FAQ: Neutron Sources

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Richard Hull
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FAQ: Neutron Sources

Post by Richard Hull » Mon Aug 09, 2004 4:32 pm

This FAQ is a defensive FAQ rather than the usual informative and HOW-TO type of FAQ.

First of all, you can probably never have, or own, an effective neutron source based on a radio-isotope.

You should certainly never entertain "rolling your own" source.

1. it will fail
2. it is illegal.
3. it is dangerous

Here is the low down on why you shouldn't even attempt this effort.

Most normalized sources for neutrons based on radioactives are of the Alpha-Be reaction or the Photo-neutron type. The absolute worst is the photo-neutron type. In this source an intense gamma source is sealed up with antimony and the photons shear off neutrons. These sources are lethal to even be near and require a minimum of a 1 Curie source. The reaction is a very inefficient one and such sources are rarely used or assembled anymore.

The classic neutron source is the Ra-Be source based on the action of alpha particles with beryllium. The alpha-Be source is just about the best there is. Still, even in the best of these sources, it is a given that a minimum of 10,000 alphas must impact the Be at full energy (~5mev) to yield one single neutron.

Let's do some math. Question: What would be a minimum alpha activity to produce a viable source that would yield a 2 neutron per minute count into a standard moderated BF3 counter? (This count is easily detectable and far above background enough to pick out immediately with no statistics.)

This is going the wrong way round in normal calculations, so we work backwards.

A BF3 tube at a range of 25cm (about the minimum distance from our counter to source), would represent a 12cm sq moderated tube at 1% efficiency.

The sphere would be 4XPiX25^2 = ~7900cm sq. of which the tube is but a 12/7900 part and so for every 660 neutrons isotropically emitted only one would enter the tube. We would need a hundred entering the tube to detect one. So 66,000 neutrons would need to be emitted every minute to get one CPM or 132,000/min to get 2 counts. this works out to about 2,200 n/sec from the source to get 2 cpm in the counter.

2,200 X10,000 = 22 million alphas per second demanded of the source. provided it is flawlessly intermixed (radium dust to Be dust) No home-made source can have this and thus most are target systems. This means a one Pi effective emitter so we will need double the activity and thus a 44 million alpha per sec source.

With 37,000 alphas/second/uCi we would need a 44,000,000/37,000 uCi source. This is about a 1.2 millicurie source!!! In terms of smoke detectors you would need 1200/0.9 or over 1330 smoke alarms.

This is for two lousy counts every minute!

DOES EVERYONE GET THE PICTURE HERE!

To continue on.... The modern N sources are all Am241-Be systems as the old Ra-Be systems were horrible gamma emitters which made even a two minute encounter with one in the open at 20 inches give you about a life time of normal exposure. Very nasty.

The Am-Be sources are still gamma emitters but of only the weakish 59kev gamma that is easily shielded either by the source container, itself, or light shielding around it.

So you can see that so far as rolling your own is concerned, it is out of the realm of possibility. Far too much isotope needs to be on hand to make an effective neutron source.

I can't control what each of you do, but I would ask that you would refrain from posting, here, any attempts at assembling a neutron source. This is mainly due to the current climate in the US and that you are fostering an illegal activity.

The success of a working fusor will supply your counters with enough activity to check them out. Beyond this, you can send your counter to Ludlum for calibrtion and check out if you are unsure of its functionality.

The best neutron source is a working fusor for it can be turned on and off. There are no storage issues and no contamination or nuclide handling problems. About as clean and sweet as it gets.

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.

Alex Aitken
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Re: FAQ: Neutron Sources

Post by Alex Aitken » Mon Aug 09, 2004 6:18 pm

According to the CRC the numbers are a little worse, 30 neutrons per million alphas for beryllium. The numbers are a little worse for boron. In terms of remebering the ratio, it works out pretty damn close to 1 neutron per second per uCi of alpha source.

Disassembling an alpha source, ie trying to remove it from the foil itself, or powdering a high activity alpha source is really stupid. The physics equivalent of a baby playing in the shit, just dont go there.

Radium clocks contain a few uCi of radium at the very most - its not useful. Aside from that the Radium/Beryllium sources used to be very unreliable, Ive read more than one report where the series of experiments had to be stopped because the radium source exploded. This seemed a common occurence in the 40's when they were still being used.

Rumor has it that for the high speed drilling of plastics you can get polonium sources (with a few month half life) as a laminated source for destroying static buildup and that these are in excess of 1mCi. Simply laying these on a beryllium or boron surface should yeild a little less than half of the the theoretical maximum neutrons youd be able to get from the source under ideal conditions. If anything is going to be detectable, that would be. Its illegal ofcourse, as its a violation of the exemption that alows the sources to be sold but its the only plausable way youd get a chemical source of neutrons at home and thats only if you can get these. Its also no more dangerous than having the plates in the first place even if you get neutrons. I cant and since theres no exemption in this country I can find for them, I'm assuming I'll never even see one. The highest source I could aspire to as an unlicenced individual here is 4Mbeq, or around 100uCi. Not useful at all for this and too dangerous to have around for no reason, I havn't enquired as to the cost and would expect it to be substantial.

A much more plausable way of getting access to a weak neutron source would be to ask someone at a university. They usually have one if only for activating a foil for undergrad decay measurements. The one I saw was in the 2nd year phy chem lab and was a black box about 20cm square (including moderator and lockable container) and produced a 2cm^2 indium foil of a dozen counts a min or so. Decidedly unimpressive but if you could get a moderatorless BF3 tube close to where the foil was that should register. I would expect anyone to object to removing the source from its moderator or tampering with it in any way. As a side note the box was located in the (locked) bench cupboard where people were working with no additional sheilding so noone was under the impression it was dangerous.

Some US universities have californium sources in the region of 10^6 n/s located in large water tanks for undergrad experiments.

To my mind explaining what you are doing, and why you want to test the neutron detector is far more plausable than being able to get hold of high activity laminated (which are the only sort you'd want to have anyway) alpha sources and a hell of a lot safer.

Nice summery on chemical neutron sources Richard, it would be nice to see one for other methods.

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Re: FAQ: Neutron Sources

Post by 3l » Mon Aug 09, 2004 6:41 pm

Hi guys:

One reason I don't have strong neutron sources at home is they are too dangerous..period. Hell the company I worked for contracted out to Los Alamos all neutron source work.
The hottest source, I have is one microcurie of strontium 90 in a test set I bought from Oak Ridge. I bag all U rocks no matter how small because I was trained to do so. An accelerator source has fewer regs due to the fact it can be shut off and put under lock and key. Some illinformed folks might gather that I'm relatively free with high neutron fluxes at my site.
You will be dead wrong literally if you think this.
I had 500 or more hours at high level work to fall back on.
Including reactors ,sources , xrays ,gamma lithography and yes special weapons.
A isotopic source cannot be shut off for hundreds of years and it is a clear and present danger to untrained people. This is a very good rule considering the radiation poisoning cases during the 1920's when you could get a gram of radium for commercial purposes. People simply buried them in the yard for disposal or worst yet left them in the basement for the next tennent to find.

This falls into the three "I"s territory.

It is not an oppressive government at work here.
It only costs seventy bucks to have Ludlum do it!
(I spent that much on coffee and creamer last year!)
It is idiotic to build any isotopic source for home use

Happy Fusoring!
Larry Leins
Fusor Tech

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Re: FAQ: Neutron Sources

Post by Richard Hull » Mon Aug 09, 2004 7:40 pm

1 gram of radium is a curie and a one curie radium source cost about $2,500 when new so they were never ever around a casual disposal situation as a capital investment item. A few may have accidentally gone surplus way back when, but now that is not the case. Under a formal Goverment recall, all significant medical and source forms of radium (>10milliCuries) were totally recalled in the 60's and replaced by Co60 sources for medical purposes. Radium as a "supplied isotope" hasn't existed in the US since 1960. Microcurie quantities are used industrially in very rare cases. There is no legal, license exempt quantity of Ra allowed under the small sources act. (save for grandfather claused items (watches, clocks, household items and instruments manufactured prior to 1965). Much of this is discussed in Bill Kolb's book, "Living With Radiation - The First Hundred Years. ( see books and refs forum)

Ra-Be sources can't normally blow up if made after WWII as they went from a simple soldered brass case to a thick stainles steel casing with an internal all welded design.

Most "pop opens" were the old sources where the 5CC/year helium production just lightly pressurized the container and popped the soft solder bead. Nothing like this could happen with most any source made by a reputable company after WWII.
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.

3l
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Re: FAQ: Neutron Sources

Post by 3l » Mon Aug 09, 2004 8:12 pm

Hi Richard:

Government contractor of aircraft instruments were the one's I was thinking of ,in Chicago the old radium dial corp bankrupted leaving 25 grams to to bank. They auctioned it away in 1918. To anyone with cash. Leo Szilard's radium came from that stash.
I know the history because we buried 10 grams of radium bromide on the 14th hole of the officer's golf course at Barksdale Airbase. After that I went to the base library to research it. Nasty crap for sure. A bunch of my exposure had come from that tiny bottle. It had sat since the early 50's in a shed out side of the Pmel building. It had pressure popped like you described. A scintilator went mad in that shed. So we used a ten foot pole to move it in short shifts. It was a sharp eyed Master Sargent who spoted it. It was a secured building but the lawn mower was stored there too. Thank God I never had to mow the lawn.

Happy Fusoring!
Larry Leins
Fusor Tech

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Re: FAQ: Neutron Sources

Post by MARK-HARRISS » Tue Aug 10, 2004 3:02 am

My Mom is the sole survivor of the Qantas Instrumentation luminising lab in Sydney from about the late 1950's. She was afraid they would find out about her being an art student as it was grounds for an immediate dismissal as they claimed all art students licked their paint brushes: She would not even scratch her nose while inside the rooms where they worked with lead aprons and glove cabinets and hand mixed the radium with the phosphors.

The only decontamination was not with radiation detectors but a UV room
which would excite up any traces of phosphor on your body so you could wash it off, in other words pretty primitive stuff. She quit after a few years as the supervisor told her it would be a good idea if she wanted any kids in future.

They used to paint all the dials for the Smith's clock factory which had to have it's store room broken up and sealed in steel drums a few years ago as a rad hazard.

Legend has it that the WW2 Aussie made fighter planes had to have their
instruments re-painted in the lab as they were too bright allowing the cockpits to be spotted some distance away by enemy fighters.

The only other surviving co-worker died recently of leukemia after surviving as an identical twin test subject in a Nazi concentration camp during WW2.

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ROM RANEY: BURSTING Neutron Sources

Post by raneyt » Sun Aug 15, 2004 11:27 am

Hi Guys,

Apparently, the most significant cause was residual water entrapped in the old neutron sources. The resulting bursting or rupturing occured if the RaBe preparation was not prepared properly & the water was not removed before incapsulation. So, the Ra activity basically electrolyzed the water & the H2 & O2 took up a little more volume than the liquid H2O.

Just a little history.

Warm Regards,

TIM

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