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

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 ²³⁵UF₄-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.