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Alchemy2
Canada
89 Posts |
 Posted - 10/31/2014 : 16:22:45
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Hi all!
I have been having mucho fun with my unit lately. I calibrated it recently with a replacement SBT-5 tube I got form Russian surplus, and this tube JUST fits into the unit, if I remove the clips or move them (I removed them and used wire direct connection). This tube is very sensitive and runs around 153CPM/uSv/h as well, FYI.
Response is a bit more sensitive in the low end of the MeV range for gammas, as these tubes are more for Uranium and daughter detection. However, you can have a LOT of fun playing (with precautions) with this tube, and use it to detect neutrons.
HOW do you ask? 
Well, one could use Cd foil (Cd=cadmium). But it's harder to find and expensive and heavy! However, In foil (In=indium metal) also works like Cd, but it has a broader neutron energy response. Basically, it absorbs neutrons, and then the radionuclide produced emits a gamma ray, which triggers the tube. Cool eh? I used 0.1mm or 100 micron thick foil and wrapped it once around the tube, using double-sided thin 3M adhesive tape. The foil wraps around sort of like a cigarette foil, and then molds to the tube. Very thin, so no big issue. You do NOT electrically connect the foil, only wrap the inner 80-90% of the active tube to have this work. In foil is easily accessible on eBay, and I got mine for ~$30 delivered. 99.995% minimum is recommended.
How do you now make neutrons? Well, the easiest way is to use a smoke detector source, which is usually 0.9uCi of Am-241, which is an alpha emitter, with a tiny fraction of gamma also emitted. I placed mine into a 1"x5mm thick acrylic disk with a 5/16" hole drilled to allow my source to sit in the middle. You then use thin aluminum or beryllium foil (I have some Be foil on order and its not here yet) and place the foil ONTO or on top of the smoke detector source. Use tongs or tweezers to avoid exposure to the hands and fingers. You then put a small piece of acrylic plastic (3mm or 1/8") thick on top of the foil to hold it very close to the source, and it also acts like a partial moderator to slow the neutrons a bit (not much). The In-foil covered tube reads noticeably higher counts with this setup than without the aluminum foil on it, signaling the presence of neutrons!
I get about 25-30 CPM higher with my setup than with the source by itself. So, something MUST be happening. BTW, the foil is thin enough to minimize beta ray reduction and gammas are unaffected by the foil covering the tube. So, my Unit #1 (I have two) GMC-320+ has now the capability of reading n, beta, and gamma radiation! Neat hey?
As with all things radioactive be careful, as its what you DON'T see or read that'll harm you.
Have fun all...
Darren |
The more I learn, the more I realize I do not know! |
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| Reply #1
Alchemy2
Canada
89 Posts |
Posted - 11/03/2014 : 09:29:38
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I decided to leave my settings for this In-foil covered tube at 153CPM/uSv/h, which is the average of the specs on the tube.
The SBT-5 is rated at 22cps/mR/h at Co-60 and 29 cps/Mr/h for Ra-226. Ba-133 gammas are close to this value. If I use 22cps/mR/h for Co-60, I get an effective 132 CPM/uSv/h. The Co-60 source I use for the calibration test setup is bang on for the gamma exposure reading if the 22 factor (or 132CPMs) is used in the calibration, even with the IN foil on it. That's a good sign. If I use 174CPMs (29 factor for the tube, per the Ra-226 specs, closer to Ba-133 calibration) then I get pretty nominal readings for the Ba-133 source disk values. So, the foil seemingly does not affect much the gamma performance (though it MAY slightly reduce the sensitivity to lower energy betas on this tube, not sure, but would make sense since it may shield them slightly.
I am still evaluating whether to leave this tube at the 132 setting, as neutron induced reactions in the In foil will generate some higher energy gammas closer to the Co-60 gamma energies. However, for general use, the unit would read falsely high. So, for now... I'll run as is at 153CPM (25.5 average) and see what I can.
I have Beryllium foil due in any day, and can use this to generate some neutrons (typically from what I see in the literature there's about 40ppm n0 (neutrons) with an alpha source and thin Be foil target. Using this number (40 n0 per million alphas), given the activity of my alpha source (Am-241 0.9uCi taken from a smoke detector, or 3.33kBq) I should see around 1.3 n0 per second, or around 78/min. Not all will hit the tube, but a good majority should if I setup the experiment properly. Also, some of the higher energy n0 won't necessarily hit the Indium foil and interact with it, and may pass through too. The source is unidirectional for the most part, so probably at least 90% of the neutrons will be heading in the direction of the detector if scattering is not too severe. Also, 116m1 Indium (the main produced isotope - produced nearly exclusively from N0 bombardment - is 80-85% abundant, and it has a 1/2 life of around 54 min, so being the primary activation product, it takes some time for this to build up. Some secondary products (116In, 116m2In with 1/2 lives of 13s and 2s respectively) are minor contributors, but may be seen in final counts. The major product though, is 116m2In. Whether I see a steady count increase to a peak, and then slow decay off after exposure is yet to be seen. Will be interesting though... I can use Pb to shield gammas and n0 pass through Pb fairly well. Thus the bulk of anything seen above background should be due to neutrons... Can't wait to see.
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The more I learn, the more I realize I do not know! |
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| Reply #2
ZLM
1261 Posts |
Posted - 11/11/2014 : 22:26:05
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No one did this before. It is a very interesting testing.
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| Reply #3
Alchemy2
Canada
89 Posts |
Posted - 01/16/2015 : 18:44:52
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I made neutrons!!! YEAH!
Using 4x ~1uCi Am241 (total activity is actually 4.3uCi - measured using my GMC320) alpha particle sources, placed in a square like pips on a "4" on dice. Epoxied into a 3/4" (20mm) tube 1cm high. Used 2x 0.15mm thick Be foils over the sources, and a 1mm lead disk to filter out the bulk of the residual gammas (~60keV gammas from the Am241).
Placed in a 5" cube electrical plastic box top 4" filled with paraffin wax as a neutron moderator. Source placed in a 1" container, and read counts using In foil in a thin plastic sheet with the SBT-11A on my Unit#1 GMC320+. Ideally, the counts should go up as neutrons transmute the In115 in the foil to In116m which decays to Sn116 with gamma and beta emissions. I did see a ~5-8CPM count increase over BG, but one of the issues I saw were surges of counts exceeding both by 2x. Example, 24CPM mean background, counts with neutrons around 33-35CPM max. Surges went to as high as 66CPM at times, and was not accounted for on any known physical process I could think of. With or without the foil in place over the GM tube, I still saw as high as 45-50CPM infrequently as bursts.
So I knew I was making neutrons and detecting them, but what the heck was going on here? Well, I think I figured it out!
The neutron path to near thermal for 5.4MeV neutrons is around 4" or 100mm. Just about the path length for the container edges from the source! When I packed up the source and foil, I set the top of the container (filled with the paraffin wax) on the bench, beside the SBT-11A tube. I get everything else put away, and all of a sudden, about 2 minutes after setting it down, counts started climbing! 40...60...85...110...125...145...180...220...245CPM over a period of about 20 minutes! What the heck?!
AH... I know! my neutrons were interacting as thermal neutrons on the case, which is most likely PVC or something similar. If so, the Cl35 is most likely undergoing a transition to Cl36, which gamma decays over about 2h (1/2 life is ~37 minutes). this correlates with the drop to near background after 3 to 4h. I was nominal CPM after 4h, so this may indeed be the process interfering with my counting of neutrons alone. So, I definitely made neutrons!
Nice to know!!
More to come as I fine tune my irradiation and measurement arrangements, thermal absorber geometry and shielding. |
The more I learn, the more I realize I do not know! |
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| Reply #4
ZLM
1261 Posts |
Posted - 01/21/2015 : 17:04:57
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| Congratulation! |
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| Reply #5
Stibnut
USA
2 Posts |
Posted - 05/02/2015 : 14:35:07
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I've been doing the same experiment, also with a GMC-320+, trying to activate indium, dysprosium, and manganese. I finally got it to work on the manganese, but it took a much stronger alpha source than I had initially counted on. I started with a 0.9 uCi smoke detector americium source like you did, but got nowhere. I only succeeded when I tried using a 5 millicurie source of polonium-210 in the form of the Nucleospot static eliminator, which is legally available on a general license and can be bought by anyone. It costs about $230. Even then, I only barely recorded readings that are statistically significantly above background.
I taped a piece of beryllium onto the Po-210 source and stacked up a bunch of 14 x 7 x 2.5 cm polyethylene blocks as follows: one layer below the source, one layer stacked around it, two layers between the source and 110 g of MnO2 in small plastic baggies, and one more loosely stacked layer on top of the baggies.
All natural manganese is stable Mn-55, while the activation product Mn-56 has a half-life of 155 minutes and decays to Fe-56 with the emission of an 846 keV gamma ray which is easy to detect. The MnO2 should be irradiated for at least 5 half-lives or about 13 hours, longer being better. After that time had passed, I took it out of the stack, taped the baggies to the underside of the GMC-320 where the GM tube is, and let it collect data for about a day.
Then I downloaded the data in CSV format and looked at it in Excel. I compared the average count for minutes 0-150 (just under one half-life) to the average of everything from minute 600 (3.87 half-lives) on, which should be near background and therefore lower than the 150-minute average). Each of three times I've done this, I got about 0.6 CPM more in the first 150 minutes than the background count after minute 600. The p-values ranged from 0.014 to 0.058; when combined the probability of having all three results come out at p<.06 is 0.0002, so I'm convinced I got it to work.
No such luck so far with a 12 x 6 cm piece of indium foil or a 10 g chunk of dysprosium, though. I'm trying different moderator configurations and shielding the GMC-320 and sample with lead as I do my count. It's currently recording data on indium now. |
Edited by - Stibnut on 05/02/2015 14:36:49 |
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Making neutrons and detecting them with the 320+
