A dead BF3 Tube? :(

[gamempire]

Hi All,

My professor, who used to be a branch head at the Naval Research Lab outside of Washington DC, took me on a visit there this past Thursday to finally test out my ratemeters (I have an Eberline ASP-1 and an Eberline E600) and my BF3 tube for neutron detection. We also took along my 4" diameter, 12" long HDPE moderator. The BF3 tube I have is from N. Wood Lab's, model G-10-5, that I picked up second hand on eBay. Specs can be found here: http://www.nwooddetectors.com/specs01.html

We were going to compare it to one of their Ludlum Model 12-4 using an AmBe neutron source. We quickly found out though that my BF3 tube was non-responsive to that source, after determining that both ratemeters were working properly when we hooked them up to their probe on the Model 12.

My question is this: what problems could cause the tube to fail and is it possible to have the tube refilled and fixed?

Thanks in advance.

-Josh

[Carl Willis]

Hi Josh,

Sorry to hear about your dud tube. I'm inclined to agree that the evidence indicates it has a problem.

Did you increase the voltage / decrease the threshold in search of some counts from the AmBe? The way I set up proportional tube detectors is to (A) increase voltage or gain or decrease the threshold until counts from a strong gamma source are picked up; (B) back off on gain (or voltage) until gamma counts disappear; and (C) check with a neutron source to make sure the neutrons are detected. Also: cable length matters. To compare the characteristics of two detectors of the same type, you must test them with the same cable.

Usually a dud tube produces no counts at all until some abrupt threshold is reached (ranging from very low to very high voltages, depending on tube pathology), at which point it engages in large-amplitude, noisy breakdown and produces a flood of counts. It's hard to find a proportional or even a Geiger region in the response of bad tubes; they act like simple spark gaps because that's effectively what they have become for one reason or another.

N. Wood can probably advise you directly on the possibility of reconditioning a tube.

I have a G-10-2 you could borrow if you need it for only a few months. It is also a 1" dia. tube.

-Carl

[gamempire]

Thanks for the reply Carl.

I did increase the voltage (up from 1500 to around 1700, tube spec is from 1500 to 1800), and decreased the threshold (it should be around 3mv). However, I wasn't able to do it while next to the neutron source, because of NRL's policy on radiation exposure to non employees. Therefore, I was only able to try a couple variations in settings, because the health physics people only had 30 minutes or so to test it. I would definitely like to be able to test the tube myself with a neutron source. However, I doubt I'll be able to get a hold of an AmBe source, even from the various isotope manufacturers, because the EE dept is already freaked out about having deuterium (even a lecture bottle) in the lab and the risk management bureaucrats are over my shoulder all the time now since first light with helium.

However, my brother has done extensive research with the radiation oncology dept at University of Maryland Med Center, and we have some good family friends down there as well. What would be a strong gamma source I could do some general testing with that would be available in a medical enviorment?

Also, thanks for the offer for the other tube, that's very kind of you. I might have to take you up on that offer just so I can safely create some neutrons by the end of this semester.

-Josh

[Richard Hull]

BE3 usually die due to a change in pressure due to exceedingly slow leaks or chemical decomposition of the gas or burying/deposition of componenets of the gas, etc. Keep uping the voltage as Carl suggests. You might be able to find some narrow proportional range that you can live with. Of course, every thing gets super critical out on this end-of-life, tiny "shelf", if you can find it.

Let us know if the good fight works for you with that tube.

Richard Hull

[gamempire]

Thanks for the reply Richard. I'll be sure to let everyone know how it goes.

I have to ask, what would be the best gamma source I can buy for testing do you think? United Nuclear has several options. I assume Cobalt60 would be the best choice because of it being a strong neutron source. I'd probably get a lead lined container to keep it in just so Health & Safety/Risk management won't freak out, or I'd just store it in the basement of my home and do the testing back in Baltimore.

-Josh

[Carl Willis]

Hi Josh,

The sources I use for gamma rays are either big chunks of very concentrated uranium ore or any of various radium strip sources. The exempt-quantity Cs-137 and Co-60 sources are detectable but much less effective for rapid threshold setup.

I should add that I can also check out your tube using my neutron sources and pulse-height spectroscopy setup to determine if the tube is good and where you need to operate it. I have done this a lot for Don Orie in the past because he's nearby in New Mexico and has occasional customers for neutron detector tubes. The disadvantage of this is that I am not an accredited calibration shop or anything like that. The advantage is that it would be free.

-Carl

[gamempire]

Not worried about calibration just yet. I just need a working detector. Heck if you did get it working, just a comparison graph of cpm between one of your detectors and mine versus distance away would be helpful. Some of the EE professors here at GWU have collaborators at NIST who said they'd gladly get it calibrated once we know its working.

Do you want me to send the ratemeter as well, or just the tube for testing?

As for the ore, I had considered it, but that would most likely scare some people here at the university. The Cobalt 60 is probably the best choice (from a university/department political standpoint) at the moment for getting my own gamma source.

Thanks again for offering to help with this. Just shoot me an email with your address and I'll get it shipped out tomorrow or Friday hopefully. Oh, and is there any special way I should store it for an airplane trip, or should I just ship it via ground transportation for that very reason?

-Josh