Neutron Source: 210Po/Beryllium

[Dennis P Brown]

While I'm waiting for my fusor window (finally shipped today!) and as I am getting ready to assemble my 120-150 kV voltage multiplier this weekend (the success with the 50 kV unit encouraged me; now I have the new diodes I will assemble a few stages and if I have time, finish it), and as I am still waiting for lower humidity levels (like that will happen any time soon) so that I can test and run my new super sized Van de Graaff, I decided to lease the 5 milli-Curie Polonium source; since I have Beryllium foil (yes, I know the toxic issues and have worked with it before and far more toxic materials, for that matter), I decided to try this route when I get back from Germany (to visit my daughter who is working at the Max Plank Inst. on their Stellarator.) I need some lead time for them to get the order, prepare it and ship, so will order it late next week so when I get back, it will arrive soon after.

This method does have a lot of appeal for me. In this manner I can test my new design neutron detector idea right away; worse case, it doesn't work and I then get a He3 detector (thanks for the headsup on the paper, Richard; also, guess Steve will also hear from me then ... ) for the fusor/accelerator and use the Po source to confirm that detector is working.

If my new detector works, I have a detector for my fusor and deuteron "ion" gun to detect neutrons (using a deuterated target.) I am a little surprised that such a source is available because I was led to believe that there were no neutron sources available to Joe-the plumber ...I mean public (lol.) Rather surprised but at least I will have a fairly easy to handle neutron source with a known emittence (yes, I know the rate of neutron flux is low since one one in a million alpha's create a neutron and we have 4-Pi Steradians after that but from seeing results with detectors on YouTube, it appears rather strong/detectable.) Should get at least a good six months of usage out of it, and maybe a year if things are sensitive enough.

[Rich Feldman]

I've been wondering if the beryllium needs to be in foil form.
If the same radioisotope alpha source faces a thicker piece of beryllium, we expect the same or greater number of alpha captures and subsequent neutron ejections.
But in the thick metal case, unreacted Be atoms will have a greater effect on the distribution of neutron angles and energies.

I happen to have Be in a 3 gram nugget, 5 gram block, and a large sheet about 0.062 inches thick. Have also been saving up smoke detector ion chambers. Am prepared to experiment with a BF3 tube, and measure in counts per week if necessary.

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[Richard Hull]

Any form of Be will work, but sheet Be is best. Thickness is of no importance whatsoever usless the foil is hyper thin (microns). .001" to a full 1 inch thick plate will be fine. Naturally, the 1 inch thick plate is a waist of Be. Once neutrons are made, they will cruise through the Be like it is not there. Alphas will stop and make neutrons in very thin foil. Regardless, you are looking at about 1 neutron per 10,000 alphas in Be. Alpha-Be source are the most effecient of all "made up" n sources.

Working with finished forms of Be is pretty much a daudle and a non-toxic issue. I had to melt 100 grams of pure Thallium metal last weekend and cut it up after casting. No probs. The skimming of the dross went in a separate container for safe keeping. (probably the most dangerous part of the work). Working with Cadmium in large melts for my bullet alloys is a bit more dangerous than the Thallium. (vapor pressure wise).

Richard Hull

[Steven Sesselmann]

When it comes to making an AmBe neutron source, has anyone tried making one in vacuum?

One would imagine a longer mean free path would make the source more efficient, especially so if you are using smoke detectors where the Am is not in direct contact with the Be.

Steven

[Dennis P Brown]

LOL- either great minds think alike or else I am not so far out there like I thought; yes, I intended to mount my source, with the Beryllium foil, in an enclosure and under vacuum. Not really sure that it would make any measurable effect but might prove interesting to test. I do like projects and experiments ...

[Richard Hull]

Intimate contact is always desired and is always obtained in the best sources. With a leased source this is not possible. All leased sources are ionizers and are built for air flow and and no intimate contact is allowed in the sealed device.

Richard Hull

[Dennis P Brown]

Richard, should they use a modulator for the silver activation?

[Richard Hull]

I assume you meant moderator..........Yes! A moderator is demanded as all neutron sources emit only fast neutrons.
All activation occurs only with thermal neutrons, though some few resonant peak activations are epi-thermal. In spite of this assume 100% of all useful activation occurs only with thermal neutrons. This means surrounding the source with about 2-4 inches of water, HMW polyethylene or paraffin. More thickness means slower and slower neutrons and more and more activation cross section, but also more scattering and loss of flux. 3" seems ideal for amateur work.


Richard Hull