The back-scatter correction for beta-ray chambers.

The ionization rate in a thin air film sandwiched between a large area β-active plaque of phosphorous-bakelite and a thick layer of inactive material has been examined by McInally and Neary (1953). They denoted by R the ratio of the increase in ionization rate caused by the presence of the inactive material, to the rate with no material present. Values of (1 + R) for a number of materials relative to acrylic resin were tabulated, and it was suggested that the results could be used to correct the “contact” readings of a β-ray ionization chamber having a solid base of material significantly different from tissue. This is true provided that the dimensions of the chamber approximate to the conditions mentioned above, i.e. that its depth h is very much less than its radius a. In cases where this is not so the correction factors will be less as is shown by the following considerations. Suppose a plane β emitter is at a distance h0 from a thick layer of inactive material. Let J0 be the ionization rate at the sur...