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: Watch those instruments!
Date: Apr 05, 12:19 pm
Poster: Richard Hull

On Apr 05, 12:19 pm, Richard Hull wrote:

>>I was running fusor III this weekend for some more doubting thomases and can always get to 1 mrem of neutrons quickly. I use a Bonner Sphere neutron counter for the plebian audiences as it doesn't give any audible indication until the fusor starts actually making neuts. One millirem, at the distance I have it placed from the fusor, represents about 55,000 n/sec @ 30-40 cpm. It makes for a great eye opening demo.
>When you say 55,000 n/sec is this a calibrated number representing the number of neutrons being produced in the fusor?. Or is it calibrated as the number of neutrons that the neutron counter is intercepting at its current location?


This has all been hashed out before on this list, but here goes again.

Normal reactor and pile researchers officially speak of "flux" this is clasically the number of neutrons per square cm per second at any specific point. This is because the density of pile and reactor neutrons is tremendous in virtually all experiments done by fission scientists.

No fusorite can use flux for the emission from a fusor is many orders of magnitude below that of a seething chain reacting pile or reactor. (piles just aren't around anymore.) So, most fusion researchers and all fusor builders have traditionally referred to the number of neutrons ISOTROPICALLY emitted from the reaction each second! The is therefore given in neutrons per second, without reference to any area, but is assumed by all to be the total isotropic emission from the core reaction each second. To get flux one would just figure the "solid angle" (4pi r squared for a 1 square centimer area at some distance from the fusor core. The math in either direction is abysmally simple.

Our job is to back figure from some point outside the fusor with a neutron instrument what the core neutron production or reaction rate is.

The mechanics of this task is not easy and requires a lot of prior knowledge and careful understanding regarding your instrumentation and extremely careful measurement of that distance from the core of the detector to the core of the reaction (poissor). The errors here would compound as the square of the error.

Above, I gave a totally lucid description of the reaction rate, albeit not a familiar one to the novice or those used to flux measurements.

I noted that at a specific range of the detector (BF3 ) tube from my fusor that it was recording a production (total isotropic emission) of 55,000 neutrons /second. I also noted that 1 mrem for my instrument represented between 30 to 40 couunts per minute.

That last sentence told the story. This was my actual count! More on this shortly.

Neutron meters can give readouts traditionally in events or counts per second or per minute. They can also be calibrated directly in flux. Rate or dose meters, are always calibrated in Rem or millirem. Neutron counters are a very rare and specific beast so far as the common Joe is concerned. All research meters are usually set up for flux or events/unit time. These virtually never find their way, surplus, into the hands of an amateur experimenter. They are too few and too valuable to just cast off. Around the fission power business, and most all neutron research facilities, there are loads of health physicists, and for each such guy there are often 20 neutron rate or survey meters. These are always calibrated in millirems.

Neutrons (fast ones) are the most dangerous and highly biologically damaging radiations known to man. Therefore, it is imperative to have regular and diverse neutron readings for the safety of operating personnel.

Meters used for this work are usually a "catchall meter" and look at the entire neutron spectrum of energies from thermal (.02ev) to fast (5mev) energies. The BF3 tube doesn't respond well to any but thermal neutrons. Even then, its efficiency is very low compared to a geiger counter where charged particle efficiencies easily approach 95%.

The manufacturers usually place a moderator of parafin or polyethylene plus a cadmium or gadolinium wrapping around the BF3 tube. This makes the averaged response appear much as human tissue responds to absorption of the full range of neutrons. Thus, a good general health figure or "Roentgen Equivalent Man" dose can be arrived at.

The manufacturers usually publishe a set of weird and often confusing curves to show the effective flux levels at a vast range of different neutron energies for their particular BF3 tube and moderation assembly.

Such charts, for my meter, is where I pulled my data to link the critical energy/flux/count data to back figure the isotropic emission from my fusor.

Amateurs will find 10,000 neutron rate meters for each neutron flux meter on the surplus market, and 10,000 geiger counters for each single neutron meter of any form found surplus! Such is their great rarity. I have trolled the hamfests for 22 years and have never seen a single neutron meter of any sort and yet have seen hundreds upon hundreds of geiger counters, proportional counters, scintillators, etc.

Our fusors are producing monoenergetic neutrons in the 2.45 mev range. Looking up on my charts, I found that my particular counter has a neutron flux of 8-9 neutrons per sq. cm per sec for each mrem/hr for 2.45mev neutrons. Another chart shows that 1mrem averaged rates result from 30-40 counts per minute.

SOOOO.......when I get 30-40 cpm or clicks per minute on my specific meter with my specific tube and its specific moderation in a monoenergetic field of 2.45mev neutrons, then I have a flux of 8-9 neutrons/sq.cm/sec at the tube.

Now....... I have my BF3 tube at a range of 24cm from the poissor. I know that I am getting a flux of ~8n/sqcm/sec at this range. What is the isotropic emission? 4pi R squared = 7238 sq cm. Thus, if one neutron per sq.cm. per second is received at this range, then 7238 neutrons had to be emitted. We received 8 per second so 8 times 7238 = 58,000 approx neutrons per second needed to be isotropically emitted from the poissor. I err on the side of caution or low end and the whole thing works out a bit lower, so 55kneuts/second is the figure for 30 cpm with my counter at that range.

This was rather "windy" and I am not checking my spelling as it is lunch time here. Please pardon such matters. This should be good for others as well who look at this process as a mystery.

Richard Hull