MEASUREMENTS OF THE THERMAL-NEUTRON CROSS-SECTION OF 90SR(N,GAMMA)91SR REACTION

The thermal neutron cross section of the Sr-90(n,gamma)Sr-91 reaction was measured in a D2O moderated reactor with an activation technique. The Cd ratio of the neutron flux al the location of the irradiation is about 30. For reduction of the Y-90 high energy beta-ray background s trontium-specific columns (Sr. Spec(TM)) were used to chemically separ ate the Y from Sr before irradiation and again after irradiation prior to commencement of counting the Sr-91 activity. The amount of Sr-91 i n a counting sample was determined from the beta activity of Y-90 with an end-window beta-ray proportional counter. This system was calibrat ed with a known Sr-90 standard sample. The neutron flux at the irradia tion position in the D2O moderated reactor was determined from a Co/Al alloy monitor irradiated simultaneously with a Sr-90 sample. The Sr-9 1 activity was measured with a high resolution Ge spectrometer. The se nsitivity to beta induced bremstrahlung was reduced by the use of luci te, cadmium, copper, and lead absorbers around the sample and the Ge d etector. The overall efficiency of the spectrometer was determined wit h a set of calibrated Cs-137 Cs-134,Ba-133, Y-88, Co-60 and Mn-54 sour ces. These measurements yielded a value of (9.7 +/- 0.7 mb) for the D2 O moderated neutron cross section of Sr-90(n,gamma)Sr-91.