Observations on the Ionization Produced by Radium Gamma Rays in Air-walled Ionization Chambers at Low Gas Pressures

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Abstract
In 1940 Sievert published curves of the measured variation of γ-ray ionization with air pressure obtained with spherical elektron metal condenser chambers with smaller inter-electrode distances (0·25–0·50 mm). These showed that as the pressure was reduced, the ionization per cm of mercury, measured at atmospheric pressure, increased very rapidly instead of remaining constant at 100 per cent as it should according to the Bragg-Gray theory of cavity ionization. A calculated curve obtained by Sievert (1940) showed that ionization by collision did not entirely account for his observations. In Fig. 1 Sievert's curves have been replotted to show the relation between observed current and pressure of air, the current per cm of mercury measured at atmospheric pressure being taken as 100. The curves are straight lines which we have extrapolated back to zero pressure. In effect, some current is still present at zero pressure when theory indicates zero current, and this for very low chamber potentials. Sievert's calculated curve for the inclusion of collision ionization shows entirely different behaviour. Sievert's paper encouraged a number of experiments in this laboratory between 1940 and 1943. A combination of more recent events, viz., (a) the description of vacuum chambers and their behaviour by Dr. Lauriston Taylor in his 1950 Sylvanus Thompson Memorial Lecture; (b) discussions which occurred during the Symposium on Radiological Units at the Sixth International Congress of Radiology; and (c) the theoretical considerations brought forward by Greening concerning possible events in chambers at low pressures (this Journal, page 163), suggests that they should be recorded.