Proceedings of the British Institute of Radiology

Experimental methods for determining ion recombination corrections are well established. However, recent measurements at NPL and elsewhere using parallel-plate chambers show effects at moderately high polarizing voltages which are not explained by present theory and which compromise the measurement of recombination corrections. Commonly used two-voltage methods, for example the half-voltage method recommended in the UK, are not able to detect these problems and can give erroneous correction factors. Results were presented for the NACP chamber type which show that, so long as certain precautions are taken, consistent results can be achieved over a wide range of dose rates. Values for the coefficient of general recombination in pulsed beams are presented for a series of nominally identical chambers. The values are shown to vary significantly from chamber-to-chamber of the same chamber type. These variations may be interpreted as differences in the chamber volumes, which allow effective electrode spacings an...