The effect of particle leaking and its implications for measurements of the (n,α) reaction on light elements by using ionisation chambers

A new technique for the spectrometry of (n,α) reactions on light elements at MeV energies has been developed and successfully used for the measurement of the 10B(n,α)7Li reaction at the 7 MV Van de Graaff accelerator of IRMM. The basic elements of the new technique are a gridded ionisation chamber, a fast waveform digitizer and advanced off-line analysis. The powerful data visualisation allowed the discovery of the effect of particle leaking. Particle leaking arises from the simultaneous emission of more than one reaction products in forward angles and the inability 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 and 3.8 MeV have been obtained by using the new technique. Compared to evaluations the IRMM cross sections are close to the JENDL-3.2 data for energies from 1.5 to 2.15 MeV and above 2.5 MeV they have values between those of 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. Forward angular distributions are truncated at large emission angles by the effect of particle leaking and appears depleted of reaction products between a kinematically determined angle θ0 and 90°. It is shown that all values of the branching ratio α0/α1 of the 10B(n,α)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 was not considered.