A Comparison of X-ray Standards

Abstract

Comparison of the old NBS chamber and the NPL chamber with the new NBS chamber has shown that the two former chambers have adequate dimensions to give correct readings below about 70 kV for moderate filtration. They are in error by the order of 2 and 3 per cent respectively between 150 and 250 kV. The smallness of the difference in the errors explains why no significant difference was found between the chambers in the 1931 comparison. An initial discrepancy between the NPL chamber and the new NBS chamber at low kilovoltages was traced to a capacitance error in the NPL compensating system due to strain in fixing the capacitor to the rest of the system. This error provides an explanation for the disagreement in the calibrations of Boag's chambers and to a large extent for those with Roth's chambers. Comparison of the calibrations of Victoreen chambers by the two laboratories has shown differences of less than 2 per cent, which are nearly within the range of the variation of the values for the correction factor found by one laboratory on different occasions. The calibrations with radium γ rays are also in satisfactory agreement. A difference of 3 or 4 per cent between the correction factor found for radium γ rays and that for X rays from a 2 MV Van de Graaff generator has been shown to be associated with the irradiation of the stems of the Victoreen chambers to a differing extent. This effect and others, such as changes of electrometer sensitivity, indicate that the Victoreen dosemeter has too many minor sources of error to be satisfactory as a means of comparing primary standard chambers.