Probing Neutron-Proton Pair Correlations

19 Nov 2010, 09:00 → 20 Nov 2010, 19:40 Asia/Tokyo

Nishina Hall (Nishina Memorial Building, RIKEN Wako-campus)

Hiroyoshi Sakurai (RIKEN), Jenny Lee (RIKEN), Nori Aoi (RIKEN)

Probing Neutron-Proton Pair Correlations – pairing models, cross section measurements and reaction mechanisms Neutron-proton pairing correlations have been investigated with tremendous efforts. However, the nature and the interplay between T=0 and T=1 pairs are still the subject of debate because of the inconclusive experimental evidences. This situation may be about to change with the recent advent of radioactive N=Z beams. The purpose of the workshop is to organize a collaborative effort between experimentalists, structure and reaction theorists to advance the systematic study of neutron-proton correlations in the nuclei that are becoming accessible through rare isotope accelerators worldwide such as RIBF. Current status in experimental and theoretical studies will be presented with considerable time allocated to discussion about the prospects and road map on this subject. Plans of experimental and theoretical actions are expected from the workshop.

Map to RIKEN from Toyoko Inn / Super Hotel Access-map.pdf

Workshop final program PROGRAM-np-WS.pdf

Workshop photo Workshop_photo.jpg

Friday, 19 November

09:00 → 09:10 Opening remark

Speaker: Prof. Hiroyoshi Sakurai (RIKEN, Japan)

09:10 → 09:55 Overview of the np-pairing problems

Speaker: Prof. Augusto O. Macchiavelli (LBNL, USA)

Slides AOM_RIKEN-NP_final.pdf (Protected)

09:55 → 10:25 break

10:25 → 11:05 Spin-triplet neutron-proton pairing and its observation

Nuclei will form pair condensates of triplet pairs between neutron and protons in extremely large N=Z nuclei, but outside the limits of the nuclear chart. This is the conclusion of a study by Luo and myself (Phys. Rev. C81 064320 (2010)) in a model study of pairing in large nuclei. In the first part of the talk, I will describe these calculations. Whether or not theory predicts the exotic pairing in nuclei that can be formed in the laboratory, it is interesting to look for it if the experimental tools have the required sensitivity. The second part of my talk will be to address sensitivity issues. Essentially, the variation in cross sections due to the presence or absence of exotic pairing should be larger than the variations due to effects of a more kinematic nature (Q-values, single-particle orbital characteristics, ...).

Speaker: Prof. George Bertsch (U. Wash, USA)

11:05 → 11:45 Spatial correlation and enhanced pair transfers

Speaker: Prof. Masayuki Matsuo (Niigate, Japan)

Slides np-pairing-2-matsuo-up.pdf

11:45 → 12:25 Shell-Like APproach ( SLAP ) for pairing correlation based on Mean Field

Speaker: Prof. Jie Meng (Peking, China)

12:25 → 13:45 Lunch

13:45 → 14:25 Structure along the N=Z line studied by interacting shell models

Owing to fruitful structure variations along the N=Z line, it is interesting to study how these changes influence the consequences of neutron-proton knockout and transfer reaction experiments. To this end, one needs to calculate the physical quantities that are directly linked to observables by using shell model wavefunctions. In this talk, we mention two kinds of interacting shell models: the spherical shell model that deals with weakly deformed nuclei and the projected shell model that can treat well deformed nuclei. Structure variations such as the shape phase transition, shape coexistence, occurrence of isomers will be highlighted. An isospin invariant Hamiltonian with the extended P+QQ terms is discussed.

Speaker: Prof. Yang Sun (SJTU, China)

Slides 1011_RIKEN.pdf (Protected)

14:25 → 15:05 Proton-neutron pairing correlations in nuclei: a shell model perspective

Correlations in nuclei are generally well described by the shell model with residual interaction. The proton-proton (p-p) and neutron-neutron (n-n) paring interaction leads to the well-known odd-even effects, but the requirement of good isospin symmetry for the residual interaction may have as outcome proton-neutron (p-n) pairing effects, especially in N~Z nuclei. In my talk I will examine potential p-n pairing signals, such as the staggering of proton occupation probabilities, suggested by recent shell mode calculations.

Speaker: Prof. Mihai Horoi (CMU, USA)

Slides MHoroi-riken-111910.pdf

15:05 → 15:35 break

15:35 → 16:15 Aligned neutron-proton pairs in N=Z nuclei

It is shown that the aligned neutron-proton pair with angular momentum J=9 and isospin T=0 plays a central role in the low-energy spectroscopy of the N~Z nuclei approaching 100Sn. This observation is made in the context of the spherical shell model on the basis of a realistic two-nucleon interaction constructed for the g9/2 orbit. Shell-model results are analyzed in terms of a variety of two-nucleon pairs corresponding to different choices of their coupled angular momentum J and isospin T. The analysis is performed exactly for four holes (96Cd) and carried further for six and eight holes (94Ag and 92Pd) by means of a mapping to an appropriate version of the interacting boson model. On the basis of these results one concludes that a realistic model can be formulated in terms of s (with J=0) and b (i.e., aligned J=9) bosons. Due to its simplicity, such a model could be of use to elucidate the main structural features of N~Z nuclei in this mass region. Examples of simple predictions of such a model will be given.

Speaker: Prof. Pieter Van Isacker (GANIL, France)

16:15 → 17:15 Discussion Section -- structure theories on np-pairing

Speaker: Prof. Kenichi Matsuyanagi (Kyoto University, Japan)

Saturday, 20 November

09:00 → 09:30 Studies of np-pairing using transfer reactions

Speaker: Prof. Augusto O. Macchiavelli (LBNL, USA)

Slides AOM_RIKEN-NP-2_final.pdf

09:30 → 10:00 Experimental prospects - Probing np-pair correlations using knockout reactions

Speaker: Dr Jenny Lee (RIKEN, Japan)

Slides np-Knockout-lee.pdf (Protected)

10:00 → 10:30 break

10:30 → 11:10 Fast two-nucleon removal reactions and their sensitivity

Speaker: Prof. Jeff Tostevin (Surrey, UK)

Slides Tostevin.pdf

11:10 → 11:50 Simultaneous and sequential contributions to two-nucleon transfer reactions

Speaker: Prof. Ian Thompson (LLNL, USA)

Slides Riken10-a2-Thompson.pdf

11:50 → 13:10 Lunch

13:10 → 13:30 Proton-neutron tensor correlation studied by measuring spin-M1 strengths in self-conjugate even-even nuclei

H. Matsubara, CNS, Univ. of Tokyo, Japan H. Nakada, Chiba Univ., Japan A. Tamii, RCNP, Osaka Univ., Japan (presenter) and the RCNP E299 collaboration. We have measured spin-M1 strength distributions in stable self-conjugate even-even nuclei by measuring proton inelastic scattering at 295 MeV at forward angles. The target nuclei are 12C, 16O, 20Ne, 24Mg, 28Si, 32S, 36Ar, 40Ca. Each of isoscalar and isovector spin-M1 strengths were summed up to the excitation energy of 16 MeV. The summed strengths were divided by the square of the bare isoscalar and isovector g-factors, respectively, and the ratios of the isoscalar to the isovector strengths were obtained. The experimental result showed a nearly constant value of ~1.5, while shell model calculations predicted a nearly constant value of 1. The value of 1 corresponds to the case when the sums of the square of the nuclear matrix elements, M(sigma) and M(sigma tau), are the same. Thus the experimental data for M(sigma) are systematically larger than M(sigma tau). The result may be closely related to p-n tensor correlation effect in the ground states. I will report on the work.

Speaker: Prof. Atsushi Tamii (Osaka, Japan)

Slides npCorrealation_tamii.pdf

13:30 → 13:50 Formation of Gamow-Teller Resonance structure in Tz=0, odd-odd f-shell nuclei

We did \beta^{-}-type high-resolution (3He,t) experiment on Tz=1 f-shell target nuclei with mass numbers A=42, 46, 50 and 54 and studied the strength distribution of Gamow-Teller transitions in the final Tz=0 nuclei. The main strength moved from low-lying region to higher Ex region as a function of A, and a Resonance-like structure was formed in the A=54 system. This suggests that the nature of the residual p-n interaction changed from attractive to repulsive.

Speaker: Prof. Yoshitaka Fujita (Osaka, Japan)

Slides RIKENpn10ys.pdf

13:50 → 14:10 Correlated pp and pn pairs probed with O16(e,e'pN)

Speaker: Prof. Carlo Barbieri (Surrey, UK)

Slides RIKEN_np-pairing_Nov2010.pdf

14:10 → 14:30 Particle-number conserving analysis of rotational bands in 247,249Cm and 249Cf

The recently observed high-spin rotational bands in odd-$A$ nuclei $^{247,249}$Cm and $^{249}$Cf [Tandel et al., Phys. Rev. C 82 (2010) 041301] are investigated by using the cranked shell model (CSM) with the pairing correlations treated by a particle-number conserving (PNC) method in which the blocking effects are taken into account exactly. The experimental moments of inertia and alignments and their variations with the rotational frequency $\omega$ are reproduced very well by the PNC-CSM calculations. By examining the $\omega$-dependence of the occupation probability of each cranked Nilsson orbital near the Fermi surface and the contributions of valence orbitals to the angular momentum alignment in each major shell, the level crossing and upbending mechanism in each nucleus is understood clearly.

Speaker: Prof. Shan-Gui Zhou (ITP/CAS, Beijing, China)

Slides Zhou_Shan-Gui_20101120.pdf

14:30 → 15:00 break

15:00 → 17:00 Plans making – cross section measurements coupled to structure and reaction theories & future experiments for np pair correlations

Speakers: Prof. George Bertsch (U. Wash, USA), Prof. Nori AOI (RIKEN, Japan)

17:00 → 17:05 Closing

Speaker: Dr Jenny Lee (RIKEN, Japan)