An experimental and theoretical study of the energy absorption from high voltage radiation by means of ionization measurements with an extrapolation type chamber

Abstract

This experimental and theoretical study aims at further investigation, by means of an extrapolation type of ionization chamber, of the ionization measurement of energy absorption from high-voltage radiations within a medium. Wavelengths ranging between 0.08 and 0.5 A0 were used. The walls of the ionization chamber were made of simple elements (graphite, aluminium and copper) or pressed bakelite - graphite mixtures which were loaded with cerium oxide in order to control the effective atomic number of the mixtures. A distinct advantage of the experimental arrangement used is the possibility of measuring the ionization per unit spacing when the air space is vanishingly small which thus eliminates the variable effects of chamber size. The results of earlier workers with chambers of fixed finite dimensions have been difficult to interpret in terms of theoretical considerations because of these effects. Furthermore, by varying the thickness of the upper plane electrode of the chamber, correction could be made for absorption of radiation in this electrode, which, at long wavelengths, maybe considerable in the media of higher atomic number. The results obtained with a chamber of graphite walls show that graphite behaves approximately as air walled material, the ionization per unit volume is constant i.e. the ionization I0 being proportional to V where V is the air volume. With walls of atomic number greater than that of air the ionization per unit spacing increases slightly as the spacing decreases up to a certain threshold spacing blow which it increases very rapidly. The ionization per unit spacing and the threshold spacing both depends upon the material of the electrodes and the wavelength of the radiation. The ionization per unit spacing at zero dimensions may be measured in two ways. Firstly, by drawing at the origin a tangent to the ionization - spacing curve and secondly by extrapolation to zero dimensions of the ionization per unit spacing - spacing curve. It was thus possible to compare these experimental observations with expectations based upon the Bragg -Gray theory of ionization within a cavity. This comparison suggests that the Bragg - Gray theory may be regarded as a satisfactory description of the facts for the range of wavelengths studied, at least for elements of atomic number up to that of aluminium (Z = 13). For copper (Z = 29) and the mixtures (depending upon the electron emission from Ce of Z = 58) the experimental results disagree with the theory except for the shortest wavelengths, and the disagreement increases with increase of wavelength. Suggestions are advanced and a modification made to Gray's equation in an attempt to correct for this disagreement. These are based upon a consideration of the sources and the energy of the photoelectrons omitted from the wall materials.