Biasing and Testing GE B-10 Neutron Counter

[Kyler Johnson]

Recently, I bought a GE B-10 lined neutron counter off of eBay and have been messing around with the voltage bias, lead shielding, etc. to determine the best operating parameters for my tube. Unfortunately I'm not equipped with a strong neutron source nor a strong gamma source so biasing the tube has been a challenge. With that said, I think I have found an optimal operating voltage for my tube where there is very little background radiation being detected by the tube and a demonstrable increase in counts when exposed to a neutron source, albeit a weak one compared to the 5mCi PoBe source often used. I am using an AmBe neutron source constructed with the foils found in Pyrotronics f3/5a smoke detectors.

I've attached pictures of my current detector setup, the voltage bias is set to 635v using a Gamma Spectacular V5. I have yet to get the remaining lead necessary to cover the rest of the tube and hopefully that will drop the remaining background being detected. I also tested the tube against a radium dial and the detector didn't show a demonstrable increase compared to background.

Below is also the results of the one of the counts I did on the neutron source and one of just the Am241 sources.

(Avg 3.6 cpm, Am source)

(Avg 6.6 cpm, AmBe source)

Is this enough to conclude that my tube is reliably detecting neutrons or am I completely off the mark? Is there merit to testing the tube against a stronger gamma source?

[Richard Hull]

I think you are within the bounds typically found of the operating voltage range of these detectors. inching the voltage up a bit might be in order to where the background maybe doubles with no source and then check with the source. The important thing is not to have the tube respond to a gamma source like radium. You did well here in lowering the voltage to where the radium source went quiet. That was a good piece of work.

Your tube maybe in the neutron detection zone right now.

Richard Hull

[JoeBallantyne]

Kyler, if I were you, I would remove the lead from the tube. Put your radium source right up against it, and very slowly raise the bias until the counts start to take off. You will hit a threshold above which the tube should start counting faster and faster as the voltage is raised only slightly. In my experience with those tubes (I have a number of them) this typically happens somewhere between 715-740V if you use a 10mV threshold in your meter.

When you find this threshold where counting takes off, then drop the voltage to just under that, to where the counts drop to just a few per minute. Like 1-5 cpm.

Then measure the counts over a 10 minute interval, and get the total. Divide that by 10 to get a reasonable estimate of your background cpm. This should be mostly just background neutrons.

Then put your tube and AmBe source inside the moderator together and measure the total counts over another 10 minute interval. Divide that by 10, and that number should be some reasonable multiple of the background neutron cpm.

I got background cpm of about 1cpm with the B10 tube, and with a ~200uCi AmBe source about 40cpm. If your Am241 alpha source is less than that, then scale the number to get an estimate of expected counts. Every tube is slightly different, so your numbers will not match mine exactly.

If the B10 tube is biased where it can detect neutrons, you will ALWAYS get a few cpm of background neutrons whether or not the tube is in the moderator. (The atmosphere is the moderator for background neutrons.) If you are detecting neutrons, then you should get a noticeable increase in neutron counts when the tube and AmBe source are together inside your moderator.

The best way to just count neutrons, is NOT to try wrapping your tube in lead. It is to adjust the bias so the threshold of your detector cuts out all counts except neutrons. As you drop the bias voltage, at some point most of the counts due to betas and gammas will be below the threshold, and you will just be counting neutrons.

Joe.

[JoeBallantyne]

Note that you should be able to use the GS Pro 5 with the gamma spectrum software to see where the neutron counts are showing up in the spectrum relative to the gammas and betas. I don't think the GS Pro has a built in threshold, since it is trying to build spectra, so it will show you the energies detected in the tube for everything, gammas, betas, and neutrons. As long as you don't go too high with the voltage where it looks just like a geiger tube for everything, you might be able to see the neutron generated peak, vs the gammas and betas from the radium. There is likely a bias point where you can distinguish between them in the spectrum generated by the software you are using. The neutron peak should be higher energy than the gammas and betas.

Joe.