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 rontgen–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 (rontgens 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 ...