A NEW THERMAL NEUTRON FLUX DENSITY STANDARD

Six Am241–Be(α, n) neutron sources are arranged around the central circumference of a graphite cylinder 28 cm in diameter by 26.5 cm high. This assembly is surrounded by 10 cm of polyethylene. The graphite cylinder contains an accessible central cavity, 5 cm in diameter by 5 cm high, for the calibration of thermal neutron detectors. The thermal neutron flux density in the cavity is measured by 4πβ–γ coincidence counting of irradiated gold foils. A flux density below cadmium cutoff of nthv0 = 8.70 × 103 cm−2 sec−1 (±1.5%) was found. The uniformity is shown to be constant within the cavity to better than ±0.5% over a cylindrical volume 5 cm in diameter by 2 cm high.