Good technique for work with proportional neutron decetors in harsh environment

[Madmax]

Hi folks,
Some time ago I was asked about how to work with He3 neutron detectors while doing a fusion research with many kilovolts equipment near neutron detectors. We know that proportional neutron detectors generates around only 0.1-0.3 pC charge per detected neutron, and this becomes a problem, also ground loops in all equipment adds their input to overall interferences.
I have made the experimental electronics for He3 counter readout with a very compact design: all HV and charge amplifier arranged near the anode terminal of He3 counter, and all transmission of the resulting signal is carried out using balanced circuit. Standard cheap USB C to C cables can be used, and immunity to external interference is very high. I tried to discharge 1000 pF ~15 kV and sparks was ~15-20 mm long near the signal cable with no visible interference with the signal.
[assistant, slides!]

HV part and charge amplifier. HV part is veru compact because of ~300 kHz sinewave resonant converter, very small capacitors needed to filter out all the ripple.


HV part and charge amplifier assembled together. No. it's not my hand too big, it's M2.5 standoffs instead of M3. All electronics fits inside 30 mm tube.


Mating part which converts balanced signal to unbalanced and eliminates any possible ground loops with DC-DC converter


Resulting signal, ~0.15 pC per volt


That's all for today :)

[Sesselmann]

Max,

This is good work, I know from personal experience how easy it is to get false counts on a neutron detector when working with high voltage circuits.

It's so important to have well shielded circuits and short cables between the tube and the preamplifier.

False counts only result in excitement followed by disappointment.

Steven

[Jim Kovalchick]

I use a CHM 56 Soviet He3 tube running in proportional mode to measure neutrons from my fusor. I don't bother with a pre-amp at all. I put in a 10 M ohm inline resistor and use a Ludlum 2200 for the high voltage and counting. I am using a cable of only about three feet. Fusors are notoriously noisy, but I still manage to get good results.

I setup the detector discrimination and threshold for proportional operation and to eliminate the inherent noise at the quiescent state and only a few background counts a minute. Then, I ran the fusor with just air up to a decent voltage level to make sure there are xrays and noise but no neutrons. At this point I adjust the threshold to get rid of any additional noise. Operating with fusion fuel gas, I verified that my settings were okay by removing the detector from the moderator and watching the neutron counts drop to nothing.

Earlier this month, I made some improvements to my system that allowed me to operate at higher voltage (-61 kV) and make an almost ten-fold increase in neutron counts from 15,000 cpm to 150,000 cpm. Naturally, I was concerned about whether the increased count rate was influenced by increased noise at the new operating conditions. I know that at least some of the increased counts are neutrons because my ability to activate material has significantly improved. However, when I remove the detector from the moderator to do a negative check, counts drop off, but about five percent of them remain unless I back the detector away from the fusor chamber. I suspect that the neutron flux is high enough that the detector is seeing thermal neutrons emitting beyond the moderator and thermalization from my hand and fingers. Next time I am in my lab I want to check for this, but this thread has given me the idea that I need to examine the pulses to see if there is any noise.

Thanks for your sharing your work.

Jim Kovalchick