Using brass/copper or other materials for radiation shielding instead of lead

[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.

[CatalinaWOW]

You might also want to think about where your background radiation is coming from.  Cosmic rays will be almost entirely from overhead, being effectively shielded by the bulk of the earth. High altitude sources will have more (less atmospheric shielding).   If you live near sources of uranium or thorium your background could have some fairly obvious directionality.  Your shield might be different types and thickness for different directions to reflect the energy spectra and direction of the various sources.

[Berni]

Lead is mostly so good because it is so dense while also being so cheap and easy to form.

Even just rock or concrete works reasonably fairly well too, or lead shot can be mixed in to make it even more dense.

Indeed the source of background radiation is an important thing to consider. It could be cosmic rays shooting down from above, it could be that you are sitting on top of a radioactive rock deposit, you could have radon gas seeping up from the ground, or you could just have a old clock with radioactive glowing dials sitting near by.

Also when it comes to background shielding you have to consider the radioactivity of the shielding material itself. For example they reclaim old steel (made before the first nuclear tests) as a valuable low radioactivity metal for building scientific equipment. It has much less radioactive impurities in it.

[Kleinstein]

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.

Bismuth is a rather odd element to include. It is rather expensive and somewhat radioactive. It is questionable if it is less toxic than lead - mainly less common. So I see no practical use at all.
The main alternative heavy element is tungsten, though a bit hard to machine and also not cheap.

Lead has the desity advantage and in addition, especially for the lower energies also a higher effect per mass. For some 100 keV 1 kg of lead may repalce some 5 kg of iron.
Also at high energies there is also a slight advantage per mass, though not that much.
Compared to rock and concrete the background of lead is relative good. For shielding a detector / sensitive experiment the radioactive contamination in the shield also becomes a factor. So contrete is limited.

The background radiation is a mix of both high and low energies. The lower engergies come from more local radioactivity and also conversions of cosmic radiation (e.g. myons or pair generation from high energy gamma).  It depends how relevant the energies are - some detectors can discriminate in the energy and thus largely ignore parts of the background.

[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.

Bismuth is a rather odd element to include. It is rather expensive and somewhat radioactive. It is questionable if it is less toxic than lead - mainly less common. So I see no practical use at all.
The main alternative heavy element is tungsten, though a bit hard to machine and also not cheap.

Lead has the desity advantage and in addition, especially for the lower energies also a higher effect per mass. For some 100 keV 1 kg of lead may repalce some 5 kg of iron.
Also at high energies there is also a slight advantage per mass, though not that much.
Compared to rock and concrete the background of lead is relative good. For shielding a detector / sensitive experiment the radioactive contamination in the shield also becomes a factor. So contrete is limited.

The background radiation is a mix of both high and low energies. The lower engergies come from more local radioactivity and also conversions of cosmic radiation (e.g. myons or pair generation from high energy gamma).  It depends how relevant the energies are - some detectors can discriminate in the energy and thus largely ignore parts of the background.

https://www.nuclear-shields.com/radiation-shielding/bismuth.html
I wasn't the creator of the idea, I've seen it in limited use elsewhere too. Though something to hold a "hot" item is a different regime than trying to achieve a low background, so if the natural isotope blend has some long-lived decaying isotopes, then yeah, it may not be suited.

And I agree that any heavy metals should have at least some care taken. But with the prevalence of bismuth subsalicylate in Pepto bismol and such, I'd expect at least some alarm bells at this point if it had toxicity that wasn't extremely low level.

Edit:I'll also add that for making a specific shape at home, sand casting, or casting into plaster of Paris (that has been baked out sufficiently) is viable, combined with the sub 300C melting point being pretty attainable also. So casting whatever shield shape you please is viable.

[Kleinstein]

If you don't like lead, maybe use mercury  .

[T3sl4co1l]

If you don't like lead, maybe use mercury  .

Even easier to form.

Tim

[jmelson]

You might also want to think about where your background radiation is coming from.  Cosmic rays will be almost entirely from overhead, being effectively shielded by the bulk of the earth. High altitude sources will have more (less atmospheric shielding).   If you live near sources of uranium or thorium your background could have some fairly obvious directionality.  Your shield might be different types and thickness for different directions to reflect the energy spectra and direction of the various sources.

Yup.  And, it is fairly impractical to shield from cosmic rays.  The energies are insane, although most of them collide with a molecule in the atmosphere first, and produce showers of lesser-energy particles at lower altitudes.  If you want to completey shield against them, you have to go down into a mine.  The usual technique is a fast scintillator above your detector that produces a veto of the main detector.
Jon