Dependence on gamma-ray incident angle of exposure rate absolutely determined by the use of cylindrical cavity chambers

The standard of gamma-ray exposure at the Electrotechnical Laboratory has been established by the use of cavity chambers of cylindrical shape. In the absolute measurement of exposure rate, the chambers are usually set in such a way that the chamber axis makes an angle of 450 to the gamma-ray incident axis to avoid distance effect. It has been found that the exposure rates absolutely determined with the chambers depend significantly on the incident angle of gamma-rays on the chamber even after the attenuation of gamma-rays in the chamber wall is corrected. The dependence is insignificant in the case of graphite chambers except for incident angles of 900 and 00, where it is impossible to correct the attenuation of gamma-rays by the method of extraporating the wall thickness. The dependence becomes significant in aluminum chambers and much larger in copper chambers. This is considered to be attributed to the assumption that the mean mass stopping power ratios are constant irrespective of the gamma-ray incident angle. The mean mass stopping power ratios of cylindrical cavity chambers, however, vary with the incident angle of gamma-rays on the chamber due to the variation of the range of Σ-rays produced in the chamber according to the incident angle. A qualitative analysis using Burlin's general cavity theory shows that in the absolute measurement of exposure it is not necessary to take into account of the dependency on the incident angle of gamma-rays except at the angles of 900 and 00 in the case of graphite cavity chambers, while it is necessary in the case of aluminum and copper chambers. (author)