The Ionization produced by High Voltage Radiation in an Extrapolation Chamber with Walls of various Atomic Numbers and its Relation to that to be expected theoretically

Measurements with an extrapolation ionization chamber are described which aim to measure the ionization within chambers with high Z walls when the chamber dimensions are extrapolated to zero. The extrapolated values of ionization per unit volume are compared with those calculated from the Bragg-Gray theory. They agree well with the theoretical values for elements at least up to Z = 13 (Al) for radiations up to 0·5Å. For elements of greater Z and for λ above about 0·10Å, agreement is not found between the experimental results and the original theory. Better agreement can be obtained if the theory is modified so as to take into account the fact that in the ejection of the photoelectrons from the K shell of the wall material, an appreciable proportion of photon energy is used. The significance of these results is discussed from the point of view of measuring the energy absorbed in media.