Verification of the Surrogate Ratio Method

TL;DR

This paper investigates the surrogate ratio method for determining neutron-induced reaction cross sections, establishing conditions for its validity without detailed spin-parity distribution knowledge, and estimating its accuracy for uranium nuclei.

Contribution

It demonstrates that the surrogate ratio method can reliably determine cross sections under specific conditions without detailed spin-parity distributions, expanding its practical applicability.

Findings

Surrogate ratio method can determine cross sections with ~5-10% accuracy.

Method remains robust despite breakup reactions.

Absolute surrogate method is less applicable for capture cross sections.

Abstract

Effects of difference in the spin and parity distributions for the surrogate and neutron-induced reactions are investigated. Without assuming specific (schematic) spin-parity distributions, it was found that the surrogate ratio method can be employed to determine neutron fission and capture cross sections if 1) weak Weisskopf-Ewing condition (defined in this paper) is satisfied, 2) there exist two surrogate reactions whose spin-parity distributions of the decaying nuclei are almost equivalent, and 3) difference of the representative spin values between the neutron-induced and surrogate reactions is no much larger than 10 $\hbar$. If these conditions are satisfied, we need not to know the spin-parity distributions populated by the surrogate method. Instead, we should just select a pair of surrogate reactions which will populate the similar spin-parity distributions, using targets having similar structure and reactions having the similar reaction mechanisms. Achievable accuracy is estimated to be around 5 and 10 % for fission and capture channels, respectively, for nuclei of the Uranium region. The surrogate absolute method, on the contrary, can be marginally applicable to determination of fission cross sections. However, there will be little hope to apply this method for capture cross section measurements unless the spin-parity distributions in the neutron-induced and surrogate reactions are fairly close to each other or the difference can be corrected theoretically. The surrogate ratio method was shown also to be a robust method in the presence of breakup reactions, again, without assuming specific breakup reaction mechanisms.