Theoretical assessment on the validity of the surrogate technique

Surrogate method is an important methodology to determine nuclear reaction properties for which direct measurements are practically impossible, such as neutron-induced cross sections of short-lived isotopes.  In the surrogate method, the same compound nucleus as the desired reaction is populated by different combination of target and projectile, and decay branching ratios are measured.  This information will be combined with population probability of the desired reaction to get information on what we need.  

Such a method is associated with uncertainties.  The biggest one is the mismatch of angular momentum distribution between the desired and surrogate reactions.  We propose a way to compensate for this difference in the surrogate "ratio" method, where the decay branching ratios are determined relative to those of known reactions.  The idea is to use 2 reactions having very similar properties in terms of nuclear structure and reaction mechanisms.  If the 2 reaction populate the compound nuclei with identical spin distributions, we can prove that the "ratio of branching ratios" does not depend on the spin distribution noticeably.  This is a good way to assess validation of the surrogate "ratio" method without knowing the underlying reaction mechanisms in detail.

Recently, a group in JAEA is actively pursuing the surrogate method using $^{18}$O projectile on heavy nuclei, $^{232}$Th, $^{238}$U, $^{242}$Cm and $^{254}$Es, to extract fission properties such as mass distribution of fission fragments over many populated compound nuclei, and Langevin theory is applied to analyze the data.  In this analysis, however, we need detailed knowledge on the population mechanisms of each compound nucleus, which are populated by multinucleon transfer reactions.  Especially, the spin distribution of each compound nucleus at specific excitation energy bin must be properly understood.  We will present our resent analysis based on antisymmetrized molecular dynamics.