Progress Committee Report for 1971

International Fluence-Rate Intercomparison for 144 and 565 keV Neutrons

“…4). If it is assumed that the cross-section has been correctly measured in the previous NPL work [16] near 565 keV (in an energy region where the cross-section is almost constant and target-scattering effects are very small), it follows that the old NPL cross-section value at 157 keV (which is near 144 keV) is 10% high, in excellent agreement with the conclusion reached by Hunt [8]. A disturbing feature is the failure of the Monte Carlo calculations to predict such a large target-scattering effect, and further more detailed calculations are clearly required as mentioned in the previous paragraph.…”

“…B. Hunt) [8] National Research Council (NRC) Ottawa, Canada (L. Van der Zwan, K. W. Geiger) [9] Physikalisch-Technische Bundesanstalt (PTB) Braunschweig, Federal Republic of Germany (H. Lesiecki, M. Cosack) [ 101…”

“…The NPL [8] used a standard long counter, which has previously been described in detail [ll]. This instrument had been very carefully calibrated with neutron sources of accurately determined output, as measured in the NPL manganese bath [12], and also by an associated target activity technique [13], and a collimated vanadyl sulphate bath [ 141.…”

“…The NPL scatter corrections were based on time-of-flight measurements, and at both energies were much larger than the Monte Carlo calculations would suggest. In the appendix to the NPL report [8] (which was written shortly after the original intercomparison result had been submitted to the evaluator) an ingenious argument was used to show that the result at 144 keV should be increased by 4.4% (the 'revised' result of Table 5). Briefly, the argument compared the present indium-activation measurements at 144 keV and 565 keV using a thin, 0.25 mm, silver target backing, with previous very similar NPL measurements [16] at nearly the same energies using a thick, 1 mm, silver target backing, and extrapolated the results to zero thickness of target backing.…”