Monte Carlo Calculations of Nuclear Evaporation Processes. III. Applications to Low-Energy Reactions

Abstract

Monte Carlo calculations of nuclear reactions in the low-energy (R(50 Mev) region are described. The calculations are based on the nuclear evaporation model of Weisskopf. Continuum theory was used for the calculation of inverse reaction cross sections. In the calculation of the level densities of excited nuclei, pairing and shell energy corrections were used in terms of characteristic level displacements. The accurate equation rather than the approximate Maxwell distribution was used for the selection of the kinetic energy of the evaporated particle. Experimentally determined Q-values for the various reactions were used. The calculations are compared with experimental measurements for about 60 excitation functions of nuclear reactions in the mass range Cr" -Se". Cameron's values for pairing energies were used at the outset; but a new set of pairing and shell energy correction values, which leads to substantially improved agreement with the experimental curves, is presented. The procedure which was used to arrive at this set is described and several features of the set are discussed. The need for a further downward correction of the level density of symmetrical (A = 2Z) nuclei is indicated. Computed excitation functions are shown for all the reactions studied as well as for several reactions for which experimental data are not yet available. Further experiments on reaction cross sections are suggested which would allow a unique determination of the pairing and shell energy corrections of level densities for any value of Z and N in the region under discussion. The existence of a unique set of these correction terms would provide strong evidence for the validity of evaporation theory for the reactions considered.

Production of energetic light fragments in extensions of the CEM and LAQGSM event generators of the Monte Carlo transport code MCNP6

“…We have performed an extensive comparison of the NASA [55], Tsang et al [58], Dostrovsky et al [54], Barashenkov and Polanski [56], GEM2 [29], and Kalbach [57] systematics for total reaction (inverse) cross sections (see also the older works [13,59,60] with similar comparisons). We conclude that the NASA approach is superior, in general, to the other available models (see Ref.…”

“…At the evaporation/fission stage of reactions described by CEM0.03 and LAQGSM0.03, which use an extension of the generalized evaporation model code GEM2 by Furihata [29], the inverse cross sections are calculated with the same functional form, but using different constants from those in the original approximations [54]. We label those different inverse cross sections as "GEM2".…”

“…The unmodified codes use the inverse cross sections σ inv , from Dostrovsky's formulas [54] for all emitted nu- …”

MCNP6 updated proton-induced fission cross section calculations at intermediate energies

“…Atchison uses the Weisskopf and Ewing statistical model [12] with an energy-independent pre-exponential factor for the level density and Dostrovsky's [13] inverse cross section for neutrons and estimates the neutron width n as…”

Recoil studies of fission products formed in the fission of 238U by protons of energies 25–85 MeV

“…Dostrovsk y et al (14,15) among others have made a very meaningf ul calculation of the evaporat ion process for many nuclei at energies up to 50 MeV. The calculati on yields the charge, mass, excitatio n energy of the residual nuclei and also the energy spectra of the differen t particle s.…”