The accurate fission cross-section of 239Pu from 0·005 eV to 0·1 eV and its reference value at 2200 m/s

A precise measurement of the fission cross-section of ²³⁹Pu was performed with a slow chopper at the Belgian BR2-reactor (energy range 0·000 to 0·1 eV). Simultaneous recordings of pulse-height and time-of-flight spectra of successively compared ²³⁹Pu layers and standard elemental boron layers were made. The detection of the ¹⁰B(n, α)⁷Li and ${}^{239}\text{Pu}\text{(n, ƒ)}$ reaction fragments is made with excellent pulse-height resolution in low geometry with a AuSi surface barrier detector. The separation between the pile-up tail produced by the α-particles of ²³⁹Pu and the fission fragment energy-distribution was very good. The amount of ¹⁰B in the elemental boron layers was determined by weighing in vacuum combined with careful chemical and isotopic analysis of witness foils. The relative amounts of ¹⁰B were checked by neutron-counting in the geometry of the experiment. The amounts of ²³⁹Pu on the fission foils were determined from precise low-geometry α-counting and the knowledge of the isotopic composition and α-half-life values. The α-activity was controlled before and after the measurements.

The fission cross-section was calculated in the energy-range from 0·005 to 0·1 eV and its absolute value deduced at 2200 m/s was 741·9 ± 3·4b. These cross-section values were used to calculate the Westcott g_/tf-factor for a 20·44°C Maxwellian neutron distribution with as results $\text{g}{}_{\mathrm{ƒ}}\text{= 1·0553 ± 0·0013}$.