Dosimetry of a bare and a D2O-moderated 252Cf source using low pressure proportional counters

Neutron and photon absorbed dose and dose equivalent were determined with tissue-equivalent proportional counters (TEPCs) in the radiation fields produced by a bare and a D2O-moderated 252Cf neutron source. The measured values of absorbed dose are related to the corresponding neutron fluence values and compared with calculated mean fluence-to-kerma conversion factors. The excellent agreement is an indication of the accuracy achievable in the absorbed dose determination with the TEPC. An additional measurement with a carbon proportional counter shows that the contribution of gamma rays produced by thermalised neutrons in the TEPC is negligible. The influence of the wall thickness and of the presence of a phantom on the dose equivalent response (ratio of evaluated dose equivalent to true dose equivalent) was studied. The comparison with theoretical fluence-to-dose equivalent conversion factors gives a 24% to 31% under-reading for the TEPC with 2.5 mm wall thickness and up to 50% under-reading for the thicker walls. The measurements in front of and at a 10 mm depth in the phantom show the suitability of the TEPC method for potential application as a reference instrument and in personal dosimetry. (author)