FAQ - He3 neutron counter system review

[Richard Hull]

For those looking for the ultimate electronic neutron counter, look no farther than the He3 system.

This FAQ will deal solely with this system.

some bullet points..........

The He3 tube, just like the BFe tube is temperature sensitive. (characteristics and operating voltages change with temperature gradients.

The He3 tube should be as large or as long as possible to obtian greater sensitivity in weak neutron flux fields.

The pressure of He3 gas within the tube must be as high as obtainable. (the higher the pressure, the more sensitive per unit volume.)

The high voltage bias supply needs to be very stable, voltage wise. The current needed is well under 1ma often under 100ua.

He3 tubes operate from a low of 1200 volts to over 3500 volts.

A special charge sensitive pre-amplifier is a must have item if you are assembling a NIM based detection system. This must be attached as close as possible to the detector tube. All cabling, tube-to-preamp, must be short, well shielded and stout or locked down and incapable of flexure in normal operation. Make absolutelty sure that your preamp is capacitively coupled and that the input capacitor has a voltage rating of at least your tube's maximum bias voltage rating, i.e., 3-5kv.

A spectroscopy amplifier or "windowed" amplifier/SCA module is also needed that will allow discrimination of the preamp output level to detect only the large neutron signals and reject or not pass any gamma or x-ray signals.

A digital ratemeter/counter/timer module is also required for the best work.

A block diagram of a NIM He3 system is attached below.

Many special Ludlum or other brand "all-in-one" insturments can help you avoid the NIM system hassle, but still must be located and read near the detector tube. You must make sure that they have an internal supply capable of adjustment to the voltage level of the tube you are using and have a special charge sensitive preamp as well as discrimination capabilities.

COSTS.....................

The He3 tubes are the tough part. They cost between $600.00 and $3000.00 new.
Allmost everyone here obtains them second hand or at surplus for prices ranging from $100.00 to $500.00. They sometimes appear on e-bay. New tubes can be found at LND on line.

A full NIM system found on e-bay or bought at surplus, containing the above items with a NIM powered buss crate, can be assembled for $200.00 to $800.00

All-in-one insturments by Ludlum and the like might be found on E-bay for $200.00 or more.

Unless you are very lucky and are a class A scrounger, be prepard to part with $500.00-$1000.00 for your He3 system.

**************Assmbly and adjustment of a NIM He3 system***************************

Assemble all the components noted in the block diagram. In addition, try and obtain a good, digital, storage oscilloscope.

Assuming you know your max tube voltage, (you better!), you need to feed the output of the preamp into the input of a good digital scope. (Point "A" in block diagram) Place a super radioactive piece of Uranium or a hot radium dialed instrument in contact with the He3 tube. Put the scope on a low voltage per division range like 0.1 volt or .05 volt/div. Slowly increase the bias voltage until you are about a few hundred volts shy of your tube's max voltage rating. Set the scope's sweep for about 10 usec/div and up the scopes screen intensity a bit.

You are now to be looking for "gamma hair" these are low level pulses or fuzz that are due to the radioactive item you have in contact with the tube. If there is no "hair " then dial up the tube's bias voltage until it appears. !!!! Do not hit or exceed you tube's maximum voltage rating!!! I adjust my system's bias voltage for 10-30 millivolts of hair out of the preamp. You may now connect the preamp back to the input of the spectro-amp.

Set the spectro amp gain to 5X ,10X or 20X gain factor and place the scope on the output of the spectro amp. (point "B" in the block diagram) The "hair" ought to be in the 500millivolt to full volt range now, due to amplification. * Note* you will see an occasional massive pulse, but just ignore it.

Your job now is to discriminate out any hair pulses using the "lower window" detect set potentiometer. If you have an upper window set pot, dial it all the way up to 1000 now. looking at the scope, raise the lower window point from 000 towards the high end slowly and when all the hair is gone, turn it a bit more. example... the hair is gone at a setting of 210. Move it to 225 and you are done. Note that the real cosmic detections and neutron pulses are spikes of many volts on the scope.

Now reconnect to the output of the spectro amp to the counter and run a ten minute count of your finished system. Be sure to put the He3 tube in its moderator assembly.
If all is well, then you should count between 20 and 150 counts over the 10 minute period depending on your location and tube fill pressure and size. With a good tube and a well adjusted system you should see between 2 and 15 counts per minute. Higher elevations will see higher counts as will warmer climates or heat extremes.

Freezing cold can completely stop a well adjusted system from counting at all! You would have to either warm the area and system back to room or calibration temperature or raise the tube voltage and perhaps redo the entire procedure above.

I have as a visual example my system image in the following, now ancient, post when I modified it for the final time with effectively zero tube to preamp distance. I did this not because I was having issues, but because I didn't want any issues to ever develop.

viewtopic.php?f=13&t=5528#p33972

I hope this will help others looking to assemble and adjust their He3 system in future.

Richard Hull

[Tyler Christensen]

It seems that most people go with a NIM system. What is the advantage to doing this versus getting an appropriate all in one unit such as a model-12 or a Ludlum 2000?

Also are there any good sources of used He3 tubes right now? I emailed Oetech a few weeks ago and he said he doesn't have any appropriate neutron detector tubes right now and is retiring soon. Hoping to find one at HEAS, but if not it'd be good to know of places to purchase one. They sure are rare on ebay!

[Richard Hull]

I noted in my post only "special" Ludlum and other all-in-one units will work. It is up to and the responsibility of the end user to ferret out appropriate systems. Tyler has noted that most real neutron counting folks serious about their system use NIM.

I finally finished the posting

Richard Hull

[Starfire]

Thanks Richard & George.