Comments on extracting the resonance strength parameter from yield data

Abstract

The F(α,n) reaction is the focus of on-going research in part because it is an important source of neutrons in the nuclear fuel cycle which can be exploited to assay nuclear materials, especially uranium in the form of UF6 [1,2]. At the present time there remains some considerable uncertainty (of the order of ±20%) in the thick target integrated over angle (α,n) yield from 19F (100% natural abundance) and its compounds as discussed in [3,4]. An important thin target cross-section measurement is that of Wrean and Kavanagh [5] who explore the region from below threshold (2.36MeV) to approximately 3.1MeV with fine energy resolution. Integration of their cross-section data over the slowing down history of a stopping α-particle allows the thick target yield to be calculated for incident energies up to 3.1MeV. This trend can then be combined with data from other sources to obtain a thick target yield curve over the wider range of interest to the fuel cycle (roughly threshold to 10MeV to include all relevant α-emitters). To estimate the thickness of the CaF2 target they used, Wrean and Kavanagh separately measured the integrated yield of the 6.129MeV γ-rays from the resonance at 340.5keV (laboratory α-particle kinetic energy) in the 19F(p,αγ) reaction. To interpret the data they adopted a resonance strength parameter of (22.3±0.8)eV based on a determination by Becker et al [6]. The value and its uncertainty directly affects the thickness estimate and the extracted (α,n) cross-section values. In their citation to Becker et al׳s work, Wrean and Kavanagh comment that they did not make use of an alternative value of (23.7±1.0)eV reported by Croft [7] because they were unable to reproduce the value from the data given in that paper. The value they calculated for the resonance strength from the thick target yield given by Croft was 21.4eV.The purpose of this communication is to revisit the paper by Croft published in this journal and specifically to explain the origin of the reported resonance strength. Fortunately the original notes spanning the period 12 January 1988 to 16 January 1990 were available to consult. In hindsight there is certainly a case of excessive brevity to rectify. In essence the step requiring explanation is how to compute the resonance strength, ωγ, from the reported thick target resonance yield Y.