Abstract
THE röntgen is defined as “that quantity of X- or gamma-radiation such that the associated corpuscular emission per 0.001293 gm. of air produces, in air, ions carrying 1 e.s.u. of charge of either sign”. An ionization chamber designed for the realization of this unit must therefore provide sufficient space around the air volume in which the primary ionization occurs to permit the corpuscles from it full range, effectively. In the parallel-plate type of chamber, this involves not only sufficient electrode spacing, but also adequate dimensions at right angles, that is, lateral to the X-ray beam. It was recognized in the design of the typical standards set up more than twenty years ago in Great Britain1 and the United States2 that the actual electrode spacing need not be so great as might at first sight be thought necessary on theoretical grounds. Thus for 200-kV. X-rays, a plate spacing approaching 70 cm. (with effectively equal lateral dimensions) is theoretically desirable, whereas, in fact, the ionization losses with dimensions considerably less than these may be negligible. Thus 10 cm. only was adopted as the plate-spacing in the chamber set up at the National Physical Laboratory, Teddington, and 12 cm. in that at the U.S. Bureau of Standards.
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Kaye, G. W. C., and Binks, W., Brit. J. Rad., 6, 530 (1933).
Taylor, L. S., and Singer, G., Radiology, 15, 637 (1930).
Kemp, L. A. W., Brit. J. Rad., 18, 107 (1945); 19, 233, 301 and 488 (1946); 24, 211 (1951).
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KEMP, L. The Röntgen: Deficiencies in Present-day Free-Air Chamber Standards. Nature 172, 35 (1953). https://doi.org/10.1038/172035a0
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DOI: https://doi.org/10.1038/172035a0
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