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| Using brass/copper or other materials for radiation shielding instead of lead |
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| daqq:
Hi, I'm playing around with some detectors and I want to make a shielded enclosure - nothing to go into a nuclear reactor, just to limit the background radiation. I've got a lot of brass and access to a good machinist who works with the stuff. With a density of 8.7g/cm3 it's about 3/4 of the density of lead. As I understand it ( and as has been shown in this paper https://www.eichrom.com/wp-content/uploads/2018/02/Gamma-Ray-Attenuation-White-Paper-by-D-M-rev-6-1-002.pdf ), this should mean that, while I shall have to use more than I'd use lead, I won't have to use that much more. Looking at table 1, to shield out some 95% radiation at high energies, working for the data for copper, I would have to use some 60mm wall thickness vs some 36mm for lead. More than I expected for both of them. But this is the data for a Co-60 source, which peaks at over 1MeV, background radiation is mostly present in the lower energies, so I should be able to get away with a smaller wall thickness for background shielding. Looking at the graph around figure 6, to when interested in mostly the background below some 500keV, where the bulk of the background is, I should be able to get away with some 20-25mm of brass to shield out ~95% of that (more effective for the lower energy stuff). Is this assumption correct? The whole thing is not for professional or safety use or anything, just playing around, seeing what I can see. Thanks, David |
| Kleinstein:
The heavy elements are more effective both at low (e.g. < 500 kev) and high energy ( > 2 MeV). For the low energies lead can be significant (e.g. > 3 x) more effective than copper or Iron and one may want more than 95% shielding. For the low energies one may be good enough anyway if the thickness is choosen also for absorbtion at higher energies. It still depends on which energy range is important / which detector is used. For a relativly small volume to shield a thicker shield also means more area. So high density can really help. If really going for low background the radioactive radiation from the shielding material itself can also be an issue. Lead is not bad, but also not ideal - for the really high end they look for old lead from the roman times, because it is less radioactive. Normally lead is reasonable available as a roofing material (e.g. 1-2 mm thick sheet). |
| Someone:
take brass to metal merchant/recycler and swap for lead? (and some cadmium if they are spicy enough to deal in it) |
| jwet:
The main gammas in CO-60 are about 1.2 MeV which brass would do well in attenuating. If Co-60 is your actual source term, then this would be good. Co-60 also has a compton edge around 200 KeV and a 75 keV X-Ray that are about half the rate but would be less well alttnuated by a lower Z material over lead. Brass will likely have a lower background level itself- a lot of lead has been crapped up over the years. The real shielding formula is the integral of an exponential e to the minus mu * x, where mu is kind of a cross section that is a function of energy. Its pretty standard Nuc Engr. course kind of calculation. The lazy man's way is half value layers at a spot energy. If you refer to a reference you can get a graph of mu's at different energies. Superposition applies. Lead is especially good at attenuating soft X-rays. It might be worth getting 1/4 of inch or so of roofing lead followed by your brass. I would experiment with brass blocks before you get your machinist involved in making something beautiful. PM me if you want to discuss further. I did these calcs routinely early in my career and can probably dig out some old references to help further. Any university library that has a Nuc Engr major would have what you need. |
| ConKbot:
Bismuth? Low enough in melting point to do some mucking around on in the shop. Most the density of lead, not much of the toxicity. |
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