A. Axelsson

Cd-98(48)50: The two-proton-hole spectrum in Sn-100(50)50

Excited states in {sup 98}Cd, two proton holes from {sup 100}Sn, were identified and studied for the first time, using in-beam spectroscopy with highly selective ancillary detectors. The structure of the ({pi}g{sub 9/2}){sup {minus}2} two-proton-hole spectrum below a T{sub 1/2}=0.48(16) {mu}s isomer is deduced and compared to shell-model predictions. A tentative I{sup {pi}}=(8{sup +}) assignment, as suggested by systematics, yields a strongly reduced B(E2,8{sup +}{r_arrow}6{sup +})=0.44({sup +20}{sub {minus}10}) W.u., corresponding to an effective proton charge of e{sub {pi}}=0.85({sup +20}{sub {minus}10}) e , which is at variance with existing theoretical predictions. {copyright} {ital 1997} {ital The American Physical Society}Excited states in Cd-98, two proton holes from Sn-100, were identified and studied for the first time, using in-beam spectroscopy with highly selective ancillary detectors. The structure of the (pi g(9/2))(-2) two-proton-hole spectrum below a T-1/2 = 0.48(16) mu s isomer is deduced and compared to shell-model predictions. A tentative I-pi = (8(+)) assignment, as suggested by systematics, yields a strongly reduced B(E2,8(+) --> 6(+)) = 0.44((+20)(-10)) W.u., corresponding to an effective proton charge of e(pi) = 0.85((+20)(-10))e, which is at variance with existing theoretical predictions.

RoSiB - a 4 pi silicon ball for charged-particle detection in EUROBALL

A 4 pi silicon bail for detection and identification of light charged particles in large multidetector gamma-arrays as EUROBALL is presented. The design is based on a N = 42 ball with 12 pentagons and 30 hexagons as used in the GASP array. The absorptive material for gamma-rays is minimized to the detector thickness of 300 or 500 mu m and a 0.63 mm ceramic backing. The geometrical coverage is designed for about 90% of 4 pi. A pulse shape discrimination method with totally depleted detectors working in the reverse mount allows identifying protons and alpha-particles above an energy threshold of about 2 MeV. The performances of the ball were rested at the tandem - booster accelerator combination of the MPI Heidelberg in two experiments using the high-recoil reaction of 228 MeV Ni-58 + Ti-46 and the low-recoil reaction of 95 MeV O-16 + Ni-58. The two-dimensional spectra of zero-crossing (ZC) versus energy confirmed an excellent discrimination of protons and alpha-particles in all the detectors at different angles. The energy spectra of protons and alpha-particles measured in the experiments are presented. too. The gamma-spectra measured in coincidence with various combinations of emitted particles showed a high selectivity of the ball. The reduced total efficiency for protons of 59% and 55% and alpha-particles of 44% and 32% measured in a nuclear spectroscopy application is analyzed in a Monte-Carlo simulation (GEANT). It is due to a combined influence of a thick target needed to stop the recoiling residual nuclei and thick absorbers needed to protect the Si-detectors from scattered beam. The results along with the GEANT extrapolation to optimum experimental conditions confirm that RoSiB is a highly efficient and selective device for identification of rare reaction channels with heavy ions

T=0 Pairing Correlations and Band Crossing Phenomena in N=Z Nuclei

The structure of the N=Z nuclei Zn-60, Ga-62, Ge-64 and Kr-72 has been investigated at GASP spectrometer through the S-32 + Ca-40 and Ca-40 + Ca-40 reactions at 140 and 160 MeV, respectively, using reaction channel selection with the ISIS Si-ball. The high spin states of such nuclei have been investigated searching for the four quasi-particle g(9/2) alignment. TRS calculations have been performed for the N=36, 38 and 40 Kr isotopes. In the case of Kr-72 th, four quasi-particle g(9/2) alignment is observed to be significantly delayed in rotational frequency with respect to the heavier Kr isotopes. Such a delay contradicts the predictions of mean field calculations and mag; be interpreted as the first sign of additional correlations in the T=0 pairing channel. (Less)

In-beam study of 102Sn

Excited states in 102Sn have been identified for the first time, in an in-beam γ-ray spectroscopic experiment. Two γ-ray transitions with energies 1472 and 497 keV following the decay of the seniority 6+ isomer with t1/2 = 1.0(5) μs were unambiguously assigned to 102Sn. Due to the very low cross section of about 2 μb for producing 102Sn in the reaction 50Cr(58Ni,1α2n), a highly selective detector setup utilizing NORDBALL ancillary detectors and a recoil catcher device was used. High γ-ray detection efficiency was achieved with two EUROBALL Ge cluster detectors.

Nuclear structure in the region of 100Sn at N≃Z

With the recent development of efficient γ-arrays with highly selective ancillary detectors, studies of shell structure and residual interaction in exotic nuclei like 100Sn have become feasible. The shell structure at N=Z=50 (100Sn) shows a remarkable similarity to N=Z=28 (56Ni), albeit recent experiments on the 100Sn neighbors 102,104Sn and 98Cd indicate substantial and puzzling deviations in the E2 and E3 polarizability of the magic core 1–3. New experimental results and their shell model interpretation are discussed.

An investigation of pairing correlations in diabatic configurations at high spin and large deformation: applications to the description of SD bands in Eu-143

The HFB cranked shell model is applied in an investigation of the selfconsistent pairing properties of the superdeformed (SD) bands in Eu-143. Results from a fully pairing and deformation selfconsistent mesh calculation of the properties of the most intense SD band in 143Eu are presented, as well as pairing selfconsistent calculations done at a fixed deformation close to the SD minimum for an excited SD configuration. The calculations are compared to unpaired calculations and to published experimental data. Fully diabatic configurations are studied and total energy surfaces are constructed for fixed values of the angular momentum. Other issues addressed include the development of the particle number distribution in the HFB wave function up to very high frequency, the consequences of using a fixed Fermi energy in high-spin calculations and the effects of the number of excited quasiparticles on the pairing properties of the superdeformed nucleus

In-beam study of Sn-102

Excited states in102Sn have been identified for the first time, in an in-beamγ-ray spectroscopic experiment. Twoγ-ray transitions with energies 1472 and 497 keV following the decay of the seniority 6+ isomer with t1/2=1.0(5)μs were unambiguously assigned to102Sn. Due to the very low cross section of about 2µb for producing102Sn in the reaction50Cr(58Ni,1α2n), a highly selective detector setup utilizing NORDBALL ancillary detectors and a recoil catcher device was used. Highγ-ray detection efficiency was achieved with two EUROBALL Ge cluster detectors.

A critical appraisal of the rediagonalization method for construction of diabatic levels

Abstract The possibilities and limitations of the rediagonalization method for detection and removal of virtual interactions at bandcrossings are explored in the framework of the cranked Nilsson–Strutinsky shell model. Improvements of the method are suggested.

Monte Carlo study of the charged-particle detectors used in NORDBALL and EUROBALL spectrometers

The ancillary charged-particle detector arrays used inside the NORDBALL and the EUROBALL spectrometers were simulated employing the Monte Carlo code GEANT3. The simulated charged-particle spectra are in good agreement with the experimental ones.

Spectroscopy at N=Z with EUROBALL III

A complete study of the nuclear structure by means of gamma spectroscopy requires, in addition to the high resolution gamma measurement and accurate DCOs or angular distributions, the information concerning the Electric or Magnetic character of the transition. This information for transitions in nuclei far from stability valley is now reachable in the new generation of Ge-arrays based in composite detectors. EUROBALL III is a good example with the high polarization sensitivity of the 90 degrees ring of Clovers. The Polarization correlations PCOs measured in coincidence with the Cluster detectors permits to investigate transitions in weakly populated nuclei. In this contribution we present results on medium mass N=Z nuclei measured with EUROBALL III coupled with light particle ancillary detectors. (Less)