Abstract
The energy deposition pattern in a parallel plate ionisation chamber made of A-150 TE plastic and filled with methane based TF gas has been calculated by the Monte Carlo method. The effect of cavity size on the sensitivity of a TE ionisation chamber is described as a function of neutron energy. It is shown that for an accurate determination of absorbed dose by a TE ionisation chamber the effect of the size of the cavity has to be considered, otherwise an error of up to 5% may be introduced.