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A few thoughts on the safe handling of radioactive rock specimens

Last Updated: 17th Apr 2010

By Antony Glauser

Summary

A lot of rockhounds and enthusiasts clearly appreciate the hazards of handling radioactive specimens well enough, but there may be some who investigate properly only after they've started collecting, and understandably (although needlessly) get a bit concerned. My intention with this short discussion is to highlight some key issues, and hopefully to encourage people to read up on the topic and inform themselves.


Everything is radioactive to some extent, and minerals are no exception. Naturally occurring isotopes of potassium, thorium and uranium are three of the most common contributors, but in the majority of specimens they are present in such low levels as to provide no noticeable increase above background. Occasionally something will turn up that is a bit 'hotter' than normal, though. For this reason, a Geiger counter is a useful addition to any collector's tool kit. It's worth investing some time in understanding the distinctions between alpha, beta and gamma radiation, and the ability of your counter to detect and report them. Most simple devices can only really give a comparative indication of the radiation level, even if the readout is quoted in microsieverts or millirems per hour [1]. For true dosimetry, you need something like an ion chamber or a spectrometer, and these instruments cost quite a lot more.

If you do decide to hold a radioactive specimen in your collection, there are a couple of things to know, besides the basic time-distance-shielding rules, and the importance of good general hygiene [2, 3]. Firstly, it’s not really the penetrating gamma radiation you need to worry about most, unless you’re planning on building a particularly large stockpile. A hundred grams or so of typical uranium minerals will give a gamma dose rate in the region of 10 microsieverts per hour a few inches from the surface of their container, taking into account self-shielding and absorption in the container wall [4]. This isn’t great – you shouldn’t sit with them in your lap for hours on end, or put them under your bed – but your instantaneous whole body dose rate is likely to be comparable with what you might get sitting on board a jet airliner, due to cosmic radiation.

A useful number here is 20 millisieverts (20,000 microsieverts). If you accumulate more than this total per year, you’re going over a limit that was set down to protect workers from the worry that any illness they may find themselves suffering from might have been caused by their occupational exposure to radioactivity [5]. In practice, you have to get more than 50 millisieverts per year (or 100 millisieverts per year, depending on which study you read) before there’s statistical evidence for increased cancer rates, and even then it's pretty hard to pick out from the background [6]. But that’s certainly no justification for allowing yourself to be exposed to 49 millisieverts, or even 49 microsieverts. Until the theory of radiation hormesis is proven (or otherwise), the official and safest line is that no dose is a good dose [7].

The second thing to be aware of is that two risks from radioactive rocks you should be more concerned about manifest themselves more like chemical than physical hazards: slow-acting poison gas, and toxic dust. It is sobering to realize that the majority of the gamma radiation emitted by a uranium mineral comes from the decay products of the fraction of radon gas that manages to remain within it [8]. The rest of the radon escapes into the air around the mineral and diffuses relatively freely, being chemically inert. It doesn’t stay in this mobile state for long, though; after the next decay (4 days later, on average) it becomes a sticky polonium ion, looking for something to bond to. Whatever it finds – hopefully not the lining of your lungs – is subjected to a protracted bombardment of alpha, beta and gamma rays, lasting 20 years or more. The alpha radiation is especially damaging to soft tissue, each decay depositing a comparatively large amount of energy over a small distance. But alphas are only a problem inside your body, where they can get close to living cells; outside, the layer of dead cells covering your skin gives you ample protection.

To quantify just how harmful radon is, it is useful to start with a figure produced by UNSCEAR (the United Nations Scientific Committee on the Effects of Atomic Radiation). Their best estimate for the effective dose rate from exposure to radon is 9 nanosieverts per hour for every becquerel of radon activity per meter cubed. (1 becquerel = 1 decay per second) [9]. In other words, standing in a room where there are a thousand radon decays per second per meter cubed would give you an effective dose rate of 9 microsieverts per hour. That sounds worrying, but not devastating. Let’s estimate the actual radon concentration around your hypothetical radioactive rock collection. 100g of high grade pitchblende, with say 50% inert matrix, contains 44g of uranium, which decays at a rate of just over half a million atoms per second. So do all of the successive steps in the decay chain, including radon, if the material is in secular equilibrium. The biggest uncertainty now is the emanation factor – the proportion of radon that escapes before decaying. Let’s be cautious and say 25%. (You might want to search the web for a better figure; this one’s for soil). That means there will be over a hundred thousand radon decays per second in the air around the minerals. How big is your hobby room? Three meters by two meters by two meters would give a volume of around twelve meters cubed, and hence over ten thousand decays per meter cubed (fifty times the HPA action limit). The UNSCEAR conversion factor translates this into a dose rate of 90 microsieverts per hour, or about 500 times your normal background dose rate [10]. This is appreciably more than the likely whole body gamma dose rate from the same rocks.

Note that radon isn't easy to detect by waving a Geiger counter around; even at problematic levels, the instantaneous activity in the air may be low. The damage comes from the decay products, which accumulate with time. As a rule of thumb, a good way to test whether you've got radon around is to check the air filter on your tumble dryer, your cooker vent or anything else that collects dust from the air regularly. If the alpha count rate goes up to more than a few times normal background, it may be worth investigating further. Another thing to keep in mind is that radon diffuses fairly easily through certain materials, including plastics. It takes a good millimetre or two of polythene or PVC to stop it, so a typical specimen box (even if glued shut) may not do the job; a plastic bag certainly won't [11]. Metal and glass work much better, although some care is needed to ensure the joints don't leak.

I haven’t mentioned much about the risks of dust, but from the above discussion it should be fairly clear that you don’t want to get this in your lungs either. (Some of the more common mica-type minerals such as torbernite and autunite shed very fine flakes when disturbed, which may then become airborne). Ingestion should be avoided for the same reasons, although the material will leave your body rather sooner. Likewise, be careful to avoid getting material trapped in cuts, or under the nails – best to wear gloves when handling specimens.

Besides alpha and gamma, there is also beta radiation to consider. Being medium-range and moderately penetrating, it will affect your hands in particular. The extremities are deemed to be proportionately less sensitive to radiation than other parts of the body, but this isn't a good reason to be less careful with them; the use of tongs or thick rubber gloves will provide valuable protection. (What the gloves absorb, your fingers won't. You can measure the difference using a Geiger counter with a mica window, but be sure to block the alphas with a thin sheet of paper or plastic food wrap first, otherwise you will overestimate the benefit). It is not easy to quote a representative figure for the beta dose rate at the surface of a typical uranium mineral, because it depends so much on the composition of the specimen, and the distribution of material within it: only particles emitted within the outer millimetre or so of the sample will escape and contribute to your dose. However, those that do will deposit most of their energy in your skin. As a general indication, a webpage from the IAEA states that the contact dose rate for a depleted uranium object is 2 millisieverts per hour, 'primarily from beta particle decay from DU progeny' [12].

If this all sounds a bit scary, it isn’t meant to be; bear in mind the doses we’re talking about. An hour in your radon filled room is still only a small fraction of your annual ‘allowance’. The important point is to know where the risks come from, how big they are relative to one another, and how to avoid them. Keep your exposure to radiation to a minimum, but don’t be unduly frightened of it. Ventilate your workspace. Open any sealed containers outside and let them breathe for a while before bringing them back in. Preferably keep rocks outside of your main living space, and use more than just a thin plastic bag to keep the radon in. And as with any risk, make sure you understand it before you expose yourself to it – if only for your own peace of mind.

References:

[1] Unit conversions: 1 millirem = 10 microsieverts.
[2] For an excellent and comprehensive guide to radioactive mineral handling and storage by Alysson Rowan, see http://www.nexus08.clara.co.uk/article.RadioactiveMineralSpecimens.A4.pdf
[3] Example Q&A on shielding at the HPS website: http://www.hps.org/publicinformation/ate/q4182.html
[4] To estimate the dose rate near to a sample, see http://www.wise-uranium.org/rdcx.html (be sure to select a representative material to get the full emission spectrum, i.e. 'U_nat++').
[5] See http://hps.org/publicinformation/ate/q8900.html for a discussion. The limit for public exposure from the specified work activity is much lower (1 mSv), presumably on the basis that there is no corresponding 'desirable human action' to offset the risk taken, however minimal it might be.
[6] The difficulty with drawing clear conclusions is that the most alarming side effect of long-term radiation exposure is sadly pretty common. It's hard to spot an increase in cancer rates amongst a given sub-group with a known exposure when so many people get it anyway. The BEIR VII report estimated that the increased cancer risk associated with a 10 microsievert dose is about one in a million, roughly equivalent to smoking a couple of cigarettes. This would be a pretty comforting figure, if we weren't all so convinced that we'll win the lottery each time we play it. For a summary of the BEIR VII report, see http://www.nap.edu/nap-cgi/report.cgi?record_id=11340&type=pdfxsum
[7] For a balanced introduction to hormesis from Wikipedia (many pages have a noticeable 'angle' on the topic), see http://en.wikipedia.org/wiki/Radiation_hormesis
[8] A handy page on the uranium decay chain, complete with decay energies, can be found at http://www.wise-uranium.org/rup.html
[9] For a review of the radon dose equivalent relationship, see http://www.radpro.com/Chen-2.pdf
[10] My simple back-of-the-envelope calculation comes out a little higher than the WISE calculator estimate: http://www.wise-uranium.org/rdcrn.html
[11] http://www.aarst.org/proceedings/2008/15-RADON_DIFFUSION_COEFFICIENT-A_MATERIAL_PROPERTY_DETERMING_THE_APPLICABILITY_OF_WATERPROOF_MEMBRANES_AS_RADON_BARRIERS.pdf
[12] See http://www.iaea.org/NewsCenter/Features/DU/du_qaa.shtml




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