The National Physical Laboratory secondary standard therapy-level x-ray exposure meter

As part of a programme directed towards the optimization of the Laboratory's calibration services—including the calibration techniques themselves—it was decided over five years ago to embark on the development of a secondary standard exposure meter. The aim was to produce an instrument whose specifications and performance would meet to the full the requirements of a secondary standard, and around which an optimized calibration service could be built (Kemp, 1970a, b). The following is a preliminary account of the outcome; a fuller description will be published in due course. It must be emphasized that no attempt has been made to produce an all-purpose instrument, since a basic proposition in the underlying philosophy is that by definition a secondary standard should have the single function of serving as the link between the primary standards and the multiplicity of field instruments. The instrument is comprised of three basic units, namely, the ionization chamber, the measuring circuit, and the stability check source. The design objectives were: (1) long-term stability of a high standard; (2) an effectively level response over as wide an X-ray energy range as possible; (3) the achievement of (1) and (2) without the necessity of adjustment during or after the manufacture of the chamber. The following are the main aspects of the design concerned with the attainment of these objectives: (1) the use of high-purity, high-density extruded graphite as the cap material (Barnard et al., 1964; Kemp and Barber, 1956).