Interesting He3 Tube Result

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Alan Sailer
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Joined: Sun Nov 24, 2019 12:54 am
Real name: Alan Sailer

Interesting He3 Tube Result

Post by Alan Sailer » Fri Jan 03, 2020 12:59 am

I was running a short experiment today which gave suggestive but rather unbelievable results.
I'd like to run it by the forum for wiser (or at least more experienced) people to comment.

The goal of this experiment was to try and learn about the SNM-18 proportional tube.
For the bias I am using an old HV DC power supply and the classic bias/measurement circuit
using a 100Mohm bias and a 2pF series cap. The output of the cap is connected to a Rigol
DS1054Z scope.

When the SNM-18 tube is biased up to about 1300V I can see an occasional 0.25 to 1 volt downward
pulse. The pulse width is about 1usec at FWHM. The rate (an average of 24 events) is about one every
17 seconds. A typical pulse is shown below.


DS1Z_QuickPrint13.jpg
DS1Z_QuickPrint13.jpg (76.49 KiB) Viewed 754 times

So far this does not seem totally crazy. The rate is higher than the background rates I have seen on
this forum. But not by a lot.

All this is with the tube in open air. When I put it into my wax moderator (8" dia by 19" high) the
counts go up dramatically. It is more difficult to count them because sometimes they come too fast but
after 15 minutes trying I got a count rate that is almost double, about one count every 9 seconds.
This seems crazy. But the only variable I am changing is the addition of the wax moderator.

Are there really that many fast neutrons ready to be moderated at sea level?

Any comments?

Cheers.

Alan Sailer
Posts: 104
Joined: Sun Nov 24, 2019 12:54 am
Real name: Alan Sailer

Re: Interesting He3 Tube Result

Post by Alan Sailer » Fri Jan 03, 2020 4:12 pm

Finally found an answer.

The keywords are, well key. I had been searching "neutron detection" and not getting results.
Using "neutron background" got me onto a thread from 2009. In it, Richard Hull gave a normal background count
(moderated He3 tube) of 4 to 10 cpm.

My count level of one every nine seconds gives a 6.7cpm rate. Right in the middle.

So I am happy that my results are not crazy. I like this scope method of doing counts. The neutron events are quite
obvious. If it isn't too much effort I would like to see if a laptop hooked up to the scope can store counts for me. I realize
EMI form a working fusor may cause problems bu we shall see.

Cheers.

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Richard Hull
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Re: Interesting He3 Tube Result

Post by Richard Hull » Sun Jan 05, 2020 8:20 am

A well biased 3He tube of 4 atm. in a water, hdpe or wax moderator will give as low as 4 to as many as 20 cpm. I am only about 300 feet above sea level here and I average about 8 CPM. During an early 2000 CME from the sun, I recorded 18 cpm background. If you are up in the mountains, your background can be a wee bit higher.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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