Abstract
The surrogate reaction method may be used to determine the cross section for neutron induced reactions not accessible through standard experimental techniques. This is achieved by creating the same compound nucleus as would be expected in the desired reaction, but through a different incident channel, generally a direct transfer reaction. So far, the surrogate technique has been applied with reasonable success to determine the fission cross section for a number of actinides, but has been less successful when applied to other reactions, e.g. neutron capture, due to a ‘spin-parity mismatch’. This mismatch, between the spin and parity distributions of the excited levels of the compound nucleus populated in the desired and surrogate channels, leads to differing decay probabilities and hence reduces the validity of using the surrogate method to infer the cross section in the desired channel. A greater theoretical understanding of the expected distribution of levels excited in both the desired and surrogate channels is therefore required in order to attempt to address this mismatch and allow the method to be utilised with greater confidence. Two neutron transfer reactions, e.g. (p,t), which allow the technique to be utilised for isotopes further removed from the line of stability, are the subject of this study. Results are presented for the calculated distribution of compound nucleus states populated in 90Zr, via the 90Zr(p,t)90Zr reaction, and are compared against measured data at an incident proton energy of 28.56 MeV.
Highlights
The majority of reactions of interest to the nuclear industry, and many relevant to astrophysics, involve the collision of an incident neutron with a target nucleus
The model developed has been applied to the case of 28.56 MeV protons incident on an isotopically enriched 92Zr target, a case for which experimental data have recently been taken by a group from the Lawrence Livermore National Laboratory using the STARLiTeR detector at Texas A&M University
The model developed makes a number of assumptions: the target nucleus is even-even, spherical; it is in its ground state with Jπ = 0+; and that the two neutrons are transferred simultaneously, in one-step, as a single spin-singlet di-neutron object, with zero intrinsic angular momentum
Summary
The majority of reactions of interest to the nuclear industry, and many relevant to astrophysics, involve the collision of an incident neutron with a target nucleus. The decay path taken from a higher excitation energy state may be different in the surrogate case compared to that of the desired reaction, it will still result in the production of fission fragments and the same (n,f ) cross section. Surrogate (n,γ) studies have shown that, for the current surrogate approach employed, a more sophisticated application of theory is required to take into account the differences in spin distribution between the desired and surrogate cases [6] Another assumption made during early applications of the surrogate method was that differences in both the type and relative strength of pre-equilibrium reactions could be ignored in both the desired and surrogate reaction channels. This developed model has first been applied to recent measurements of the 92Zr(p,t)90Zr reaction
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