Primary Fragment Mass Research Articles

Monte Carlo simulation of the measurement by the 2E technique of the average prompt neutron multiplicity as a function of the mass of fragments from thermal neutron-induced fission of 239Pu

Using a Monte Carlo method, we simulate the measurement, by the 2E technique, of the average prompt neutron multiplicity as a function of the mass of fragments from the thermal neutron-induced fission of 239Pu. The input data for the simulation, associated with the primary fragment mass (A), consist of the yield (Y), the distribution of the total kinetic energy characterized by its average ((TKE) ̅) and its standard deviation (σ_TKE), the average prompt neutron multiplicity (ν ̅_s, a sawtooth approach of experimental data), and the slope of neutron multiplicity against total kinetic energy (dν_s/d<TKE>). The output data, associated with the simulated as the fragment mass measured by the 2E technique (µ), consist of the yield (y), the distribution of the total kinetic energy characterized by its average ((tke) ̅) and its standard deviation (σ_tke), and the average prompt neutron multiplicity (ν ̅_µ). In the mass regions A≈115 and A>150, ν ̅_µ is higher than ν ̅_s. This result suggests that, in those mass regions, the 2E experimental values associated with the average neutron multiplicity are overestimated, referred to the corresponding to the primary fragments.

Quantal diffusion description of multinucleon transfers in heavy-ion collisions

Employing the stochastic mean-field (SMF) approach, we develop a quantal diffusion description of the multi-nucleon transfer in heavy-ion collisions at finite impact parameters. The quantal transport coefficients are determined by the occupied single-particle wave functions of the time-dependent Hartree-Fock equations. As a result, the primary fragment mass and charge distribution functions are determined entirely in terms of the mean-field properties. This powerful description does not involve any adjustable parameter, includes the effects of shell structure and is consistent with the fluctuation-dissipation theorem of the non-equilibrium statistical mechanics. As a first application of the approach, we analyze the fragment mass distribution in $^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ collisions at the bombarding energy $E_{\text{c.m.}}=193$ MeV and compare the calculations with the experimental data.

Partition of total excitation energy between fragment pairs in asymmetric and symmetric fission modes

Primary and secondary fragment mass and fragment total kinetic energy were measured by the time-of-flight technique in the $12\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ proton induced fission of $^{232}\mathrm{Th}$. The average neutron multiplicity was obtained from the difference of the primary fragment mass and the secondary one as a function of mass and total kinetic energy of fission fragments. Simulation calculations were applied for correction of observed fission data affected by instrumental fluctuations in measurements of flight time and kinetic energies of fragments. Anomalous behaviors of the average neutron multiplicity as a function of total kinetic energy in the mass region of $98--107$ for the lighter fragment and $126--135$ for the heavier were observed and they were found to correlate with two fission modes, symmetric and asymmetric fission. The overall trend of the average neutron multiplicity was discussed in terms of the two fission modes. Partition of the total excitation energy between fragment pairs deduced from the neutron multiplicity data was evaluated for each fission mode for the first time.

Heat partition in the E/A=8.5 MeV 74Ge+165Ho reaction.

The excitation-energy distribution of projectilelike fragments formed in the E/A=8.5 MeV $^{74}\mathrm{d}$/rGe${+}^{165}$Ho reaction has been obtained by applying a statistical evaporation calculation to the difference between primary fragment mass yields derived from kinematic coincidence measurements and directly measured yields. A gradual transition from approximate equipartition of excitation energy for small energy losses to a division that favors the targetlike fragment for highly damped events is observed, in agreement with previous measurements. For a given energy loss, heat partition is found to correlate with the direction of net nucleon transfer. The variances of the excitation-energy distributions are shown to increase monotonically as a function of energy loss. The average data are compared with predictions of the nucleon exchange transport model. The results suggest that energy-loss mechanisms in addition to nucleon exchange may be important in the early stages of the collision.

Precision measurements of mean kinetic energy release in thermal-neutron-induced fission of 233U, 235U and 239Pu

Abstract The values of the total kinetic energy (TKE) of fission fragments published until now show discrenancies up to several MeV's depending on the type of experiment. Therefore, a novel method was developed to get more reliable figures of high precision by avoiding systematic errors of earlier measurements. Moreover, the determination of primary fragment mass- and energy-distributions is feasible.

Evidence for nucleon pair breaking even in the coldest scission configurations of 234U and 236U

Primary fragment mass yields, measured at the highest values of the kinetic energy released in the thermal neutron induced fission of 233U and 235U, do not reveal any preference for splits into two even-mass nuclei. Moreover our data suggest that the mechanism of pair breaking would take place before scission, namely that the low energy fission would be a strongly dissipative process.