Candidates for fast neutron standards among neutron producing reactions

Neutron fields from charged particle reactions are examined for their suitableness as standards. Very accurate differential neutron production cross sections are available from measurements of the associated charged particles, e.g. for the reaction 3H(p,3He)n, 2H(d,3He)n and 2H(t,α)n. However, these cross sections are usually not available at those energies and angles needed in fast neutron work. Therefore, measured neutron data over the ranges of interest must be used. By applying the ratio (or quasi-absolute) method reference cross sections of reactions involving the same target were combined and by exchanging projectile and target nuclei the two target types were combined, too. Thus unified differential cross sections for the monoenergetic neutron production by the hydrogen isotopes are available which use simultaneously all associated charged particle data of the three reactions as cross section reference. An energy dependent evaluation of all available data yielded cross sections with a minimum total absolute error of less than 2%. This small uncertainty justifies the attempt to choose some of these cross sections as fast neutron flux standard. The procedure selecting these standards is fully discussed. In addition it is shown how these proposed standards can be connected through time reversal with the n-6Li and n-10B standard. Aside from absolute standards there is a need of relative ones which can be used for easy calibration of the energy response of neutron detectors. Presently, data on the candidates for such standards are so sparse that one can only consider them for future use. At the moment a point-by-point measurement of the energy response using differential cross sections of the monoenergetic neutron production by the hydrogen isotopes is state-of-the-art. By selecting optimum excitation functions and angular distributions this procedure can be improved and standardized. Proposals toward this end are presented which take into account the accuracy of the reference cross sections as well as the practicability of their application