Posted on by maxfomitchev

I have acquired this beautiful thermal neutron spectrum – Fig. 1 – using helium-3 filled SNM-18 proportional counter tube coupled to MCA-PRO 16. Thermal peak at 764 keV is clearly visible (with excellent FWHM of 7%), wall effect kinks at 191 keV (due to protons) and at 573 keV (due to tritons) are also clearly distinguishable. For clarity I have included an annotated smoothed-out (16x moving average) spectrum on Fig.2.

I have biased the tube at 1300V, which gave me average pulse amplitude around 15 mV. The neutron source was Po-Be. Although higher bias voltage increased the pulse amplitude, it distorted the shape of the spectrum by lowering the magnitude of the thermal peak.

Fig. 1. Thermal neutron spectrum acquired using SHM-18 proportional counter.
Fig. 2. Smoothed-out annotated thermal neutron spectrum.

For comparison I have also captured SI-19N thermal neutron spectrum (1450V bias), Fig. 3 and SNM-17 thermal neutron spectrum (1500V bias), Fig. 4. In both cases average pulse magnitude was around 20 mV.

Fig. 3. SI-19N thermal neutron spectrum (1450V bias).
Fig. 4. SNM-17 thermal neutron spectrum (1500V bias).

Fig. 5 compares SI-19N and SNM-17 thermal neutron spectra: one can clearly see how the smaller diameter SNM-17 tube produces wider and less defined thermal neutron peak, which is entirely expected. One has to use a large diameter helium-3 tube in order to get good energy resolution.

Fig. 5. Scaled comparison of the SI-19N and SNM-17 thermal neutron spectrum. One can clearly see how the smaller diameter SNM-17 tube produces wider and less defined thermal neutron peak.