The Experimental Realisation of the Röntgen

An account is given of the X-ray standardisation equipment at the National Physical Laboratory for the realisation of the röntgen–the unit of quantity of X rays. The X rays, which are excited by constant voltage, are “photometered” by the aid of the graphite parallel-plate ionisation chamber of the “free-air” type, which is adopted as a primary standard. A theoretical and experimental study has been made of the factors involved in the determination of ionometric intensity, that is, the ionisation equivalent of the X-ray energy absorbed in unit volume of air in unit time (röntgens per second). Particular attention is directed to the influence of the diaphragam system and to the estimation of the volume of air ionised. In the absence of an ideal source of X rays, the nature of the diaphragm system employed is of importance. By reason of the non-uniformity of the focal spot, or in other cases, of the presence of exteraneous radiation, it is established that the inverse square law of radiation is not exactly obeyed. Two independent methods of estimating the volume of air ionised have been investigated and found to be in accordance. Under suitable conditions the accuracy with which the röntgen can be realised practically by the present N.P.L. equipment is of the order of 0·5 percent, which would appear to be adequate, at any rate for present purposes.