Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: New neutron counter
Date: Nov 16, 8:40 pm
Poster: Tom Ligon

On Nov 16, 8:40 pm, Tom Ligon wrote:

>I had once held the illusion of using Indium foil and neutron activation to get at the count rate for a non-electronic solution, but the flux is so low from the low end amateur fusor that such methods would demand even more sophisticated and costly instruments than a full blown neutron detector. The indium foil, even then, might only offer a crude order of magnitude idea about the neutron yield.
>Richard Hull

I did get a couple of runs on a 15 kV Fusor which did get statistically significant counts from neutron activation of indium. We used less than an ounce of indium (I know you have about 6 pounds of it or so) rolled to thin foil strips, moderated in water, and counted with a cheap "Alert 4" GM counter. The runs we detected were making short bursts at around an estimated 100,000 neutrons per second total reaction.

Indium has a HUGE resonance crossection for neutrons as the energy drops thru the epithermal region during moderation, something like 30,000 barns! The crossection is so high the indium must be thin foil or fine particles, because a neutron will only travel something like 0.002 inches before being absorbed. Thus, it is pretty efficient. The activation produces a half-life on the order of half an hour.

I would recommend this as a back-up method to an electronic counter (PMT and BiCron 720 as a case in point. The electronic methods are subject to false positives when the "pulses anomolous discharges" occur on the inner grid.

My preferred system would suspend fine indium powder in parafin wax or a comparable moderator, then put a GM tube on a computerized counter in the middle of this. I would do 5-minute counts for at least half an hour, and look for a statistically significant fall-off compared against a nice, long background count.