7] M. Lettmann, doctoral thesis, TU Darmstadt (2018).

• 15:30
Approaching the Fifth Island of Inversion from the north 15m

Islands of inversion (IoIs) appear close to semi-magic or even doubly magic nuclei and are certain areas of the nuclear chart where the nuclei expected to be spherical in their ground states become deformed. They are interpreted as strong nuclear quadrupole-quadrupole interactions producing a shape transition with highly correlated many-particles-many-holes configurations. When reducing the proton number along the N=50 chain towards and beyond the 78Ni doubly-magic nuclei, a competition between the single-particle and collective characteristics with the isotones is expected, thus forming the Fifth IoI of the nuclear chart [1]. To date, experimental approaches have reached the western and northern ends of this region. In the former study, the constantly low 2+ excitation energies of 66Cr and 70,72Fe, measured by means of the in-beam gamma-ray spectroscopy technique, revealed the possibility of the extension of the island of inversion at N=40 towards N=50 [2]. For the latter, investigating the doubly magic nature of 78Ni implied a possible shape co-existence composed of competing spherical ground-state and deformed bands. This study reinforced the hypothesis of the collapse of the N=50 (, and even Z=28,) shell gap(s) beyond the anchor point, 78Ni.

In this presentation, I would like to discuss the physics case and the feasibility of an experimental study on the lifetime measurement of the excited states in 80Zn to investigate indications of disruption of the N=50 shell-closure by approaching from the northern side of the proposed Fifth IoI. While the first measurement of the first excited states by Coulomb excitation [4] indicated a high excitation energy and a low transition probability of 2_1+ state, which supports the doubly magic nature towards 78Ni, a more recent study performed at the RIBF facility illustrates rather complicated levels in the isotope. By reflecting the discussion of the measurement of 78Ni [3], such entangled states in 80Zn can be considered as the competition between the persisting spherical magic ground-state band and the descending intruder deformed band. In the case of 80Zn, the level at 2627 keV, feeding directly to the ground state [5] may be equivalent to the one at 2910 keV in 78Ni, which is assigned as a (2_2+) deformed state decaying into the ground state directly [3].

More specifically, the lifetime of the 2_2+ level is assumed to be an order of 10 ps because of the low transition probability B(E2) between the spherical ground state and the deformed excited state. The (p,2p) and (p,3p) reactions are better to be compared as well since the different population of the states was observed in the case for 78Ni. To achieve the lifetime measurement, the thickness of the target to be optimized, which will be discussed in the presentation as well.

[1] F. Nowacki et al., PRL 117, 272501 (2016).
[2] C. Santamaria et al., PRL 115, 192501 (2015).
[3] R. Taniuchi et al., Nature in press. / R. Taniuchi, PhD thesis, The University of Tokyo.
[4] J. Van de Walle et al., PRL 99, 142501 (2007).
[5] Y. Shiga et al., PRC 93, 024320 (2016).

Speaker: Dr Ryo Taniuchi
• 15:45 16:15
Coffee 30m
• 16:15 18:00
Proposals: Physics IV
Convener: Peter Reiter
• 16:15
Spectroscopic Factors in the Neutron-Rich Ca Isotopes 15m

The neutron-rich calcium isotopes have been a focus both experimentally and theoretically, as a key isotopic chain with clear examples of evolving shell structure and a test-bed for microscopically-based interactions and large-space $ab-initio$ calculations. While spectroscopy has extended quite far along the Ca isotopes, out to $^{56}$Ca at least, the evolution of single-particle occupancies has only been explored in detail to $^{50}$Ca through direct nucleon removal reactions. This can now be extended to $^{52,54}$Ca with the high-resolution array at RIBF, to explore both proton and neutron occupancies for the most neutron-rich Ca. We would like to propose spectroscopic factor measurements for both neutron and proton knockout on a solid Be target from $^{51,52,53}$Ca. This will allow us to explore the potential breaking of the $Z$=20 proton core, indicated by the change in charge radii in $^{50,52}$Ca relative to $^{48}$Ca, as well as the changing occupancies in the neutron sector as a function of isospin.

Speaker: Heather Crawford (Lawrence Berkeley National Laboratory)
• 16:30
Single-particle and collective structure of neutron-rich N=40 nuclei 15m

The increase in collectivity in neutron-rich isotopes around N=40 has been attributed to the enhanced occupation of neutron intruder orbitals from above N=40 [1]. At the center of this island of inversion lies 64Cr. We propose to measure neutron and proton knockout from 64Cr to quantify the neutron g9/2 and d5/2 intruder orbital occupations. The proton knockout will yield the relative location of the f and p proton states which drive the evolution of collectivity in this region. Spectroscopic factors will be compared to state-of-the art shell model calculations.
In parallel, lifetimes of excited states will be measured using the line-shape method. The same experimental technique has been applied already to 66Fe at NSCL employing GRETINA [2]. The extracted transition strength will shed light on the evolution of collectivity at N=40.
The expected level density in 63Cr and 63V is high requiring the use of a high resolution gamma-ray spectrometer for the in-beam spectroscopy. Few states are known in 63Cr from beta decay [3], no excited state is established in 63V. A recent decay experiment at RIBF found no isomeric states in either nuclei [4] thus isomer tagging is not required for the experiment.

[1] S. M. Lenzi et al., Phys. Rev. C 82 (2010) 054301
[2] K. Wimmer et al., to be published.
[3] S. Suchyta et al., Phys. Rev. C 89 (2014) 034317.
[4] K. Wimmer et al., Phys. Lett. B 792 (2019) 16.

Speaker: Kathrin Wimmer (The University of Tokyo)
• 16:45
Spectroscopy of $^{63}$V and Spectroscopic Factors in the N=40 Island of Inversion 15m

We propose proton knockout from $^{64}$Cr to $^{63}$V. For a deformed ground state (as may be expected) the odd proton can be removed from low-lying Nilsson states for example the $\Omega = 3/2$ levels originating from the $d_{3/2}$ or $f_{7/2}$ shell model states. The spectroscopy will provide an important measure of the relative energies of the proton states, which is sensitive to the deformation, spherical gap, and pairing. Indeed the odd system can often offer more insight into the underlying structure than the even core. The spectroscopic factors (overlap between $^{64}$Cr and $^{63}$V) provide a measure of the occupancies and a test of the model predicted wavefunctions, either Nilsson or shell-model derived. This allows a comparison between these differing starting points.

Speaker: Paul Fallon (Lawrence Berkeley Laboratory)
• 17:00
Single-particle structure of 55Ti and 57Ti 15m

Beyond the new magic number N = 34, the region around the neutron-rich Ca isotopes, continues to attract a lot of attention. Recently, the energy of the first 2+ state of 56Ca was measured to have an unexpectedly high value [ChePC]. The excitation energies of the 2+ states of the N = 34 nuclei 56Ti and 52Ar indicate that the shell closure occurs only at and below Ca [Liu]. To clarify the evolution of the single-particle structure of N = 34 and N = 36 nuclei, we propose to perform the spectroscopy of 55Ti and 57Ti via one-neutron knockout reactions.
Parallel momentum distributions will allow us to determine the spin-parity of the populated states. This will clarify the ordering between f5/2 and p1/2 orbitals, which is pointed out by [Ste13], and investigate the transition into the island of inversion at N = 40.

[ChePC] S. Chen, et al., private communication.
[Liu19] H. Liu, et al., Phys. Rev. Lett. 122, 072502 (2019).
[Ste13] D. Steppenbeck, et al., Nature 502, 207-210 (2013).

Speaker: Takuma Koiwai (University of Tokyo)
• 17:15
Experiments with the High Resolution gamma-ray Array at RIBF 15m

Understanding the nuclear structure and dynamics in terms of the underlying fundamental interactions between protons and neutrons is one of the overarching goals of the nuclear science community. To this end, nuclear theory is currently developing nuclear interactions and Hamiltonians derived from chiral Effective Field Theory (EFT) employing 2- (NN) and 3-body (3N) forces. This new approach provides an exciting link to the theory of the strong interaction, Quantum Chromodynamics, and hence a unique opportunity to understand the nuclear structure and its evolution from first principles.

The neutron-rich C and O isotopes lie at the limit of accessibility of many ab initio techniques, while first calculations indicate a significant sensitivity of their electromagnetic (EM) structure to the underlying nuclear interaction and in particular to the inclusion of 3N forces. Therefore, experimental studies of these nuclei provide a critical testing ground for newly-developed chiral interactions, as well as new many-body techniques. I will propose to:
• re-measure the lifetime of the 2+ state of 20C with increased precision and accuracy,
• perform a Coulomb excitation of 22O (this potentially will benefit from the use of DALI instead).

Ab initio theory is working towards extending its calculations of transition rates to heavier medium mass and open-shell nuclei. These calculations are under development by e.g. applying the in medium similarity renormalization group (IM-SRG) for valence-shell Hamiltonians and extending it to new multi-reference formulations. I will propose experiments in two regions of the nuclear chart that will play a critical role in testing the newly developed ab initio calculations, the so-called “Island of Inversion” at N=20 and the neutron-rich Ca isotopic chain. In particular, I will propose to:
• study the transitional nucleus 30Na using neutron knockout reactions and study in detail its excitation spectrum, from lifetimes to gamma-ray polarization measurements,
• extract lifetimes of excited states in neutron-rich Ca isotopes (both odd and even), namely 48-54Ca using nucleon-removal reactions to study at the same time their single-particle structure.

Speaker: Marina Petri
• 17:30
High resolution spectroscopy beyond 132Sn 15m

We will are considering to propose experiments to clarify the anomalous structures in the neutron rich nuclei around and beyond 132Sn in continuation of our previous study.
One of the interests is the decoupling between proton and neutron motion in the low-lying excited states of the nuclei around 132Sn. The other is the evolution of collectivity along Z~50 towards N~90. We are considering more comprehensive study including higher excited states and odd-mass nuclei by using high resolution apparatus.

Speaker: Nori AOI (RCNP, Osaka Univ.)
• 17:45
Breaking of the isospin symmetry in the A=71 mirror nuclei: Knock-out reactions in 71Kr-71Br nuclei (and neighbours) using Miniball at Riken. 15m

Isospin formalism, which describes the neutron and the proton as two states of the same particle, the nucleon, is amongst the essential descriptive tools of a broad range of nuclear phenomena. The success of the isospin symmetry concept belies its broken nature. Not only is the symmetry broken by the proton-neutron mass difference and the Coulomb interaction, but also by the nucleon-nucleon interaction itself. The investigation of isospin symmetry conservation and breaking effects has revealed a wealth of nuclear structure information. The so-called mirror energy differences (MED) are defined by the differences in the excitation energies of analogue states and are regarded as a measure of isospin symmetry breaking in an effective interaction that includes the Coulomb force. The MED have been extensively studied for pairs of mirror nuclei in the upper sd- and the lower fp-shell regions. In both cases a remarkable agreement between experimental data and shell model calculations has been achieved allowing a clear identification of the origin of the MED based on the Coulomb force and on an isospin non-conserving term of the nuclear interaction, which turns out to be essential for a quantitative description of the data [1, 2, 3]. These studies allow one to extract several nuclear structure properties, such as the nature of particle angular- momentum re-coupling along yrast structures (e.g.backbending), the evolution of the nuclear radius or deformation and the identification of pure single-particle configurations.

An important aspect that can strongly influence the MED in the upper fp shell concerns the possible breaking of the isospin symmetry due to shape coexistence, which refers to the ability of some atomic nuclei to assume competing mean-field shapes at low excitation energies. A variety of nuclear shapes is expected around $A\sim70$ due to the complexity of the Nilsson diagram which shows pronounced subshell gaps at nucleon numbers 34 and 36 (oblate), 34 and 38 (prolate), and 40 (spherical). Several studies have shown that the even-even neutron-deficient Kr isotopes represent one of the unique regions where the ground-state deformation appears to change from prolate to oblate shapes as the self-conjugate nucleus $^{72}$Kr is approached. The occurrence of the oblate ground state in $^{72}$Kr is of particular interest in relation to the well-known open question about the pre-dominance of prolate deformation in well-deformed nuclei [4]. Recent results [5] on the structure of $^{72}$Kr obtained at NSCL with the Gretina array seems to confirm this picture. Further studies of the ground state shape have also been made recently with the TAS technique [6]. $^{72}$Kr appears therefore to be a transitional nucleus in which oblate shapes are predicted to dominate at low energy, though coexisting with prolate ones. This is supported by shell model calculations, presented recently by Zuker and co-workers [7].

The presence of deformed shell gaps at oblate and prolate deformation may give rise to opposite shapes even in mirror nuclei. An anomalous (negative) behavior of the Coulomb Energy Differences (CED) between 70Br and 70Se seems to indicate the presence of different deformation mixing for analogue states [8]. On the other hand shell model calculations predict both 70Br and 70Se to have the same shape and suggest that the origin of the unexpected trend of the CED is in the spin-orbit contribution [9]. To pin down such possible breaking of the isospin symmetry and to disentangle the different contributions we plan to investigate the Mirror Energy Differences (MED) in the mirror nuclei $^{71}$Kr−$^{71}$Br populated from $^{72}$Kr using knock-out reactions. Shape coexistence has been reported in $^{72}$Kr. Experimentally the level scheme is well known and it is characterized by the presence of a Jπ = 9/2+ isomeric state at 759 keV with $T_{1/2}= 33$ ns. Related studies can be found in refs. [10, 11, 12, 13]. The latest results from Fischer et al. [13] suggest the presence of eight distinct Nilsson bandheads below ~ 1 MeV, corresponding to different deformation, from oblate to prolate. In this work the level scheme of $^{71}$Br has been extended up to $J\sim 41/2$.

Regarding $^{71}$Kr, the only information available comes from β decay studies. The spin assignment to the g.s. was made first by Arrison [10] and Oinonen [11] based on mirror symmetry. Urkedal and Hamamoto pointed out in [12] that this could be the first case of mirror nuclei where the isospin symmetry is broken in the g.s.. This was deduced from Nilsson model calculations and a change in the spins of the lowest states was proposed. Finally Fischer et al. [13] changed back the spin of the first excited state in $^{71}$Br following observations of the angular distribution of the 397 keV transition and so the g.s. spin of $^{71}$Kr was proposed to be $5/2^-$.

Excited levels in $^{71}$Kr have been investigated over many years using different techniques and setups. Three attempts used the $^{40}$Ca + $^{40}$Ca reaction at 160 or 180 MeV. There were experiments at LNL with GASP + ISIS and EUROBALL + ISIS and at ORNL using the recoil separator plus HYBALL (CP array) and the CLARION gamma-ray array. It was possible to observe the $^{71}$Br lines from the 2α1p channel but there was no convincing evidence for the observation of excited states in $^{71}$Kr. Those experiments performed with coupled gamma-particle spectrometers (GASP + ISIS and EUROBALL + ISIS) suffered heavily from the poor channel selectivity as a consequence of the lack of tagging of the neutrons evaporated in the reaction. The experiment at ORNL reported a limited resolution in Z in the ion chamber of the recoil separator as a consequence of the low velocity of the residual nuclei. This poor channel resolution in Z translated into contaminated spectra where background subtraction had to be used extensively in order to highlight the transitions of interest. This challenging procedure compromises the identification of the most exotic nuclei.
Similar limitations also affected an experiment performed using Gammasphere coupled to the FMA recoil detector, that has a limited channel selection for recoils of such large mass. In this measurement the $^{36}$Ar + $^{40}$Ca reaction at 103 or 105 MeV was used.

Recently the $^{71}$Kr isomeric decay has been studied at RIKEN. Assuming, based on the mirror symmetry, the existence of a longer living isomer, the $^{71}$Kr ions produced by fragmentation after mass selection were implanted at the focal plane of the zero degree spectrometer. No gamma rays could be clearly identified as belonging to the $^{71}$Kr decay indicating that the expected isomer, if it exists, has to be significantly shorter than the flight-time in the spectrometer.The RIKEN RIB facility, combining the largest production of unstable isotopes with the highest selectivity of the reaction products, should allow us to clarify this longstanding issue. We propose to study excited states in $^{71}$Kr exploiting the unique channel selectivity provided by the BigRIPS magnetic spectrometer allowing full identification of the incoming and outcoming particles. A $^{72}$Kr secondary beam will be used to populate, through neutron and proton knock-out, the $^{71}$Kr-$^{71}$Br mirror nuclei identified event-by-event in the zero degree spectrometer. Prompt gamma rays de-exciting excited states will be detected in the Miniball array. The high resolution of the Ge detectors will be essential to disentangle the complex level scheme. Due to the mixing of the oblate and prolate components in the ground state of $^{72}$Kr, we expect to have access to the different shape structures in the final products. Mirror fragmentation (populating at the same time $^{71}$Kr and $^{71}$Br) will be used to control the populated excited states allowing a direct indication of the possible breaking of the mirror symmetry. For delayed gamma radiation at the focal plane we propose to mount the Total Absorption Spectrometer DTAS [14] of the Gamma Spectroscopy group from IFIC (Valencia) at the focal plane after the zero degree spectrometer recording delayed coincidences. We know that the sensitivity to a possible short living isomer will be better here than in the previous experiment where the $^{71}$Kr beam had to travel along the full BigRIPS spectrometer in order to reach the focal plane at F11. The DTAS will be available at Riken during the Miniball campaign.

We propose to use a primary $^{78}$Kr beam ($^{124}$Xe is an alternative) at 345 MeV/u with an intensity of 300 pnA. In order to help the spin-parity assignment we plan to use longitudinal momentum analysis of $^{71}$Kr. To this purpose the acceptance of BigRIPS can be reduced to $0.5\%$ to obtain the needed resolution in the momentum transfer. Even with such small acceptance we expect to have a secondary beam with an intensity of 3.5 104 pps at F8. The expected beam purity for $^{72}$Kr is $90\%$.

Assuming an efficiency of $3\%$ for the Miniball array we expect a gamma/particle counting rate of 1 and 2 Hz, respectively, for the lowest lying transitions of the mirror nuclei. Therefore we expect to be able to identify the main prompt gamma transitions in one day of beam time and to have sufficient statistics for a gamma-gamma correlation analysis.

The large statistics, high precision, measurement of knockout reaction towards $^{71}$Kr and $^{71}$Br will allow the verification of the symmetry rules that are expected to hold for mirror knockout reactions. Differences in the cross section for the population of isospin analogue states will be exploited to highlight differences in the structure of such states.

References:

[1] A.P. Zuker et al., Phys. Rev. Lett. 89, 142502 (2002).
[2] M.A. Bentley and S.M. Lenzi, Prog. Part. and Nucl. Phys. 59, 497 (2007).
[3] M. A. Bentley et al., Phys. Rev. C 92, 024310 (2015).
[4] I. Hamamoto and B.R. Mottelson, Phys. Rev. C 79, 034317 (2009).
[5] H. Iwasaki et al., Phys. Rev. Lett. 112, 142502 (2014).
[6] J.A. Briz et al., Phys. Rev. C 92, 054326 (2015).
[7] A.P. Zuker, A. Poves, F. Nowacki, S.M. Lenzi, Phys. Rev. C 92, 024320 (2015).
[8] B.S. Nara Singh et al., Phys. Rev. C 75, 061301 (2007).
[9] K. Kaneko et al., PRL 109, 092504 (2012).
[10] J.W. Arrison et al., Phys. Lett. B 248, 39 (1990).
[11] M. Oinonen et al., Phys. Rev. C 56, 745 (1997).
[12] P. Urkedal and I. Hamamoto, Phys. Rev. C 58, 1892(R) (1998).
[13] S.M. Fischer et al., Phys. Rev. C 72, 024321 (2005).
[14] V. Guadilla et al., Nucl. Instrum. Meth. A910, 79 (2018).

Speaker: Francesco Recchia (INFN LNL)
• Friday, 12 April
• 09:00 10:30
Proposals: Physics V
Convener: Anna Corsi (CEA Saclay)
• 09:00
Nature of proton and neutron motions in neutron-rich Te isotopes 15m

The understanding of the few-nucleon pair motions in an atomic nucleus is one of the ever-evolving topics for the many-body quantum system within a finite potential space. The Te nuclide with $N$ > 82 is the best laboratory to investigate the motion and interaction of a single proton pair in the neutron-rich environment like helium over the robustly closed core of $^{132}$Sn. The neutron-rich $^{136}$Te isotope with two proton and neutron pairs over the inert core has been studied for a long time. From several experiments, the collective motion of $^{136}$Te is revealed to be dominated by a neutron pair rather than a proton pair or equal footing of both pairs. However, the only interactions of two proton and neutron pairs in $^{136}$Te have been investigated due to the limitation of the experimental instruments. Consequently, the future experiments on the Coulomb excitations of ground band levels in $^{138, 140, 142}$Te with more neutrons will play essential roles on the nucleon interactions and motions with the extreme neutron-to-proton ratios.
The symmetric feature of the collective motions based on the neutron shell closure at $N$ = 82 in the Te isotopic chain is one of striking questions as reaching to the neutron drip line. The unique pattern of the nuclear structure evolution may be one of nature's riddle to be solved to understand behaviors of valence nucleons over the doubly closed shell structure. The in-depth investigation with the help of state-of-art theoretical models will answer this arising question by comparing with the experimental results on the level energies and reduced transition probabilities.
From the proposal, behaviors of valence nucleon pairs in the neutron-rich circumstance will be deeply studied with the following questions. Do neutron pairs still lead the entire motion of the nucleus with more neutrons? How many neutron pairs lead the entire motion and deformation? How much a proton pair contribute to the collective motion? This proposal will provide the way how the neutron pairs dominate the quantum system over the stable core and the actions of a pair of protons.

Speaker: Dr Byul Moon (Center for Extreme Nuclear Matters, Korea University)
• 09:15
Pairing in weakly bound systems 15m

The evolution of pairing correlations in exotic nuclei is a subject which has received much attention in recent years, as RIB’s accelerator facilities are providing unique isotopes to study. Of particular interest is the role of pairing in neutron-rich systems where the appearance of a low-density surface may induce a transition from the well known BCS Cooper-pairing mechanism to a BEC di-neutron condensate[1].
While the observation of this interesting crossover might not be within the current reach of RIBF, Skyrme-Hartree-Fock mean field and continuum RPA calculations predict a significant change in the neutron pair-addition/removal strength to low-lying excited 0+ states (pairing vibrations) in the Sn nuclei for $N$ = 82 – 90 nuclei [2].
Here we propose to study $^{A}$Sn (p,p2n) and (-2n) KO reactions combining FRANKENBall with MINOS, SAMURAI, and NEBULA to carry-out exclusive cross-section and momentum distribution measurements to low-lying 0+ and 2+ states in the (A-2) system. (Relative) Comparison of QFS and KO results should also serve as a proxy of the pair-correlation length in the volume and the surface.
Preliminary estimates suggest that for $^{136}$Sn, delivered at ~400 pps on target, we will obtain approximately 400 counts/day for gs-to-gs transitions and 40/day to the 2+, a feasible experiment.

[1] M. Matsuo PRC 73, 044309 (2006)
[2] H. Shimoyama and M. Matsuo. Phys. Rev. C, 84:044317, 2011.

Speaker: Heather Crawford (Lawrence Berkeley National Laboratory)
• 09:30
Single-particle states in the N=82 nucleus 129Ag 15m

129Ag is a single magic N=82 nucleus. With three protons holes below 132Sn is neutron-rich, and any experimental information to be obtained on its structure is directly applicable for the understanding of lighter N=82 nuclei on the r-process waiting path. Proton knockout from 130Cd on the MINOS liquid hydrogen target will be used to populate 129Ag.

130Cd in it is ground-state has two proton holes in g9/2. Therefore, in (p,2p) reaction one would populate predominantly single-particle states in 129Ag: the 9/2+ g9/2 ground-state and the low lying excited states with p1/2, p3/2, f5/2 proton-hole characters (all coupled to (g9/2^2)0+). Several shell model calculations were performed: with NA14 [1], CSnhp [2] and SM28 [3] in full proton hole model space(g9/2, p, f5/2) and reduced space (g9/2, p1/2) (SM28). Effective operators e = 1.5 e, gs = 0.7 gsfree were adopted. A 132Sn core is assumed. The results are shown in figure 1. The calculated transition strengths using NA14 were used to predict the decay properties of the single-particle dominated states. Both the 5/2- f5/2 (predicted half-life T1/2<1ps) and the 3/2- p3/2 (~4ps) will decay into the yrast ½-. Some of the positive parity-states, with predominantly g9/2^3 character are expected to have longer half-lives. The population of these might be enhanced in reactions involving the removal of more particles. Line shape analysis [8] of the measured gamma using MINIBALL++ will be used to determine some of these half-lives and therefore transition strengths.

129Ag was populated before, and its beta-decay half-life measured at RIKEN [4]. No excited state were ever directly observed, however from systematics the ½- state is expected at 20+-20 keV [5]. For comparison, in the heavier N=82 isotone 131In, the 1/2- p1/2 state is 365 keV above the 9/2+ g9/2 ground-state, while the 3/2- p3/2 was recently measured at 1353 keV [1].

Fig. 1: Low-lying levels calculated for 129Ag with three different interactions. Note that SM28 has a reduced model space, i.e. no p3/2 (and f5/2) single hole states.

Predicted beam intensity of 130Cd is 15 particle/s. This assumes 40pnA 238U beam at 345 MeV/u. 5mm Be target, sigma=1.12e-4 mb, transmission 0.85%. This should be optimized. This value fits well with yields observed in previous measurements in this mass region [6,7].

[1] J. Taprogge et al., Phys. Rev. Lett. 112, 132501 (2014) ; A. Gargano, priv. communication (2013)
[2] A. Jungclaus et al., PRC 93, 041301 (2016) ; A. Gargano, priv. communication (2014)
[3] F. Naqvi et al., PRC 82, 034323 (2010)
[4] G. Lorusso et al., Phys. Rev. Lett. 114, 192501 (2015)
[5] J. Timar and Z. Elekes, B. Singh, Nucl. Data Sheets 121, 143 (2014)
[6] H. Watanabe et al., PRL 111, 152501 (2013)
[7] H. Wang et al., Chin. Phys. Lett. 30, 042501 (2013)
[8] P. Doornenbal et al., Nuclear Instruments and Methods in Physics Research A 613, 218 (2010)

Speaker: Zsolt Podolyak (University of Surrey)
• 09:45
High-resolution spectroscopy around N=90 15m
• 10:00
γ(γ)-spectroscopy and lifetime measurements in the 132 Sn region following (p,2p) reactions 15m

Compared to studies of neutron-rich nuclei (A,Z) employing isomeric decay or Coulomb
excitation which require the isotope of interest as secondary beam or spectroscopy
following β-decay which even requires the more neutron-rich (A,Z-1) isotope, (p,2p)
reactions start from the less exotic isotope (A+1,Z+1). This reaction mechnism has been
exploited successfully already in the SEASTAR campaign with the liquid hydrogen target
MINOS at RIBF. Also the population via alternative reaction paths, e.g. (p,pn), can be
investigated. The reaction mechanism itself, hence the extracted cross sections, offers
additional sensitivity on the structure of the populated states.
The nuclei in the region of 132 Sn are just at the heavy border of what can be done at RIBF.
Various decay studies with EURICA, but also in-beam spectroscopy, e.g. of 136 Te, have
been done. New experimental information will challenge modern nuclear theory
calculations, the shell model as well as beyond-mean-field approaches.
Cd isotopic chain
Cd lies two protons below of Sn. The Cd isotopes up to 132 Cd (N=82) have been
investigated by mass measurements, laser spectroscopy and decay spectroscopy,
however in-beam studies are scarce and B(E2) values have often large errors. The level
schemes of the odd isotopes from 123 Cd and heavier have to be revised as mass
measurement revealed a wrong placement of the 11/2 - isomers. No excited states in odd
isotopes are known above N=82.
The Cs isomers will be populted from the (A+1) In isotones. The production cross section
for 130 In is about 10 -2 mb. In the same setting Sn isotopes are strong side channels giving
Sn isotopic chain above 132 Sn
The 6+ seniority isomers in 134,136,138 Sn have been studied with EURICA. Below the
isomers, only the B(E2, 0 → 2) value in 134 Sn has been measured with a large error. For
the odd isotopes, no excited states are known for 135 Sn and above. The Sn isotopes will
be populated from the (A+1) Sb isotones.The lightest Sb isotope with published production
cross section is 139 Sb (about 10 -4 mb). In the same setting there will be Te side channels
giving acces to Sb isotopes.
Embryonic rate estimate ... feasible at all?
10 pnA 238 U and a 3 mm Be primary target results in 2.3·10 5 part/s secondary beam for 0.1
mb production cross section. Assuming MINOS with 10 cm thickness and a cross section
for (p,2p) of 1 mb, 10 5 part/s result in 20 reactions/s. With 5% efficiency the γ(γ)-rates are
3600(180)/h. For the quite rare channel 138 Sn, these numbers would result in about 100
counts/day γ-rate.
Electromagnetic transition matrix elements are of paramount importance for the
understanding of the nuclear structure. The high velocities at RIBF naturally cause that the
point of emiison of the decay g-ray is not the reaction vertex for lifetimes above some 10
ps. This can be exploited to extract lifetimes. The better energy resolution compared to
DALI2 allows for lifetime measurements in the region of about τ = 10 ps -100 ps. The exponential decay curves are folded with distribution of reaction vertices along the target
and the slowing down (within the target) before decay. The analysis is similar to DSAM.
Some examplary lieftimes of the first 2 + states in even-even isotopes in the region of
interest are given in Table 1.
126
E(2 + )[keV]
τ [ps]
Cd
652
128
Cd
646
1
130
Cd
618
2
134
Sn
726
136
Sn
688
2
138
Sn
715
3
14.9
19.9
67.7
42.9
24.6 3
12.9 2
59.1 3
64.5 3
Table 1: 1 direct lifetime measurement; 2 B(E2) from Coulomb excitation; 3 theory predictions
The result of a simple and very schematic simulation is shown in Fig. 1. Assumed is a
MINOS target of 10cm length and a distance of the detector from the centre of MINOS of
20 cm. No uncertainties for the direction and the velocity of the incoming beam, entrance
windows, straggling inside the target (constant dE/dx is assumed), background, intrinsic
resolution and opening angle (θ Lab =45°, infinite position resolution) of the γ-detector are
included. More realistic simulations e.g. with APCAD have to confirm the region of
sensitivity.
Fig. 1: Doppler-shifted γ-ray energy for reactions at the centre of MINOS ± 5 mm (E γ0 =600
keV, E beam =200 MeV/u, A=128). Shown are lineshapes for τ =10 ps, 25 ps, 50 ps and 100
ps with 10 6 events each.

• 10:15
Study of octupole deformation in neutron-rich Ba isotopes using Coulomb excitation at intermediate energies 15m

With this proposal we intend to continue our experimental program to study neutron-rich nuclei in the region around doubly-magic 132Sn using in-beam γ-ray spectroscopy at relativistic energies, which we initiated with experiment NP1306-RIBF98R1 in 2015. In particular, we propose to study the octupole deformation of neutron-rich Ba isotopes by measuring the transition probabilities to the first excited 3- states, B(E3; 0+→3-), in 144,146,148,150Ba using the technique of intermediate-energy Coulomb excitation. The results of this experiment will allow to test the predictions of various microscopic calculations which establish the Ba isotopes as low-Z boarder of the region of octupole deformation above the Z=50 and N=82 shell closures.

Speaker: Andrea Jungclaus (IEM-CSIC)
• 10:30 11:00
Coffee 30m
• 11:00 12:30
Proposals: Physics VI
• 11:00
Exploring the collectivity around the N = 82 shell closure via relativistic Coulomb excitation of the Z = 50 130,136Sn isotopes 15m

We propose to study excited states in the isotopes 130,136Sn the direct neighbors of 132Sn by gamma-ray spectroscopy following relativistic Coulomb excitation. The experiment aims to investigate the evolution of collectivity and nuclear structure around and the magic-shell closure at N = 82 for tin isotopes (Z = 50) via the determination of the reduced transition probabilities, in particular B(E2; 0+ - 2+). The fragments of interest are produced by impinging a 345 A MeV 238U beam onto a 9Be target and the following identification with the BigRIPS spectrometer. The identification of beam-like and target-like particles after the secondary target is performed by the ZeroDegree spectrometer. Gamma-rays from the deexcitation of beam and target-like particles are detected with the MINIBALL array.

Speaker: Peter Reiter
• 11:15
Disentangling collective and single-particle structures in neutron-rich Se isotopes 15m

The quadrant of the nuclear chart north-east of the doubly magic $^{78}$Ni is expected to be rife with competition between single-particle and collective degrees of freedom. Of the nuclei in the $28\lt Z<50$ and $50\lt N<82$ valence spaces, the neutron-rich Se isotopes have been suggested by theory and experiment to exhibit shape co-existence of spherical, prolate and oblate shapes, and prone to gamma-softness [1, 2]. In addition, in-elastic scattering studies of $^{90,92}$Se indicate a lowering of the $3^-$ state, in-line with global systematics [3]. This observation is coincident with the correspondence of these nuclei to the predicted “octupole magic numbers”, $Z=34$ and $N=56$, leaving open the possibility of strong octupole correlations. From an astrophysical perspective, electron capture on nuclei with $Z<40$ and $N>40$ in type II supernovae collapse is expected to be strongly suppressed as the equivalent neutron orbitals of the proton valence space are fully occupied. Should, however, there be a significant mixing between the $pf$ and $sdg$ neutron shells, the Fermi-blocking effect could be diminished shell causing nuclear electron captures to dominate over free proton electron capture [4]. To shine light on these matters, we propose the study of the Se isotopic chain with $53≤N≤58$.

In the odd-mass Se isotopes the locations of the states of which comprise of single-particle configuration, as part of a multiplet or purely single-nucleon, how they evolve, and their composition shall be studied through the single-nucleon knockouts of the respective even-mass isotopes. The locations of these orbits will guide shell model calculations and help disentangle the collective and single-particle contributions, as well as provide evidence for the $pf$-$sdg$ mixing relevant for the electron captures in novae. A previous study [5] claims to have located 3 such states in $^{87}$Se belonging to the f$_{5/2}^3$ multiplet, the verification of these states can be carried out more robustly in the work proposed here. The importance of a s$_{1/2}$ neutron state, predicted in $^{87}$Se to be 0.97 MeV, is highlighted in Ref. [5], with knockout reactions, this and other single-particle states will be populated for study. To probe in a more experimentally quantifiable way the shapes of the even-even nuclei inferred from a previous study [1] , Coulomb excitation of the even-mass isotopes on a high-$Z$ target is proposed to obtain reliable $B(E2)$ values. In addition to the population of the 2$^+$ states, the yrast 3$^-$ states, are expected to be populated. The extraction of the $B(E3)$ values to these states will provide a direct measurement of the octupole correlations present in this supposedly doubly ocuptole-magic region.

To address the issues above, very specific experimental conditions are required. Due to the exotic nature of the nuclei proposed for study here, high-intensity RI beams are required. Additionally, since excitation states in the odd-even nuclei are expected to be of high level-density with some transitions having rather similar energy, excellent $\gamma$-ray resolution is required. This will also allow unambiguous measurement of parallel momentum distributions in coincidence with the de-excitation $\gamma$ rays allowing for firm classification of the orbital configuration of the state populated. From these constraints, it is evident that the optimum configuration to perform these experiments will be with the proposed hybrid array of HPGe detectors at the RIBF.

[1] S. Chen, et al., Phys. Rev. C 95, 041302(R) (2017).
[2] K. Nomura, R. Rodríguez-Guzmán and L. M. Robledo, Phys. Rev. C 95, 064310 (2017).
[3] S. Chen et al. in preparation.
[4] K. Langake et al. Phys. Rev. Lett. 90, 241102 (2003).
[5] T. Rząca-Urban et al. Phys. Rev. C 88, 034302 (2013).

Speaker: Frank Browne (RIKEN Nishina Center)
• 11:30
Shape coexistence studies in neutron-rich krypton isotopes around N=60 15m

Even-even strontium and zirconium nuclei in the A=100 region show a sudden onset of deformation at N=60 while the lighter isotopes up to N=58 are rather spherical. Unlike, the even krypton isotopes exhibit a smooth onset of collectivity up to N=60 [1]. Recent high-resolution gamma-spectroscopy results on 96Kr [2] following projectile fission confirmed the energy of the yrast 2+ --> 0+ transition [1] and reported for the first time on the yrast 4+ --> 2+ transition. Further on, recent results from the SEASTAR 2015 campaign on the extremely exotic 98,100Kr isotopes, populated in nucleon knockout reactions of radioactive beams, reported a decrease in the energy of the yrast 2+ states suggesting a continuation of the smooth shape transition beyond N=60 [3]. The side product data on the neutron-rich 96Kr collected from the same campaign suggested new low-lying excited states [4], which unlike the oblate shape g.s., could correspond to the prolate minimum of the potential energy surface (see ref.[5] for example). Unfortunately the low-resolution data from DALI2 together with the possible lineshape effects due to state lifetimes, made it difficult to establish a reliable level scheme. We performed a subsequent experiment to excite the lowest non-yrast states of 96Kr using Coulomb-nuclear excitation reactions at HIE-ISOLDE, CERN, which suffered from a reduced radioactive beam intensity and increased stable beam contamination.

As shown in the neighbouring nuclei of higher Z, the prolate shape is related to the lowering of the neutron g9/2 orbital. Thus studies of the odd-neutron krypton isotopes 95,97Kr are complementary and of similar importance. The existence of a T1/2 = 1.4 us isomer in 95Kr [6] made it mandatory to measure delayed gamma-rays at the end of Zero Degree spectrometer, which we did in the SEASTAR 2015 campain. The correlation of delayed and prompt gamma-rays worked nice, but again due to the low-resultion of DALI2 and the increased line density of the prompt spectrum only a preliminary level scheme could be built [7].

From the point of view of reaction mechanism and the production yeilds, the RIBF is currently the only place in the world where these shape-coexisting states in the krypton isotopes around N=60 could be studied.

Thus, in order to obtain definite results on the gamma-energies and their coincidence relations, as well as have a more sensitive analysis of the lineshapes, we propose to study the neutron-rich krypton isotopes around N=60 with high resolution gamma-ray spectroscopy at the RIBF populated via nucleon knockout reactions.

[1] M. Albers et al., Phys. Rev. Lett. 108, 62701 (2012)

[2] J. Dudouet et al., Phys. Rev. Lett. 118, 162501 (2017)

[3] F. Flavigny et al., Phys. Rev. Lett. 118, 242501 (2017)

[4] K. Moschner et al., in preparation.

[5] K. Nomura et al., Phys. Rev. C 96, 034310 (2017)

[6] J. Genevey et al., Phys. Rev. C 73, 37308 (2006)

[7] R.-B. Gerst et al., in preparation.

Speaker: Andrey Blazhev (Institute for Nuclear Physics, University of Cologne, Germany)
• 11:45
Coulomb and quantum bubbles in heavy nuclei 15m

This is a letter of intent to study the proton shell structure near Z = 82 in the forthcoming Miniball campaign at the RIBF. The depletion of the central proton density, which characterizes the so-called bubble structure, is expected to arise from two different mechanisms. One is for a given atomic nucleus to minimize its Coulomb energy and the other to optimize the nuclear shell energy. While the shell evolution generates the bubble structure, for instance, in neutron-rich 34Si, the repulsive Coulomb potential is increasingly important to form bubbles in heavier nuclei. The two mechanisms are predicted to cross over in the region near Z = 82. The proton structure will be studied via in-beam gamma-ray spectroscopy.

Speaker: daisuke suzuki
• 12:00
Single particle structure coupled to the second 0+ state in 32Mg 15m

More than 40 years have passed since the first discovery of the island of inversion [1]. In the
shell model point of view the phenomenon is attributed to the change of the effective single
particle energy, which gives rise to the quenched N=20 shell gap [2]. On the other hand the
single particle structures of 31Mg [3] and 33Mg [4] can be well understood in the Nilsson
diagram, which suggests that the quadrupole deformation is important and the magicity loss is
the result of the deformation. One cannot disentangle the effect of the quadrupole deformation
and the tensor correlation experimentally when single particle structure is coupled to the core of
the deformed ground state. The studies of the single particle structure coupled to the spherical
core is desired.
Around the island of inversion, in the even-even nuclei the second 0+ state were observed.
They are regarded to have different shape of that of the ground state. For example, in the case
of 32Mg, the second 0+ state is considered to be spherical [5].
Though it is almost impossible to study the transfer reaction on the second 0+ state because
of the short lifetime, there is an alternative way to study the single particle structure coupled to
the excited state. In the past, the inelastic channel from the isobaric analog resonance were
studied extensively for the single particle structure [6]. The inelastic decay widths can tell us the
spectroscopic factor coupled to the excited state.
We would like to propose to measure the inelastic channel of the isobaric analog resonances
of 33Mg in particular to the second 0+ state. The isobaric analog resonances will be searched
by the proton resonance scattering.The resonance is expected to appear above 3 MeV/u. The
energy degraded RI beams will be directed upon the hydrogen target. The two cascade
de-excitation gamma rays from the second 0+ states of 32Mg will be measured in the coincident
with the recoil energy, which enables us to identify the inelastic chanel. In this talk, we will
discuss the details of the experimental setup.
[1] C. Thibault et al., Phys. Rev. C 12, 644 (1975).
[2] T. Otsuka et al., Phys. Rev. Lett. 104, 012501 (2010).
[3] G. Neyens et al., Phys. Rev. Lett. 94 022501 (2005).
[4] D.T. Yordanov et al., Phys. Rev. Lett. 99, 212501 (2007).
[5] K. Wimmer et al., Phys. Rev. Lett. 105, 252501 (2010).
[6] S.A.A. Zaidi, P. von Brentano, D. Riek, J.P. Wurn, Phys. Lett. 19, 45 (1965).

Speaker: Nobuaki IMAI (CNS, Univ. of Tokyo)
• 12:15
Magnetic moments of short-lived excited states at projectile-fragmentation energies. Would the recoil in gas technique work? 15m

Nuclear moment measurements of short lived excited states are in general very challenging due to the requirement to obtain strong enough magnetic fields, capable to provide a sizeable perturbation of the nuclear spin ensemble within the lifetime of the state of interest. Kilotesla’s magnetic fields are usually needed for nuclear states of picosecond lifetimes. Hyperfine fields were often utilized for this purpose. Those fields could rarely be estimated from first principles and empirical calibrations are needed.
An approach that has been attempted for these measurements at projectile-fragmentation energies (tens to hundreds of MeV/u) is the Transient Field technique. However, even at relatively low energies (~40 MeV/u) and not too high Z (Z~30) the observed effective fields are quite small and are expected to weaken further for higher Z values [1]. Therefore, it is considered that the TF technique wouldn’t be applicable for higher-Z ions at higher velocities.
Here we are investigating the possibility of applying the Recoils In Gas (RIG) approach for higher-Z ions at energies in the range of 100 – 150 MeV/u. The basic ideas of the RIG technique will be presented and its possible applicability at RIKEN conditions would be discussed.
The Recoil In Gas technique would use a Plunger device and could therefore be integrated in a longer campaign or set of experiments aiming at lifetime measurements of exotic nuclei at RIKEN.
[1] E. Fiori et al., Phys. Rev. C85, 034334 (2012)

Speaker: Dr Joa Ljungvall (CSNSM, Orsay, France)
• 12:30 14:00
Lunch 1h 30m
• 14:00 15:00
Discussion
Conveners: Pieter Doornenbal (RIKEN), Kathrin Wimmer (The University of Tokyo)
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