FAQ - Details on Be- Alpha neutron generators

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Richard Hull
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FAQ - Details on Be- Alpha neutron generators

Post by Richard Hull » Fri Apr 25, 2003 9:25 pm

Here is the low down on Alpha-Be neutron generators.

The reaction is Be9 +He4 = C12 + N

The key is to intermix perfectly (impossible) the alpha emitter and the beryllium. The components are normally in a near dust form and are tumbled or compressed air mixed for uniformity of product and then immediately dropped into a sintering die and sintered to a pellet. In this special state of near perfect intermix the best sources are made.

The above reaction only results in a .008% yield per alpha stop in a mix. Thus, for every 12,500 alphas emitted only one neutron will be released in a loose amateur mix. In a sintered pellet the there can be as many as 1 neutron in 8000 alphas stopped. A one micro curie source if intermixed perfectly would only yeild about 4 or 5 neutrons/sec/uCi.

For easy calc's we can settle on an average of 10,000 alphas producing only one neutron.

A full Curie of Am241 should produce about 3X106 n/sec, but this is rarely achieved. A full curie of Radium will easily produce this amount due to growth of the daughter products within the source over a couple of months after manufacture.

The Am241 generator produces no gamma that can escape its capsule. The Radium based generator would present a very dangerous gamma hazard. So it is very rare to find a Ra-Be neutron source today. I doubt if they are still made.

Thus, amateur neutron sources are somewhat doomed due to the inability to secure a decent alpha emitter in the amount needed.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: Details on Be- Alpha neutron generators

Post by ChrisSmolinski » Tue Jun 03, 2003 11:56 am

Richard Hull wrote:
> A full Curie of Am241 should produce about 3X106 n/sec, but this is rarely achieved.

While skimming through an Amersham catalog, I took a look at their Am241/Be neutron sources. They claim 2.2x10^6 n/sec for 1 Ci. Interestingly, half a mCi of Cf252 has the same output. I suspect the exempt qty of Cf252 is zero ;-)

They also point out something I knew vaguely of years ago when I routinely worked with Am241 gamma sources, the production of neutrons from alpha reactions with the low Z stuff in the souce holder. They claim 10^4 n/sec per Ci. Much worse (by an order of magnitude) was the Cm244 we also used.

I'd like to try my hand at building a BF3 neutron counter, but there don't seem to be any neutron sources for the hobbyist (which may be a good thing), and I am not at the point of building a fusor.

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Re: Details on Be- Alpha neutron generators

Post by Richard Hull » Tue Jun 03, 2003 1:48 pm

Cf252's very short half life compared to Am241 coupled with its natural tendency to decay by fission without needing a co-reactant like Beryllium make it much richer in neuts per unit mass.

You are right. ZIP Cf252 is allowed without NRC license.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Details on Be- Alpha neutron generators

Post by JohnCuthbert » Tue Jun 03, 2003 9:47 pm

Never say never. I have not been able to check if this story is real or a hoax. It does seem to indicate that an enthusiastic amateur could make a neutron source.
Google "radioactive boy scout" and see what you get.

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Re: Details on Be- Alpha neutron generators

Post by jlheidecker » Tue Jun 03, 2003 10:03 pm

I used a neutron source at caltech that used the Be-Alpha reaction. A
huge particle accelerator created an alpha current that blasted a Be
source. Man, that thing could really produce neutrons!

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Re: Details on Be- Alpha neutron generators

Post by Richard Hull » Wed Jun 04, 2003 3:19 pm

The Radioactive Boy Scout is an old article written by a non-technical wirter (Ken Silverstein) who got a lot of his facts just plain wrong. (or at least stretched and magnified without specific numbers.) It was an excellent and exciting read with a lot of humor for us tech types but also a lot of horror and hype for the great unwashed masses.

First appearing in Harper's magazine in 1998 as "The Nuke Kid on the Block" and then within the year in a condensed, expurgated version in Reader's Digest as "The Radioactive Boy Scout", the article played on fear, and knee jerk public reaction, but was written in a humorous vein.

I have about 4 copies of the article in my library and keep one on the coffee table and another in my briefcase to amuse others with. It is a great read, but just a lot of classic over reaction and freaking out on most everyone's behalf.

You gotta' give th' kid, (David Hahn), credit. He focused on a task and went about it in the finest of American traditions. Adversity drove the effort and hurdles were overcome, bypassed or worked around to learn, study and reach a goal. The goal was just not what he was supposed to achieve by the common, down-at-heel standard of the numbed, median IQ, public understanding of what a boy his age might seek to accomplish.

Basically, the kid concentrated radium sources which could go particulate once taken from their normal carrier (locked down in dial paint). The americium in smoke alarms is on the order of 1uc in old devices and under 0.5uci in what few current ones are still being made with the stuff. The material is locked down under a gold vapor deposit and tough to re-extract and concentrate. I seriously doubt there was any neutron hazard in his device at all. Most of the hazard came from his radium concentration's gamma rays and the possibility of ingestion, mainly, into his body.

The ever intensifying radiation from his source mentioned in the article is not through neutron activation, but from the collection of radon daughters which builds up to as much as 4 times the radium radiation over a period of a month or two every time a radium source is opened to the air and then closed again. Common stuff that any student in first year nuc. physics would know and learn (maybe not, the way they teach today.)

His piece of "swiped" beryllium, as used, could not form the required homogenious mixture of reactants in an efficient manner. Therefore, any nuets would be minimal from a health standpoint........or even detection standpoint!

So enjoy the read and the hype. I believe I put up the URL a long time ago on the Songs board, but it can, of course, be serched for.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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