Observation of yrast states in neutron-rich41Cl

Abstract

The yrast decay sequence of 41 Cl, populated in deep inelastic processes produced by the interaction of 234 MeV 37 Cl ions with a 160 Gd target, has been studied using the highly sensitive EUROBALL IV ␥-ray detector array. Yrast states are established for the first time at energies of 130 and 891 keV.Studies of the properties of nuclei far from stability provide a unique opportunity to increase our understanding of nuclear interactions in extreme conditions and often challenge our theoretical models. An example is the well-known deformation near singly magic 32 Mg ͓1͔. Theoretical calculations by Werner et al. have indicated the possibility of deformation in the region near singly magic (Nϭ28) 44 S ͓2͔.The experimental quadrupole deformation ␤ 2 values obtained from B(E2:0 ϩ →2 ϩ ) measurements for 40 S ͑0.284͒ ͓3͔ and 42 Ar ͑0.273͒ ͓4͔ provide evidence for the deformation of these isotones of 41 Cl. However, experimental information on the nuclear structure in this region has been quite limited, primarily due to the inability of most traditional methods to produce these nuclei. There is little spectroscopic information available currently for the neutron-rich NϾ23 isotopes of Cl. In particular, for the Tϭ7/2 41 Cl nucleus, no published work exists on the experimental level structure ͓5͔. Spectroscopic measurements can reveal details of the underlying microscopic structures and have proved essential for understanding properties of nuclei far from stability. Furthermore, they potentially provide a stringent test of modern large scale shell-model calculations.Neutron-rich 17 41 Cl 24 has three proton holes in the sd shell and four neutrons in the f p shell. It is thus a good candidate for testing cross-shell shell-model interactions that have a strong implication on the ordering of the first 3/2 ϩ and 1/2 ϩ states ͓6͔. The ground state of 41 Cl was assigned a J value ͓7͔ of either 1/2 ϩ or 3/2 ϩ by Gurach et al. on the basis of an allowed and dominant ␤ Ϫ decay branch to the J ϭ1/2 ϩ 1.87 MeV level in 41 Ar. Considering the 41 Cl excited states, we have observed in the present work two decay transitions at energies of 130 and 761 keV.A number of neutron-rich nuclei were populated in the interaction of a 234 MeV beam of 37 Cl ions, delivered by the VIVITRON at IReS, Strasbourg, with a 160 Gd target. The target, isotopically enriched to 98.2% in 160 Gd, was of thickness 12 mg/cm 2 and was backed with 40 mg/cm 2 of isotopically enriched 208 Pb (99.47%). The backed target was sufficiently thick to stop all forward-recoiling reaction fragments. The ␥ decay of excited states with lifetimes comparable with or smaller than the slowing-down time (ϳ1 ps) of recoiling nuclei in the target material will not readily be observed because of Doppler effects. For projectilelike species the v/c value is estimated to be of the order of 10%. The present experiment is thus sensitive to the ␥ decay of states with longer lifetimes. Study of the ␥ deexcitation of the products of deep-inelastic reactions is difficult because yields are gen...

Particle-core coupling inS37

“…3, there are two very broad γ -ray photopeak structures at energies of around 600 and 950 keV, which correspond to γ rays from the reaction partner 207 Pb direct single neutron pickup reaction [15], and they correspond to fairly pure neutron hole states, 2f Pb are also populated, albeit weakly. Neutron evaporation from the initially formed target-like and projectile-like fragments occurs; this was noted in previous work, e.g., [16].…”

“…The results of an 0 ω sd-pf shell-model calculation which uses the SDPF-U effective interaction [22] are also presented in the figure. The full sd(pf ) valence space was used for protons (neutrons) above an 16 O core. Within the configuration space of the calculation, only negative parity states can be calculated.…”

γ-ray spectroscopy of neutron-richS40

“…For the isotopes of potassium, the total monopole shift between 1d 3/2 and 2s 1/2 proton binding is about 350 keV per 1f 7/2 neutron [36]. This behavior has also been observed in the phosphorus [37] and chlorine [38] isotopes, but here the behavior of the energy splitting as a function of neutron number is different, in detail, from that for the isotopes of potassium. The significance of the near-degeneracy of the π (1d 3/2 ) and π (2s 1/2 ) orbitals with increasing neutron number is that this results in a deformation-driving force related to the pseudo-SU(3) symmetry of the two orbits [39].…”

“…The photopeaks at energies of 904 and 1352 keV correspond to the previously observed 2 + → 0 + and (4 + ) → 2 + transitions [15][16][17], respectively. To illustrate the population characteristics of binary grazing reactions, the γ -ray spectrum corresponding to the deexcitation of states in 38 S, also populated in this work, is presented in Fig. 3.…”

“…3. Transitions within the known 38 S yrast decay sequence of 1292, 1533, and 849 keV, corresponding to the 2 + → 0 + , 4 + → 2 + , and (6 + ) → 4 + γ -ray transitions [26][27][28], respectively, correspond to the most intense photopeaks observed in the spectrum of 4 yrast states, we would expect, in a simple shellmodel picture, protons to fill the 1d 5/2 and 2s 1/2 single-particle states. To obtain a more informed microscopic description of the present experimental results, we have performed largescale 0hω sd-pf shell-model calculations for 40 S using the ANTOINE code [29,30].…”