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 activation scare/non-issue
Date: Feb 18, 09:57 am
Poster: Richard Hull

On Feb 18, 09:57 am, Richard Hull wrote:


Doug Fix has a point on this matter, although it is really "much a' do about nothing."

"Professional" Fusioneers seeking to keep their ranks unsullied by amateurs will often try and frighten such perceived dilettantes off with a number of fear tactics. Some of what they profer is valid, some hysterical scare tactics. Most such pro's would drool over an amateur's ability to quickly and dangerously activate a small fusor chamber, as they can't do it themselve and it would indicate a level of operational success they are seeking desparately.

It is as Sir Frances Bacon once noted and I paraphrase regarding your reading of anything......"Read not to confute nor confound, nor to blindly accept or give idle discourse, but instead, to weight and consider".

First of all, all materials will neutron activate over time. The degree of activation and the activities involved are related to the degree of neutron thermalization, the cross section of the material, flux density and the time of exposure.

At our basement level of amateur operation and due to the short experimention run periods involved, the only significant activiation, if any, will be of ultra short lived isotopic materials of sizable neutron cross section. Gadolinium, indium and the like are superb examples and can be considered valuable instrumentational aids. The actual flux density in our devices will be extremely low. The operational run time at any level will be, hopefully, very short (research runs). Our chambers are small, mostly of low cross section material and thin. They will thermalize and capture very few fast neuts per unit time. In short, unless you leave one of these things running at 10KW input power for hours and hours, there will be little activation of the chamber to create some sort of hazard should you ever get bold enough or silly enough to actually disgard the entire system you have spent a small fortune making!

Time is a factor. As you neutron activate for X time period, whatever isotopes might form in any significant quatities (activity) will also decay very rapidily. (on the order of the exposure time) What microscopic quatities of longer lived isotopes are produced, will be at activity levels and be so uniformly distributed throughout the volume irradiated that the actual rad output from such materials will be trivially insignificant.

Still, if anyone is in the business of fusion experimentation, they must assume responsibilities for all the hazards of monkeying around with such processes. This is where Doug's commentary is wise.

One is a fool to rush into this thing wide eyed and excited without first securing a grounding in the numerous disciplines involved. There are so many ways to kill onself playing with fusors. The least dangerous of all is neutron activation!!!!

The number one death threat we live under and greatest danger is electrocution. Careless handling of D2 gas is a definite explosion hazard. Welding and machining can burn and mangle limbs. Vacuum stuff (especially glass) can implode blinding and slashing the neophyte. The number one rad hazard is never neutrons or secondary radiation from neutron activated materials!!!!! It is X-radiation! This will boil off in larger amounts and much earlier in amateur experiments far out of proportion to any neutron or activation threat.

Most dangerous neutron activation by amateur fusors will unfortunately be within the bodies of stupid, careless and doltish amateurs who take their fusors way beyond the point of reasonable experiment. They will not shield or thermalize the neutron radiation properly and their big bag of water bodies will do it for them. I doubt seriously if one of us will ever arrive at this point.

90% of all fusors built by amateurs thus far and of those to be built are and will be demo models and will never, ever produce the first neutron.

My earliest measured neut producing was 10E3/sec total production with old fusor II and I was unable to detect neutron activation after a 5 minute run on indium foil at a range of 15" from the poissor. The reason.........poor/unknown thermalization, short exposure time and virtually zero flux DENSITY at that range!

Instrumentation and knowledge are the keys to safe operation of low level neutron producing systems. Neutron activation is way, way down on our list of worries. When it becomes a problem, we will be standing behind 2 ton lead shields to save us from X-rays, have large 1 ton oil tanked 100kv++ HV supplies and 2 foot diameter fusors working off of turbo molecular pump systems.

I stress all the above in my safety tips on Jochen Kronjaeger's web page.

When I stop fusin', the chamber may lay around the lab for a few years and all the micro quantities of isotopes will have coasted through 50-100 half lives and I'll sell to another experimenter or junk it.

Richard Hull