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

Abstract

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 Pu(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 g ƒ = 1·0553 ± 0·0013 .