Comparison of Burlin cavity theory with LiF TLD measurements for cobalt-60 gamma rays.

Cobalt-60 gamma-ray absorbed dose measurements were carried out using LiF thermoluminescent (TL) dosemeters to obtain additional insight into cavity theory as applied to dosemeters of a size intermediate relative to the range of secondary electrons. Stacks of dosemeters (0.4 mm x 3 mm x 3 mm, hot-pressed LiF TLD-100) were surrounded by homogeneous LiF, polystyrene, aluminium, copper or lead. The absorbed dose in the stack (0.1 to 0.7 g cm-2 thick) was derived from TL readings of the individual dosemeters, and was compared with calculations based on the Burlin cavity theory. A relatively strong asymmetry in the experimental absorbed-dose distribution was found between the front, rear and middle dosemeters of the stack, especially when lead was the surrounding medium. Cavity theory shows good agreement with the experimental results obtained in polystyrene, while theory tends to overestimate the dose in the LiF dosemeters enclosed in Al, Cu, and Pb. The general trend predicted by the Burlin theory is somewhat steeper relative to cavity size than that observed experimentally. The theory was modified to make it more applicable to the dose at the centre of the stack. For lead this clearly agreed more closely with experimental results than did the conventional theory.