A SIMPLE EXPERIMENTAL METHOD FOR DETERMINING EFFECTIVE THRESHOLD ENERGIES AND CROSS SECTIONS

A facility was designed and fabricated to study fastneutron activation techniques. With this facility, the neutron-energy spectrum from the BRR can be modified by means of liquids or solids surrounding the specimen at the irradiation position. The resonance and fast-neutron flux and energy spectra were measured as a function of distances through air, water, and monoisopropylbiphenyl (oil) with threshold detectors whose threshold energies are well known. Traverses were then made with foils whose reaction threshold energies and effective cross sections are nat well established. By plotting the slope of the curves for corrected-saturation activity vs. distance against neutron energy for all reactions, the threshold energies of the unknown reactions were determined. Since the neutron spectrum had been accurately determined wlth known threshold detectors, the effective cross sections of the new detectors could be evaluated by dividing their saturated activity by the number of neutrons above their threshold energies. The results were as follows: Ti/sup 47/(n,p)Sc/ sup 47/, E/sub ef/f = 2.40 Mev, sigma /sub e/ i/sub ff/ = 0.085 barn; Ni/sup 58/ (n,p)Co/sup 58/, E/sub eff/ = 2.55 Mev, sigma /sub ef/ i = 0.226 barn; CI/ sup 35/(n, alpha )P/sup 32/, E/sub eff/= 4.10 Mev, sigma /sub eff/ =more » 0.178 barn; Fe/sup 54/(n,p)Mn/sup 54/, E/sub eff/ = 5.10 Mev, sigma /sub eff/= 1.16 barn; I/ sup 127/(n,2 n)l/sup 126/, E/sub eff/ = 10.70 Mev, sigma /sub eff/ = 1.28 barns; and F/sup 19/(n,2n)F/sup 18/, E/sub e / f>i/sub f/ = 13.10 Mev, sigma /sub eff/ = 0.121 barn. Some results of resonance-flux traverses through air and water are also included. (auth)« less