CF-252 FISSION SPECTRUM IRRADIATION FACILITY FOR NEUTRON REACTION-RATE MEASUREMENTS

Spontaneous fission sources of **2**5**2Cf, lightly encapsulated and with neutron source strengths approaching 10**1**0 n/s, have been developed especially for integral cross-section measurements and neutron reaction rate calibrations. An irradiation facility at the National Bureau of Standards makes use of these sources in two well-investigated geometries. Neutron scattering effects in the source capsule and support structures and neutron return from concrete and earth boundaries have been calculated and investigated experimentally. In the worst case, they contribute plus or minus 0. 7% to the total flux response uncertainty for all observed neutron reaction rates, including those with sensitivity to low-energy neutrons.