Hi all,
It has been a while since I posted an update. I started a Master's program, so I have had less free time, but since my last post I have been trying to get a neutron detector system together and working. This will likely be a long post, so I'll split it into sections.
I. Neutron Detection Struggles:
I started with a N. Woods tube from an Eberline PNC-4 detector. I previously reported getting a background with the tube on a Ludlum 2200 that seemed in the right ball park. I tested this system over several weeks with a test source, and I concluded that either the tube or the Ludlum was having issues, or my source wasn't as strong as I calculated. I also tested the tube with a Ludlum 2221, but I still couldn't make heads or tails of the results. I was stuck for a while trying to decide if I should take the plunge into a nice shiny (and expensive) fully functional neutron system, or if I should continue to diagnose electronics that were slightly beyond my current capabilities. As is well known, if you don't have the luck to stumble on a working system, or the smarts to build one, you will pay dearly (with your wallet)!
II. A New Detection System:
There are a couple of different options on the market for computerized neutron detection systems. I was more drawn to MaximusEnergy's offering simply due to the datasheets and blogs he has on his website. For many of you, this would be the first red flag, as I also found out that there are some other forum users that have used the Neutron-LITE system and have either had very significant noise issues, or have not bothered to correctly follow the documentation and take unfiltered pulses as significant data. I can follow instructions, but I was worried about the noise issues of the infamous Russian He3 corona tubes. I emailed the founder of MaximusEnergy about my worries, and he happened to be working on a new noise-resistant system that would be done in a few months. He added something of a Faraday enclosure around the tube, as well as filtering electronics. Admittedly, any filtering beyond simple band pass filters and capacitors is beyond me, so if you have any questions about it, you will have to contact him.
I must stress that this is not a paid advertisement of this system. I paid for this system with my own money and conducted my own testing.
As an amateur, I couldn't help but be dazzled by the capabilities of a system like this with all the bells and whistles of modern software control. It consists of a SI-19N He3 tube, which on his website is quoted as 47 CPS/nv, but that product description is the only source I could find on the tube efficiency.
III. Source Safety:
Most of the testing of this system revolves around a neutron source. I estimated mine to output around 100 n/s in perfect conditions. For those worried, I ensured that I had the proper equipment to measure contamination (Ludlum 44-9 and 43-90, and a beautiful Berthold LB 1210D Contamination Monitor) before I started anything, as well as actual lab experience. All testing took place with ALARA and TDS principles. The testing area was shielded with 2" leaded acrylic and 1/8" lead sheet where needed. This easily stopped the low energy gammas from the source. Gamma dose rate monitoring and surveying was done with every change in setup. When conducting the gamma rejection test, the source was placed behind 4" of lead bricks on the side facing towards me, again with gamma surveying. This was important as the (sealed) radium gauge has a contact dose rate of ~500 uSv/h, measured from a Radiacode 102. Again, precautions were in place for handling and using the source, including wipe tests.
IV. System Testing:
I am not an expert in this sort of testing, but I will present my methodologies and results. I welcome any feedback and improvements I can make. As I had grown accustomed to, my tests consisted of mainly six configurations:
1). No moderator, and no source - This will get the unmoderated background for the system.
2). Detector in moderator, no source - This will get the moderated background.
3). No moderator, with source - This will test the detector efficiency for fast neutrons.
4). Detector in moderator with source - This will test the efficiency for thermal neutrons.
5). Gamma Rejection - This tests the gamma sensitivity of the detector.
6). EMI Rejection - This tests the sensitivity to EMI.
The moderated tests consisted of about 4" of paraffin wax on all sides. The test with the source and moderator had the source and detector very close together, both surrounded with the moderator. The gamma rejection test was done with the face of the gauge as close to the tube as was reasonable. The EMI rejection test is not perfect, as it is not the fusor power supply I will be using, but I thought it would be good enough for some initial data. It consisted of a small (~5") table top tesla coil running about 8" away from the tube of the detector. The test began and I ramped up the power of the coil, as well as doing a frequency sweep from ~3 Hz to ~100 Hz. This had enough power to light up a fluorescent lamp at about the same distance as the detector.

- Small Tesla Coil operating near the detector tube for the EMI Rejection Test
All of these tests had an integration time of 3600 seconds EXCEPT the EMI rejection test, as I could not run the tesla coil for that long, so this was done for 600 s.

- Gamma Rejection Test with gamma source (Dial face mounted on the yellow-ish box) behind the grey lead bricks
For those that just want raw data, here is a table:
One of the nice things about this system is that the SI-19N is running in proportional mode, so a spectrum can be attained with the included MCA. I will include three of the tests here: Moderator and Source, Gamma Rejection, and EMI Rejection. The other spectra look mostly the same.

- Spectrum of the Moderator and Source Test

- Spectrum of the Gamma Rejection Test

- Spectrum of the EMI Rejection Test
For the gamma rejection test, the tube is reacting to the intense gammas, but the software is rejecting those pulses, leaving a background neutron count rate. For the EMI rejection, the tube seems to not even notice the tesla coil operating near it, as those pulses aren't even detected. More tests are definitely needed to characterize the system's performance with a fusor.
The most critical test that can be performed is to place the detector in place for detecting fusion events and run the fusor exactly as you would for a fusion run, but with
NO deuterium. If there are any counts above background, it is detecting noise, as no fusion is happening. The exact same run should be done with deuterium and compared to the previous run. This NEEDS to be done before any claim of fusion, as too many times have people attributed noise to achieving fusion.
I am very pleased with the static performance of this system, as well as its operation in a high EMI environment. My next step is to revert to a different power supply and rewire some of the fusor as it is not in running condition at the moment. Then, I can test this detector with the fusor power supply.