B. Fornal

Nature of yrast excitations near N=40: Level structure of

Excited states in 67 Ni were populated in deep-inelastic reactions of a 64 Ni beam at 430 MeV on a thick 238 U target. A level scheme built on the previously known 13-µs isomer has been delineated up to an excitation energy of 5.3 MeV and a tentative spin and parity of (21/2 − ). Shell model calculations have been carried out using two effective interactions in the f5/2pg9/2 model space with a 56 Ni core. Satisfactory agreement between experiment and theory is achieved for the measured transition energies and branching ratios. The calculations indicate that the yrast states are associated with rather complex configurations, herewith demonstrating the relative weakness of the N = 40 subshell gap and the importance of multi particle-hole excitations involving the g9/2 neutron orbital.

^{67}Ni

High-energy gamma-rays emitted in the decay of the giant dipole resonance built on excited states of 55Mn nucleus were measured with a bismuth germanate detector. Resonance parameters were extracted from the fits to the experimental spectra. Two new high-energy calibration points originating from slow-neutron capture in germanium isotopes of bismuth germanate crystal are proposed for large-volume BGO detectors.

Giant dipole resonance in 55 Mn * studied with the BGO detector

The reactions induced by 143 MeV32S on58Ni have been studied detecting discrete γ-rays in coincidence with projectile-like fragments (PLF). Information on PLF excitation probability and sequential decay of target-like fragments (TLF) has been obtained. For the28Si+62Zn outgoing channel at small energy loss (¦Q¦<20 MeV), both PLF and TLF data indicate that thermal equilibrium is not attained. The hypothesis of an equal excitation energy partition between the two reaction fragments does not describe properly experimental TLF data. A dependence of PLF excitation probability on the outgoing channel is found for the two final channels32S+58Ni and28Si+62Zn. The values of the spin alignment parameterPzz, derived for PLF and TLF from measurements ofγ-rays anisotropy, are in disagreement with the expectations of the transport theory for dissipative collisions.

Discrete γ-rays in the reactions of 143 MeV32S with58Ni

The study of nuclear cluster states bound by valence neutrons is a field of recent large interest. In particular, it is interesting to study the pre-formation of α-clusters in α-conjugate nuclei and the dynamical condensation of clusters during nuclear reactions. The NUCL-EX collaboration (INFN, Italy) is carrying out a research campaign studying pre-equilibrium emission of light charged particles and cluster properties of light and medium-mass nuclei. For this purpose, a comparative study of the three nuclear reactions: 16O + 30Si, 18O + 28Si and 19F + 27Al, has been recently carried out using the GARFIELD+RCo 4π setup. After a general introduction on the experimental campaign, the preliminary results for the three systems are presented.

Pre-equilibrium emission and clustering in medium-mass nuclei: 46Ti from 16O + 30Si, 18O + 28Si, 19F + 27Al

S. Valdréa,b, S. Piantellib, G. Casinib, S. Barlinia,b, M. Ciemałac M. Kmiecikc, A. Majc, K. Mazurekc, M. Cinauserod F. Gramegnad, V.L. Kravchuke, L. Morellif , T. Marchig S. Appannababuh, G. Baioccoi, P. Bednarczykc, G. Benzonij M. Binia,b, N. Blasij, A. Braccoj,k, S. Brambillaj, M. Brunof F. Cameraj,k, M. Cicerchiad,l, M. Colonnam, F.C.L. Crespij,k M. D’Agostinof , M. Degerliern, D. Fabriso, B. Fornalc O.V. Fotinap, A. Giazj,k, M. Krzysiekc, S. Leonij,k, J. Mabialad M. Matejska-Mindac, W. Męczyńskic, B. Millionj D. Montanarij,k, S. Myalskic, R. Nicolinij,k, A. Olmib G. Pasqualia,b, G. Preted, J. Styczeńc, B. Szpakc, E. Vardacir B. Wasilewskac, O. Wielandj, M. Zięblińskic

Constraining hot sources in central heavy-ion collisions below 20 MeV/u

SummaryWe have measured the high-energy γ-ray spectra from the fusion reactions 130 MeV16O +98Mo and 240 MeV48Ti +64Ni, populating the114Sn and112Sn compound nuclei at the excitation energy of 110 MeV. The comparison of the spectra shows a ~36% enhancement of the α-ray yield in the Giant Dipole Resonance region (Eγ > 8 MeV) when the 16 O-induced reaction is considered. This enhancement is interpreted as due to entrance channel effects related to theN/Z asymmetry between the target and the projectile.

Giant dipole emission inN/Z asymmetric heavy-ion reactions

The 48 Ti on 40 Ca reactions have been studied at 300 and 600 MeV focusing on the fusion-evaporation (FE) and fusion-fission (FF) exit channels. Energy spectra and multiplicities of the emitted light charged particles have been compared to Monte Carlo simulations based on the statistical model. Indeed, in this mass region (A 100) models predict that shape transitions can occur at high spin values and relatively scarce data exist in the literature about coincidence mea- surements between evaporation residues and light charged particles. Signals of shape transitions can be found in the variations of the lineshape of high energy gamma rays emitted from the de-excitation of GDR states gated on di erent region of angular mo- menta. For this purpose it is important to keep under control the FE and FF processes, to regulate the statistical model parameters and to control the onset of possible pre- equilibrium emissions from 300 to 600 MeV bombarding energy.

Measurement of light charged particles in the decay channels of medium-mass excited compound nuclei

Lifetimes of high-spin states in 69As have been measured using Doppler shift attenuation technique with the GASP and RFD setup. The determined transition probabilities indicate large deformation associated with some rotational bands in this nucleus.

Lifetime measurements of high-lying short lived states in 69As

Level structures in {sup 60,62}Fe{sub 34,36} and adjacent nuclei were studied with Gammasphere using the reaction of a 430-MeV {sup 64}Ni beam on a 55-mg/cm{sup 2} thick {sup 238}U target. High-spin level schemes were deduced from singly- and doubly-gated prompt coincidence events. Levels populated in the {beta} decay of {sup 60,62}Mn isomers were studied in singly- and doubly-gated delayed coincidences. Spin and parity assignments were deduced from angular correlations of {gamma} rays observed in both nuclei, leading to firm positive-parity yrast assignments up through 8{sup +} in {sup 62}Fe. The observed levels for {sup 60}Fe up to {approx}3.5 MeV compare well with pf shell-model calculations available in the literature. Calculations performed in a limited pfg basis exhibit reasonable agreement for the 2{sup +}, 4{sup +}, and 6{sup +} levels in {sup 62}Fe. However, theoretical descriptions that include the newly identified negative-parity levels in {sup 62}Fe require a much larger basis, necessarily including g{sub 9/2} neutrons, as well as improved interactions leading to negative-parity levels.

Structure of {sup 60,62}Fe and the onset of {nu}g{sub 9/2} occupancy

Excited states in the N=30 neutron-rich isotope {sup 49}K have been studied using multinucleon transfer reactions with thin targets and the PRISMA-CLARA spectrometer combined with thick-target {gamma}-coincidence data from Gammasphere. The d{sub 3/2} proton-hole state is located 92 keV above the s{sub 1/2} ground state, and the proton-particle f{sub 7/2} state is suggested at 2104 keV. Three other levels are established as involving the coupling to 2{sup +} of two neutrons above the N=28 shell. The measured or estimated lifetimes served to reinforce the interpretation of the observed level structure, which is found to be in satisfactory agreement with shell-model calculations.