Measurements of displacement factors in a neutron beam using activation dosemeters

Displacement factors to account for the presence of a cavity when making dosimetry measurements in a phantom with ionisation chambers have been measured in a neutron beam. Activation dosemeters were used in cylindrical cavities between 5 and 25 mm in diameter at a depth of 5 cm in the phantom. Measurements were also made with one of the cavities at four depths between 2.5 and 20 cm. A linear relationship was found between cavity radius and the activation ratio for the 27Al(n,p)27Mg reaction and no change in this ratio was found with depth. When measurements were made with the 27Al(n,α)24Na and 115In(n,n')115Insup(m) reactions changes in activation ratios were observed due to changes in spectrum. Estimates of the spectral changes were made in order to calculate ratios in kerma which can be equated to the true displacement factor. This was found to be given by ksub(d) = 1.0-0.0045r, where r (mm) is the cavity radius. Measurements were also made in a simulated Extradin T-2 ionisation chamber cavity and the equivalent cylindrical radius was found to be 1.9 +- 0.45 mm. (author)