Measured influence of the central electrode diameter and material on the response of a graphite ionisation chamber to cobalt-60 gamma rays

Abstract

An investigation was performed on the response to ⁶⁰Co radiation of a cylindrical chamber having wall and air cavity dimensions similar to those of the Farmer graphite chamber type 2505. Cylindrical aluminium and graphite electrodes of various diameters ranging from 1.35 to 5 mm were mounted in the cavity. The air volumes were accurately determined and the respective ionisation yields were measured relative to the kerma. The following results were obtained with graphite central electrodes; the ionisation per unit air volume, J/V, decreased by 0.4+or-0.4% as the electrode diameter was increased from 1.35 to 5 mm; with aluminium electrodes J/V increased by 3.5+or-0.4% for the same variation in diameter. The collision kerma response, defined as the absorbed dose to the air-filled cavity per unit collision air kerma, was found to be 0.983+or-0.011 for the chamber with a 1.35 mm diameter graphite electrode, a result close to the expected value.