Evolution of the N = 20 shell gap

Stucture of N = 17,18 nuclei has been investigated via proton inelastic scattering and neutron removal reactions up to the neutron dripline in inverse kinematics by use of radioctive ion beams provided by the RIKEN isotope separator, RIPS. Low energy excited states have been found in all the 26,27F and 27Ne nuclei, which are out of the neutron sd shell configuration space and are considered as intruder states arisng from cross shell excitations. Monte-Carlo shell model calculations with the sdpf-m interaction predict low energy negative parity excited states from neutron cross shell excitation at N = 17,18, which is in agreement with our experimental results. Observation of intruder states even at N = 17, especially the p3/2 state in 27Ne at 765 keV is a clear indication of a vanishing N = 20 shell gap at Z = 8 as predicted by the sdpf-m shell model calculations.

[1]  T. Kubo,et al.  Vanishing N = 20 shell gap: study of excited states in (27,28)Ne. , 2006, Physical review letters.

[2]  T. Kubo,et al.  Neutron-dominant quadrupole collective motion in C16 , 2006 .

[3]  B. Jurado,et al.  Shell gap reduction in neutron-rich N = 17 nuclei , 2006 .

[4]  S. Shimoura,et al.  Liquid hydrogen and helium targets for radioisotope beams at RIKEN , 2005 .

[5]  J. Duprat,et al.  Search for neutron excitations across the N=20 shell gap in Ne25-29 , 2005 .

[6]  T. Kubo,et al.  Quadrupole collectivity of 28Ne and the boundary of the island of inversion , 2005 .

[7]  B. A. Brown,et al.  ‘Magic’ nucleus 42Si , 2005, Nature.

[8]  P. Mantica,et al.  29Na: defining the edge of the island of inversion for Z=11. , 2004, Physical review letters.

[9]  S. Takeuchi,et al.  Bound excited states in 27F , 2004 .

[10]  D. Warner.  Nuclear physics: Not-so-magic numbers , 2004, Nature.

[11]  T. Otsuka,et al.  Onset of intruder ground state in exotic Na isotopes and evolution of the N=20 shell gap , 2004, nucl-th/0407082.

[12]  Á. Horváth,et al.  Decoupling of valence neutrons from the core in 17B , 2004 .

[13]  B. A. Brown,et al.  N=14 and 16 shell gaps in neutron-rich oxygen isotopes , 2004 .

[14]  T. Otsuka,et al.  Extreme location of F drip line and disappearance of the N = 20 magic structure , 2001 .

[15]  F. Nowacki,et al.  Shell model studies of neutron-rich nuclei , 2000, nucl-th/0011010.

[16]  J. Berger,et al.  Evolution of the N = 20 and N = 28 shell closures in neutron-rich nuclei , 2000 .

[17]  T. Otsuka,et al.  Varying shell gap and deformation inN∼20unstable nuclei studied by the Monte Carlo shell model , 1999 .

[18]  R. Ronningen,et al.  Role of intruder configurations in Ne and Mg , 1999 .

[19]  Yusuke Watanabe,et al.  Evidence for particle stability of 31 F and particle instability of 25 N and 28 O , 1999 .

[20]  F. Nowacki,et al.  Shell model study of the neutron rich nuclei around N = 28 , 1996, nucl-th/9608003.

[21]  Isao Tanihata,et al.  The RIKEN radioactive beam facility , 1992 .

[22]  R. Varner,et al.  A global nucleon optical model potential , 1991 .

[23]  Brown,et al.  Mass systematics for A=29-44 nuclei: The deformed A~32 region. , 1990, Physical review. C, Nuclear physics.

[24]  M. Storm,et al.  Crossing of single-particle energy levels resulting from neutron excess in the sd shell , 1983 .

[25]  B. Wildenthal,et al.  Collapse of the conventional shell-model ordering in the very-neutron-rich isotopes of Na and Mg , 1980 .

[26]  V. A. Madsen,et al.  Isospin decomposition of nuclear multipole matrix elements from. gamma. decay rates of mirror transitions: Test of values obtained with hadronic probes , 1979 .