Neutron activation with 10 mCi Po + Be

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Neutron activation with 10 mCi Po + Be

Postby Michael Hughes » Mon Mar 27, 2017 4:26 am

While it's a long-term goal of mine, I don’t have the resources or technical know-how to make a fusor yet, but I still wanted to play with a few neutrons and activate some stuff. The best way to do this would be Carl Willis’s AmBe source, but I don’t know where to get a whole bunch of 60+ uCi smoke detectors and I’m not sure on the legality of this anyway.

So I did the next best thing: I leased two 5 mCi P-2042 Po-210 static eliminators and bought a couple of pieces of an old beryllium X-ray window from ebay, which I cut down to size. For people who haven’t seen the P-2042 before, there is a little wire mesh about 2 mm above the gold/polonium surface of the ionizer, so direct contact between the Po/Au and the Be isn’t possible without either violating the law and removing the mesh, or grinding the Be up into powder, which would both be bad ideas. ;)

I taped the Be to the P-2042 so that the whole area of the P-2042’s mesh is covered; hopefully I’m not losing too many alphas and alpha energy to the mesh and the ~2 mm of air. I got some 12”x12”x0.75” HDPE sheets and some smaller 0.75” thick HDPE blocks to moderate the neutrons. Four of the sheets are on the bottom, then there are the sources and the material(s) to be activated surrounded by the smaller blocks, and then five more HDPE sheets are on top. I usually leave things in the neutron oven for several half-lives.

For detection, my mainstay is the GMC-320. It’s awesome - a Geiger counter for only $120 with built-in second-by-second data logging, and the battery lasts for ~4 days on a charge. The data can then be exported as a CSV and analyzed with Excel. I take the stuff I’ve activated, usually in baggies if a powder, open up the GMC, and tape the baggies or foil directly to the tube so that the GM tube is surrounded as much as possible by the (hopefully) activated material.

Then I wrap several ~1.5 mm thick sheets of lead around it to cut down on background, stick it under the bed, and wait for at least six half-lives of the activation product. I take the average of everything past about 4 or 5 half-lives to be background, and subtract that from the number of counts recorded each minute. If the average of the counts for the first half-life is above background in a statistically significant way (preferably p << .01), then I have some good evidence that my neutrons did something.

For things with half-lives above 12 hours (e.g. tungsten, gold), that method won’t work as well. I’ll probably use the old Inspector pancake Geiger counter I picked up for a bargain on ebay. Its main drawback is that it doesn’t connect to any electronic devices, so no data logging. It will count for a set amount of time, though, so I can take periodic 1-hour readings every so often and see if those change over time. I’ve started with tungsten, but got a negative result there.

Using the GMC-320, I can say with a high level of confidence that I’ve activated indium and manganese. A bar of gold and some dysprosium oxide are currently in the neutron oven, and europium oxide is currently taped to the Geiger counter in the lead sandwich under my bed and is being counted. I was, however, not able to detect Mn-56’s 850 keV gamma with the cheap Russian-made gamma spec I have. I’m hoping to have better luck with Au or maybe a second try with In.

Here are my results from the GMC-320 for In (17 g, mostly foil) and Mn (110 g of MnO2). As you can see, the increase above background is not huge but is definitely statistically significant for the first half-life (p = .00003 for In, p < 10^-10 for Mn). Mn is significantly (p < .0001) above background for its second half-life as well, and In is marginally above (p=.04). They both return statistically to background levels by the third half-life.

The GM tube is fairly small and not all that sensitive, so background is only about 14 cpm within the lead sandwich; an increase of 3 cpm is about a 21% increase overall. I’m posting the 15-minute running average of the CPM for each one to smooth out some of the random wiggles. Both graphs are chopped off; I continued the Mn one out to 10 half-lives and the In one to 15; the averages from 5 half-lives on were averaged and made up the background, and nothing like the spike at the beginning of each occurs in the second half.

Image

So, I have a few questions. First, when alpha particles hit beryllium, is the radiation of the neutrons isotropic, or are neutrons emitted in some anisotropic way? Related to that, once the neutrons hit the hydrogens in the moderator a few times and thermalize, what does the distribution of thermal neutrons look like? Is it more or less isotropic with greatest density right next to the source and diminishing in inverse-square fashion after that, or does it look different? If not, is it possible that putting a layer or two of HDPE between the sources and the target is a good idea?
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Re: Neutron activation with 10 mCi Po + Be

Postby Michael Hughes » Mon Mar 27, 2017 4:47 am

I have thought about the total neutron flux, and it's obviously going to be low: the best case would be 70 n/10^6 alphas * 5 mCi (the half directed upwards) * 37x10^6 Bq/mCi = 12950 n/s, and in reality probably lower because of alphas hitting the mesh and reduced alpha energy as it hits the Be. So obviously any results I'm getting are going to be modest, but as I have hopefully shown with my first results and as Carl Willis showed much more convincingly with his activations of Mn and Au with his AmBe source, a little bit of detectable activation is possible with things that have high cross sections.

Also I'd like to report another success: I checked my lead-lined GM counter after three days of counting a 100g sample of Eu2O3, and I got pretty much the same results albeit a little less strong than the manganese or indium. The first half-life (a bit over 9 hours for Eu-152m) averaged 16.05 cpm as opposed to 15.38 for the average of the last day. A one-tailed t-test gave me p=.0003, so I'll take it. The standard errors for both the first half-life and the background are quite low because of the large number of one-minute samples, so statistical significance was achieved. While it's always possible that e.g. cosmic rays were stronger at the beginning than at the end, or something like that, three consecutive statistically significant successes with this method is convincing me I'm probably actually detecting something.

25g of Dy2O3 is the next target. It was in the oven all of yesterday and is now taped to the GM tube and in the lead sandwich wrap being counted. :D
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Re: Neutron activation with 10 mCi Po + Be

Postby Michael Hughes » Tue Mar 28, 2017 7:45 am

One thing that occurs to me is that my PoBe sources, while weak, have the ability to activate things that have half-lives on the order of days to weeks to even a couple months, which is a certain advantage over fusors because fusors require very short run times, restricting neutron activation to short-lived isotopes like those of silver, indium, and rhodium.

One promising idea would be to activate substances like tantalum (half-life for Ta-182 is 114 days, with a variety of gammas and mid-energy betas). I've already been activating gold for about a week now and will look at it soon, since Au-198 has a strong, clear gamma peak along with good medium-energy betas. One other possible idea might be scandium - 27.5 barns, half-life of Sc-46 is 84 d, and it has two nice hard gammas a little to the left of the Co-60 ones.

Do these sound like promising targets, and does anyone have any other suggestions?
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Re: Neutron activation with 10 mCi Po + Be

Postby Michael Hughes » Wed Mar 29, 2017 4:34 am

Another success: I activated Dy using a baggie of Dy2O3. Details are in my post in the dysprosium thread here.

Next up: A 1 oz bar of gold has been cooking for over a week. Time to test it out, although this time I'll start with a 10-hour gamma spec run and then use hourlong runs of a pancake counter every so often over the course of the next week or so.

I'm hoping that I don't flunk out of alchemy school for transmuting gold to mercury and that the price of gold goes up between when I bought it and when I'm done with it and ready to sell. I won't tell the buyer about the few atoms I transmuted. ;)
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Re: Neutron activation with 10 mCi Po + Be

Postby Bruce Meagher » Tue Apr 04, 2017 9:19 pm

To increase your neutron production you might consider placing the static eliminators, beryllium target, and sample in a small HDPE vacuum chamber. I was able boost the neutron output by about 70% by making a small HDPE vacuum chamber using a simple kf-40 o-ring seal. My vacuum pressure was slightly below 100 mTorr using a mechanical pump. This setup proved very useful testing out different He3, BF3, and B-10 lined detectors.
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HDPE Vacuum Chamber.JPG
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