On the Congruity of N.P.L. Exposure Standards

Abstract

In the use of a graphite cavity chamber as a National reference standard for 2 MV X-rays, the roentgen realized for 2 MV X-rays must be consistent with the roentgen realized for, say, 250 KV X-rays by the use of a standard free-air chamber as a National reference standard for medium energy X-rays. The paper describes the work undertaken to assess the extent of agreement between the two sets of reference standards. For obvious reasons, it is not possible to use existing standard free-air chambers to measure exposure with 2 MV X-rays; but it has now become possible to use a graphite cavity chamber to measure exposure with 250 KV X-rays. The results of independent measurements by the cavity chamber and the free-air chamber may thus be compared.However, the analytical studies of graphite chambers reveal that with 250 KV X-rays more than 90% of the total ionization arises from photon absorption in an inner skin about 0·001 in. thick surrounding the cavity. Hence, very special precautions have to be taken to avoid contamination of the inner wall of the chamber. Contamination, due to dust from the wear of steel tools, that would be quite insignificant with 2 MV X-rays, can have a profound effect on the behaviour of the chamber when it is used with 250 KV X-rays. A clean graphite chamber was constructed with diamond and ceramic tools.In spite of uncertainties about the X-ray spectrum used, it is shown that with 250 KV X-rays the N.P.L. exposure standards are consistent to within ± 1%.