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: Neutron counters
Date: Jul 20, 9:17 am
Poster: Richard Hull
On Jul 20, 9:17 am, Richard Hull wrote:
I have now amassed about 10 tomes dealing entirely or in a very thorough manner with neutron detectors. After reading and rereading them for the last year or so, it appears there doesn't exist a good, high efficiency neutron detector for small flux, fast neutrons, PERIOD!
The absolute best detector for our purposes is the good old BF3 tube. ( I cover this and the issue of counters in the tape #2). This is the oldest of the neutron detectors and is still used.
A special arrangement called a long counter is the absolute best fast neutron counter and with calibrated and fixed moderation can approach 15-22% efficiencies for a specific fast neutron energy. In a way this is good. We are dealing with what is effect a point source of mono-energetic, isotropically emitted neutrons, albeit a weak one.
The long counter relys on a longer than normal BF3 tube (up to 18 inches) and the neuts are allowed to shine in on the tube axis paralllel to the tube walls. Around the tube is a moderator of polyethylene or such and the fast neutrons are moderated and scattered, hopefully, uniformly over a long distance along the tube. Many more thermalized neuts now have an opportunity to enter the tube and cause a count.
I have a long conter tube and will fiddle with the thing as time permits.
The BC-720 proton recoil system (also seen on tape #2) is fine, but is in the .5% efficiency range and only comes out of the noise floor at about 1000n/sec area and only 5,000n/sec will give a sloppy person a definite and obvious smack in the face signal.
I currently rely heavily on the eberline rem ball counter and nuclear associates neutron ratemeter I have for low level neutron work. with the larger 5" PMT BC-720 still in construction.
Those who are really serious about fusion and hands on work WILL face these issues! you WILL need to readup and start looking for something to count with. My original indium detector idea is just a distant memory but might be usable to a limited degree at 10e7n/sec in the future.
Always thinking, doing, and trying to share the data. (often hard won)
- Re: Neutron counters - Mark Harriss Jul 21, 8:48 am