A Californium-252 Fission Spectrum Irradiation Facility for Neutron Reaction Rate Measurements

Spontaneous fission sources of ²⁵²Cf, lightly encapsulated and with neutron source strengths approaching 10¹⁰ 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. A free-field neutron flux in the range of 10⁷ n/(cm² s) (10⁵ n/mm² · s) and fluences of up to 10¹³ n/cm² (10¹¹ n/mm²) are established at the facility based only on a distance measurement and the absolute source strength of the national standard Ra-Be photoneutron source. The error in the ²⁵²Cf source strength (±1.1%) dominates the total free-field flux uncertainty of ±1.4% (1σ). 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 ±0.7% to the total flux response uncertainty for all observed neutron reaction rates, including those with sensitivity to low-energy neutrons.