Measurement of kilovoltage by the penetrameter - theoretical aspects

The penetrameter method makes use of the attenuation in copper of a heavily filtered X-ray beam to predict the generating kVp. This is derived from the matching step or copper reference number (CRN) of a film density matching process in which a fast and slow screen combination is employed. In the present work, theoretical spectra resulting from chosen representative kV and mA wave-forms have been derived by computer analysis, assuming Kramers' expression for the bremsstrahlung emission from a thick target. The CRN appropriate to each spectrum has been found and its relation to kVp recorded. The uncertainty in the use of the penetrameter is expressed in terms of the spread of the CRN as a function of kVp and waveform. The computer predictions agree well with experiment, provided a fall in the fast-slow screen ratio with energy is postulated - a fall verified by separate experimental work in screen ratios. Since Kramers' expression relates only to bremsstrahlung radiation, the theoretical analysis has been extended to include the effects of K characteristic and extrafocal radiation. An expression has also been derived for the sensitivity of the penetrameter.