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: clarification
Date: Aug 11, 10:20 am
Poster: Richard Hull

On Aug 11, 10:20 am, Richard Hull wrote:

I was told that when using parafin with a Fusor the scattering would be so great that you would essentially get zero hits even if a decent flux of mixed energy neuts from the fusor was present.

First of all, there is no gas in any neutron detector tube which will detect fast neutrons better than thermals. All, that is 100% of fill gases have far greater cross sections for thermal neutrons as do most materials.

For Thermal neuts. The He3 tube is better than a BF3 tube as the pressurized He3 not only has a greater cross section, but high gas density. So the He3 tube laying naked on a table is one of the best thermal neutron detectors. They are much more expensive than BF3 tubes and are a bit rarer in the hands of common neutron measuring systems.

For counting fast neuts both tubes need moderation to slow or hopefully nearly thermalize the fast neutrons.

It is also true that all moderators just scatter the hell out of fast neutrons. After a thick moderator, it would be tough to tell the direction of the neutrons came from. That is why with a well done long counter the scattering is turned to advantage and the average based on good calibration allows good detection of fast neuts. Even the most efficient fast neutron counters using BF3 or He3 tubes hovers around 1%-5% RH
>Richard, have you found the BF3 tube to be that efficient? I have an Amperex type 75 (900 volt) BF3 that I have tried on numerous occaioions without success (used with a Bicron analyst detector) in situations where a small thermal flux was present. I have never been able to get adequate readings with it. Perhaps it is defective or more likelyt is from my lack of skill with configuring a good setup.

While it is true that neutron counting is more of an art than a science, the BF3 tube is a pretty good counter device for the impoverished, but determined fusioneer. The problem is how much flux and do you know it to be thermal.

A nice 20uc Am241 source with beryillium (fast neut source) in contact with a 3" thick moderator with a BF3 tube on the other side barely rises about a long background test.

What was your source of thermals?

Richard Hull