THE RESPONSE OF A CARBON-WALLED PROPORTIONAL COUNTER TO 14 MEV NEUTRONS

The response of a carbon-walled spherical proportional counter filled with a methane-based tissue-equivalent gas mixture at low pressure and irradiated with 14 MeV neutrons is first measured experimentally and is then calculated theoretically by using an analytical model. The model, called the "CISS" model, is derived from a consideration of four basic modes of interaction of charged particles generated in neutron-nucleus reactions with the spherical cavity of the detector. Since several quantities which have application in neutron dosimetry, radiation protection, and radiation biology make direct use of such spectra, it is desirable to have the ability to theoretically predict what is expected experimentally. Thus, a comparison between the two response curves is made. The discrepancy between them is investigated by considering several physical phenomena occurring within the detector wall which tend to distort the experimental response curve. In particular, the C(n,n',3(alpha)) reaction occurring in the detector wall gives rise to "multiple events", that is, the situation in which two or more charged particles originating from a single neutron interaction in the wall simultaneously strike the detector cavity, and are recorded as a single larger event in an experimental spectrum. In the analytic model, the simultaneous entry of two charged particles into the cavity is scored as two separate smaller events, uncorrelated in their production.; In this work, an effort is made to modify the analytic model prediction of the response curve by correcting for the multiple events which occur. The results show marked improvement in the agreement between the theoretical detector response and the experimental response curves, indicating the significance of this effect. The CISS model is also used to demonstrate that under certain conditions related to the gas pressure in the cavity, the carbon-walled proportional chamber behaves (theoretically) as a Bragg-Gray cavity. Finally, the CISS model is used to compute mass stopping power corrections for this inhomogeneous detector.