Cross section for thermal-neutron absorption and resonance absorption integral in Pu²⁴⁰

Abstract

Using the oscillating reactor method the cross section for the absorption of thermal neutrons neutrons and the resonance absorption integral in the isotope Pu²⁴⁰ have been measured. The measurements were performed at two positions inside the heavy-water reactor of the Academy of Sciences, USSR; in the lattice and in the thermal column. Plutonium samples with three different abundances of the isotopes Pu²³⁹, Pu²⁴⁰ and Pu²⁴¹ were used. The oscillations in the power level produced by excursions of the samples were recorded with a recording potentiometer. To avoid self-shielding effects, the dependence of sample effect on plutoniuni weight was investigated. To determine the spectral composition of the neutron ilux in the lattice and in the column the cadmium ratio for indium was measured. The values for the cross sections in Pu²⁴⁰ are as follows in the column — (560 ± 35) barns, in the lattice — (1010 ± 120) barns. The cross section for absorption of thermal neutrons in Pu²⁴⁰ was found to be σT = (460 ± 45) barns and the resonance absorption integral — Σ = (10,000 ± 2,800) barns. The value obtained for the thermal neutron cross section agrees with a contribution in the thermal cross section from a resonance level at E₀ ≃ 1 ev. This indicates the absence of strong levels in Pu²⁴⁰ in the energy region from 1 ev down to negative values and also indicates that in the thermal-neutron region the cross section in Pu²⁴⁰ varies according to a 1/v law.