The simple perturbation technique has been used to determine effective capture crosssections for a large number of materials in the small experimental fast reactor ZEPHYR. Measurements have been made not onlywith the majority of the natural elements, but also with a considerable number of enriched isotopes. The results provide interesting information concerning the dependence of the fast-neutron capture cross-sections upon nucleon number and shell structure.
1. Introduction
AN extensive programme of perturbation measurements has beencarriedoutat Harwell in the small experimental fast reactor ZEPHYR. This system, which has been described in detail in the paper by HOLMES et al. (1954) consists essentially of a plutonium core surrounded by a natural uranium envelope. Although the reactor is operated at most at only a few watts of fission power, the corresponding neutron flux at the core centre is nearly IO9 n/cm2/sec. The energy spectrum of the neutrons in the core region is approximately constant, with an average energy about I.5 MeV, being only slightly degraded below that of a fission spectrum.
Perturbation studies have been performed at the centre of the ZEPHYR core with the majority of the natural elements, in order to investigate their capturing properties for the core neutrons. The results have already been reported briefly at the Geneva conference (HOLMES et al., 1955). Significant variations were observed for the measured capture cross-sections. Marked reductions occur for elements whose mean neutron numbers are close to the magic numbers, whilst, in general, odd-Z elements show larger capture cross-sections than corresponding even-Z elements. These conclusions are in qualitative agreement with those reported recently by OKRENT et al. for GODIVA which is a bare U235 fast reactor whose neutron-energy spectrum is similar to that in ZEPHYR, and by SNYDER (1955) for SAPL PPA-5, an intermediate system. Because of these interesting variations, the ZEPHYR program has been extended to include a considerable number of enriched isotopes, in an effort to determine the variation of fast-neutron capture as a function of nucleon number.
This article describes, in detail, the measurements which have been carried out with nonfissionable materials at the centre of the ZEPHYR core, both for the natural elements and the enriched isotopes. It is emphasized that the effective capture crosssections which are reported here do not represent the effects of neutron absorption only. In that they show marked dependence upon proton and neutron number, however, they provide interesting information concerning odd-even and shellstructure variations.
2. THEORY
The energy spectrum of neutrons over the ZEPHYR core remains approximately constant.
It seems reasonable, therefore, to use a simple one-group theory for an