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Neutron Source (Fusor vs. Alpha-Beryllium) and Neutron shielding

Posted: 25 Oct 2017, 17:28
by ZeeCaptain

i want to experiment a bit with neutron activation and for this i need a neutron source.
The two options i am seing here are a Alpha-Beryllium source or building myself a Farnsworth-Hirsch-Fusor.

I would love to have my own fusor but the protabillity of the Alpha-Beryllium source is also quite nice so maybe i am going to buld both.

In the internet i can't find information about the doserate or energy of the neutron (or gamma) radiation produced by such fusors and i have no real experience with it. Is there good literature about how fusors exactly work and what radation is to expect?
I think Steve did build a few so maybe he/you can help me out here.
Also, how many deuterium gas do i roughly need for running it for say 10 minutes or is there a more commen/cheaper gas when the target is to produce neutron radiation?

For the other option i got some Am-241 Ionisation chambers for fire alarms here and thinking of buying a piece of beryllium and kinda "tape" it onto the source. Did anybody of you ever open a Ionisation chamber before? I am a bit hesitating working with exposed alpha emitters...

For the shielding i thought about buying a piece of cadmium and shielding the generated gammas with lead. But cadmium is ludicrously expensive (like 150$ for 1mm think 10x10cm piece of cadmium) and i am considering buying old defective cadmium batteries shoving the powder out of them and sticking it onto a piece of metal.

I also took a look at boron carbide. I could fill a plate like container with the powder and shield the gammas with lead but i am not sure if i can get it cheap and in the quantities i need.

Does anybody of you experimented with neutron activation, and neutron shielding and has some tipps for me?

Re: Neutron Source (Fusor vs. Alpha-Beryllium) and Neutron shielding

Posted: 25 Oct 2017, 20:23
by luuk
Hello Felix,

You can buy Boron loaded Polyethylene to shield for thermal Neutrons, you can easy slow(moderate) the fast neutrons with normal Polyethylene or other “plastics” that contains a lot of water.
To open an smoke detector source holder source is simple but be carefully although they claim that these sources are “wipe proof” this is not always the case, you don’t want any contamination with alpha’s!!
Use cloves and try not to touch the real active area only the small metal frame the source in mounted in.
If you look at a test I did with different Alpha sources you can see how these sources look, you can find it here:

( viewtopic.php?f=5&t=30 )

Never used a Am/Be as neutron source but I don’t expect you will get a lot of Neutrons from it or you need lots of Alpha’s, maybe someone else uses this kind of source.


Re: Neutron Source (Fusor vs. Alpha-Beryllium) and Neutron shielding

Posted: 26 Oct 2017, 01:25
by MartinM
Really no need to mess with toxic Cadmium when you can use Boron. There is a lot of documents around describing neutron cross sections for absorption in various materials. Gadolinium is the best I think, but also expensive.
I think Carl Willis was doing some experiments with neutron production and various Am-Be. Check out his very interesting blogs:

Re: Neutron Source (Fusor vs. Alpha-Beryllium) and Neutron shielding

Posted: 26 Oct 2017, 04:44
by sgt_bear
If you need the source just for experimenting, go with a Alpha-Be source, since building a working fusor can be a technical and financial experiment on its own :)

Re: Neutron Source (Fusor vs. Alpha-Beryllium) and Neutron shielding

Posted: 28 Oct 2017, 06:10
by MaxGaspa
I would say the AmBe neutron source is really not recommended. From this kind of source the neutron yield is quite low, typically 0.05 neutron / ( second kBq), and in case you are using high Am activity there is also an energetic gamma ray from the source (4.4 MeV). Moreover the average energy of the generated neutron is 4 Mev, quite not useful for activation. You need thermal neutron for activation.

The worst thing is "playing" with Am241 sources. Smoke detector sources are safe in theory but can become dangerous easily.

In the past I dismantled, very carefully, some cheap smoke detectors and with a simple visual inspection, carried out using a microscope, of the gold/metal plated surface I found damaged sources. Alpha contamination may be dangerous and the manufacturing quality of cheap components can be very low.

Please avoid to accumulate Am241 sources trying to get more neutron form Be.

Moreover Beryllium is a toxic metal...

So please avoid....unless you are skilled enough to cope with that parts. Forgive me but the simple fact you are trying to understand "how to do that" with Internet (googling....) is a clear sign that you are "at risk", in my humble opinion..

Re: Neutron Source (Fusor vs. Alpha-Beryllium) and Neutron shielding

Posted: 28 Oct 2017, 12:40
by Steven Sesselmann
This post be ZeeCaptain must have slipped through the cracks, as I just noticed it now.

Yes, building a fusor isa relatively safe way to generate neutrons, but it's not for the faint hearted. If it is a first time build, expect that it will take at least 1 year and cost anywhere from $5000 upwards (depending on how patient you are). Cheap components come up on eBay, but you wait a long time for the right stuff. Everything you want to know about building a fusor is on the fusor forum. ... a29b6d2f90

The great thing about a fusor is you can turn off the switch and it's perfectly safe to have around the house, this is not the case with an AmBe source, if you seal it to make it safe you have a nasty source emitting neutrons 24/7 and it eventually becomes an issue what to do with it. Homemade AmBe sources are usually very weak, because high intensity alpha sources are not easy to find (fortunately). I suppose I am lucky, because the University of Sydney have a 2 x 10^6 AmBe source which I am allowed to come and use if I need to test my detectors.

The absolute safest way to catch neutrons is just to sit and wait for cosmic background neutrons, they arrive at a constant flux of around 1 neutron per minute per sq/cm. Varies by location, so here is a calculator.


Re: Neutron Source (Fusor vs. Alpha-Beryllium) and Neutron shielding

Posted: 30 Oct 2017, 07:48
by stamasd
I have a neutron source that I built myself. It's not your run-of-the-mill source... It's made with a 5mCi 210-Polonium source and 1mm thick Be sheet. I made it primarily to calibrate my neutron detection setup, but since then I found myself starting to use it for other things like activation experiments. I don't have great results yet but I'm only beginning to do some real gamma work. I can probably say I have proof that I've activated silver with it. :)
If you're in the US (and ONLY in the US) you can get access to the above-mentioned alpha source. You can't buy it but you can lease it for a year for approximately $200. Look for "static eliminator P2042" from NRC. You don't need any special license to lease it. But you're legally obligated to return it at the end of the lease (and by that time it will have only 1/8 of its original activity as the t1/2 is 138days). Or for about the same sum you can purchase a 0.5mCi Polonium source that you will own - but of course it decays at the same rate. I chose the first option.
I use a moderator composed of several 1-inch thick sheets of polyethylene and jugs of mineral oil (unopened car engine oil bottles more specifically). I haven't found references of mineral oil used for moderation of neutrons but it seems to work pretty well. For shielding I use blocks of borated paraffin. And a few boxes of borax from the grocery store. :)