Absolute Measurement of the Cross Section for 23 Kev Neutron Activation of Thorium.

Abstract

Measurement has been made of the thorium radiative capture cross section using Sb-Be photoneutrons and an absolute activation method. The average of two determinations gave a value of 604 mb at 23 keV neutron energy with a 4% estimated error from all sources ((+OR-)1(sigma)). In each experiment, a cylindrical shell target of natural thorium metal was irradiated for approximately two weeks by a centrally-positioned Sb-Be source. The accumulated 27-day half-life ('233)Pa, activated by neutron capture in thorium, was recovered by solvent extraction with diisobutylcarbinol after first dissolving the target in hydrochloric acid. Ninety percent recovery of protactinium was obtained, as measured by isotopic tracing with ('232)Pa. The 1.3-day half-life ('232)Pa had to be produced immediately before each use. A thin thorium foil was irradiated with 11 MeV protons using a t and em van de Graaff accelerator. The ('232)Pa was then recovered by chemical separation. ('233)Pa activity was measured by counting the 312-keV decay gamma with a Ge(Li) detector. Detector efficiency was determined by indirect calibration against a ('237)Np deposit assayed by alpha counting at the National Bureau of St and ards. The Sb-Be source strength was measured by the manganese bath method, using a ('252)Cf source for calibration, traceable to the NBS II neutron st and ard. Flux per unit source strength was determined by Monte Carlo modelling of the source/target irradiation geometry.