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Interesting He3 Tube Result

Posted: Fri Jan 03, 2020 12:59 am
by Alan Sailer
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 (76.49 KiB) Viewed 756 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?


Re: Interesting He3 Tube Result

Posted: Fri Jan 03, 2020 4:12 pm
by Alan Sailer
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.


Re: Interesting He3 Tube Result

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