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: Neutron rate meters.
Date: Jan 23, 2:03 pm
Poster: Richard Hull

On Jan 23, 2:03 pm, Richard Hull wrote:

I have two neutron ratemeters. These are specifically designed to give an averaged dose of neutrons from thermal energies in the .01ev to fast neutrons in the 5mev range. As such, tradeoffs are necessary in the moderator surrounding the BF3 tube contained in each. The BF3 tube relies on a THERMAL neutron hitting a Boron 10 gas atom and producing an alpha particle in the gas which can then be detected by gas amplification. We are hunting only fast neutrons (2.45mev) and for the BF3 tube to detect these, they must be "thermalized" or "moderated". This being said, I regard these two instruments as being more qualitative than quantitative.

The general upshot is that someone, at sometime in the past defined a curve called an RPG curve (radiation protection guide) based on the RBE (relative biological effectiveness) of neutrons from thermal to fast neutron energies. The object of the moderator is to mime the "inverse" RPG curve over a broad range of energies to give an averaged energy independent dose in millirem/hr of neutrons. WHEW! If you followed that, you were better off than I was when I first read it!!! Stuff like this slowly settles into place in my old brain by study and use in calculation and empirical study.

T.W. Bonner of Rice Institute was one of the first to utilize a detector within a spherical polyethylene ball to mime this curve, and D.E. Hankins of Los Alamos National Lab was the first to codify and specify the idealized 9-10" sphereical polyethylene detector arrangement as a fully functional neutron ratemeter/dosemeter and general survey instrument. His paper, "New Methods of Neutron-Dose-Rate Evaluation", presented at the December 1962, Harwell England, International Conference on Neutron Dosimetry is a classic.

Eberline, Nuclear Chicago, Victoreen and others quickly produced a vast line of "Bonner Sphere" neutron survey meter dosimeters. Perhaps the most famous and infamous of these was the Eberline PNR-4 with its 9" poly sphere and log-linear twin needled meter single channel indicator. I recently acquired one of these classic instruments. About a month ago, I also picked up a Victoreen-Nuclear Associates more modern, non-spherical equivalent. Thus, I now have two professional neutron rate/survey meters calibrated in millirems/hr dose rate. Both are relatively well calibrated with the non-spherical Victoreen unit being orientation sensitive. In general, "Bonner sphere" based instruments are not orientation sensitive.

Remember, I still have a fine, self made, quantitative neutron counter which is well known and calibrated to detect the precise, characteristic 2.45 mev neutron from D-D fusion. This is my plus ultra instrument for neutron measurement.

The acquistion of the two recent neutron survey meters was to give me an idea of the biological dosage that I am receiving and to act as a crude cross check on the proton recoil Bicron BC-720 scintillator instrument.

This weekend I setup the two survey meters near fusor III (~25cm from the fusor core, inner grid.)

I decided to adjust the fusion rate to indicate 1mrem/hr as close as possible on both instruments. This was achieved at about 20.5kv @ 10ma. The longer time constant on the Victoreen-Nuclear Associates instrument caused a slight slippage in the values such that the Eberline instrument was indicating about 1.25mrem as the slower reponse Victoreen unit was dead on 1mrem.

It is my guess that the Eberline was made in the 70s and that the Victoreen unit is an 80s creation. Both are transistorized, battery powered units.

Now to back figure from dose rate to isotropic, point source, neutron production is possible provided some key data is known. I have operators/service manuals for both instruments.

The key item is that 1 mrem for each is considered to be 30cpm or .5cps. Now how does this relate to flux and what are the efficiencies of the counters for the 2.45mev neutrons??!! There will be some hand waving and approximations here by the manufacturers as the system is not made for specific energy detection but quite the opposite. It is designed to smear the energies to arrive at a time ordered average dose based on RBE.

It just so happens that both The Hankins paper and another by I.O. Andersson and J Braun at the same conference give a relative efficientcy of the bonner sphere to 2.5mev neutrons as being about .5 counts/sec per neutron/square cm for 80mm polyehtylene moderation. Given the generalizations thus far, we can now do some crude order of magnitude calcs to get at isotropic emission from the fusor.

As I had the instruments rather close at 25cm, the spherical area of the measurement is 4 X pi X 25e2 or about 7850 sq. cm. As we were getting .5cps from the meters, there were on the order of 7850 neutrons being emitted per second, isotropically. Or, with a small stretch, 10e4n/sec. This is very close to me earlier reports in Jan/Feb 99 on this BBS using my Bicron detector which is highly accurate.

Nice, huh?

I am getting a bit more comfortable with neutron measurment now and while no expert, do have a handle on things, I feel. It is nice to be able to cross check, and even nicer to be able to get neutrons at will with a functional fusor.

I would never attempt to rely on the BF3 tube measurements in the rate/survey meters for precise results, but you can at least, (1) get an order of magnitude idea about what you are doing, (2) prove that you are producing neutrons to any doubting Thomas idiot observer, (3) watch out for dangerous levels of neutrons in order to prepare shielding and (4) check the biological effectivenss of that shielding.

Note these instruments were very lucky finds and relatively expensive ~$500 each. I am sure that someone will be able to beat these figures on a lucky strike in the surplus market.
Richard Hull