The Effect of Particle leaking and its Implications for Measurements of the (n,alpha) reaction on Light Elements Using Ionisation Chambers

Abstract

A new technique for the spectrometry of (n,alpha) reactions on light elements at MeV energies has been developed and successfully used for the measurement of the 10B(n,alpha)7Li reaction at the 7MV Van de Graaff accelerator of IRMM. The basic elements of the new method are a gridded ionisation chamber, a fast waveform digitizer and advanced off-line analysis. The powerful data visualistion allowed the discovery of the effect of particle leaking. Particle leaking arises from the simultaneous emision of reaction products in forwards angles and the inablity of the detector to resolve multiple particles. It is an inherent property of all GIC spectrometers used for the study of (n,charged particle) reactions on light-element solid targets. The measurement of the cross section strongly benefits from it but the determination of other measurables is negatively affected. Cross sections at seven energies between 1.5 MeV and 3.8 MeV have been obtained using the new technique. Compared to evaluations the IRMM cross sections are close to the JENDL 3.2 and JEF 2.2 but strongly deviate from the ENDF/B-VI data with the exception of very good agreement at 2.5 MeV. Forwards anagular distrivbutions are truncated at large emission angles by the effect of particle leaking and look like depleted of reaction products between a kinematically determined angle theta0 and 90°. it is shown that all values of the branching ratio alpha0/alpha1 of the 10B(n,alpha)7Li reaction published up to now in refereed journals and obtained by using ionisation chambers and face to face surface barrier detectors contain inaccuracies caused by particle leaking which has not been considered.JRC.D.5-Neutron physic