Gamma08

3 Apr 2008, 09:00 → 5 Apr 2008, 21:00 Asia/Tokyo

Nishina Hall (RIKEN Nishina Center)

Eiji Ideguchi (CNS, University of Tokyo)

CNS-RIKEN Joint International Symposium on Frontier of gamma-ray spectroscopy and Perspectives for Nuclear Structure Studies (gamma08)

Bus from Wako to Narita wakoshi-narita_bus.pdf

Important Information intro-info.pdf

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Thursday 3 April

Registration

1 Welcome

Welcom by Chair

Speakers: Dr Eiji Ideguchi (CNS, University of Tokyo), Prof. Takaharu Otsuka (Department of physics, The University of Tokyo, RIKEN, CNS/Tokyo, MSU), Prof. Tohru Motobayashi (RIKEN Nishina Center for Accelerator-Based Science)

Collectivities and Shell effects in neutron/proton-rich nuclei: I

2 In-beam gamma-ray spectroscopy with fast beams of rare isotopes

Gamma-rays emitted from bound excited states carry valuable information on the structure of the nucleus. At present, in-beam gamma-ray spectroscopy with fast beams combined with a set of robust experimental techniques contributes in many ways to new discoveries and insights into the structure of nuclei far from stability. An overview of in-beam gamma-ray experimental techniques, illustrated with examples of recent results, will be presented. A number of new promising developments addressing some of the challenges inherent to in-beam gamma-ray experiments with fast-beams of rare isotopes will be discussed.

Speaker: Dr Przemyslaw ADRICH (NSCL)

3 Collective deformation of neutron distribution in nuclei probed by proton inelastic scattering

\author{ S. Okamoto$^{1}$, T. Furumoto$^{1}$, M. Takashina$^{2}$ and Y.Sakuragi$^{1,3}$, } \affiliation{ $^{1}$Department of Physics, Osaka City University, Osaka 558-8585, Japan\\ $^{2}$Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan\\ $^{3}$RIKEN Nishina Center, RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan } The electromagnetic probes such as electrons and $\gamma$-rays are very useful tool to study proton distribution and its deformation in nuclei. However, the neutron distribution and its deformation are almost insensitive to the electromagnetic probes and one needs to use hadronic probes, such as protons and composite nuclei. In this paper, we propose a grobal method to extract information about the deformation of neutron distribution in nuclei, over the whole range of nuclear chart, using proton inelastic scattering which is analyzed by a microscopic coupled-channels method based on a complex G-matrix interactions. In this method, a collective model is assumed for transition densities for protons and neutrons and the deformation length for neutrons is the only parameter to be determined from the comparison of calculation with experimental data for the proton inelastic scattering, whereas the proton deformation length is expected to be known independently from electric transition probability, $B(\rm E \lambda)$, obtained e.g. from the $\gamma$-ray measurements. All the diagonal/transition pontetial for the proton inelastic scattering are calculated by folding the JLM complex G-matrix interaction [1] with the diagonal/transition densities. The proton and neutron density distributions are assumed to have a simple Fermi form factor and its geometrical parameters are fixed so as to reproduce the rms charge radii for protons, while for neutrons the parameters are fixed so as to adjust the rms radii given by a Hartree-Fock calculation [2]. Therefore, no free parameter is left in this method except for the neutron deformation length to be determinded from the comparison with the proton inelastic-scattering cross sections. We have tested this method in proton inelastic scattering by various stable and unstable nuclei, typical examples of which are shown in Figures. The solid curves for $^{208}$Pb target and the dotted ones for $^{20}$O target are the results with $M_n/M_p=N/Z$, where $M_n$ ($M_p$) denotes the $r^2$-moment of the neutron (proton) transition density. The solid curves for $^{20}$O target show the result with $M_n/M_p=3.78$ which is much larger than $N/Z$, indicating an extra deformation of neutrons in $^{20}$O, which is consistent with the result by other method [3]. [1] J.P Jeukenne, A.Lejeune, and C. Mahaux Phys Rev C {\bf 16}, 80 (1977) [2] N.Tajima, S.Takahara, N.Onishi, Nucl. Phys. A{\bf 603}, 23 (1996), and online at \\ \hspace*{5mm}http://serv.apphy.fukui-u.ac.jp/~tajima/hfs3/table/index.html [3] Dao T.Khoa, Phys Rev C {\bf 68}, 011601 (2003)

Speaker: Prof. Yukinori Sakuragi (Department of Physics, Osaka City University)

10:40 break

4 New states in 32Mg and 34Si

We have studied the spectroscopy of neutron rich nuclei in the region of N=20 with a new method, using different reactions to produce the same nucleus. A composite secondary beam formed by several nuclei in the region of the island of inversion was produced by fragmentation at GANIL with the SISSI device, at an energy close to 30 MeV/nucleon. It was transported to the VAMOS spectrometer beam line and each incident particle was identified by time of flight and energy loss. The reactions of these nuclei on a CD¬2 target were measured simultaneously: (d,d’) inelastic scattering, (d,p), (d,t) and (d,3He) transfer reactions, and fragmentation reactions. The spectroscopy of 32Mg, 32,33,34Al, 33,34,35Si was studied by the deexcitation γ’s measured with the EXOGAM clover array, in coincidence with the ejectile at the focal plane of VAMOS. New level schemes are proposed for these nuclei, based on γ−γ coincidences, and angular distributions are obtained for the most strongly populated transitions. In particular, for 34Si, the angular distributions for the two most strongly populated transitions at 3.3 MeV (2+–>0+gs) and 929 keV were obtained (see Fig.1), confirming the E1 character of the latter and therefore the 3- spin/parity assignment for the state at 4.2 MeV, which was previously only indirectly attributed, through population considerations from β-decay. Furthermore the second excited 0+ state was searched for. The previous assignment obtained from in-flight excitation was ruled out by the present γ−γ coincidence studies and a new candidate is proposed at 4.0 MeV. In the case of 32Mg, we determined the multipolarity of the 1.43 MeV transition and therefore the spin/parity of the state at 2.32 MeV for which conflicting results existed. The γ−γ data show some evidence for a new isomeric state around 3.9 MeV. The experimental method will be explained and the results obtained for the N=20 benchmark nuclei 32Mg and 34Si will be discussed, together with their consequences on the understanding of the neutron rich nuclei in the so-called “island of inversion” region.

Speaker: Mrs Patricia Roussel-Chomaz (GANIL)

5 Study of High-spin States in $^{49-51}$Ti

In nuclei near closed shell, shell structure can be studied by looking at high-spin states. At excited states with certain amount of spins, these nuclei need to promote nucleons across the shell gap in order to gain larger angular momentum. Such core excitation across the N=28 shell gap have been previously discussed in even-even nuclei of N=28 isotones both experimentally and theoretically [1,2]. In order to study shell structure in neutron-rich Ti isotopes, $^{49-51}$Ti, we have performed in-beam $\gamma$-ray spectroscopy. In order to realize high-spin studies in neutron rich nuclei by fusion-evaporation reaction, we need to use neutron-rich secondary beam, since it is difficult to access high-spin states in such nuclei with stable beam and stable target combinations. The experiment was performed at the RIPS beam line in RIKEN [3]. A $^{46}$Ar beam was produced by projectile fragmentation of $^{48}$Ca at the energy of 64 MeV/A, and its energy was lowered to a few MeV/A using an aluminum degraders. A $^{9}$Be target of 1.84 mg/cm$^{2}$ was used to bombard the $^{46}$Ar beam to make fusion reaction, $^{9}{\rm Be} \left(^{46}{\rm Ar},xn \right) {}^{55-x}{\rm Ti}$. Details on the production of low-energy secondary beam are reported in ref. [4]. Gamma rays emitted from high-spin states of evaporation residues were detected by the CNS Gamma-Ray detector Array with Position and Energy sensitivity (CNS-GRAPE) [5]. The experiment involved the measurements of excitation functions, $\gamma$-ray angular distributions and $\gamma$-$\gamma$ coincidences, and high-spin levels up to (21/2), (11+), and (17/2) states in $^{49-51}$Ti were identified, respectively. Shell-model calculations suggest that these high-spin levels were created by promoting one neutron across the N=28 shell gap. In the talk, shell structure around N=28 will be discussed. [1] B. Gass et al.: Phys. Rev. Lett. 40, 1313 (1978). [2] M. Honma et al., Phys. Rev. C 69, 034335 (2004). [3] T. Kubo et al., Nucl. Instrum. and Methods. B 461, 309 (1992). [4] E. Ideguchi. et al., AIP Conf. Proc. 764, 136 (2004). [5] S. Shimoura, Nucl. Instrum. and Methods. A 525, 188 (2004).

Speaker: Mr Megumi NIIKURA (CNS, University of Tokyo)

11:55 Lunch

Exotic Deformation: I

6 High Spin Physics - Recent Achievments and Perspectives

An overview of selected recent experimental results is given. Their impact on our understanding of nuclear structure is discussed. The following subjects are touched: Termination of regular bands and their recurrance, hyperdeformation, triaxial strongly deformed bands and wobbling, chirality, isospin breaking in mirror nuclei, isomers in the heaviest elements and their implications for superheavy elements, octupole phonon condensation.

Speaker: Prof. Stefan Frauendorf (Research Center Dresden-Rossendorf, Germany)

7 Multiple band structures of 131,133Cs isotopes

The nuclei in the mass $A\sim130$ region below N = 82 shell closure are soft to gamma-deformation at low and medium spins, and exhibit various intriguing phenomena. Depending upon the coupling of the angular momenta of valance neutrons and protons with that of the even-even core different types of excitations [1 - 4], namely, magnetic rotation, chiral twin bands and recently predicted chopstick configurations have been discussed for these nuclei. The structure of $^{131,133}$Cs isotopes at moderate and high spins were investigated in the present work in search of $\Delta I = 1$ doublet bands and quasi-vibrational structures. The excited states of $^{131}$Cs and $^{133}$Cs were populated in through the $^{124}$Sn($^{11}$B,4n)$^{131}$Cs and $^{130}$Te($^7$Li,4n)$^{133}$Cs reactions, respectively. Ion beams of $^{11}$B and $^{7}$Li were obtained from the 14-UD pelletron accelerator at T.I.F.R., Mumbai. The experimental set up consisted of 8 Compton-suppressed Clover detectors placed in the horizotal plane at 60$^{o}$, 90$^{o}$, 120$^{o}$, 150$^{o}$, 210$^{o}$, 250$^{o}$, 285$^{o}$, and 325$^{o}$ with respect to the beam direction [2]. A 14-element NaI(Tl) multiplicity filter in the form of two clusters were also placed above and below the horizontal plane. The master trigger for collecting gamma-gamma coincidence data was generated with the condition that at least two Clover detectors and two NaI(Tl) detectors fired in coincidence. The present level scheme of $^{131}$Cs consisting of fifteen bands has been found to exhibit a variety of collective structures in this nucleus at intermediate spin [5]. Apart from the rotational bands based on $\pi g_{7/2}$, $\pi d_{5/2}$, and $\pi h_{11/2}$, dipole bands with strong M1 transitions and gamma vibrational bands have been established. The excitation energies of the observed levels in different bands and the corresponding ratios of various transition strengths have been compared with the results of projected deformed Hartree-Fock (PHF) calculations based on various quasiparticle configurations [6]. Based on comparison of the experimental data with the theoretical model calculations, a strongly coupled band has been reassigned a high-K three quasiparticle configuration. This band is found to be fed by another weakly populated side band with similar excitation energy. There are indications of strong $\pi h_{11/2} \otimes \nu h_{11/2}$ component in these two bands. Gamma vibrational bands coupled to the $\pi h_{11/2}$ and $\pi g_{7/2}$ single particle configurations have also been observed in this nucleus. New bands based on different quasiparticle structures have been identified in $^{133}$Cs. Observed features of the other new bands in these nuclei will be discussed. Future possibilties of investigation of moderate and high spin states of neighbouring nuclei with the upgraded Indian National Gamma array consisting of 24 Clover detectors coupled with a charged particle detector array will be discussed. References: 1. S. Frauendorf, Rev. Mod. Phys. \textbf{73}, 463 (2001). 2. S. Lakshmi, {\it et. al.} , Nucl. Phys. \textbf{A761}, 1 (2005). 3. K. Starosta, {\it et. al.,} Phys. Rev. Lett. \textbf{86}, 971 (2001). 4. K. Higashiyama, N. Yoshinaga, and K. Tanabe, Phys. Rev. C \textbf{72}, 024315 (2005). 5. S. Sihotra, R. Palit, {\it et. al.} , (submitted). 6. Z. Naik, and C. R. Praharaj, Phys. Rev. C \textbf{67}, 054318 (2003).

Speaker: Dr Rudrajyoti Palit (Tata Institute of Fundamental Research)

14:20 Coffee break

8 Testing Chirality in the Nd Nuclei

Speaker: Prof. Umesh Garg (University of Notre Dame)

9 Lifetime Measurement in $^{103,104}$Rh with RDDS Method in Inverse Kinematics: A Test for Nuclear Chirality

A spontaneous symmetry breaking is a common keyword in modern physics. Chiral symmetry breaking in nuclear structure has recently been debated. This phenomenon is related to time reversal and arises from mutually perpendicular angular momenta of the valence proton, valence neutron, and core rotation in the body fixed frame. The nuclear chirality results in a nearly degenerate pair of $\Delta I = 1$ rotational bands, called chiral doublets. First experimental step is to identify such a twin rotational bands, which are observed several odd-odd and a few odd-$A$ nuclei in the $A\sim$130 and $A\sim$100 regions. However, in the case of $^{134}$Pr, $B(M1)$ and $B(E2)$ values are clearly different between both bands, that is, their electromagnetic properties are very different. Hence, lifetime measurement for chiral candidates were one of the major requirements for establishing nuclear chirality. Indeed, a few candidates are measured, and the $B(E2)$ and $B(M1)$ values between both twin bands in $^{135}$Nd are very similar. A similar measurement in the other mass region of $A\sim$100 has been much anticipated. Chiral candidates for $^{103,104}$Rh, are considered one of the best cases to study by lifetime measurements via the recoil distance Doppler shift (RDDS) method. The high spin states of $^{103,104}$Rh were populated in the $^{11}$B($^{96}$Zr,4(3)n)$^{103(104)}$Rh reactions at the beam energy of 330-MeV provided by the Argonne Tandem Linear Accelerator System (ATLAS). The Cologne plunger device and GAMMASPHERE array were used. The data were collected for 7 distances between the target and the degrader; 8, 15, 23, 35, 50, 75, 100~$\mu$m. On average, a total of approximately $4 \times 10^8$ unfolded events were sorted into $\gamma - \gamma$ matrices for each distance. The germanium detectors are grouped in 17 rings by the same polar angles. However, only 7 rings having forward or backward angles with respect to the beam axis were analyzed. About 80 matrices were analyzed, and three and four lifetimes of the levels suspected for chiral nature are measured in $^{103}$Rh and $^{104}$Rh, respectively. The results of analysis will be presented and discussed.

Speaker: Dr Tomokazu Suzuki (Department of Physics, Osaka univ.)

10 Recent Results from Gammasphere

Gammasphere has been in operation for over 10 years. In the initial years, it was used almost exclusively to study nuclei produced in fusion evaporation reactions at high spin. For example, successful campaigns to study and characterize superdeformation throughout the nucleonic chart were performed. In recent years, many experiments have been directed at the study of nuclei near the proton drip line or in the neutron rich region. These studies have utilized different reaction types and in many instances ancillary devices such as the FMA. In this talk, I will present several recent examples of the use of Gammasphere to study both proton- and neutron-rich nuclei.

Speaker: Dr Michael Carpenter (Argonne National Laboratory)

11 Spectroscopy of A=30~40 Neutron-Rich Nuclei via Fusion-Evaporation Reactions at JAEA

Progress of the RI beam physics has revealed the new interesting phenomena such as neutron halo and island of inversion. Theoretically, the breakdown of the well-known N = 20 magic number in the island of inversion is interpreted in term of the lowering of 2p-2h energy gap. Experimentally, however, the knowledge on the nuclear structure is so far limited to ground states and lowest-lying states due to their production methods. In order to explore further the nuclear structure, the findings on the high-spin states is also indispensable since the shell structure is also strongly dependent on the nuclear rotation and deformation. Furthermore, the nuclear deformation can rapidly evolve as the angular momentum increases. Thus, the systematic high-spin study towards the island of inversion is of great interest. We will present some experimental results on Tz ~< 2 nuclei in this region produced by heavy-ion induced fusion evaporation reactions.

Speaker: Dr Tsuneyasu Morikawa (Department of Physics, Kyushu University)

16:20 Coffee break

Collectivities and Shell effects in neutron/proton-rich nuclei: II

12 Evolution of deformation in neutron rich Ti and Cr isotopes.

Speaker: Dr Nori Aoi (RIKEN Nishina Center)

13 Deformation around neutron-rich Cr isotopes in axially symmetric Skyrme-Hatree-Fock-Bogoliubov method

Recent experiments on the neutron rich Cr isotopes with N=36,38 suggest a new region of quadrupole deformation[1][2]. In this presentation we analyse the deformation mechanism in this region by means of a deformed Skyrme-Hartree-Fock-Bogoliubov code based on a 2D mesh representation in the cylindrical coordinate system assuming axial symmetric deformations[3]. It is found that when we adopt the Skyrme parameter set SkM* the deformation energy curves in the isotopes N=32-44 exhibit an onset of a large quadrupole deformation around N$\sim\$38-42, but the potential energy surface is quite flat up to $\beta \sim$0.3. From analysis of the neutron Nilsson diagram obtained with constrained HFB calculation, we found that the deformation is sensitive to the N=38 deformed gap which arises from the down-sloping $\nu$g9/2 orbits. However the Fe (Z=26) isotopes with the same neutron number show a similarly soft potential but with smaller deformation, and the Ti isotopes (Z=22) do not exhibit deformation, indicating a combined effect of protons and neutrons. By comparing with other Skyrme parameter sets, we shall demonstrate that the deformation in this region emerges as a consequence of a delicate competition between spherical and deformed configurations, which is governed by the position of $\nu$g9/2. The deformation properties in the n-rich Cr isotopes provide us with a rather strong constraint for a proper choice of the Skyrme parameter sets. References: [1] O. Sorlin et al., Eur. Phys. J. A16. 55(2003). [2] N. Aoi et al., Nucl. Phys. A in press. [3] H. Oba, M. Matsuo, preprint 2008 Feb.

Speaker: Mr Hiroshi Oba (Graduate School of Science and Technology, Niigata University)

14 Continuum and pairing effects for rotational excitations in neutron rich nuclei

We discuss the unique role of pairing correlations for low-lying rotational excitations in neutron rich Be, Mg and Cr isotopes. Appearance of di-neutron correlation due to the neutron skin and continuum effects, and its influence on the moment of inertia are investigated. We also point out the necessity of the isospin density dependence in the pairing effective force; the usual isoscalar density dependence is inappropriate for describing the collective phenomena in neutron rich nuclei. At first, we discuss the puzzle of the E2 properties in neutron rich Be isotopes; the energy of the first 2+ state in 14Be is lower than that in 12Be while its deformation length is shorter. We calculate the moments of inertia by the cranking formula based on the coordinate space Hartree-Fock-Bogoliubov formalism. The mixed type density dependent contact force is used for the pairing channel. By an appropriate choice of the deformation parameters of the Woods-Saxon potential in the p-h channel, both the deformation lengths and the 2+ excitation energies can be well reproduced. This is because the neutron pairing correlation is weakened by the large neutron deformed shell gap at N=10. As a result, the moment of inertia in 14Be becomes larger in spite of the smaller quadrupole deformation. We extensively investigated the pairing effect for the first 2+ states in neutron rich Mg and Cr isotopes. We show that the di-neutron correlation appears if the pairing force has the strong density dependence such as the surface pairing. The nature of the neutron pairing strongly reflects the systematic trend of the 2+ energies. We also point out that the usual isoscalar density dependence in the pairing force is inappropriate for describing the collective phenomena in neutron rich nuclei. With the isospin density dependence for the surface pairing force, the 2+ energies shift upward at the neutron drip line while the di-neutron properties remain unchanged. Therefore we conclude that the systematic information of the rotational 2+ states can be utilized to explore the unique role played by the pairing correlation in neutron rich nuclei.

Speaker: Dr Masayuki Yamagami (RIKEN Nishina Center)

Poster

15 A coupled-channel approach to Hartree-Fock-Bogoliubov mean-field theory for deformed neutron rich nuclei

The coordinate-space Hartree-Fock-Bogoliubov (HFB) method plus the continuum quasi-particle random phase approximation (QRPA)[1] is useful schemes to describe nuclei close to the neutron drip line as the methods allows us to treat properly the asymptotic behaviours of quasi-particle wave functions of weakly bound and unbound orbits. However, no complete formulation has been given once nuclear mean-field is deformed. Attempting to formulate a continuum QRPA for deformed nuclei, we investigate in the present work the coupled-channel formalism of the deformed HFB[2][3], where the quasi-particle states are represented as a coupled radial wave functions for channels with different quantum numbers $ljm$. Using a deformed Woods-Saxon potential and the density-dependent delta-type pair interaction, we succeeded for the first time in obtaining the self-consistent pair potential and the associated quasi-particle states. We confirm that the exponential tail of the density and pair density is accurately described. This is because the coupled-channel representation allows us to adopt a very small discretization $\Delta r=0.2$ fm for the radial wave functions. It is straightforward to implement out-going wave boundary condition using a closed form construction of the HFB Green function for the coupled channels. References: [1] M. Matsuo, Nucl. Phys. A696, 371(2001). [2] K. Hagino et al., Nucl. Phys. A731, 264(2004). [3] I. Hamamoto, Phys. Rev. C71, 037302(2005).

Speaker: Mr Hiroshi Oba (Graduate School of Science and Technology, Niigata University)

16 Atomic picture versus covalent picture for the breaking of N=8 magic number in 12Be

The breaking of N=8 magic number in 12Be is suggested by recent experimental observations (anormalous deformation length, and low-lying 1- and excited 0+ and so on). In the present report, we will discuss the low-lying states of this nucleius based on the microscopic cluster model of alpha+alpha+4N. We will investigate the breaking of N=8 magic number from the viewpoints of the covalent picture and the atomic or ionic one. In the covalent picuture, we employ the so-called molecular orbital (MO) model which is successul to describe the many kinds of properties in 9,10Be. In the MO picture, the competition between the normal configuration ((0p)6) and the 2hw one ((0p)4(sd)2) occurs, and the latter state becomes the ground states due to the strong effect of the spin-orbit interaction. On the other hand, in the atomic (ionic) picutre, we perform the coupled channel calculation among the 4He+8He, 6He+6He, and 5He+7He. We will show the former two configurations (4He+8He and 6He6He) is insufficient to describe the normal and 2hw configuration simultaeously, and the odd-odd cluster, 5He+7He, plays important role for the formation of a pair of the 0+ state. We will also discuss the monopole transition between the ground 0+ and excited one from the viewpoint of the covalen picutre where the delicate competition between the normal configuration and the 2hw one occurs.

Speaker: Dr Makoto Ito (RIKEN Nishina Center Accelerator based Science, RIKEN)

17 Continuum effects for the low-lying states of drip-line oxygen isotopes

Recently, neutron unbound low-lying states in oxygen isotopes near the drip line have been measured in several experiments. The conventional　nuclear shell-model calculation, one of the most reliable methods in　this mass region, cannot sufficiently explain the results. We therefore　investigate the continuum effects, which have not been considered　directly in the conventional shell model, for the low-lying　states in oxygen isotopes near the drip line. A continuum single-particle basis is introduced and shell-model calculation is performed using　this basis in a small model space. The calculated excitation strengths　of the low-lying states in $^{23-26}$O　are compared to those of the experiments which have recently been performed. If we restrict the single-particle wave functions　to those of the harmonic oscillator, the results come back to those of　the conventional shell-model calculation. Thus our method can evaluate　the deference between shell model with and without continuum effects.　The importance of the contribution from the continuum　states, especially from the resonance state, is clearly seen. The　many-body perturbation theory to derive the effective interactions from the bare　nucleon-nucleon potential is also discussed. Gamow basis, which　includes not only bound states but also resonant states and　non-resonant scattering states, is used for the construction of the　effective interaction in several model spaces. The present state of　this approach is shown.

Speaker: Mr Koshiroh Tsukiyama (Department of physics, The University of Tokyo)

18 Effects of thermal fluctuations and angular momentum on nuclear pairing properties

The effects of angular momentum on pairing in nuclei are studied within the so-called FTBCS1 theory, which is an extension of the finite-temperature BCS theory including the thermal fluctuations such as quasiparticle-number fluctuations (QNF). For simplicity, the angular momentum projection M is introduced instead of the total one. The numerical calculations are carried out within a doubly degenerate equidistant model with a constant pairing interaction G. The results obtained show that, at constant values of M, the pairing gaps given by the FTBCS1 theory do not collapse at critical temperature Tc as predicted by the conventional FTBCS one but decrease monotonously with increasing temperature T. At high enough values of M, there appears the so-called thermally assisted pairing correlation or anomalous pairing, in which the pairing gap is zero at T < Tc1 and reappears at T = Tc1 and remains finite at T > Tc1. These features are caused by the QNF within the FTBCS1 theory.

Speaker: Mr Nguyen Quang Hung (Heavy Ion Nuclear Physics Laboratory, RIKEN Nishina Center)

19 Pygmy resonance in neutron-rich Ne isotopes

Microscopic structure of the low-lying isovector dipole excitation mode in neutron-rich $^{26,28,30}$Ne is investigated by performing deformed quasiparticle-random-phase-approximation (QRPA) calculations. The particle-hole residual interaction is derived from a Skyrme force through a Landau-Migdal approximation. We have obtained the low-lying resonance in $^{26}$Ne at around 8.5 MeV. It is found that the isovector dipole strength at $E_{x}<10$ MeV exhausts about 6.0\% of the classical Thomas-Reiche-Kuhn dipole sum rule. This excitation mode is composed of several QRPA eigenmodes, one is generated by a $\nu(2s^{-1}_{1/2} 2p_{3/2})$ transition dominantly, and the other mostly by a $\nu(2s^{-1}_{1/2} 2p_{1/2})$ transition. The neutron excitations take place outside of the nuclear surface reflecting the spatially extended structure of the $2s_{1/2}$ wave function. In $^{30}$Ne, the deformation splitting of the giant resonance is large, and the low-lying resonance is overlapping with the giant resonance.

Speaker: Dr Kenichi Yoshida (Department of Physics, Kyoto University and Institut de Physique Nucleaire, Universite Paris Sud)

20 Lifetime measurements of excited states in the weakly-bound nucleus 17C

Lifetime measurements have been performed for excited states in the neutron-rich carbon isotope 17C. Recently, several experimental studies have been dedicated to investigate the low-lying structure of 17C, revealing a couple of unique features inherent in the weakly-bound states located below the extremely low neutron emission threshold at 730 keV. The spin-parity of the ground state was assigned to be 3/2+ [1,2], which is in contradiction with the naive shell model expectation that the ground state of an odd nucleus with N=11 should have the spin-parity of 5/2+. Close to the anomalous ground state, two excited states were found to be almost degenerate at excitation energies of about 220 keV and 330 keV, respectively [3]. In contrast with the degeneracy, however, a large asymmetry of the excitation cross-sections was observed in a study using proton inelastic scattering [4], where the excitation strength for the first excited state is much smaller than that for the second excited state. These experimental findings suggest an emergence of an intriguing structure in the low-lying states of 17C. The present work aims to elucidate the low-lying structure of 17C by studying the electromagnetic transitions between the bound states. The electromagnetic deexcitation strengths for the first and second excited states have been determined by means of the gamma-decay lifetime measurements. We successfully conducted the measurements by employing the recoil shadow method (RSM) with intermediate-energy radioactive-isotope beams [5,6]. The experiment was performed at the RIPS facility in RIKEN. In the present study, we populated the excited states of 17C by break-up reactions of 18C at 79 AMeV on a 9Be target. A 110-AMeV 22Ne primary beam impinged on a 1.02-g/cm^2 9Be production target for projectile fragmentation reactions. A high-intensity 18C beam of about 2.3x10^4 counts per second was obtained, and directed onto a 370-mg/cm^2 9Be reaction target set at the final focal plane of RIPS. Outgoing particles were detected and identified by means of the TOF-dE-E method using a plastic scintillator hodoscope, located 3.8 m downstream of the target. The deexcitation gamma rays were detected by 130 NaI(Tl) detectors in coincidence with scattered 17C particles. Energy thresholds of the detectors were set to as low as 150~200 keV for gamma rays in the projectile frame. In order to implement the RSM, a 5 cm-thick lead slab was installed close to the secondary target. In the present talk, the measured lifetimes of the first and second excited states will be presented. The reduced electromagnetic transition probabilities for the two deexcitations will be discussed in relation to the low-lying structure of 17C. References [1] J. P. Dufour, et al., Z.Phys. A324 (1986) 487. [2] H. Ueno, et al., Nucl.Phys. A738 (2004) 211. [3] M. Stanoiu, et al., Eur.Phys.J. A20 (2004) 95. [4] Z. Elekes, et al., Phys.Lett. B614 (2005) 174. [5] N. Imai, et al., Phys.Rev.Lett. 92 (2004) 062501. [6] H. J. Ong, et al., submitted to Physical Review C; arXiv:0711.4062 [nucl-ex].

Speaker: Mr Daisuke Suzuki (Department of Physics, University of Tokyo)

21 High-spin structure of neutron rich Se and Ge isotopes

The band structure of the Se and Ge isotopes around the mass 80 is studied in terms of the full-fledged shell model. The monopole and quadrupole pairing plus quadrupole-quadrupole interaction is employed as an effective interaction. As for single-particle levels, all the four orbitals in the major shell between the magic numbers 28 and 50 are taken into account for both neutrons and protons. The calculation reproduces the experimental energy levels of high-spin states as well as low-lying states well. The shell model results are examined in a pair-truncated shell model, where the full shell model space is restricted within the subspace of collective pairs. The results of these calculations will be presented and discussed in this symposium.

Speaker: Dr Koji Higashiyama (Chiba Institute of Technology)

22 Systematic study of electric dipole excitations with fully self-consistent Skyrme HF+RPA from light to medium-mass deformed nuclei

Radioactive isotope facilities of the new generation enable us to access unexplored territories of unstable nuclei with large neutron excess. Theoretical studies and predictions of properties of unknown nuclei are important subjects in nuclear structure physics. In order to clarify peculiarities of these exotic nuclei, we need systematic calculations across the entire nuclear chart. The nuclear density-functional approach provides a promising tool for this purpose. So far, the ground-state properties of even-even nuclei have been investigated with use of the Hartree-Fock-Bogoliubov calculation. Systematic investigations for excited 2^+ states have been recently done with a few groups using different methods and different functionals We have carried out systematic calculations of the electric dipole modes of excitation up to Ni isotopes in the self-consistent Skyrme-Hartree-Fock (SHF) plus RPA approach. We solve the equations in the three-dimensional Cartesian-coordinate-mesh representation without any spatial symmetries. The fully self-consistent RPA are realized using an iterative method we have recently developed; the finite amplitude method (FAM). The method allows us to treat both spherical and deformed nuclei on an equal footing and simultaneously to avoid explicit evaluations of complex residual fields. We will show systematics of the centroid energies, widths, and deformation splitting of the giant dipole resonances, in comparison with experiments. We also discuss variations of the low-lying dipole mode in neutron-rich and proton-rich nuclei.

Speaker: Dr Tsunenori Inakura (Univ. fo Tsukuba)

23 Real-time calculation of response function with TDHF+BCS

We present a theoretical study of nuclear response using a method that is applicable to systems from light to heavy nuclei systematically; The real-time calculation of the time-dependent Hartree-Fock+BCS(TDHF+BCS) in the 3-dimensional mesh representation. The method is a simple extension of the well-known TDHF method but is able to describe various responses of deformed nuclei with pairing effects. We show results of our first application to isoscalar monopole and quadrupole modes of excitation for even-even nuclei restricted to N=Z, from Carbon 12 to Zinc 60. Coupling between monopole and quadrupole giant resonances in deformed nuclei is clearly seen in our systematic analysis of the monopole resonance.

Speaker: Mr Shuichiro Ebata (Institute for Physics, Univ. of Tsukuba)

24 Isomeric states in stable and neutron-rich odd-A Sb and I isotopes

Isomeric states in atomic nuclei are unique probes which reveal various aspects of the microscopic structure of the nucleus and the nature of nuclear interactions. This presentation will focus on characteristic isomers in stable and neutron-rich odd-A antimony and iodine isotopes. The nuclei of interest were investigated by means of time-correlated gamma-ray spectroscopy with the GAMMASPHERE array, in combination with deep-inelastic reactions with 136Xe beams incident on thick targets. New results achieved in the present work include the identification of new isomers in 131,133I and odd-A Sb isotopes with the mass range A=121-127. For the stable nucleus 121Sb, complementary experiments were carried out using 7Li beams at ANU. These features will be discussed.

Speaker: Dr Hiroshi Watanabe (RIKEN)

25 Quadrupole collectivity in $^{20}$Mg

The Coulomb excitation of the very proton-rich nucleus $^{20}$Mg was studied using a radioactive $^{20}$Mg beam at 58 $A$ MeV impinging on a lead target. The reduced transition probability $B$(E2; $0^+_{gs}\rightarrow 2^+_1$) was extracted. The ratio of the neutron-to-proton multipole matrix elements $M_n/M_p$ was deduced by comparing the $B$(E2) with that for the mirror nucleus $^{20}$O assuming the isospin symmetry. The results confirm the large $M_n/M_p$ in $^{20}$O, leading to a large isovector component for the transition.

Speaker: Prof. Naohito Iwasa (Department of Physics, Tohoku University)

26 New formulation of Interacting Boson Model and the structure of exotic nuclei

Recently, various exotic nuclei with intriguing collectivities have been created, which motivate us to investigate the properties of neutron-rich A$\geq$200 region nuclei far from the stability line. We study the low-lying collective states, especially the quadrupole deformations, of these heavy unstable nuclei within the interacting boson model (IBM). A conventional way for the derivation of the IBM Hamiltonian has been based on the nuclear shell model. However, the effective interaction of the shell model is so sensitive to the precise structure of nuclear force, which is not important to describe quadrupole collective states, that it's difficult to discuss only the properties of quadrupole collective modes robustly. In this study, we demonstrate that constrained Hartree-Fock+BCS method (HF+BCS) is more suitable for constructing a model of Hamiltonian for the description of the low-energy collective excitations. The parameters of the IBM Hamiltonian are determined so that its expectation value reproduces the potential energy surface calculated by HF+BCS with Skyrme-type interaction. This procedure is a new derivation of the IBM Hamiltonian, and quantitatively describes the low-lying states of medium-heavy nuclei in wide area of the nuclear chart, including unstable region at issue. Excitation spectra of several nuclei with U(5), SU(3) and O(6) dynamical symmetries, along with the newly proposed X(5) and E(5) critical-point symmetries, are well described. We suggest the manifestations of transitions from spherical to rotational shapes in neutron-rich (A$\geq$200) Os and W isotopes and the corresponding critical point. We also discuss the further applications, for instance, to the configuration mixing in light Pb-Hg-Pt region isotopes.

Speaker: Mr Kosuke Nomura (University of Tokyo)

27 High-spin states of 93Nb

High spin isomers are known in N=83 isotones systematically. These isomers are considered to be shape isomers caused by sudden shape changes from near spherical to oblate shapes. In order to search for high-spin isomers in other mass region, the 90Zr region was selected. Comparing the spherical single particle orbits near Fermi surfaces of nuclei in these two regions, there are similarities both for protons and neutrons. Then the existence of the isomers with the same origin as those in N=83 isotones may be expected in nuclei of 90Zr region. The high-spin states of 93Nb were studied via the 82Se(16O, p4n)93Nb reaction. The level scheme of 93Nb was extended up to 11.0 MeV in excitation energy. Twenty-three g-rays and 19 levels were newly found using gg coincidence data. States lying near the yrast line were interpreted using a weak coupling picture of a g9/2 proton to the excited states of a 92Zr core. An M1 rotational band was found starting from the 37/2(-) state. This band shows characteristics of a collective oblate band.

Speaker: Dr Yasuo Wakabayashi (Center for Nuclear Study, University of Tokyo)

Friday 4 April

Collectivities and Shell effects in neutron/proton-rich nuclei: III

28 Spherical or deformed - prolate or oblate ?

Speaker: Prof. I. Hamamoto

29 TBA

Speaker: Prof. G. de France (GANIL)

30 TBA

Speaker: Prof. D.C. Radford

10:30 Coffee break

Isomeric states

31 Isomeric decay spectroscopy at RISING

Decay spectroscopy following projectile fragmentation constitutes one of the most sensitive methods of studying exotic nuclei. It enables nuclear structure investigations for species produced with very low yields (few nuclei an hour). Exotic nuclei were synthesised using relativistic projectile fragmentation. The fragments were separated and unambiguously identified event-by-event using the GSI FRagment Separator. The final reaction products were stopped either in a passive stopper or a stack of double-sided Si strip detectors at the final focal point of the FRS and viewed by the high-efficiency, high granularity Stopped RISING gamma-ray spectrometer, consisting of 15 Euroball cluster Ge-detectors. Time-correlated gamma decays from individually identified nuclear species have been measured, allowing the clean identification of isomeric decays in a wide range of exotic nuclei both at the proton drip-line and in heavy, neutron-rich systems. Highlights of the experimental programme include: (i) the first observation of excited states in the N=126 closed-shell nuclei 205Au, 204Pt and 203Ir following the internal gamma-ray decay from isomeric states. (ii) internal conversion electron spectroscopy in 205Au from a long-lived (seconds) isomeric state. (iii) isomeric decay in the probably oblate nucleus 198Os. (iv) decays from the previously reported isomeric I=27 and I=(49/2) states in 148Tb and 147Gd, respectively. These isomeric decays represent the highest discrete spin states observed to date following a projectile fragmentation reaction, and opens up the possibility of doing high-spin physics using this technique. (v) internal isomer decay in the r-process waiting point nucleus 130Cd [1] (vi) studies of medium-mass N~Z nuclei from both internal [2] and beta-delayed gamma-ray decay. The experimental setup as well as selected highlights of the experimental results, together with theoretical interpretations, from these highly successful experiments will be presented. [1] A. Jungclaus et al., Phys. Rev. Lett. 99(2007) 132501. [2] A.B. Garnsworthy et al, Phys. Lett. B660 (2008) 326.

Speaker: Zsolt Podolyak (University of Surrey)

32 Gamma-ray spectroscopy at Osaka

We are performing the experimental studies on high-spin isomers [1]. These isomers have been reported in $N$=83 isotones with $Z$=60$-$67. The spins of these isomers are 49/2$^+$ and 27$^+$ for odd and odd-odd nuclei, respectively. Their shapes were known to be oblate with a deformation parameter $\beta$$\sim$$-$0.2. These isomerism were understood to be caused by the sudden shape change from near spherical to oblate shapes. The experiments to search for the isomers of the same origin in other isotopes of $A$$\sim$90 and $A$$\sim$150 near $\beta$-stability line were made using the secondary beam line, EN course [2] at RCNP, Osaka University. Although EN course was developed as the fragment mass separator, experiments using low-energy heavy ion beams have been successfully performed. Experiment to search for isomers in nuclei near $A$$\sim$90 based on the recoil catcher method was carried out using EN course as a recoil mass separator. Reaction products produced by the fusion reaction of $^{13}$C + $^{86}$Kr with the beam energy of 7.4 MeV/u were transported through $\sim$16m down to a catcher with flight time of $\sim$500ns. A Ge detector ball which consists of 14 coaxial-type detectors was installed at the catcher position. Gamma-rays emitted from isomers and daughter nuclei of the $\beta$ decays were clearly observed. The RI beam of $^{17}$N was also developed to search for isomers in nuclei near $A$$\sim$150. The neutron rich RI beam is necessary to produce high-spin states by the fusion reaction in nuclei near $\beta$-stability line. The $^{17}$N beam was produced by the $^9$Be($^{18}$O, $^{17}$N)$^{10}$B reaction with the primary beam energy of 9.1 MeV/u. This secondary beam of 5.3 MeV/u was separated from the primary $^{18}$O beam and was transported up to secondary achromatic focal plane. The $^{17}$N beam intensity was $\sim$10$^4$$\sim$10$^5$ pps. The recent experimental results obtained by using the secondary beam line EN course and Ge ball at RCNP will be presented. [1] A. Odahara, et al., Phys. Rev. {\bf C 72} ( 2005 ) 061303( R ). [2] S. Mitsuoka et al., Nucl. Inst. and Meth. {\bf A 372} ( 1996 ) 489. T. Shimoda et al., Nucl. Inst. and Meth. {\bf B 70} ( 1992 ) 320.

Speaker: Atsuko Odahara (Department of Physics, Osaka University)

33 High-spin isomers in nuclei around the N=82 shell sclosure

The recent observation of high-spin isomers in Neodymium nuclei around the N=82 shell closure triggered new experimental investigations aiming to the identification of other similar isomers in the neighboring nuclei. The results of lifetime measurements for the 6-qp 20+ isomer in 140-Nd and for the 3-qp isomer above the 19/2+ state in 139-Nd represent a strong support to the cranked Nilsson-Strutinsky calculations. New experiments for the search for high-spin isomeric states in the nuclei around the N=82 shell closure will be discussed.

Speaker: Prof. Costel Petrache (Institute de Physique Nucleaire, CNRS-IN2P3 and University Paris Sud)

12:00 Lunch

Advanced gamma-ray detector: Ⅰ

Advanced gamma-ray detector

34 The 8pi and TIGRESS Gamma-Ray Facilities at TRIUMF-ISAC

The Isotope Separator and Accelerator (ISAC) produces and extracts radioactive ion beams by the ISOL technique. These radioactive-ion beams are used at a variety of experimental stations for nuclear structure, nuclear astrophysics, or material science studies. Two large arrays of high energy-resolution HPGe detectors are sited at ISAC. Low-energy beams can be delievered to the 8pi spectrometer that has been retrofitted for decay spectroscopy with an emphasis on high-precision lifetime and branching ratio measurements of superallowed Fermi beta decay. Reactions with accelerated beams, to energies beyond the Coulomb barrier, are studied with the TIGRESS array that has completed data-taking for two spectroscopy measurements to date. This talk will summarize the configuration and performance of the arrays, with discussion of specific scientific highlights and some future initiatives.

Speaker: Dr Greg Hackman (TRIUMF)

Slides g.ppt

35 Possibility of gamma-ray spectroscopy on hypernuclear resonance states with heavy ion collisions

Recently, experiments to study hypernuclei by using induced reactions of stable heavy ion beams and rare isotope beams have been proposed at GSI by the HypHI collaboration. In the HypHI experiments, hypernuclei are formed by coalescence of hyperon(s) produced in the participant region of heavy ion collisions into projectile fragments, therefore, it is possible to produce neutron/proton rich hypernuclei. The HypHI collaboration currently prepares for the first Phase 0 experiment for light hypernuclei, which should take place in the beginning of 2009. Because of large momentum transfer to produced hypernuclei in heavy ion collisions, collective and dynamical excitation of hypernuclei could be populated, which can not be achieved by conventional hypernuclear productions with meson and electron beams. Since the velocity of produced hypernuclei is as large as the one of projectiles (at least 94 % of speed of light), it is impractical to perform gamma-ray spectroscopy for low energy collective motion by observing discrete gamma-lines, while the gamma-ray spectroscopy for the resonance states such as GDR by observing high energy gamma-rays by BaF2 detectors could be feasible. In the presentation, over view of the hypernuclear spectroscopy with heavy ion collisions and the idea of the spectroscopy for resonance states for neutron/proton rich hypernuclei will be discussed.

Speaker: Dr Takehiko Saito (GSI)

36 Development of a waveform readout system for Ge detectors in hypernuclear gamma-ray spectroscopy

For a hypernuclear gamma-ray spectroscopy experiment at the J-PARC K1.8 beam line, a new Ge detector array (Hyperball-J) is under construction. Hyperball-J consists of 32 Ge detectors that have a transistor-reset preamp. The reset-type preamp is necessary to withstand the high energy deposit rate in the experiment caused by penetrations of high energy beam particles. The maximum beam intensity of J-PARC is expected to reach to 10 MHz which is 5 times larger than that of KEK-PS. Current readout circuit including pulse height ADC will fail from increased baseline shifts after the preamp reset and pileup events. As a new readout method, a waveform readout system is being developed. At the present stage, We have succeeded in reading correct energy by fitting to a digitized waveforms. When shaping time is 2 usec, energy resolution is FWHM 3.5 keV for 1 MeV gamma ray. The minimum separation of two pulses that can be resolved is 1 usec. In this presentation, I will discuss an algorithm that has been developed and report on the performance of the new readout method for J-PARC experiment.

Speaker: Mr kenji hosomi (Department of Physics, Tohoku University)

14:30 break

37 Medium-resolution array DALI and its use at RIBF

Speaker: Dr Tohru Motobayashi (RIKEN Nishina Center)

38 Development of CNS GRAPE and experiments at RIBF

We have been developing a position sensitive germanium (Ge) detector array, CNS GRAPE (Gamma-Ray detector Array with Position and Energy sensitivity) for high-resolution in-beam gamma-ray spectroscopy using RI beams. In order to correct for the Doppler broadening effect from the fast moving reaction products, the array was designed to have position sensitivities in the Ge crystal by using the pulse shape analysis techniques. The total array consists of 18 detectors and each of which contains two Ge planar crystals with effective radius of 3 cm and thickness of 2 cm. The outer side of each crystal has 3×3 electrodes [1]. The planar structure and the segmenting electrodes bring us different pulse shapes depending on the depth of interaction. The resolution of less than 1% for v/c = 0.3 can be achieved after Doppler shift correction. The total efficiency of 5 % for 1 MeV gamma ray is expected. This array will be used at RIBF facility for the in-beam gamma-ray spectroscopy. In the talk, present status of CNS GRAPE and experiments and future develpment will be discussed. [1] S. Shimoura, Nucl. Instrum. Methods A 525 (2004) 188.

Speaker: Dr Eiji Ideguchi (CNS, University of Tokyo)

15:35 Coffee break

Advanced gamma-ray detector: Ⅱ

Advanced gamma-ray detector

39 Status and Future Plan of GRETINA/GRETA

GRETINA, the US gamma-ray tracking detector project, will use segmented Ge detectors to cover 1/4 of the 4-pi solid angle. It is scheduled to be completed in 2011. The current status, including the production and testing of segmented detectors, the use of digital electronics for data acquisition, results of signal decomposition and tracking, will be reported. A plan for GRETA, an array with 4-pi coverage, has been developled by the US nuclear structure community and was included in the 2007 US DOE/NSF Long Range Plan. Directions of future development including international collaboration will be discussed.

Speaker: Dr I-Yang Lee (Lawrence Berkeley National Laboratory)

40 in-beam gamma spectroscopy at intermediate energies: future Instruments and methods

From the experience gained in in-beam gamma spectroscopy using reactions at Intermediate energy and induced by rare isotopes, I will try to describe the various possibilities that will be offered in the future at RIKEN. I will present few experimental methods in which some future state of the art gamma detection instruments, such as AGATA, PARIS etc.., can be used to address important physics cases.

Speaker: Prof. faisal Azaiez (IPN-Orsay)

41 PARIS: novel scintillator array for medium resolution gamma spectroscopy

The measurement of high energy gamma rays with high resolution has always been experimentally challenging, with the best resolution obtainable from a scintillator detector being around 10% from sodium iodide. The novel scintillator material LaBr3(Ce) promises a step-change in what is achievable using scintillator detectors with an unprecedentedly high resolution of <3%. The PARIS array [1] is intended to comprise a double shell of this novel material and more conventional scintillator material. The array could be used in a stand-alone mode or in conjunction with an inner particle detection system (GASPARD, FAZIA), or with high-purity germanium arrays such as EXOGAM or AGATA. Initial designs and simulations for PARIS will be discussed as well as the potential Physics opportunities. The latter will focus on aspects such as the study of giant resonances, as well as study of intermediate-energy fragmentation and Coulomb excitations. The host laboratory for PARIS will be GANIL, nevertheless dedicated campaigns elsewhere (for example FAIR or RIKEN) are envisaged. [1] http://paris.ifj.edu.pl

Speaker: Prof. Adam Maj (IFJ PAN)

42 Discussion

18:00 Party

Saturday 5 April

Stability of Very Heavy Nuclei

43 Spectroscopy of Very Heavy Elements

A stringent test for predictive power of current nuclear structure theories is provided by the study of deformed nuclei in the region of 254No. These nuclei are the heaviest for which detailed in-beam and decay spectroscopy can be performed. Initial in-beam measurements in the region focussed on gamma-ray spectroscopy of even-even nuclei (e.g. 252,254No, 250Fm), studying the ground-state yrast bands and allowing extraction of parameters such as the moments of inertia, and proving the deformed nature of these nuclei [1-4]. More recently, attention has switched to odd-mass nuclei such as 253No, 251Md and 255Lr, the latter being the heaviest nucleus so far studied in-beam [5-7]. Rotational bands have been observed in all these nuclei. The success of such in-beam gamma-ray spectroscopic studies is strongly dependent on the gK value of the odd particle, as the M1/E2 branching ratio is determined by (gK-gR/Q0). If the configuration is such that M1 transitions dominate, strong internal conversion precludes the observation of gamma-rays. Non-yrast and K-isomeric states have recently been observed in 252,254No and 250Fm through the use of both in-beam and focal plane decay spectroscopy [8-11]. The studies employed a calorimetric technique suggested by Jones, whereby the summed energy from a cascade of conversion electrons is detected in a DSSSD detector and used as a “tag” for gamma-rays detected in the various germanium detectors [12]. These experiments have yielded data which can be used to determine the excitation energies and configurations of two-quasiparticle states in the region, and compared to the predictions of various theories. Many challenges for in-beam spectroscopy of these heavy nuclei still lie ahead and developments of new spectrometers and associated electronics are emerging from development. Current results and future perspectives of these devices will be discussed. An overview of the most recent results and the experimental techniques used will be presented.

Speaker: Dr Pete Jones (University of Jyväskylä)

44 Gamma-ray spectroscopy of heavy-actinide and transactinide nuclei: toward more neutron-rich and heavier nuclei

Gamma-ray spectroscopy of heavy-actinide and transactinide nuclei is extremely difficult because of their very small production cross sections and severe backgrounds mainly arising from fission channel. Recently, some experimental approaches have overcome these difficulties, and opened a door to detailed nuclear structure studies for such heavy nuclei. For example, unsafe Coulomb excitation experiments with actinide targets and GAMMASPHERE successfully studied high-spin states in heavy actinide nuclei up to Cm [1]. In-beam gamma-ray spectroscopy with a recoil-decay tagging technique using recoil separators and cold-fusion reactions extended the frontier of gamma-ray spectroscopy up to No isotopes [2]. An isomer-gamma coincidence technique using recoil separators is also very powerful to observe excited states in heavier nuclei than Fm [2], and alpha-gamma coincidence measurements using recoil separators have achieved decay studies of Rf and Sg isotopes [3]. The latter three experiments utilize recoil separators, which restricts available production reactions to cold-fusion reactions that can produce nuclei only in the neutron-deficient side. To study more neutron-rich and heavier nuclei, we have performed two kinds of experiments at the JAEA tandem accelerator facility; one is alpha-gamma coincidence spectroscopy using a gas-jet transport technique and hot-fusion reactions with actinide targets, and the other is in-beam gamma-ray spectroscopy using transfer reactions with heavy actinide targets. Alpha-coincident gamma rays were successfully measured in alpha decays of 255,257,259No and 261Rf, and spin-parities and neutron single-particle configurations of their ground states as well as excited states in their daughter nuclei were identified [4]. This is the first spin-parity assignments for the nuclei in the Z > 101 and N > 153 region. In the in-beam gamma-ray spectroscopy, ground-state bands of neutron-rich 236Th, 240,242U, 246Pu, and 250Cm nuclei were established for the first time, and excited states in some odd-mass nuclei were also established [5]. In this talk, some interesting results and future plans of our experiments will be presented. References [1] I. Wiedenhöver et al., Phys. Rev. Lett. 83, 2143 (1999). [2] R. Julin, Acta Phys. Pol. B 38, 1515 (2007), and references therein. [3] B. Štreicher et al., Acta Phys. Pol. B 38, 1561 (2007). [4] M. Asai et al., Phys. Rev. Lett. 95, 102502 (2005). [5] T. Ishii et al., Phys. Rev. C 72, 021301(R) (2005); J. Phys. Soc. Jpn. 75, 043201 (2006); Phys. Rev. C 76, 011303 (2007); H. Makii et al., Phys. Rev. C 76, 061301(R) (2007).

Speaker: Dr Masato Asai (Japan Atomic Energy Agency)

45 Isomer Spectroscopy of the Heaviest Elements

The existence of superheavy elements implies that there are substantial shell effects, beyond the macroscopic liquid drop energy, which stabilize the nucleus against fission. The specific “magic” proton and neutron numbers, representing major spherical shell gaps, are a matter of considerable debate. Shell gaps can also occur when the nucleus distorts to non-spherical shapes leading to enhanced stability at particular deformations. It is well established that nuclei near Z=100, N=152 (252Fm) have well-deformed prolate shapes. Orbitals that originate from above the predicted shell gaps can intrude close to the Fermi surface of these deformed nuclei. There are also many high-K orbitals, which lie close to both the proton and neutron Fermi surfaces. This favors the occurrence of high-K multi-quasiparticle isomeric states at low excitation energy. By identifying such high-K states, and studying their decay to states with lower-K, we can learn about the single-particle structure, pairing correlations, and excitation modes in the heaviest nuclei. Experiments were carried out at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory and used the Berkeley Gas-filled Separator (BGS). High-K isomeric states have been identified in 255Lr (Z=103) and 256Rf (Z=104), representing the highest odd-Z and even-Z nuclei to be studied in this manner, to date. Detailed gamma-ray and electron decay spectroscopy has been performed. Three isomeric states have been discovered in 256Rf and their decay properties are strikingly different from the high-K states seen in the lighter N=152 isotones. A three quasi-particle isomer has been identified in 255Lr and its decay populates lower-lying rotational structures. The behaviors of these rotational bands provide new information on single-particle assignments and pairing properties. The implications of all the recent experimental studies (including these new LBNL results) on the structure of transfermium nuclei will be discussed.

Speaker: Dr Roderick Clark (LBNL)

46 Collective motion, rotation alignment, and K-isomeric states in very heavy nuclei: A theoretical study

Recent experimental advances have made it possible to study spectroscopy in very heavy nuclei. It has been suggested [1] that by studying the transfermium isotopes, in particular their excited structure, one can gain useful information on relevant single-particle states, which is the key to locating the anticipated `island of stability'. The study of rotation alignment of quasiparticles in superheavy elements sensitively probes the single particle states of the high- $j$ intruder orbits. These orbits are caused by the spin-orbit interaction, which is ultimately linked to the question of energy gaps in the single particle spectrum. We extend the applicability of the Projected Shell Model (PSM) [2] to the transfermium region. We study rotation alignment and the corresponding band-crossing phenomenon in Cf, Fm, and No isotopic chains, and propose observables to test the picture. Isomeric states in very heavy nuclei are particularly interesting because of their relatively long lifetimes. Xu {\it et al.} suggested [3] that the occurrence of isomeric states can enhance the stability of superheavy nuclei because the multi-quasiparticle excitations decrease the probability for both fission and $\alpha$-decay. Based on the successful PSM description for the $^{254}$No isomers [1], we give a systematical prediction for K-isomeric states in very heavy nuclei. Knowledge on vibrational states is useful for this less known mass region because of the interpretation of observed low-lying spectroscopy. We further show that in very heavy nuclei collective vibrations systematically appear as low-energy excitation modes [4]. We make a detailed prediction on $\gamma$-vibrational states and their E2 transition probabilities to the ground state band in the isotopes where active structure research is going on. Octupole effects on single particle and collective motions will also be discussed. [1] R.-D. Herzberg {\it et al.}, Nature {\bf 442}, 896 (2006). [2] K. Hara and Y. Sun, Int. J. Mod. Phys. E {\bf 4}, 637 (1995). [3] F.-R. Xu {\it et al.}, Phys. Rev. Lett. {\bf 92}, 252501 (2004). [4] Y. Sun {\it et al.}, submitted for publication.

Speaker: Prof. Yang Sun (Shanghai Jiao Tong University)

10:40 Coffee break

Exotic Deformation: II

47 Observation/Confirmation of hindered E2 strengths in 16,18C

The reduced E2 transition probability from the first excited 2+ (2^+_1) state to the ground state --- B(E2) --- of an even-even nucleus is an important observable that reflects proton collectivity. Recently, an anomalously small B(E2) was reported for the neutron-rich 16C nucleus [1]. The result points to a suppressed proton contribution to the transition strength. This finding raises an intriguing question as to whether or not the neutron contribution is similarly small for the relevant quadrupole excitation. Results from two subsequent experiments [2,3] suggest that the neutron contribution is ``normal''. When combined with the B(E2) value, the results indicate a neutron-dominant quadrupole excitation in 16C. To shed light on the exotic phenomenon exhibited by 16C and to explore the structural evolution of the carbon isotopes towards the neutron dripline, we have carried out an experiment to determine the B(E2) value for the neighboring 18C nucleus. To determine the B(E2) value for 18C, we have performed an experiment to measure the mean lifetime of the 2^+_1 in 18C using an upgraded recoil shadow method (RSM) [1]. The experiment was performed at the RIKEN Nishina Center using the RIPS beamline [4]. Besides 18C, we have also remeasured the mean lifetime of the 2^+_1 in 16C. The B(E2) values for 16,18C thus determined were about seven and five times smaller than the empirical values, indicating that the anomalously hindered E2 transition observed in 16C persists in 18C. Details of the experiment will be presented and the results will be discussed. Reference [1] N. Imai et al., Phys. Rev. Lett. 92 (2004) 062501. [2] Z. Elekes et al., Phys. Lett. B586 (2004) 34. [3] H. J. Ong et al., Phys. Rev. C 73 (2006) 024610. [4] T. Kubo et al., Nucl. Instrum. Methods B73 (1992) 309.

Speaker: Dr Hooi Jin Ong (RCNP, Osaka University)

48 Collectivity in light neutron-rich nuclei

The observation of either enhanced or reduced transition strengths (collectivity) can signal unexpected changes in nuclear structure. In light neutron-rich nuclei both phenomena have been reported and both have led to unexpected results and new insights. An example of the former is found in the “island of inversion”. First observed through an increased binding in 31Na, the N=20 isotones 30Ne, 32Mg, and 31Na are now known to exhibit large quadrupole transition strengths, B(E2)’s, consistent with large quadrupole (charge) ground-state deformations. This is despite N=20 being a pronounced spherical shell gap for stable nuclei (e.g. 40Ca). The onset of collectivity (deformed ground-states) for n-rich N=20 isotones is currently understood as a weakening of the N=20 shell gap due to the neutron-proton interaction. An example of the latter (reduced collectivity) has been reported in carbon and boron isotopes where reduced B(E2) values have been interpreted as a “decoupling'' of neutrons and protons. While the definition of decoupling may turn out to be subjective, the evolution of B(E2) values in neutron-rich carbon isotopes has received a great deal of attention recently, both experimental and theoretical. The B(E2) trend towards the drip-line is sensitive to, for example, the isospin dependence of effective charges, changes in shell gaps and possible differences in proton and neutron deformations. We will present data on both phenomena. • Data from experiments carried out at the NSCL to study the transition from “spherical” to “deformed” ground-states in Na and Ne nuclei will be discussed. The focus will be on the 2-proton knockout from 32Mg to 30Ne, both are considered to have deformed (intruder) ground states. 2-p knockout (from d5/2) has a distinctive spin dependence and we use thus to identify the 4+ state for the first time. Also for the first time we have used the fully mixed MSCM wavefunctions to calculate 2-p knockout cross-sections. The measured (quenched) 2p knockout cross-section, when compared to theory, may suggest a significant difference in the neutron intruder content between 32Mg and 30Ne, contrary to current shell models. • Data will be presented from a new lifetime measurement for the first-excited 2+ state in 16C. The experiment was carried out at the LBNL 88-Inch Cyclotron using the Recoil Distance Method and 9Be(9Be,2p) fusion-evaporation reaction. The mean lifetime was found to be 11.7(20) ps corresponding to a B(E2) of 4.15(73) e2fm4, consistent with other even-even closed shell nuclei and neighboring systematic, and provides an important benchmark for theory.

Speaker: Dr Paul Fallon (Lawrence Berkeley Laboratory)

49 Study of neutron decoupling phenomenon in 20C

In recent years, significantly different contribution of neutrons and protons to the first excited states has been observed in several, light, neutron-rich nuclei. Especially, carbon isotopes have been intensively studied, but 20C, the heaviest isotope reachable with the present experimental facilities, was not investigated. In order to make the measurements complete in the carbon isotopic chain, we have performed inelastic scattering experiments using proton and lead targets via gamma spectroscopic methods. Comparing the determined cross sections with those of coupled channel calculations, the neutron and proton deformation lengths have been derived. From these, the multipole neutron and proton transition matrix elements and consequently the contribution of neutrons and protons to the first excited state of 20C have been deduced.

Speaker: Dr Zoltan Elekes (MTA ATOMKI)

50 Proton Intruder State in $^{13}$B

We have measured the proton transfer reaction of $^{4}$He($^{12}$Be,$^{13}$B$\gamma$) with ``Gamma-Ray detector Array with Position and Energy sensitivity (CNS-GRAPE)'' in RIKEN. Analyzing the angular differential cross section, we assigned the $J^{\pi}$ of 4.83~MeV excited state to be $1/2{+}$ with $C^{2}S=0.2$, for the first time. This state is interpreted as a proton intruder state from the $sd$ shell. A shell model calculation with an interaction including the effect of the tensor force cannot represent this state, while the deformation explains its low excitation nergy and spectroscopic factor. The observed proton intruder state shows the change of the proton shell structure and indicates the importance of the deformation.

Speaker: Mr Shinsuke Ota (Center for Nuclear Study, University of Tokyo)

12:20 Lunch

Collectivities and Shell effects in neutron/proton-rich nuclei: Ⅳ

51 Perspectives of the shell model

Speaker: Prof. Takaharu Otsuka (CNS / Univ. of Tokyo)

52 Normal occupancy of deeply bound valence neutrons in 37Ca

The ground state and first excited 2+ state in proton-rich 36Ca have been studied at Ganil by gamma-ray spectroscopy of one-neutron knock-out reactions from deeply bound states in 37Ca at intermediate energy. The 2+ energy in 36Ca was found to be consistent with a sizeable N=16 gap similar to the Z=16 one observed in the mirror nucleus 36S. Partial cross-sections and momentum distributions of the knock-out reactions to both the ground state and the first excited 2+ state have been measured and the angular momentum of the two populated states identified. In contrast with previously reported cases, the extracted spectroscopic factors and their comparison to shell-model spectroscopic factors are found to be consistent with the trend observed for stable and near-magic nuclei.

Speaker: Serge Franchoo (IPN Orsay)

53 Spectroscopy of neutron-rich nuclei with MINIBALL

Experiments with exotic nuclei are the essential tool to investigate nuclear structure far-off stability. In particular, our research programme focuses on the study of the evolution of magic shell closures. These may change or even (dis)appear globally as function of isospin or locally because of a variation of the residual interactions. An important instrument for our studies employing gamma-ray spectroscopy is the highly efficient MINIBALL spectrometer consisting of 8 triple clusters of six-fold segmented HPGe crystals, each of them encapsulated individually in a vacuum-tight Al can. The clusters can be arranged in different configurations allowing to adapt the set-up to the experimental requirements. Furthermore, MINIBALL pioneered the use of digital electronics for gamma-ray spectroscopy. As experimental tools we utilised "safe'' Coulomb excitation and nucleon transfer reactions as well as nucleon knockout reactions at relativistic beam energies. The experiments have been performed at REX-ISOLDE (CERN, Switzerland) and GSI (Darmstadt, Germany). This contribution centres on recent results from the study of the region of the "island of inversion'", neutron-rich Ca and Ti isotopes for which a new shell closure is predicted at N=34 (or 32), the shell evolution from N=40 to N=50 for neutron-rich Ni, Cu and Zn isotopes, and the quadrupole collectivity of nuclei in the vicinity of the doubly-magic 132Sn. We will present the status of the research programmes and discuss the perspectives for future experiments. * This work is supported by the German BMBF under grant 06MT238, by the EU through EURONS (contract No. RII3-CT-2004-506065), and by the DFG cluster of excellence Origin and Structure of the Universe (www.universe-cluster.de).

Speaker: Dr Thorsten Kroell (TU Muenchen)

54 Microscopic description of shape coexistence/mixing phenomena in the A=80-100 region

We have investigated the oblate-prolate shape coexistence/mixing phenomena in proton-rich nuclei around 68Se and 72Kr by means of the adiabatic self-consistent collective coordinate (ASCC) method. For 68Se and 72Kr, it is shown that the collective path extracted from the TDHFB phase space of large dimensions runs approximately along the valley that exists in the triaxially deformed region and connects the oblate and prolate local minima in the collective potential energy surface. On the basis of the ASCC method, we have derived the quantum collective Hamiltonian which describes the coupled collective motion of the large-amplitude vibration responsible for the oblate-prolate shape mixing and the three-dimensional rotation of the triaxial shape. The calculation produces the oblate ground-state band and the excited prolate band. The basic pattern of shape coexistence/mixing phenomena is reproduced using the one-dimensional collective path in the two-dimensional beta-gamma plane. It is also shown that the shape mixing decreases as the angular momentum increases. Preliminary results of ASCC calculation for low-lying states of N=Z nuclei, such as the triaxial nucleus 64Ge and the spherical-oblate-prolate shape coexisting nucleus 80Zr, and the transitional nuclei around 100Mo will also be presented.

Speaker: Mr Nobuo HINOHARA (Kyoto University)

14:50 Coffee break

Exotic Deformation: Ⅲ

55 Pairing effect to band termination of Superdeformed band of 36-Ar

Superdeformed states have been populated exclusively with the fusion-evaporation reactions, which is suitable for a productin of very high-spin states. This is possible through neutron evapolations that do not take angular momentum away from the system. However, as a consequence, such a high-spin states can be produced only for neutron-deficient systems. A $N=Z$ nucleus near the magic numbers, such as $^{40}$Ca, has its ground state with a spherical shape. The Fermi levels sit in the last orbit of the so-called sd-shell, and most of the single-particle levels inside the sd-shell are filled. Hence, any excited states of this nucleus need to cross the shell gap beyond the sd-shell. But, once the gap is broken, substantial amounts of single-particle orbitals are available in the fp-shell, enabling strong collectivity. Consequently and surprisingly, this double-magic nucleus shows a contrasting deformation to the ground-state shape: neutron-richest superdeformation (so far) with $\beta\simeq 0.6$ identified by Ideguchi et al. in 2001 [1]. Prior to the one in $^{40}$Ca, a similar superdeformed structure was found in $^{36}$Ar [2]. A theoretical analysis based on the cranked Nilsson model predicts quickly growing triaxial deformation of the superdeformed state as the total angular momentum increases. The result was interpreted as the band termination caused by the full alignments of the the active valence orbitals. However, it is known that the rotational alignment is strongly influenced by the pairing correlation, which acts against the Coriolis force. Therefore, it is important to check the pairing effect to the band termination process. Such an investigation is possible with the Hartree-Fock-Bogoliubov method. Through numerical calcluations the pairing effect is studied in terms of how the band termination process is influenced. In my talk, the details of my analysis will be presented. [1] E. Ideguchi, et al., Phys. Rev. Lett. 87, 222501 (2001). [2] C. E. Sevensson, et al., Phys. Rev. Lett. 85, 2693 (2000).

Speaker: Dr Makito Oi (Department of Physics, University of Surrey)

56 Clustering and deformation in 40Ca

We have studied clustering and deformations in $^{40}$Ca using the antisymmetrized molecular dynamics (AMD) and generator coordinate method (GCM). The GCM basis are obtained by energy variation with two kinds of constraints for clustering and deformations, respectively. Superposing obtained wave functions, which form mean-field-type, $\alpha$-$^{36}$Ar, and $^{12}$C-$^{28}$Si structures, we obtain normal-deformed (ND), superdeformed (SD), and $\alpha$-$^{36}$Ar higher nodal states. We find that main components of the ND and SD states form triaxial shapes and the $K^\pi = 2^+$ side bands for the ND and SD states exist due to the triaxiality. Quadrupole electric transition strength $B(E2)$ and moments of inertia for intraband agree with experimental data. We find that ND and SD state contain $\alpha$-$^{36}$Ar and $^{12}$C-$^{28}$Si cluster structure component, respectively. $\alpha$-$^{36}$Ar higher-nodal states is obtained owing to excitation of inter-cluster motion between $\alpha$ and $^{36}$Ar clusters in ND state. This results suggest that clustering correlations are important for mechanism of excitations in medium-weight nucleus.

Speaker: Yasutaka TANIGUCHI (Kyoto Univ. (-March, 2008))

57 Structure of neutron-rich Mg isotopes studied through $\beta$-delayed $\gamma$-decay of polarized Na isotopes

Evolution of shell structures in a wide range of nuclear chart is one of the most important subjects in nuclear physics. In particular, breakdown of the $N=20$ shell closure and onset of collectivity as increasing neutron number in neutron-rich nuclei in the region of so-called ``island of inversion". In spite of intensive investigations for many years, little is known on the spin-parity of the levels in neutron-rich Mg isotopes. This situation has been preventing quantitative understanding of their structures. \par We have started systematic studies of neutron-rich Mg isotopes by using an effective method to assign spin-parity of their excited states. The essential is a use of spin-polarized Na isotopes. The allowed $\beta$-decay asymmetry, which strongly depends on the spins of the initial and final states, determines unambiguously the spins of the daughter states. The first application of this method was successfully made for the $\beta$-delayed neutron-$\gamma$ spectroscopy of polarized $^{11}$Li at the state-of-the-art ISOL-based radioactive nuclear beam facility ISAC of TRIUMF, which provides highly polarized alkali beams. Detailed analyses enabled firm spin-parity assignments of six levels in $^{11}$Be for the first time \cite{Hirayama05}. \par As the first step of the systematic studies on Mg isotopes, $\beta$-delayed $\gamma$-decays from polarized $^{28}$Na and $^{29}$Na have been observed in November 2007. Preliminary analyses suggested that a new level and a revised spin assignment for $^{28}$Mg and determined spin-parity of two levels in $^{29}$Mg for the first time. \par The results will be presented and comparison with shell model predictions will be discussed.

Speaker: Prof. Tadashi Shimoda (Department of Physics, Osaka University)

58 Closing

Speaker: Prof. Susumu Shimoura (CNS, University of Tokyo)