Test of a BF3 and disaster with the He3

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Dennis P Brown
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Test of a BF3 and disaster with the He3

Post by Dennis P Brown »

I decided to search for the parameters to operate my ancient BF3 tube (about 1.5 inch diameter and over 12 inches long - can't see the type since it is enclosed in a steel tube now.)

The good news, I rediscovered the old parameters - 1850 volts (agrees with sources but couldn't find exact unit type so wasn't sure - as I now have found out, that can lead to disasters believing such specs anyway.) The detector electronics was my surplus Ludlum 2000 (The discriminator was set about 3/4 full turn.) As Richard pointed out, the Ludlum's 2000 series are really good units for running neutron detector tubes.

The tube responded nicely to the in/out of the moderator as SOP proof. Also, decreasing the voltage (below 25 kV) - no counts and when I used the D2 control (pressure dropped from 6 microns to 'zero') this also caused the counts to drop to zero as well. As I raised these controls back to their normal settings, the count rate returned to normal.

The response of the BF3 tube was 250 -300 counts/min @ 31 kV and 30 ma. When I ran at 27 kV and 20 ma, the counts dropped to 90 counts/min. A rather believable decrease (not going to be linear for certain.) Not anywhere as good as the He3 (use to) operates but certainly a good unit.

I did more research (and others here noted this as well) that my He3 detector was operating at a very low voltage (700 volts.) I checked spec's on line and since it was a CHM 32, 700 volts was its 'starting voltage' but 1500- 3000 volts (!) was its std operating voltage.

So, I ran it at 700 volts and obtained excellent response. So I slowly raised the voltage up - nothing appeared to have issues till I reached 1400 volts. Then the 2000 stopped raising the voltage above that point even as I increased the potentiometer. The unit was (I assume) shorting across the gas in the tube (?).

I dialed the voltage back to 700 but the damage was done. The unit no longer really works. At 700 volts I get zero counts (at the normal discr. setting.) If I raise the discriminator setting, the tube doesn't respond properly and the counts run wild. Apparently, something very bad happen. The tube is supposed to operate at the higher voltage (due to its make) but that certainly wasn't the case this time.

Aside: the Ludlum still operates the BF3 tube fine so I do not think the Ludlum is at fault here. So, a terrible disaster but at least I still have a working neutron detector (BF3). I guess working on my old PMT/fast neutron detector now takes on major significance - ugh. :(
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Re: Test of a BF3 and disaster with the He3

Post by Frank Sanns »

Neutron detection can be a difficult proposition. The biggest problem being a certified, stable neutron source. Yes, the Fusor is great for generating neutrons but it takes attention away from dialing in a neutron detector.

Then there is the benefit or detriment of it generating fast neutrons. Putting moderators in and out of the detection is a great A and B comparison provided of course moving the detector tube does not change any connections or other electrical characteristics in the cable/tube/instrument.

The tubes that we have are usually surplus and not factory fresh and the entire history is know. No drops or overvoltages on improper equipment, no long service life, etc. Many unknowns here too.

To address your issue with your particular tube, it is unlikely that a short over voltage would cause any damage to your tube. Consider that the Ludlum has a fairly paltry power output. It is not a pole pig but rather a precision instrument. I am not exactly sure what the power output is from the HV connector but thinking about it another way may answer it. Consider a tube that is counting in a high radiation field. This would be true of any proportional counter and not just a neutron one. As the counts go way up, they start to approach the current of a short circuit. Admittedly it is spread over the entire volume of the tube but tubes don't get hot in normal service. This implies that the total power is probably in the single watts region. Not enough to really damage anything especially for a brief duration.

Don't give up on the tube and look closely at all other factors that may have are are giving you the results that you are seeing. I personally have never seen any of my He3 tubes detect anything at 700 volts. They all run well in excess of 1,800 volts. I also can attest to raising voltages up until it shorts in some early tubes to see how much gain I could get out of them. They all worked normally after the excursion.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Dennis P Brown
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Re: Test of a BF3 and disaster with the He3

Post by Dennis P Brown »

Good points and thanks for the post. I'm rather glad to hear that the voltage (and total power) isn't likely to create such issues or shouldn't. I am very puzzled and frankly, found it hard to believe all those issues I was experiencing where due to a simple over voltage - the low operating voltage of He3 was certainly puzzling, and the shorting of the Ludlum supply above 1400 volts. Even through that appears valid as far as the Ludlum power supply - it has no issues with the BF3 going to 2250 volts. I'll keep experimenting and trying to determine a cause but rather in the dark. Your post does shine some badly needed light on the issues I'm seeing.
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Dennis P Brown
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Re: Test of a BF3 and disaster with the He3

Post by Dennis P Brown »

Frank - you were right on the trail of the developing problem in the neutron detector system. The HV cable I was using is worthless and was shorting. Long story, made short. Tried another cable and the He3 tube handles 2500 kV. The bad news is the Ludlum is dead now (the detector circuit.) Now it doesn't even respond with a Geiger tube and uranium sample. I might try repairing it IF I can locate the parts to replace the entire pre-amp assembly (I have to assume that is the issue; otherwise, not likely a repair is possible deeper in the unit.)

If unable to replace the chips and transistor, except for the fast detector rebuild that I will try in the near future, likely I'll have to be out of the business in detecting neutrons.

This might be telling me to quit and move on but will have to think about it for a while. Certainly, as I see what is involved with the Ludlum repairs and the fast PMT rebuild (that will now need a new crystal - just gets more complex.)

All over a lousy HV cable. Guess the old saying "A Kingdom was lost for the want of a horse" (I think that was from Shakespeare's 'King Richard the III'.)
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