P. Pavan

A 4π light-charged particle Si detector as a trigger device for in-beam γ-ray spectroscopy

A 4π Si-ball detector system has been constructed as a trigger device for the γ-ray spectrometer GASP. The construction and operation of the detector is reported, together with a brief description of the electronics and of the data acquisition system used. Results are presented for the 32S + 40Ca reaction at 140 MeV of32S beam energy.

In-band M1 and E2 transition rates and collective structures in 128Ba

Abstract Subpicosecond mean lifetimes of eight excited states in 128Ba populated via the 96Zr(36S,4n) reaction were measured by the Doppler-shift attenuation (DSA) technique using a line-shape analysis. The differential decay-curve method (DDCM) was applied for the lifetime determination. The B(E2) values in the yrast band indicate that the first band-crossing is with a proton S-band. The configuration πh 11 2 d 5 2 of the negative-parity semi-decoupled bands is confirmed by the measured B(E2, I → I − 2) and B(M1, I → I − 1) transition strengths. The higher-lying “dipole” band in 128Ba can be described as a high-K four-quasiparticle band built on the prolate configuration (πh 11 2 d 5 2 ) ⊗ (νh 11 2 g 7 2 ) .

Delayed g9/2 Alignment in the N=Z Nucleus 72Kr

The structure of Kr-72 has been investigated at GASP through the Ca-40(Ca-40,2 alpha) reaction at a beam energy of 160 MeV using the 4 pi ISIS Si-ball for reaction channel selection. The level scheme has been extended up to an excitation energy of approximate to 8.5 MeV. The spins and parities of the observed levels are assigned tentatively. The observed band shows the predicted change from oblate to prolate shape. The four quasi-particle g(9/2) alignment is found to be significantly delayed in rotational frequency with respect to the heavier Kr isotopes. Such a delay contradicts the predictions of standard mean-field calculations and may reflect either additional correlations in the T = 0 pairing channel or coupling to vibrational degrees of freedom or both

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.

Lifetimes of a shears band in 139Sm

Abstract Lifetimes have been measured in 139Sm with the DSAM method, using the reaction 62Ni(81Br,p3n)139Sm at 350 MeV. In the negative-parity regular dipole band based on the 25 2 − state at 3325 keV values of 0.60(21), 0.40(14) and 0.25(8) ps were found for the 337, 409 and 473 keV transitions, respectively. The deduced B(M1) and B(E2) are in agreement with the predictions of the tilted axis cranking model, allowing a triaxial equilibrium shape for the assumed νh 11 2 ⊗ πh 11 2 2 configuration.

Excited states in {sup 52}Fe and the origin of the yrast trap at I{sup {pi}}=12{sup +}

Excited states in {sup 52}Fe have been studied up to spin 10{h_bar} in the reaction {sup 28}Si+{sup 28}Si at 115 MeV beam energy by using in-beam {gamma}-ray spectroscopy methods at the GASP array. The excitation energy of the yrast 10{sup +} state is 7.381 MeV, almost 0.5 MeV above the well known {beta}{sup +}-decaying yrast 12{sup +} state. Experimental upper limits for the B(E4) transition probabilities from the 12{sup +} isomer to the 8{sub 1}{sup +} and 8{sub 2}{sup +} states have been determined. The mean lifetimes of five excited states have been measured by using the Doppler shift attenuation method. Complete diagonalizations in the pf major shell lead to very good agreement with the experimental level scheme and transition probabilities. The lifetime, log ft value, branching ratios, and B(E4) values are calculated for the 12{sup +} isomer. The positive parity states are also interpreted in terms of a Nilsson projected method. The structure of the yrast levels of {sup 52}Fe is compared with those of its cross conjugate {sup 44}Ti. {copyright} {ital 1998} {ital The American Physical Society}

Smooth crossings between the N = 4 and N = 6 orbits and adiabatic configuration changes at high angular momentum in 134Nd

Abstract Rotational structures in 134 Nd have been investigated using the GASP array via the 110 Pd( 28 Si,4n) reaction at 130 MeV. Each of the observed high-spin bands experiences a change in structure, that involves the promotion of a particle from the N = 4 into the N = 6 orbital. The adiabatic character of the corresponding bandcrossings, indicated by vertical or gradual upbendings of the aligned angular momentum, gives evidence of a sizeable mixing between the N = 4 and N = 6 configurations. The successive occupation of the first and second N = 6 intruder orbitals indicates that, at difference from the paired alignment exhibited by normal-deformed nuclei between the ground-state band and the S -band, in highly-deformed nuclei, the unpaired alignment is the favored mechanism for generating high angular momenta.

Evolution from spherical to various deformed shapes in the odd-odd 59136Pr77 nucleus

Abstract The level scheme of the doubly odd 136Pr nucleus has been investigated via the 136Ce(p,n), 122Sn(19F,5n) and 110Pd(30Si,p3n) reactions at beam energies of 14, 84 and 125 MeV, respectively. A complex decay scheme has been established for the 90 ns isomeric level with Ex = 594.7 keV and Jπ = 6+. The two previously reported bands, one based on the πh 11 2 ⊗ νh 11 2 configuration and one with suggested negative parity consisting of only dipole transitions, have been extended at higher spins and connected firmly to the 6+ isomer. Six new bands have been also identified: two consist of strong dipole and crossover quadrupole transitions, whereas the other four, very weakly populated, consist of only quadrupole transitions. The observed level structures are compared to theoretical calculations performed within the interacting boson model for odd-odd nuclei (IBFFM) and the cranked shell model. The experimental B (M1) B (E2) ratios of the yrast band are well reproduced by the IBFFM calculations, while those of the other semi-decoupled structures are in good agreement with the values obtained from the semiclassical model of Donau.

Rotational quenching of the N = 72 shell gap and the role of the νi132 intruder orbital in 132Nd

Abstract Rotational structures in 132Nd have been investigated via the 104 Pd ( 32 S ,2p2n) reaction at 160 MeV. Four highly-deformed bands have been observed, out of which three have been linked to the normal-deformed states. Two of them are signature partners. A mixing at high spin is observed between levels of one highly-deformed band and the ground-state band allowing for an unambiguous assignment of the spins and parity of that band. The present data brings new evidence on the role of the N = 72 shell gap to the stabilization of the highly-deformed minimum in the A = 130 mass region and on the importance of the νi 13 2 intruder orbital in the structure of the highly-deformed bands.

Stopping power of Nd ions in Pb determined from γ-ray lineshape analysis in Coulomb excitation

Abstract The stopping power of Nd ions in Pb, in the range from 7 to 35 MeV, has been determined using the γ-ray lineshape analysis of the 696.6 keV transition de-exciting the first 2+ level in 144Nd, populated by Coulomb excitation using a 90 MeV 28Si beam. The data are compared with the most commonly used parameterizations.