N. Blasi

OCTUPOLE EXCITATIONS IN VIBRATIONAL NUCLEI AND THE SDF INTERACTING BOSON MODEL

Abstract Proton and deuteron inelastic scattering experiments, performed with an energy resolution of 12–15 keV, have been used to study negative-parity states of vibrational and transitional nuclei with mass between 98 and 150. The analysis has been focussed on the isovector components, on the quadrupole-octupole two-phonon states and on the fragmentation of the octupole strength. This latter displays a regular dependence on the product of proton and neutron valence particle numbers and is satisfactorily reproduced by IBM-1 + f-boson calculations. Other features of the experimental spectra, as the relative positions of the 3− states, exhibit a dependence on the ratio of valence particle numbers and indicate that a IBM-2 approach might be more appropriate.

Linking transitions between the highly deformed states and the yrast states of normal deformation in 133Nd

Abstract Transitions linking the highly deformed band to low deformation states have been unambiguously established in 133 Nd in the very first experiment performed with the GASP array. the highly deformed band becomes yrast at 29 2 + , where it mixes with the positive parity band built upon the [404] 7 2 Nilsson state. The experimentally deduced interaction amounts to 13 keV. The measurement of the spins in the highly deformed intruder band sheds some light upon the reported backbend anomaly in this mass region.

Structural Evolution in the Neutron-Rich Nuclei ^{106}Zr and ^{108}Zr

The low-lying states in ¹⁰⁶Zr and ¹⁰⁸Zr have been investigated by means of β-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹⁰⁸Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64. The deformed ground state of ¹⁰⁸Zr indicates that a spherical subshell gap predicted at N = 70 is not large enough to change the ground state of ¹⁰⁸Zr to the spherical shape. The possibility of a tetrahedral shape isomer in ¹⁰⁸Zr is also discussed.

HEXADECAPOLE STRENGTH DISTRIBUTIONS OF VIBRATIONAL NUCLEI IN THE A = 100 MASS REGION

Abstract Proton and deuteron inelastic scattering data taken in a high-energy resolution measurement have been used to study J π =4 + states of some vibrational and transitional nuclei with mass between 94 and 112. Most of the levels up to about 4 MeV excitation energy have been resolved and several new 4 + assignments are proposed by comparing measured angular distributions with coupled-channels calculations. The hexadecapole strength is found to be highly fragmented and tends to concentrate in a cluster of levels around 2.5–3 MeV in approaching the more collective nuclei in an isotopic series. This is in contrast with the behaviour of the quadrupole and octupole strengths in the same nuclei which are mostly concentrated in the first 2 + and 3 − levels, respectively. The hexadecapole distributions are compared with IBM-1 calculations in the sdg boson scheme.

Neutron transition moments and quadrupole T- or F-vector states

A study of low-lying 2+ states by proton and deuteron inelastic scattering from 104Pd, 110Pd, 112Cd, 146Nd, 150Nd and 150Sm is described. Evidence of neutron components in the excitation of 2+ states lying between 2.2 and 3.0 MeV in Pd and Cd isotopes has been obtained. This result is consistent with the predictions of the IBM-2 model for mixed-symmetry states.

STRENGTH DISTRIBUTIONS IN NEODYMIUM ISOTOPES - A TEST OF COLLECTIVE NUCLEAR-MODELS

Abstract Excited states in even Nd isotopes, up to excitation energies of 3–4 MeV, were investigated in proton- and deuteron-scattering experiments performed with high-energy resolution. More than 300 transitions were studied. For several new excited states spin and parity assignments have been suggested. Reduced transition probabilities were extracted for natural-parity states from O + up to 6 + . The experimental strength distributions have been compared with the predictions of the interacting boson model (IBM) and of the quasi-particle-phonon model (QPM). The octupole transition probabilities are well described in both models as produced by the fragmentation of the f-boson or of E3 phonons. IBM-sdf calculations seem to account also for the transitions to the low-lying 1 − states. Quadrupole and hexadecapole distributions are well described in the QPM. The leading configurations are due to 6–8 low-lying one-phonon states. The two- and three-phonon states play an important role especially in 146 Nd. The failure of IBM quadrupole and hexadecapole calculations clearly points out the need of introducing additional bosons lying at high excitation energies. QPM evaluations account also for other features of the experimental data, as the E5 and E6 strength distributions and the isovector components. The limits of the two models are discussed.

Levels in 109Rh

The levels in 109Rh have been investigated via the γ-rays following the β− decay of 34.5 s 109Ru. The ruthenium activity was produced in the thermal neutron-induced fission of 149Cf and separated chemically from the fission product mixture with the on-line centrifuge system SISAK. The emitted γ-rays were studied by γ singles and γγ(t) coincidence measurements. In addition, the 110Pd(d, 3He) reaction was investigated at Ed = 50 MeV to study the proton-hole states in 109Rh. From the comparison of the measured angular distributions with DWBA calculations l-transfers and spectroscopic factors were deduced. The level scheme of 109Rh that was obtained is discussed and compared with neighbouring nuclei and with recent calculations in the framework of the interacting bosonfermion model (IBFM-1). An interpretation of the observed candidates for intruder states as a rotational band built upon the [431] Nilsson configuration is proposed.

High-spin 1p-1h configurations in sn-116 and their fragmentation as seen in the reactions sn-116(-]p,p'), sn-116(e,e'), in-115(3he,d) and in-115(alpha, t)

Stretched spin configurations in 116Sn have been studied via the reactions 116Sn(p, p′), 116Sn(e, e′), 115In(3He, d) and 115In(α, t). The high-spin negative parity two-neutron quasiparticle states within the N = 51 − 82 major shell appear to be little fragmented. The most prominent examples are the Iπ = 9- state at Ex = 3.522 MeV and two 7− states at 2.909 and 3.120 MeV. It is found that in contrast the proton configurations (g−192, h112) and (g−192 g72) are strongly fragmented. Large-basis BCS shell model calculation, using a SkE-force have been made and DWBA analyses of the (e, e′) data and of the (p, p′) data are presented.

Study of 103Tc and the onset of deformation around N = 60

Abstract The 104 Ru(d, 3 He) 103 Tc reaction has been investigated at E d = 50 MeV. Transitions to 22 states in 103 Tc were studied. The measured angular distributions were compared with DWBA calculations, from which l -transfers and spectroscopic factors were deduced. To investigate the possible onset of quadrupole deformation around N = 60, the potential energy curves for the oddmass 97–103 Tc isotopes was calculated. Excited states were studied with the IBFA model.

Position sensitivity of large volume LaBr 3 :Ce detectors

The position sensitivity of two large volume (1” × 1” and 3” × 3”) LaBr<inf>3</inf>:Ce crystals has been measured using collimated beams of 662 keV γ-rays. The crystals have been coupled to a Position Sensitive Photo Multiplier Tube (PSPMT) and/or shielded phototube. The results indicate that, in the case of 662 keV γ -rays, both crystals should allow identification of the the position of the γ-ray first interaction with a 1-2 cm resolution. Similar tests have been done in the past using 140 keV γ-rays and no sensitivity was found. Using GEANT4 simulations we confirmed these results.