Dosimetry of planar and punctiform beta sources using an automated extrapolation chamber and radiochromic dye films

A Loevinger extrapolation chamber has been automated and used to determine detailed dose distributions around beta emitting sources of varied geometry. The absolute dose estimates provided by the extrapolation chamber have been compared with those obtained from radiochromic dye film dosemeters. For large area planar sources, extrapolation chamber estimates obtained by conventional linear extrapolation of the ionisation current/electrode spacing characteristic are in reasonable agreement with the radiochromic film values. The shape of the curve for punctiform sources, however, indicates that significant curvature may still be present at the smallest practically attainable electrode separations. In such situations non-linear extrapolation of the current/spacing curve appears to provide estimates which are in better agreement with those obtained using radiochromic film. This is in accord with the predictions of a simple theoretical model for an idealised point source. (author)