Secondary standard calibration of neutron radiation

The methods of secondary neutron standardization applied in Czechoslovakia are reviewed. Equipment is described for the reproduction of the unit of thermal (approx. 0.025 eV) neutron flux density, consisting of a graphite and polyethylene moderated 241Am-Be source having an emission of 8.3x106s-1. The thermal neutron flux density in the irradiation channels is determined by the activation of gold foils both in a cadmium jacket and without cadmium. The induced 198Au activity is measured by the 4π beta-gamma coincidence method. The equipment for the reproduction of the unit of intermediate (up to 50 keV) neutron flux density consists of a polyethylene cylinder (200 mm in dia, 200 mm in height, a coaxial cylindrical bore of 25 mm in dia) and a 241Am-Be neutron source having an activity of 8.3x106s-1 placed in the middle of the bore. The intermediate neutron flux density on the cylinder surface normally to the cylinder axis is 1.021x107 m-2s-1, with an error of +-4%. The gold foil induced activity is measured by a Ge-Li detector with a 512-channel amplitude analyzer. The fast (above 50 keV) neutron flux density is determined using a set of four polyethylene spheres (Bonner spheres) with diameters of 380, 280, 150, and 100 mm. A 6Li(Eu) scintillator or other detectors are placed in the sphere centres as thermal neutron detectors. Standard equipment is being developed for the reproduction of the unit of absorbed neutron dose consisting of two differential Bragg-Gray ionization chambers, one of tissue-equivalent material and filled with a tissue-equivalent gas, the other, insensitive to neutrons, of graphite and filled with CO2. (B.S.)