GE B10 tube gamma response

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Rich Gorski
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GE B10 tube gamma response

Post by Rich Gorski »

Hi Everyone,
I wanted to share these test results showing the gamma response of an old General Electric Boron 10 proportional tube that I have purchased on Ebay for about $100. Many on Fusor.net have said that these tubes are difficult to use for neutron detection because of their skyrocketing gamma response that occurs unless the bias voltage is set within a critical window that maybe as small as a few 10s of volts around the 750 volt level. In this test the B10 tube was irradiated with X-rays from a Dr. Lohmann X-ray tube at running at 40kV. The counts per minute response was plotted versus the bias voltage applied to the tube from a Ludlam model 3 rate meter. This was done with the B10 tube shielded with 1/8” thick lead sheet and also in the unshielded condition. The basic results were (as many here have indicated) that the tube response to 40 keV X-rays skyrocketed at Vbias = 740V while in the unshielded condition but only registered background counts with the tube shielded with 1/8” lead sheet across a large range of Vbias from 740 volts to 900 volts. The plot below shows these results.
B10-gamma.jpg
Gaseous B10 and He3 tubes will detect gamma rays as well as neutrons and therefore must be able to discriminate between them electronically or simply by shielding against the gamma rays. In this test we are trying to show that the B10 tube when connected to a Ludlam M3 rate meter (which cannot electronically distinguish between gammas and neutron signals) can detect neutrons effectively if completely shielded from gammas by materials such as lead. With 1/8” or 1/16” of lead essentially all gammas are blocked from the detector while neutrons are not affected by the shield. Attenuation calculators on the NIST website indicates that neutron transmission (both thermal and 2.4MeV) thru 1/8” lead will be > 99% while the gamma transmission at 40 keV will be essentially zero.

Conclusions:
1. In the unshielded condition the x-ray response of the B10 tube quickly jumped from zero CPM at 740 volts to over 100,000 at 760 volts. This indicates that for neutron counting in the unshielded condition this particular tube should be biased at around 730 volts.
2. In the shielded condition the B10 tube showed only background counts of around 250-300 CPM over a Vbias range from 740V to 900V (the Plateau). To detect only neutrons it should be valid to operate the shielded B10 tube in the plateau region between 740V and 900V and subtract the background count for the true neutron total. For the typical fusor there should be no other radiation of concern. It is important that the B10 tube is totally encased in the lead shield so that scattered and reflected gammas can’t find their way into the tube from the sides or from behind.
3. This B10 tube showed gas breakdown at Vbias > 950V.

Photos of test setup below:

Setup with safety shields removed
b10-1.jpg
setup with safety shields in place for personnel protection. Note small X-ray window (ruler passing through) to expose B10 tube
b10-2.jpg
X-ray tube closeup
b10-3.jpg
B10 tube with lead shield
b10-4.jpg
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Richard Hull
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Re: GE B10 tube gamma response

Post by Richard Hull »

Nice test and good results via experiment.

The original complaint was that the average idiot amateur with no radiation experience or even electronic experience, trying to bring one of these boron lined tubes on line to detect neutrons would throw it in a moderator, naked, and bias it such that it would detect both x-rays, normal background radiation and neutrons. Your background in the chart of 200+ cpm is still incredibly high and more in line with a common GM tube which is what the B10 lined counter is not. A well done true, naked, neutron counter tube should never read more than 5-9 CPM or it is, indeed, counting something else. Naked beta/gamma GM tube backgrounds are commonly from a low of 30cpm to a high of 100 cpm. (Depending on where one lives on this planet.) Alpha/beta/gamma, mica windowed GM tubes count a tiny bit higher backgrounds.

We had a lot of folks back in the early 2000s claiming neutron production with their fusors using these boron line tubes, which were common on e-bay then. They were claiming huge numbers of counts and we knew that was wrong. Thus, they were looked at with suspicion when in the hands of a person who built a fusor with detection system in 3-6 months from scratch. It has never been the B10 lined tube, but the amateurish application of it as a neutron counter that was suspect.

I really did not like your high background count with the B-10 tube. Try testing it with a real thermal neutron source with a dropped bias where the background in less than 10 cpm. I am sure the count will go up due to only neutrons. I think you will find, it is tricky to bias such that, in a moderator, a naked B-10 lined tube can be made to count only neutrons.

The whole idea to to gain the full advantage of a super low background in a properly biased boron10 or 3He tube such that only a 100% increase in neutron background from 8cpm to 20cpm will be statistically significant.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: GE B10 tube gamma response

Post by Jim Kovalchick »

The good news is that it appears that your tube may still work despite it's age.

I can't offer much more than what Richard said in the way of advice. His comment about exposing it to a reliable thermal neutron source.
Is right on. I have "played" with a number of neutron detectors and I believe from experience that you won't get anywhere important with a given neutron detector's adjustments until you show it neutrons. You may even find that threshold adjustments are specific to the application. I find that the xray field of an advanced fusor is so great that pile up is hard to deal with. This means that a higher threshold is needed and you may even need to clip into the low end of the neutron count peak. The best check is to run it with a fusor you believe works and then do a moderator removal test.

Jim K
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Rich Gorski
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Re: GE B10 tube gamma response

Post by Rich Gorski »

Thanks for the replies. Yes, I hear you. In order to stay within my budget my plan is to use this neutron detector setup biased as critically as I can and use it in a simple yes/no neutron emission test. For a yes condition the count rate in the lead shielded condition will have to be at least 3 times the background count. Once my fusor is giving me that result my plan is to spend some money and lease a Ludlam M12-4 or M2241 From Permafix environmental for a few weeks ($120/wk) to get some real neutron count numbers and possibly calibrate by setup. I'm also looking at purchasing a 5mC alpha source (with Beryllium) for a neutron source. The alpha source is expensive too at > $300.

Thanks,
Rich G.
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Re: GE B10 tube gamma response

Post by JoeBallantyne »

RIch, I have a few of those GE B10 tubes that I bought from solanotraders off of Ebay. (I suspect that is who you bought yours from as well.)

What I would suggest you do is the following.

WIth the x-ray source turned on and the tube NOT shielded, slowly drop the bias on your tube down from 740V until you are getting counts in the 5-10cpm range. Then turn off the xray source, and verify that with it turned off you are still getting counts in the 0-4cpm range. On average about 2 cpm or so. Those should all be background neutron counts. Run your tube at that bias.

I also tried shielding those tubes with lead (not as much as you used) and found - like you did - that they count quite vigorously even when shielded when the bias voltage gets above 730-740V.

You will be much better off getting the bias tuned correctly, than you will be trying to shield the tube.

I found that background neutron count rates for those tubes when biased so that they would not respond significantly to Am241 gammas and U238 betas were typically 0-4cpm. With an average of about 2cpm. I found that the proper bias level was usually between 715 and 725V, and that even 5V difference in bias could significantly affect the sensitivity to EM (xray/gamma) and beta radiation. Note that I was using a threshold level of 10mV in the meter/detector when doing this setup. I don't know if you can adjust the threshold on your meter, but if you can I would suggest setting it to about 10mV.

Basically you need to run those GE B10 tubes at the point where they are barely turning on.

Joe.
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Re: GE B10 tube gamma response

Post by Richard Hull »

Joe's advice is sage. Take his advice! For many years I have noted that the B10 lined tube is great in the right hands, but rarely ends up there for those on fusor.net due to having no real neutron source to set it up. In short, once the bias is found that detects only neutrons, you will find there is not much bias room about this ideal point where you go from no detection of anything to having the tube move towards becoming a GM counter.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: GE B10 tube gamma response

Post by Jim Kovalchick »

I do not prefer to use bias to discriminate neutrons from xrays. Because the tube is operating proportionally, operating at a lower bias will influence sensitivity. Better to use peak height to discriminate. Also, reducing voltage makes it harder to discriminate using peak height. On the high end of bias you just need to watch you don't go into Geiger mode.

All that being said, if your setup doesn't have the ability to finely adjust threshold at low pulse heights, then bias is the only way to do it.

Jim K
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Rich Gorski
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Re: GE B10 tube gamma response

Post by Rich Gorski »

Thanks all for your help with this B10 tube.

As Joe suggested I ran a more detailed response curve between 740 and 750 volts both unshielded and shielded. Results below.
B10-gamma#2.jpg
In the unshielded condition the plot shows that I would need to keep the bias voltage of between 745 and 746. Maintaining better than 1/2 volt out of 750 may be a problem. In the shielded condition it seems reasonable to live within a larger range of 745 to 748 volts.

My conclusion is to use this tube in the shielded condition since it would easier to maintain the voltage within 1 or 2 volts.

About the Ludlam M3 discriminator setting. The M3 manual indicates a fixed setting of 45mV (why that high ??) ... and does not have a user accessible discriminator control. The comparator voltage in the circuit is set by a simple resistor divider on one input so maybe I can add a pot here to allow some control. However, I should look into getting a Nim bin and some essential electronics.

Rich G.
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Richard Hull
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Re: GE B10 tube gamma response

Post by Richard Hull »

No Boron lined tube is that sensitive to +/- 1 volt! There is a range of useful bias. It is just rather narrow compared to other detection tubes. Once you get a real neutron source, you will see what we mean.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: GE B10 tube gamma response

Post by Richard Hester »

Has anyone done similar experiments with the boron-lined Russian corona counter tubes?
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Re: GE B10 tube gamma response

Post by JoeBallantyne »

Rich -

If you bias that tube at 740V, and you let it run for 10 minutes with no x-ray source on, what is the total count you get?

What about if you run it at the same voltage for 10 minutes with the x-ray source on, what is your total count?

If you get 10-30 counts total across each 10 minute run, then those are most likely neutrons, and I would run your tube at 740V.

If you get no counts at all, then I would investigate whether you can adjust your threshold down from 45mV to 10mV and see if that opens up the window where that tube works OK. It will likely be lower than 740V with the lower threshold voltage.

It is not clear from your graph what your actual data is at the lower end of the scale - whether the counts are truly zero, or are just almost zero.

Background neutron count rates with large 22-24 inch long He3 tubes are typically 6-8 cpm (sometimes a little higher, but usually not much higher). With the GE B10 tube, IME, background count rates are 1-3cpm. So even 1 click per minute means the tube may be working fine for counting neutrons.

The resolution of your graph on CPM doesn't tell me whether you are getting non zero counts on the lower voltages or not. Since 1, 2 ,3, 4 will basically look exactly like zero on your scale. Any counts higher than 20 cpm are really irrelevant for the purposes of determining where you should bias the tube.

A table with the actual count data would be more useful than the graph.

Joe.
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Rich Gorski
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Re: GE B10 tube gamma response

Post by Rich Gorski »

Joe,
Table below. The data comes from watching the analog meter on the Ludlam M3. There is no counter for totals. So, the error bars are quite large especially for the single digit values. The data near zero is based on simply counting the audio pulses over about 30 seconds at each voltage level. Thats the best I can do with what I have. I did perform one additional test as you suggested.

Listening for counts over a 10 minute period with the x-ray tube off and biased at 740V (no Pb shield) showed a total of 10 counts. So, I guess the tube is working OK and is counting background neutrons (or cosmic rays?). I didn't want to fire up the X-ray tube for that length of time due to lack of water cooling. So, no other data than the table below which are over about a 30 second time period. So, zero in the table may not really be zero but shouldn't be higher than a few CPM. The higher values in the table are reading off the analog meter in either the 0.1X of 1X scale.

Unless you can think of some other test to do with the equipment I have I think it's time to let it go until I have a neutron source (either the fusor running or a Po/Be) to test with.

Rich G.

no lead shield
(CPM over ~ 30 seconds)
Vbias bkgnd Xray on
740 0 0
741 0 0
742 0 0
743 0 0
744 0 0
745 5 0
746 10 20
747 15 70
748 20 100
749 45 400
750 100 750
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Re: GE B10 tube gamma response

Post by JoeBallantyne »

Rich -

Based on your table, and the fact that based on your 10 minute unshielded count you are getting on average ~1cpm at 740V, if I were you, I would simply run the tube unshielded at 740V.

Possibly 742V.

If you do that, you can be confident that it is counting neutrons, not fusor xrays.

My 2 cents.

Joe.
Last edited by JoeBallantyne on Sun Mar 19, 2023 2:01 pm, edited 1 time in total.
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Re: GE B10 tube gamma response

Post by JoeBallantyne »

One other thing you can do, is try running the tube at 730V, and see if you still get 10 counts over 10 minutes. If you do, try 720V for 10 minutes. Keep lowering the bias voltage until you get no counts in 10 minutes.

At some bias voltage lower than 740V, your counts will go down to zero over 10 minutes. At which point the tube is basically off due to insufficient bias to count anything.

Knowing at what voltage the tube starts counting background neutrons, is a useful number to know. It will give you an idea of the range of voltage you have to work with.

I suspect you might find that at ~735V or a little less, the tube stops counting at all. That would give you about the same ~10V working range that I have measured with my tubes.

Joe.
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Re: GE B10 tube gamma response

Post by Peter Schmelcher »

Rich my first old used Russian tube probably a bad internal spot weld. In hindsight a vector network analyzer would have instantly reveal a bad internal spot weld.
It was a while ago but I think the tube was microphonic. Tapping with my finger nail would change the scope pulse waveform or rate or something else that was odd, while the tube seemed to be somewhat working. At the same time I was also testing the tube with a few different charge amp designs. Eventually I designed and built my own amp and absolutely knew the tube was bad when I connected my amp to a new old stock He3 tube.
Life is hard....
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Rich Gorski
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Re: GE B10 tube gamma response

Post by Rich Gorski »

So, my conclusion at this point is that my B10 tube is working OK and that I should bias it near the 740 volt point. This tube biased at 740 still picks up betas from a thorium source but it doesn't appear to detect alphas from a weak smoke detector Am source and seems to be dead to 40keV X-rays. The alphas likely cannot penetrate the Al tube wall.

Based on my sensitivity calculation for a typical B10 tube of 7.8CPM/nv (from GE Reuter Stokes document for a 12" long 1" diameter tube) and 30cm distant from a fusor putting out 1x10^6 n/s should yield a fluence of 88 nv or around 7.8*88 = 686 CPM. My B10 is larger so I might expect a somewhat higher count.

Thanks all for your help on this.

Rich G.
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Jim Kovalchick
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Re: GE B10 tube gamma response

Post by Jim Kovalchick »

Rich G,
Be careful trying to predict the detector count rate for a given neutron production rate. Your production is fast neutrons. You will only count thermal neutrons. It is not a good assumption that thermal flux is the same as fast flux adjusted for distance. A lot depends on the geometry of the moderator. Monte Carlo is typically used to predict it.

Jim K
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Re: GE B10 tube gamma response

Post by Richard Hull »

A thorium source!? Thorium, if it is over about 30 years old, such as an old Th chemical, thorium lamp mantel or rock ore, is a great source of 2+ MeV gamma rays! It is not a pure Beta source. 2 MeV gamma will be detected by most anything, due all of the commonly known methods Photo electric, Compton scattering, and pair production off any metal shelled detection tube.

A nuclear metrologist should know everything about any source they use in any testing, calibration or contact in the field.

This is why the "nuclear children's park" near my home can be detected from inside my car, outside of the park with a TSA scintillator! The park has lots of thorium minerals with large boulders and sand in a creek bed. A sample of the creek sand and some rock on my gamma spec., says its thorium. (That 2 MeV+ gamma travels a long way!) I have posted here before on my discovery of this small county park.

Thorium ore, like Uranium ore spews out all forms of the ever popular radiations....Alpha, beta, gamma as each have a full load of radioactive daughter isotopes boiling away inside them that have been accumulating ever since the two neutron stars collided many billions of years ago that made the heavy elements that made up the solar nebula that became the sun, Earth and the other planets.

Thorium is a fast daughter producer in that pure thorium, whether produced 10 billion years ago or purified in a lab 40 years ago, has a full compliment of its deadly daughters. Uranium is a slow daughter producer taking less than a couple of million years to acquire all of its nasty daughters from the purified state.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Rich Gorski
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Re: GE B10 tube gamma response

Post by Rich Gorski »

Jim,

Yes, my mistake. The number I quoted was tube sensitivity to thermal neutrons not system sensitivity to fast neutrons.

Thanks for the correction.

Rich G.
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