Proportional Counters Used as Fast-Neutron Spectrometers

3 Proportional Counters Used as Fast-Neutron Spectrometers 

The relationship between the neutron spectrum and the measured pulse-height distribution is given by Eq. 14.28. The response function k(E, E ) (Eq. 14.29) may be measured or calculated. In either case, the following effects have to be taken into account in obtaining k(E, ) i - 7 27.28 

Wall-and-end effects. Tracks of protons generated close to the wall or close to the ends of the counter have a high probability for incomplete energy deposition and collection of ionization. Proton-recoil tracks close to the wall are truncated by collisions with the wall material before the struck proton deposits all its energy in the counter. Protons being produced close to the end of the counter 

As a result of wall-and-end effects, lower energy pulses are generated that tend to increase the response function toward the lower energies (Fig. 14.11). Corrections for this effect have been calculated, but unfortunately each calculation applies only to a specific geometry. 

The magnitude of wall-and-end effects increases as the size of the counter decreases. It also increases as the neutron energy increases. In fact, for neutrons in the MeV range, the distortion of the spectrum due to these effects becomes so significant that it sets the upper energy limit ( 2 MeV) for the use of a proportional counter as a spectrometer. 

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