A. Taccone

Semi-empirical determination of the shell correction temperature and spin dependence by means of nuclear fission

We study the temperature and angular momentum dependence of the shell effects in the fissioning nucleus by means of a theoretical analysis of the total fission probability. We find clear evidence of the disappearance of the shell correction to the fission barrier with both increasing spin and temperature and propose a general method to determine the level density at the saddle-point configuration. The results of the present study might explain the differences between the fission barriers extracted from the heavy-ion reactions and those obtained using light-particle induced fission.

Delay-advance phenomenon observed by bremsstrahlung spectrum of the 12C + p collision

Abstract A new analysis of the experimental data obtained by the 12C(p, p) reaction at the proton scattering angle q ƒ = 80° near two superimposed 3.511 MeV, 3 2 − and 3.547 MeV, 5 2 + resonance states of the 13N compound nucleus was carried out by using the bremsstrahlung method and considering the interference between the background (direct) and resonant (compound nucleus) processes. A general expression of the collision duration is obtained for the case of superimposed resonances with different values of the spin and parity distorted by the non-resonant background. The results of calculations performed using this expression for the case of the two 3.511 and 3.547 MeV 13N states are compared with the experimental data. The total collision duration versus outgoing photon energy is deduced.

Fission time in the 28Si + natPt reaction

The experimental fission times are analyzed for excited nuclei produced in the 28Si + natPt reaction. Experimental lifetimes obtained by the crystal-blocking technique range between 10−17 and 10−18 s at bombarding energies between 140 and 170 MeV, respectively. Experimental data are analyzed within the statistical theory of nuclear reactions and the double-humped-fission-barrier model with allowance for preequilibrium processes and the nuclear-dissipation phenomenon. It is shown that fission barriers retain their double-humped structure for nuclear temperatures up to about 1.7–1.8 MeV and that the lifetimes of excited strongly deformed states in the second potential well contribute substantially to the observed delay times in the fission decay channel.

A WAY TO EVALUATE THE LEVEL DENSITY, TOTAL WIDTH AND COMPOUND-NUCLEUS DECAY FUNCTIONS IN THE RANGE OF UNRESOLVED RESONANCES

A new relationship between temporal and resonance characteristics for a nuclear reaction in the range of unresolved resonances is derived and analyzed. By using this relationship, we describe a procedure to determine the level density and total resonance width, starting from the experimental data on the cross-sections and some duration functions and obtaining the values not only of the mean durations but also of their variances.

On the Broad First Excited 5He State

The reaction 7 Li( d , αα) n was studied by detecting αα coincidences with a forward-forward and a forward-backward geometry at 1.4, 2.1 and 2.5 MeV incident deuteron energies. The analysis of the αα bidimensional spectra allowed us to isolate the first excited 5 He state from other reaction products. The values of (3.8±0.3) MeV and of (3.1±0.5) MeV for the excitation energy and width of this state were found, respectively.

Experimental evidence of shell effects in the fission time of heavy nuclei

The crystal blocking technique has been used to measure delay times in the fission channel for excited nuclei produced in the 28 Si+ n a t Pt reaction. Experimental results range from 10 -17 to 10 -18 s at bombarding energies from 140 to 170 MeV. It is shown that fission barriers keep their double-humped structure for nuclear temperature up to 1.7–1.8 MeV, and that the lifetimes of excited states in the second potential well contribute essentially to the observed delay times in the fission channel.

Sensitivity of the evaporation residue production to dynamical effects in highly fissile compound nuclei

Abstract We investigate the sensitivity of statistical model calculations of individual evaporation residue (ER) production on the nuclear shape evolution and viscosity. To this aim we analyze the de-excitation of the 217 Ac compound nucleus formed at the excitation energies between 36 and 90 MeV. We find a strong dependence of the calculated results on the nuclear dissipation and shape evolution, which make evaporation residues a useful tool for investigating dynamical effects. We indicate a new method of estimating the nuclear viscosity and claim that the statistical model interpretation of the ERs does not exclude high values for the viscosity parameter.

Reaction mechanism and nuclear-rainbow effect in the3He+12C interaction atE(3He)=98 MeV

SummaryNuclear-rainbow effects in elastic and inelastic scattering and transfer reactions by the3He+12C interaction atE(3He)=98 MeV are observed, analyzing the angular distributions of the3He,2H and4He particles. The data are analyzed by a near/far-side formalism using Fullers model. We also observe the different characteristics of the rainbow effects and of the shape of the angular distributions for various reaction mechanisms.

A novel experimental technique of nuclear lifetime measurements

Abstract In the present paper a new experimental method to measure nuclear reaction time in the 10−15–10−10 s region is presented. Measurements of the lifetimes of low-lying and long-lived states of 19F and 20Ne decaying via α-channel were carried out with the aim of checking the feasibility of the method. The results obtained in this way are compared with the lifetimes known from different techniques.

Level density parameter behaviour at high excitation energy

Abstract We present a formalism to calculate the intrinsic (without collective effects) and effective (with collective effects) level density parameters over a wide range of excitation energy up to 180 MeV. The behaviour of a int and a eff as an energy function is shown for several typical nuclei ( 115 Cd, 129 Te, 148 Pm, 173 Yb, 192 Ir and 248 Cm). Moreover, local systematics of the parameter a eff as a function of the neutron number N , also for nuclei extremely far from the β -line, is shown for some typical nuclei (Rb, Pd, Sn, Ba and Hg) at excitation energies of 15, 80 and 150 MeV.