The accurate fission cross-section of 235U from 0·002 to 0·15 eV and its reference value at 2200 m/s

Abstract

A new precise measurement of the 2200 m/s fission cross-section of 235U was performed with a slow chopper at the BR2 reactor (energy-range from 0·002 to 0·15 eV). Simultaneous recordings of pulse-height and time-of-flight spectra of successively compared 235UF 4-layers and “standard” elemental boron-layers were made. The detection of the 10B( n, α) 7Li- and 235U( n, f) reaction fragments is made with excellent pulse-height resolution in low geometry with a surface-barrier detector. The absolute 10B contents were determined from direct weighing in vacuum combined with careful chemical and isotopic analysis of witness foils. The relative 10B contents of the standard boron-foils were checked by their neutron induced reaction rates in the geometry of the fission experiment. The amount of uranium on the fissile foils is determined by precise low geometry α-counting making use of the most recent and accurate α half-life values. The fission foils were also checked by α- and fission fragment counting in the geometry of the experiment. The fission cross-section was calculated in the energy-range from 0·002 to 0·15 eV and the absolute value deduced at 2200 m/s was 587·6 ± 2·6 barn. The value at 25 × 10 −3 eV is 591·8 ± 2·7 barn. These fission cross-section values were used to calculate the Westcott- g φ -factor for a 20·44°C Maxwellian-distribution with as result: g f = 0·9780 ± 0·0010.