Collectivity in 44S

The energy and reduced transition probability B(E2; Oz, + 2+) for the lowest excited state in the neutron-rich isotope $$ls were measured by intermediate-energy Coulomb excitation. The excitation energy is E(Z+) = 1297( 18) keV and the reduced transition probability is B(E2;0?& + 2:) = 314(M) e2fm4. The experimental results are compared with self-consistent mean-field calculations and shell model calculations with empirical interactions. The shell model calculations indicate that the large B(E2) value in ‘% is vibrational in origin, while the neighboring isotopes 4o,42S are statically deformed. @ 1997 Elsevier Science B.V. PACS: 27.4O.+z; 21.6O.C~; 25.70.De; 27.30.-l-t In this Letter, we report the first measurement of the energy and reduced transition probability B(E2; 0;‘. --t 2+) of the lowest 2? state in the neutron-rich radioactive N = 28 isotope 44S. This state was excited with the technique of intermediateenergy Coulomb excitation of a radioactive ‘% beam having an intensity of only approximately 15 particles/s. The results indicate that the 2: state in 44S has a collective nature. With the present measurement, the chain of even-Z N = 28 isotones with measured electromagnetic matrix elements B(E2; O&, -+ 2+) has been extended from iron (Z = 26) to sulfur 1 Permanent address: Drake University, Physics and Astronomy Department, Des Moines, IA 503 11, USA. (Z = 16)) allowing a systematic understanding of the effects of the N = 28 major shell closure on the structure of these nuclei. With the sole exception of 48Ca, the N = 28 isotones are collective, though generally not as much as 44S We compare the present data on 44S and previously reported data on neighboring nuclei to self-consistent mean-field calculations by Werner et al. [ 1 ] and to our own shell model calculations using empirical interactions obtained from nuclei close to the beta-stability line. Intermediate-energy Coulomb excitation [ 21 of radioactive beams has been used recently to populate low-lying states of 38,40,42S and 44,46Ar [ 31, as well as states in several A < 14 nuclei [ 41 and 32Mg [ 51. 0370.2693/97/$17.00 @ 1997 El sevier Science B.V. All rights reserved. PII 50370-2693 (97) 00077-4 164 Z Glasmacher et al./Physics Letters B 395 (1997) 163-168 The work reported here was performed at the National Superconducting Cyclotron Laboratory at Michigan State University. A primary beam of 48Ca12+ at an energy of 70 MeVlnucleon and an intensity of 25 particle-nA was produced with the NSCL room temperature electron cyclotron resonance ion source and the K1200 cyclotron. The high intensity 48Ca beam was produced by online reduction from CaO (enriched to about 70% in 48Ca) with the technique described in Ref. [ 61. The secondary ‘?S beam was obtained via projectile fragmentation in a 379 mg/cm2 9Be primary target located at the mid-acceptance target position of the A1200 fragment separator [7]. A thin degrader ( 10 mg/cm2 carbon) was placed at the second intermediate dispersive image of the A1200 to reduce the number of light fragments that reach the A1200 focal plane and subsequently the experimental setup. The magnetic field settings of the A1200 fragment separator were optimized for the transmission of 44S. Since the production cross section (four proton stripping) for 44S is very small, its yield at the experimental station was approximately 1.5 particles/s and accounted for only about 0.4% of the secondary beam fragments which reached the Coulomb excitation target. The mixed beam allowed the simultaneous measurement of Coulomb excitation of many different fragments, as the isotopes were identified event-byevent during the off-line analysis. Positive mass and element identification of each fragment came from the measurement of the time of flight between a thin plastic scintillator located after the Al 200 focal plane and a parallel plate avalanche counter (PPAC) located in front of the secondary target in addition to a measurement of the fragment’s energy loss/total energy in a cylindrical fast/slow plastic phoswich detector located after the secondary target. This O”-detector restricted the observed range of beam particles scattered from the 533 mg/cm2 secondary r9’Au target to have laboratory scattering angles of less than tit& = 4.05” and thus provided an almost exclusive selection of Coulomb excited nuclei (due to the small cross section for nuclear excitation at such forward angles [ 31) . A schematic view of the detector arrangement in the secondary target area is shown in Fig. 1. Photons were measured in coincidence with the scattered secondary beam particles in an array of 38 position sensitive NaI( Tl) detectors [ 81. The NaI( Tl) crystals were cylindrical, 18 cm long, 5.75 cm in diamg0 tekmye AE and %A iiGi Fig. 1. Schematic setup of the secondary target area. The NaI( Tl) detectors and their relative positions to the target are indicated. The distance between the secondary target and the O” telescope is 72 cm and limits the acceptance of scattered particles to scattering angles of &,b < 4.05’. The lead shielding and two parallel plate avalanche counters used for beam tracking located 1 m and 2 m before the secondary target are not shown. eter, enclosed in 0.45 mm thick aluminum shields. The detectors were oriented parallel to the beam direction around a 10.2 cm diameter beam pipe in three concentric rings, and the target was located at the midpoint of the detectors. Photomultiplier tubes were located at both ends of each detector, and the coincident signals from the two photomultiplier tubes were used to determine both the energy of the detected photon and the location of the photon interaction in the detector. Several y-ray sources ( 22Na, ‘*Y, 152Eu and 228Th) were used to establish position-dependent energy calibrations and efficiencies of each detector. The energy resolution of the detectors was typically 8% at 662 keV, and the position resolution was approximately 2 cm, providing an angular resolution of better than 10’ for each detected photon. The angular information was used to correct for the large Doppler shifts of the photons emitted from the secondary beam. The entire NaI( Tl) array was shielded from photons produced in the phoswich detector and PPACs, and from room background by a 16.6 cm layer of low-background lead bricks. The time difference between the detection of a photon in the NaI(T1) detectors and the detection of the secondary beam particle in the phoswich detector was recorded for each event to reduce accidental coincidences. The photons emitted from secondary beam particles, which had velocities of approximately u = 0.276 c (corresponding to a secondary beam energy of 35 MeV/nucleon in the middle of the gold target), could be distinguished from those from the t9’Au target by their Doppler shifts. Fig. 2 shows the y-ray spectrum in coincidence with 44S in the laboratoryZ Glasmacher et al./Physics Letters B 395 (1997) 163-168 165