Rotational and neutron-hole states inS43via the neutron knockout and fragmentation reactions

Abstract

The recent assertion that shape coexistence occurs in the neutron-rich isotope 43 S implies that a state observed at 940 keV in a previous study is a rotational excitation of the deformed ground state. Here we use results from two intermediate-energy reactions to demonstrate that this state-assigned an energy of 971 keV in the present work-is indeed a rotational state. This result strengthens the case for shape coexistence in 43 The neutron-rich nuclei in the vicinity of 44 S presently provide a critical testing ground for the idea that neutron shell structure is significantly different close to the neutron dripline than it is in the valley of stability. Gaudefroy et al. [1] recently published the results of a g-factor measurement of an isomer in 43 S at 320 keV and argued on the basis of their result that spherical and deformed shapes coexist in that nucleus.Gaudefroy et al. interpret the 320 keV isomer as a spherical f 7/2 single-neutron hole state (with J π = 7/2 − ), and assign the ground state J π = 3/2 − on the basis of the E2 transition deexciting the isomer. They further argue that the ground state must be deformed to have this J π value. Finally, they suggest that a state observed by Ibbotson et al. [2] and assigned an energy of 940 keV is a J π = 7/2 − rotational excitation of the ground state. In the present work, we report on a test of this interpretation of the state observed by Ibbotson et al. using both the direct single-neutron knockout reaction 9 Be( 44 S, 43 S)X and the reaction 9 Be( 45 Cl, 43 S)X. The latter involves both direct and nondirect removal of a neutron and proton, and is referred to here as a fragmentation reaction. The relatively small cross sections for population of this state observed in these reactions supports its interpretation as a rotational excitation built on the ground state and strengthens the case for shape coexistence in 43 S.The experiment was performed at the Coupled-Cyclotron Facility of the National Superconducting Cyclotron Laboratory at Michigan State University. A cocktail beam comprising 84% 44 S and 14% 45 Cl was produced by fragmentation of a 140 MeV/nucleon 48 Ca primary beam incident on a 705 mg/cm 2 9 Be fragmentation target. Components of the secondary beam were separated in the A1900 fragment separator [3] and delivered to a 376 mg/cm 2 thick 9 Be reaction target mounted at the target position of the S800 magnetic spectrograph [4]. In the one-neutron knockout measurement, 1.5 × 10 8 44 S beam particles were incident on the reaction target. The midtarget beam energy was 92 MeV/nucleon. The fragmentation measurement was performed with a total of 1.6 × 10 8 45 Cl beam particles and a midtarget beam energy of 98 MeV/nucleon. Incoming beam particles were identified from their time-of-flight difference measured between scintillators mounted at the extended focal plane of the A1900 and at the object of the S800 analysis line. Projectile-like reaction products were identified by the time of flight to the focal plane of the S800 and energy loss in the S800 i...

Shell model study ofN≃28neutron-rich nuclei

“…In 43 16 S, g-factor measurement strongly points toward the intruder nature of the ground state at Z = 16, the natural (νf 7/2 ) −1 state lying 320 keV in excitation energy [19]. These results in 43 S have been partly confirmed by Riley and co-workers [16]. Within the shell model framework these spectroscopic data are the result of the progressive reduction of the N = 28 shell gap and the increase of correlation energy from stability toward exotic nuclei [13].…”

“…Mass measurements also indicated the reduction of the N = 28 gap [6,7], as well as the results from in-beam γ -ray spectroscopy [8][9][10]. More recently, various nuclei have been studied by means of direct reactions [11][12][13][14][15][16]. A global analysis of these data shows the reduction of the N = 28 shell gap in exotic nuclei.…”

γ-ray spectroscopy of one-proton knockout from45Cl

“…The theoretical distributions have been transformed to the laboratory frame and folded with the measured momentum distribution of the incoming 45 Cl beam. The measured distribution exhibits a low-momentum tail below 18.3 GeV/c typically observed in knockout measurements [12,15,[26][27][28][29]. This phenomenon, discussed in detail in Ref.…”

“…The neutron-rich exotic isotones near 42 Si have attracted considerable attention because of the novel role that neutron shell structure -and the narrowing or collapse of the N = 28 major shell closure -plays in causing deformation in these nuclei [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. However, proton shell structure must also be involved [18][19][20].…”

In-beam γ -ray spectroscopy of S38–42

“…These effects are included in the SDPF-MU effective shell-model interaction introduced in [3] and the resulting predictions for the 40,41,42 S level schemes will be tested in the present work. The sulfur isotopes between N = 20 and N = 28 have been studied with a variety of experimental techniques [10][11][12][13][14][15][16][17][18][19][20], however, information on the level schemes even at low excitation energy is still scarce. Beyond N = 28, very few excited states have been reported in the S isotopic chain [21,22].…”