Search Results (7 total)

Fission and binary fragmentation of the excited nuclear systems of Z=116 and 124 were investigated using the reactions induced by ⁸⁰Se beams on ²⁰⁸Pb and ²³²Th targets at bombarding energies ranging from 470 to 630 MeV. The mass and kinetic energy of the binary reaction products were reconstructed by measuring their velocities by the time-of-flight method and the angles of emission using multiwire proportional chambers. Total neutron multiplicities were measured in coincidence with the fragments, using an array of neutron detectors. The fragment mass-energy correlation was studied for the two systems. The average total kinetic energy (TKE) of fragments for the ⁸⁰Se+²⁰⁸Pb system agrees with earlier measurements and with Viola's systematics in the mass symmetric region for compound nucleus fission, whereas for the ⁸⁰Se+²³²Th system, the TKE values are significantly lower. This is also consistent with higher values of total neutron multiplicities observed for the case of ⁸⁰Se+²³²Th at comparable available energies. From an extrapolation of the measured total neutron multiplicities for the mass symmetric region to zero compound nucleus excitation energy, the average number of prompt neutrons expected to be emitted in the spontaneous fission of the superheavy Z=116 has been estimated to be ν_tot^sf=10±2, which is consistent with the value derived for the same compound nucleus populated in the ⁵⁶Fe+²³²Th reaction in an earlier work. In the case of the ⁸⁰Se+²³²Th system, similar analysis was carried out by taking the average TKE from Viola's systematics for estimating the available energy for particle emission corresponding to compound nucleus fission. In this way, by extrapolating the observed neutron multiplicities to zero compound nucleus excitation energy, a value of ν_tot^sf=15±2 was obtained for the spontaneous fission of the superheavy Z=124 nucleus. The increase in the average number of neutrons emitted in fission as a function of the atomic number of the nucleus in the superheavy mass region was confirmed by comparing the results of the present work with published data.

The binary fragmentation of the excited superheavy system of Z=116 formed in the reaction 372 MeV ⁵⁶Fe+²³²Th has been investigated. The fragment masses and kinetic energies were determined through measurement of the fragment velocities by the time-of-flight method. Neutron spectra were measured in coincidence with the fragments at several angles with respect to the fragment direction, which were analyzed to deduce the total as well as the pre-scission neutron multiplicities. We analyzed the correlations between fragment mass and kinetic energy, gated and nongated by the coincidence with neutrons, to learn about the dynamics of the reaction with respect to the two-body exit channels. The events in the near-mass-symmetric valley region appear to receive a significant contribution from the asymmetric mode of fission of the superheavy compound nucleus Z=116, which can be due to the influence of the closed proton and neutron shells of Z=50 and N=82 in the light fragments. From the observed number of pre-scission neutrons, it is inferred that the time scales of the fissionlike reactions leading to near-mass-symmetric splits are rather large, of the order of several times 10^-20  s. The average number of prompt neutrons emitted in the spontaneous fission of such a superheavy nucleus is ν=(12±1), as deduced from the neutron measurements.

The transfer-induced fission channel has been studied in the collision of 340 MeV ²⁸Si on ²³²Th as a function of the atomic number of the projectilelike fragments (PLF’s) by using a 4π detector array. It is found that the energy loss of the transfer reaction increases as a function of the net charge transfer ΔZ from the projectile to the target nucleus, going from quasielastic to deep-inelastic regimes. The average excitation energy of the targetlike fragment (TLF) is derived from the measured energy loss, whereas its angular momentum has been obtained from the angular distribution of fission fragments. It is found that the populated TLF nuclei with Z_TLF=90–96 (ΔZ=0–6) have average excitation energies up to about 100 MeV and angular momenta up to about 40ħ. The measured ratio of transfer-fission yield to PLF singles, Y_f, first increases with increasing net charge transfer up to ΔZ=4 and then shows a plateau around the values Y_f=0.4–0.6 followed by a decrease for higher ΔZ transfers. This ratio can be identified as the cumulative fission probability of the populated nuclei for net charge transfers up to ΔZ<~6, where a two-body mechanism for the first reaction step is supported by the experimental data. This result suggests a significant survival probability against fission of these TLF nuclei, in marked disagreement with the standard statistical model predictions. The observed survival probability implies that there is a strong hindrance to fission in the early stages of deexcitation, as also indicated by the large fission times (t_f=10–100 zs) derived from earlier neutron measurements in fusion-fission reactions. The importance of such effects in the population of nuclei in the heavy and superheavy mass regions by transfer reactions is discussed.

The emission of two coincident energetic (E>~8 MeV) gamma rays has been observed in the 187 MeV ³⁷Cl+¹²⁰Sn reaction by using the cluster detectors of the EUROBALL III array. Those events are attributed to the decay of the double giant dipole resonance built on highly excited states.

The multiplicities of energetic γ rays have been measured in coincidence with both evaporation residues and fission fragments in the reactions of ¹⁹F on ¹⁸¹Ta at projectile energies of 90, 100, 110, 120, 130, and 140 MeV. The experimental data have been compared with results of statistical calculations, which take into account dynamical fission hindrance. The damping of the fission partial width has been determined as a function of excitation energy in ²⁰⁰Pb.

The alpha particle emission from 90 to 140 MeV ¹⁹F+¹⁸¹Ta fusion-evaporation reactions has been studied. The comparisons of the experimental spectral shapes and multiplicities with statistical model predictions indicate a need to use an excitation energy dependent level-density parameter a=A/K in which K increases with excitation energy. This increase is more rapid than that in lower mass nuclei. The effect of this change in level density on the prescission multiplicities in fission is significant.

Inclusive neutron spectra from the decay of ¹⁵⁶Er compound nucleus populated at E_x=47 MeV excitation energy by the ¹²C+¹⁴⁴Sm and ⁶⁴Ni+⁹²Zr reactions have been measured. The neutron spectra from the ⁶⁴induced reaction are strongly contaminated by other reactions different from fusion evaporation so that inclusive spectra cannot be used to test nonstatistical or entrance channel effects in the compound nucleus decay.