The effect of thermal neutron temperature upon 3He detector response

The count rate of thermal neutron 3He detectors is influenced by the temperature of the incident thermal neutrons. Fundamentally this is caused by the temperature dependence of the Maxwellian distribution and by changes in the detector efficiency as a function of temperature. These combine to produce two effects for increasing temperature. The first is an increase of the rate at which neutrons cross the surface of the detector and the second is a diminution of the detector efficiency. These two effects tend to compensate each other but they do not cancel. The residual effect can be very significant for sufficiently high temperatures. Knowledge of the detector dependence upon thermal neutron temperature can be important in applications such as well logging because the detectors are operated over a wide range of temperatures. Calculations reveal that formation porosity measurements can be in error by almost 40%, on a relative basis, at 2000C if corrections are not made for changes in thermal neutron temperature. (author)