Limiting excitation energies in fusion evaporation reactions

Abstract

Abstract. The mass dependence of the critical excitation energy above which a hot nucleus does not decay any more by particle evaporation, has been studied in the mass range 75 < A_< 130. The experimental values of the critical excitation energy have been obtained by means of a new integral method based on the analysis of the widths of the evaporation residue mass distributions as a function of the residue velocity. The obtained mass dependence appears to be stronger than expected by various models and a predicted dependence on the Z/A ratio of the decaying nucleus seems to be confirmed.

Maximum excitation energy in fusion-evaporation reactions from light particle multiplicity measurements in32S+58Ni at 25 MeV/A

“…In the present work we measured, by means of 4re detector devices, the multiplicities of all the evaporated light particles (n, p, d, c 0 emitted in coincidence with the ER in order to check if these multiplicities, as a function of the residue velocity, exhibit a saturation similar to that previously observed [8] for the width of the ER mass distribution. We note that, as mentioned before, in [8] the maximum excitation energy ECRIT has been extracted solely from the saturation velocity VCR, by means of a transformation which uses energy and momentum conservation assuming a participant-spectator mechanism.…”

“…An increase of excitation energy leads to a larger number of statistically evaporated particles producing an increase of the width of the ER mass distribution. The centroid of this distribution has only a weak dependence on the residue velocity (at least for the reaction considered in the present work and those studied in [8]), and cannot be used as an index for the degree of momentum transfer. Therefore, if different ICF reactions up to complete fusion can occur, and if the formed compound nuclei decay mainly by light particle evaporation, the average multiplicities of the evaporated particles (n, p, d, ~) and the width of the ER mass distribution are expected to increase with the residue velocity VE~ at least up to the velocity for complete fusion VCN.…”

“…Under these experimental conditions, if the predicted maximum excitation energy ECR~T for the decay by light particle evaporation exists, we expect that the trend of the evaporated particle multiplicities and of the mass width as a function of the residue velocity should show a saturation at a critical velocity Vck below the velocity for complete fusion VcN. This critical velocity is related to the maximum momentum transfer that leads to fusion-evaporation reactions and the excitation energy of the corresponding compound nucleus (whose mass will be indicated in the following as ACRIT) is our maximum excitation energy EcRm As discussed in detail in [8], assuming a participant-spectator mechanism and considering the conservation of energy and momentum, the ER velocity can be related to the excitation energy of the original compound nucleus, and the values of ECR~T and AcR~T can be extracted from the experimental critical velocity VCR. Following this procedure one has to take into account in the energy and momentum conservation, that the projectile fragments are not cold and are emitted with a velocity smaller than the beam velocity at an angle different from 0 ~ ((0VLF)= (10+2) ~ and (V~Lv) = (93 _+1)% of VBEAM for 32S + 58Ni at E(32S) = 828 MeV).…”

“…We note that, as mentioned before, in [8] the maximum excitation energy ECRIT has been extracted solely from the saturation velocity VCR, by means of a transformation which uses energy and momentum conservation assuming a participant-spectator mechanism. In the present case the information on EcRIT and ACR~T can also be extracted directly from the measured multiplicities of the evaporated particles, therefore, one has not to rely only on the result of the above mentioned transformation with its associated uncertainties.…”

“…In order to investigate the existence of a limiting excitation energy for the evaporation decay mode, some systematic measurements have been recently performed [8], with the aim to extract the maximum excitation energy ECR~T reached in fusion-evaporation reactions for different colliding systems, studying the trend of the width of the ER mass distributions as a function of the residue velocity. in the present work the same information, for the system 32S + 58Ni at E(32S) ~ 820 MeV, has been extracted, in *Supported by the European Community Programme "Human Capital and Mobility" a more direct way, from the measured multiplicities of the evaporated light particles.…”