Monte Carlo simulation of γ and fission transfer-induced probabilities using extended 𝓡-matrix theory: Application to the ²³⁷U^∗ system

Abstract

Abstract. This paper deals with simultaneous neutron-induced average partial cross sections and surrogatelike probability simulations over several excitation and de-excitation channels of the compound nucleus. Present calculations, based on one-dimensional fission barrier extended R-matrix theory using Monte Carlo samplings of both first and second well resonance parameters, avoid the surrogate-reaction method historically taken for surrogate data analyses that proved to be very poor in terms of extrapolated neutron-induced capture cross sections. Present theoretical approach is portrayed and subsequent results can be compared for the first time with experimental γ -decay probabilities; thanks to brand new simultaneous 238 U( 3 He, 4 Heγ ) and 238 U( 3 He, 4 He f) surrogate measurements. Future integration of our strategy in standard neutron cross section data evaluation remains tied to the developments made in terms of direct reaction population probability calculations. Context and objectivesThe idea to supplement neutron-induced cross-section data for actinides and higher transuranic nuclides with particle-transfer-induced reactions has been raised a long time ago [1]. Over the years, a variety of surrogate vectors have been used as stripping (d,p) and pickup (p,d) reactions, ( 3 He,p), ( 3 He,d) and ( 3 He,t) chargeexchange reactions or even two-neutron transfer reactions as (t,p) and (p,t) reactions. Analytical calculations of these measured direct-reaction fission probabilities were performed under several simplifications contained in the so-called surrogate-reaction method [2]. Early promising comparisons [1] made between extrapolated neutron fission cross sections and cross sections directly measured by neutron spectroscopy led to agreement within 10% to 20% at neutron energy above the nucleus pairing energy although exhibiting larger deviations at lower energies. Major limitations in surrogate extrapolation were promptly noticed [1,3] with the difficulty to estimate the compound nucleus formation cross section by neutron absorption, the possible influence of angular momentum differences between neutron capture and direct reactions and the validity of the Weisskopf-Ewing (WE) hypothesis related to fission decay probability spin-parity independence [4].A decade ago, surrogate reactions received renewed interest in terms either of simulation [5,6] or experimental investigation (initiated by study [7]; reviewed by Ref. [8]) to infer in addition to fission, neutron-induced capture cross sections. Newly measured decay capture probabilities were readily analyzed within the surrogatereaction method that proved to be very poor in terms of extrapolated neutron-induced capture cross sections [2]. a e-mail: olivier.bouland@cea.fr Indeed the surrogate-reaction method was understandable in the seventies because of computer limitations, lack of precise information on nuclear level densities across the deformation and difficulties for achieving confident optical model calculations over a large range of nuclides. Nowada...

Investigation of the surrogate-reaction method via the simultaneous measurement of gamma-emission and fission probabilities

“…The preliminary calculated decay probabilities obtained in [10] show a dependence on the spin and parity for both fission and gamma emission, although, contrary to the calculations presented in [5], this dependence is much weaker for fission. As shown in [10], it is possible to reproduce our measured decay probabilities fairly well by weighting these calculated decay probabilities with the J π probability distributions of the 238 U( 3 He, 4 He) reaction predicted by I. Thompson and J. Escher [11]. The latter distributions are very different for positive and negative parities and significantly differ from the neutron-induced ones.…”

“…It lies in the fact that the values of J and π that are more strongly populated in the 238 U( 3 He, 4 He) surrogate reaction are precisely the ones that lead to similar gamma-emission probabilities, and that these probabilities are rather close to our results. This gives confidence on both the calculated J π distributions of [11] and the statistical-model calculations of [10].…”

“…This suggests that none of the limiting situations described in the introduction can be invoked to explain the good agreement found for the fission probabilities of 239 U at low excitation energy. Contrary to the calculations used for the 239 U case, the preliminary fission probabilities of 237 U obtained in [10] show a rather weak dependence on J π , which explains the good agreement observed between the transfer-and neutroninduced fission probabilities of 237 U. However, similarly to the 239 U case, the gamma-emission probabilities of 237 U are very sensitive to J π .…”

“…Bouland et al [10] have performed a theoretical study of our results for the decay probabilities induced in the 238 U( 3 He, 4 He) reaction based on the one-dimensional fission barrier extended R-matrix theory with Monte Carlo samplings of class I and class II state parameters. The preliminary calculated decay probabilities obtained in [10] show a dependence on the spin and parity for both fission and gamma emission, although, contrary to the calculations presented in [5], this dependence is much weaker for fission.…”

“…Bouland et al [10] have performed a theoretical study of our results for the decay probabilities induced in the 238 U( 3 He, 4 He) reaction based on the one-dimensional fission barrier extended R-matrix theory with Monte Carlo samplings of class I and class II state parameters. The preliminary calculated decay probabilities obtained in [10] show a dependence on the spin and parity for both fission and gamma emission, although, contrary to the calculations presented in [5], this dependence is much weaker for fission. As shown in [10], it is possible to reproduce our measured decay probabilities fairly well by weighting these calculated decay probabilities with the J π probability distributions of the 238 U( 3 He, 4 He) reaction predicted by I. Thompson and J. Escher [11].…”