The status of x-ray standards in the United Kingdom.
Recent intercomparisons between the medium voltage X-ray standards of N.P.L. and of Sweden and USA (National Bureau of Standards) are described. The results confirm the work of Kemp and indicate that the original N.P.L. and N.B.S. standard free-air chambers have inadequate dimensions for the measurement of X-rays excited above 100 kV. In the case of the N.P.L. chamber the error has been about 0.4 per cent at 100 kV and 3 per cent at 200 kV. After allowing for the errors revealed by these intercomparisons, the calibration of secondary by the three laboratories concerned was found to be in agreement to about 1 per cent.Details are given of the installation at N.P.L. of a 2 MV Van de Graaff X-ray generator and of the dose measuring equipment constructed for use with this machine. Comparisons of the calibration of Victoreen chambers when exposed to X-rays generated at 1 to 2 MV or to radium gamma rays at N.P.L. and to 60Co gamma rays at N.B.S. indicate agreement to 1 per cent provided the conditions of stem i...
The Röntgen: Deficiencies in Present-day Free-Air Chamber Standards
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.
Comparison of the Electrostatic and the Electro-Magnetic Methods of X-Ray Measurement
Much attention has been directed in recent years to the measurement of X-ray dosage, and it will be recalled that it was the chief subject under discussion in the Physics Section of the First International Congress of Radiology held in London in 1925. It was felt at the time that the first step in the measurement of X-ray dosage was the establishment of a suitable unit in terms of which the dose could be estimated, this unit to be such that it could be adopted internationally and thus enable the work carried out in various countries to correlated. An International Committee was set up to investigate the possibility of establishing such a unit, and at the Second International Congress of Radiology held in Stockholm last year, this committee put forward a number of recommendations relating to an international X-ray unit which were adopted by the Congress.