This is a re-post of Carl's trading post exposition on some LND 3He tubes he acquired. This might also apply to some Russian 3He tubes often seen on e-bay. It is a good example of what to look for and how Carl tested these tubes. Thus, I have moved it to this forum as it is informative and relates to neutron measurement. I have edited out the sales info he had as he sold his extras. R.H.
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LND 25169 He-3 neutron detector tubes
There was a recent post about an eBay auction for these 20-bar He-3 tubes. They were apparently "pulls" or factory rejects from some piece of equipment, perhaps equipment made by SAIC. In any case, they were gone within minutes of being noticed by a Fusor.net member, and my take was four of them at $35 apiece.
They arrived today, I checked them out with my neutron source, and they all are good, sensitive neutron detectors.
Why am I bringing these tubes up again, especially with the original post missing and perhaps deleted? Obviously, I bring it up to gloat. Yep, that was a steal, and a stroke of good luck genius! But I also bring it up because I happen to not need four of these things. I was interested in ONE that worked, and I bought four hoping to come out with at least one good one.
These tubes seem to work properly as a quite-sensitive neutron detector, comparable in sensitivity to other high-pressure miniature He-3 tubes I have experienced, with good gamma rejection. All four of these tubes have the same sensitivity within maybe 20% statistical uncertainty, but they seem to like slightly different operating voltages between maybe 1000 and 1200 V. I do not know if they meet original factory spec and make no guarantees about that. Here's the manufacturer's description:
http://lndinc.com/products/pdf/534/
My test is depicted in the attached photos. It's essentially the same test I have described here many times before for setting up neutron detector tubes. The pulse-height threshold control on the electronics is set at its lowest setting and the bias voltage to the tube raised until counts appear when an intense gamma radiation source is approached to the tube (in this case, a vial of "Undark" radium paint). Then the threshold is raised until the gamma counts disappear. That happens quite abruptly with these tubes, which is convenient. At this point the tube is ready to use. Finally, you can see what happens when the tube is placed next to a moderated / reflected AmBe source--counts on the counter, about 400 cpm (background might be 1 cpm or less). The countrate falls to near background when the tube and source are lifted together out of the plastic moderator.
-Carl
Carl Willis posting on E-Bay 3He tubes
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Carl Willis posting on E-Bay 3He tubes
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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: Carl Willis posting on E-Bay 3He tubes
I am very grateful to Carl for selling me one for our NWNC teaching consortium. It definately rounds out a group of detectors, and will provide one of the best neutron detectors available. I am going to encase it in a thin brass tube to protect it without decreasing its sensitivity, and of course get a BNC attached to it, before I release itt to the group. Thanks again, Carl!