Competition between few nucleon emission and?-ray deexcitation for the reaction systems65Cu+87Rb and40Ar+110Pd

Abstract

Both y-ray and neutron emission have been studied for the reaction systems 65Cu(237MeV)+87Rb~t52Dy* and 4~ MeV)+ ax~176 By using a sum spectrometer in coincidence with neutron counters, Ge(Li) or NaI detectors, we have measured the total y-ray energy and the average total y-multiplicity distributions as well as the neutron spectra for various exit channels. These measurements provide strong evidence for thermal equilibrium in reactions involving a small number of emitted neutrons (i.e. 87Rb(65Cu, n or 2n)) at rather high excitation energy (~54MeV). This statistical emission of only a few neutrons is controlled by very strong 7-ray competition: the 7-entry line is found not to be parallel to the yrast line. Instead the energy gap is about 8 MeV for J~27h and rises to at least 13 MeV for J~36h. There are some indications that the main part of the energy from this gap is removed by statistical y-ray cascades. The main features of the experimental data for both entrance channels are well reproduced by statistical model calculations with proper attention to the yrast line position and an adjustement of the dipole y-ray normalization coefficient. It is conceivable that the y-ray enhancement that we introduce may be related to a lack of knowledge of the absolute level densities at high energy and spin, or possibly to the presence of new or additional degrees of freedom that may enter into the competition between neutron and y-ray emission.

Influence of the giant dipole resonance in the statistical decay of the compound nucleus160Er

“…Such a small value for ~el implies that 7 emission becomes important only when particle decay is energetically forbidden. In agreement with the results of many previous works [29][30][31] such values for the two parameters lead to the prediction of too many evaporated neutrons. For example, the maximum of the calculated 4n excitation function is shifted by ~ 5 MeV towards lower energy compared to the experimental data.…”

“…To reproduce these widths in the present mass region requires ~E1 ~3-6 x 10 -7 [29,31,321. Increasing the V competion by using such values for the v-strength function slightly improves the agreement.…”

Thermal characteristics of composite systems formed in fusion ofSi28with

“…For example, the experimental particle multiplicities for the reaction Sn+ Si at various bombarding energies were found by Stwertka et aj]l. [7,8] to be systematically lower than predicted by statistical evaporation codes, employing sets of input parameters established in earlier studies covering a wide range of nuclear systems. In related work, Janssens et al [11] have studied the two systems 9 30 to 70 MeV. It was noted in their study that, while the statistical model describes quite accurately the average multiplicities of neutrons from the asymmetric, C-induced reaction, it systematically underpredicts the emission of neutrons from the more symmetric, Ni-induced reaction.…”

Unexpected Entrance-Channel Effects in the Decay of the Compound NucleusEr156

“…The average spin A/ ns per nonstatistical photon was determined by fitting the y-ray angular distributions with a series of Legendre polynomials. The attenuation coefficient a 2 sst 0JS reported 11 for 154 Er has been used. The multiplicity-tospin conversion is described in detail by Fischer et ai n Since most of the residues have isomeric states, 11 distributions for individual decay branches populating or bypassing isomers have been summed to obtain the spin distribution for a given residue.…”