Proportional Counter Spectrum Identification
When working with proportional counter neutron detectors, it is important to understand the energy spectra of each gas tube type to aid in setup and diagnostics using our multichannel analyzer. These spectra are functions of the tube itself and not the actual neutron events. Some features of the various spectra are the wall effect, gamma radiation, noise, and the neutron peak. Following are typical spectra for Helium-3, Boron Tri-Fluoride, and Boron lined tubes. It is important to note that both the independent and dependent axis are scaled, and it is only necessary to recognize the features of each spectrum in order to set the discriminators on the neutron detector.
Boron Trifluoride is similar to Helium-3 with some notable differences. Just after the gamma region is a small peak that contains some gamma pulses overlapped with some neutron pulses. Because of this overlap, the lower discriminator will likely be set above this region to ensure neutron-only counts. The wall effect is again visible as a plateau from ~bin 30 to ~bin 70. There are then two neutron peaks. One large peak and a smaller peak, here at bins 80 and 100, are present. The presence of the two peaks is due to the physics of the neutron capture reactions in the BF3 gas.
Boron 10 lined tubes generate an interesting spectra in that it is entirely due to the wall effect. Gamma pulses are again overlapped with neutron pulses below ~bin 20, and these are rejected using the lower discriminator. The rest of the spectrum is entirely due to the wall effect of the tube, and noise above ~bin 120 is rejected with the upper discriminator.
When setting up your neutron monitor, it is important to set the lower and upper discriminators on the device to reject gamma rays and noise, while still capturing the maximum possible signal from neutron events occurring in the gas tube. With traditional analog neutron pulse monitors, this is done using the “plateau” method, where the electronic gain and lower discriminator is fixed and the voltage is changed in steps over a several hundred volt range surrounding the tube manufacturer’s recommended operating voltage. At each voltage the count rate is recorded. Without a radiation source, the operator may have to wait many minutes at each voltage point to obtain enough counts. The final operating voltage is selected by choosing a voltage where the change in count rate due to a change in voltage is sufficiently small. To optimize the neutron detector operation, often the operator will make additional changes to electronic gain and discriminator values, and repeat the “plateau” exercise. Setting up a neutron detector in the field can sometimes take hours.
Quaesta’s integrated multichannel analyzer allows one to bypass the “plateau” exercise. In addition to counting neutron pulses, a Quaesta neutron pulse monitor automatically creates a histogram of the tube spectrum as neutrons are counted, allowing the user to visually determine where the lower and upper discriminators should lie. This feature is invaluable in low counting environments and with Boron lined tubes, where traditional setup may take hours. With minimal experience, a user can set up their device with our multichannel analyzer in as little as 5 minutes, even without a radiation source. Our multichannel analyzer will save you valuable hours in the lab and field, and reduce downtime while calibrating portal monitoring systems.
In addition, the Quaesta integrated multichannel analyzer allows for continual monitoring of the neutron detector system health. The number of detector site verification and calibration exercises can be greatly reduced or even eliminated.
There are three main options currently available for tubes to use in proportional counter neutron detectors: Helium-3 gas, Boron Trifluoride gas, and Boron lined. Of the three, Helium-3 is by far the most sensitive at roughly twice as sensitive as Boron Trifluoride and up to 10 times as sensitive as a Boron lined tube for tubes of roughly equivalent size. However, Helium-3 is drastically increasing in price due to a major shortage of the gas. For systems with equivalent counting rates, a Helium-3 system can currently be up to three times the price of a Boron Trifluoride system. By the time enough Boron lined tubes are compiled to match the count rates of a Helium-3 system, the price of the system can as much as match the price of a Helium-3 system.
Proportional Counter Gas Tube Selection
Here the gamma region is shown on the far left side of the spectrum. The wall effect creates a “shelf” that in this spectrum goes from ~bin 18 to ~bin 82. The peak is located just above bin 90, and noise due to alpha particles are visible as full scale pulses. To optimally count all neutron events, the lower discriminator is set to count neutron events while eliminating noise due to gamma radiation, and the upper discriminator is set to eliminate noise due to alpha particles.