FAQ - The Helium 3 (3He) tube- a discussion

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Richard Hull
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FAQ - The Helium 3 (3He) tube- a discussion

Post by Richard Hull »

It is unfortunate that the current availability of these tubes is now limited and that, when found in good, proven condition, they are rather high priced. Nonetheless, a good high pressure 3He tube is the most sensitive neutron detector that an amateur might hope to obtain. BF3 tubes are fine, but just do not measure up to the results of the best 3He tubes. There are a myriad of other, less efficient real time neutron detectors that have been discussed in these forums, but we will limit our discussion here to only this tube mentioned in this post's title.

Note* These tubes used to be readily available surplus from any number of sources. These sources, as of 2010, have more or less "dried up". A recent shortage of 3He gas has also seen the new tube prices soar a bit with some lead times faced in delivery.

General characteristics

1. The 3He tube is almost always filled with this gas at atmospheric pressure or higher. Most often, they are filled to 3 or 4 atmospheres
2. The tube is almost always made of stainless steel
3. The most common diameter is ~ 1 inch or 25mm
4. The most common electronic connector is the HN connector on the 1" tubes
5. The tube constuction mimes or is similar to a GM counter, but is considered and operated as a proportional detector.
6. The tube requires a high voltage bias that varies tube to tube over a rather wide range. (700 volts to 3000 volts with most tubes operating in the 1400-2000 volt range.)
7. The tube usually requires a high quality charge sensitive preamplifier and a stable high voltage supply.
8. As with most all electronic neutron detectors, the 3He dector only detects thermal neutrons. ( To detect fast neutrons, they must first be externally thermalized.)


Discussion:

The key feature of the 3He detector is that it can be "tuned" via diameter, length and fill pressure to handle and efficiently detect any desired neutron flux. However, the 3He tube is most often used where there is an small, unknown flux, especially in portable measurement applications.

For the amateur fusioneer, or just the amateur scientist investigating small to near non-existant fast neutron fluxes, the ideal tube would be a long, high pressure 3He tube in a large moderator.

Most amateurs in 2010 would have great difficulty finding a warranted good tube of the above type surplus. If desiring a new tube, they would be forced to spend over $1000.00 for a naked tube of the type mentioned above and might have to wait for it to be produced. (late 2010 - Some main line suppliers are currently no longer accepting orders for new 3He tubes! All are headed for homeland security!)

Once a tube is in hand, another whole raft of issues are faced. The preamplifier is an issue for many who are forced to "roll" their own system from scratch. Fortunately, there are any number of good surplus charge sensitive preamps of the "NIM" type that appear on E-bay from time to time. The same is true of NIM compatible HV bias power supplies.

Far rarer, but highly desireable, are complete "turnkey" neutron counter systems that might be for sale. Unfortunately most of these are older systems and are constructed around a less than desireable, small BF3 detector tube, sensitive to only full fledged fluxes of the type encountered in and around a real nuclear reactor environment. We fusioneers, especially early in the game, need highly sensitive neutron counting capabilities as we produce few neutrons, at our lower, early operating voltages.

I have only seen one or two 3He based turnkey systems offered for sale in my liftime. The reason is obvious to even the meanest intelligence. (You get a good neutron counting system, you never let it go.) Probably one of the best systems I have ever seen for sale was offered up at my last HEAS conference.....Not by me!

In general, the larger the 3He tube and the higher the pressure, the more sensitive it is to any given flux. This also usually means a higher bias voltage. A 1 inch daimeter tube of 21 inches in length with 2 atmospheres of 3He pressure might be expected to require a bias voltage of between 1800 and 2200 volts.

Ideally, the sensitive preamplifier should be located as close to the 3He detector tube as is reasonably and mechanically possible to avoid externally induced noise into the system, (false counts).

Once the foregoing is obtained, the preamp output signal needs to be conditioned and "windowed". This is often handled by another NIM module such as an SCA, (single channel amplifier). Here, the signal is pulse shaped and temporally expanded or limited as well as discrimated from other lesser or greater non-neutron related event pulses such that only amplified, shaped and bonafide neutron detection pulses appear at the output of the SCA.

From here, the SCA ouput pulses are simply counted using a simple ratemeter/timer/scaler. (another NIM module). Note** they can also be counted using a simple standard electronic digital counter and a stop watch.

Unknown tubes of unknown fill pressures with no stated operating bias voltages must be carefully probed with some skill and care in a known functional counting system with a good variable voltage bias supply. Needless to say a good neutron source is also demanded for this careful work. A good oscilloscope is a virtual "must have" here as well. One must never allow a tube to enter the townsend breakdown or arc breakdown region as damage or destruction of the tube as a neutron detector may result.

A complete video of the basic 3He setup procedure is currently provided by Carl Willis on line at You Tube. Check our his superlative recent FAQ with URLs to these and other neutron measurement processes at..........

viewtopic.php?f=13&t=6053#p34497

I am open for further discussion related to this detector.

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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Doug Coulter
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Re: FAQ - The Helium 3 (3He) tube- a discussion

Post by Doug Coulter »

Carl's videos are highly recommended, that's the good stuff and eye opening if you've not done it yourself, and maybe even if you have.

This link:
viewtopic.php?f=13&t=6071#p34515
here mentions use of 3He for high energy neutrons as well. They see them some, just not very well, and even give proportional output vs energy. I don't think Jon has corrected for the 1/v detection quantum efficiency on those plots yet either, but Jon's looking at a pretty loud source (his fusor) to see that high energy stuff and maybe just doesn't need to do that other than by eye.

I have a tube just like his, and love it. (Thanks BillF for finding that!). The only trouble I have with this and other gas tubes (BF3) is that they are too slow to resolve individual hits during a fierce pulse of neutrons in the mode I am currently running the fusor in, so I'm adding some other approaches here.
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Re: FAQ - The Helium 3 (3He) tube- a discussion

Post by Starfire »

Great information Richard and top marks to Carl

I have a number of tubes and my question is will a moderater placed on the outside of the tubes work? source - tube - moderator instead of source - moderator - tube ?
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Re: FAQ - The Helium 3 (3He) tube- a discussion

Post by Starfire »

Richard

I have constructed a crown to carry six Neutron tubes for my B11 experiment. In my rush I forgot to include a moderator [ I should have known better but I do make mistakes and was thinking of the electronic circuit ]

Will the shield [ two meter of water surrounding the experiment, in the pond ] act as a moderator and will it reflect Neutrons back to the counters or do I need to include a HDPE moderator surrounding the crown? - I have included two pockets on the crown for BTI's since the photos were taken.

Now that I have built a crown I am reluctent to build again

The crown fits arround the experiment and goes down the well which is a long SS pipe
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Re: FAQ - The Helium 3 (3He) tube- a discussion

Post by Frank Sanns »

Water is a great moderator so you will be fine.

Tell me again why you are taking an already difficult job of building a fusor and doing it in a bottle (under water)?

Frank Sanns

EDIT: I just realized this is occuring under FAQs so I will leave my post for the day and remove it tonight so as to not distract from the original post.
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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Richard Hull
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Re: FAQ - The Helium 3 (3He) tube- a discussion

Post by Richard Hull »

I use only water moderation in my counting system. It is just fine.

Neutrons diffuse throughout moderators and part of this component is reflection, albeit inefficient.

Discussions on moderators and moderation are sprinkled throughout this forum. Carl Willis has produced the most useful moderation posts with statistics, etc. Jon Rosenstiel has given a number of hints as to practical moderation in some of his experimental reports. I have also commented on this subject. I suggest you ferret them out.

When I see a valuable posting in these forums, I print it out and later place it in any number of specialized, loose-leaf, bound reference volumes I assemble on specific subjects.

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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