Myth or Fact? – Will Alcohol Help Protect Against Radiation?

Dear Dr. Zoomie – I was watching a movie about a Russian submarine that had a reactor accident. In the movie they told the crew to drink vodka to help protect against the radiation. Is this BS or is it real?

I saw the movie you’re talking about – I think it was called K-19, the Widow-maker. Not a bad movie, and the incident it refers to actually happened. A Soviet nuclear submarine had catastrophic reactor accident not far off the American coast. The movie took some liberties with the science and engineering of course, but it was a fun one to watch.

This picture is said to be the photo the U.S. NAVY had of the K-19

A U.S. NAVY photo of the actual K-19 submarine of the 658 class of submarines

As far as your question goes, there’s a specific way to look at it and more general way. Let me try to tackle both.

The specific question is “Will alcohol help protect against radiation damage, or will it just keep me from caring as much?”

The answer is a little of both – but more of the latter. But let’s backtrack a little bit and talk about how radiation harms the body. Then we can see how alcohol (and other measures) can – or cannot – help.

When radiation interacts with our cells it can do one of two things – it can strip an electron off of an atom, creating a pair of charged particles (the negatively charged electron and the remaining positively charged atom, now called an ion) that we call an ion pair. This ion pair can recombine, or it can go on to cause chemical changes in the cell that lead to the formation of active (and potentially damaging) molecules called free radicals. These free radicals can then go on to damage the DNA. Alternately, the radiation might hit the DNA directly, causing one of the two strands of DNA to break. Our bodies have ways to repair both of these types of damage – there are many sources of free radicals besides radiation, and DNA is under attack all the time. But there’s only so much capacity for our DNA repair mechanisms – eventually they’ll be overwhelmed and damage will start to accumulate. And if too much damage accumulates too rapidly then we start to see the ill effects – we might have a higher risk of cancer over the next few decades, or if we’re really blasted with a huge dose of radiation (as happened on the Soviet sub) we can see changes in our blood cell counts, we can experience skin burns, develop radiation sickness, or even die.

Now – before you worry too much, it takes a LOT of radiation to get to the point of seeing physical damage – a minimum of 25 rem in a short period of time to see changes in blood cell counts, over 100 rem to start to feel ill, and over 400 rem before we really start worrying about death. So these are things that CAN happen – but they usually don’t. But let’s get back to the main question: If I’m blasted with high levels of radiation exposure, will a glass of vodka save my life? The answer here is no. Having said that, there is some science that alcohol can help to scavenge free radicals. But it’s not a strong scavenger and if we’re exposed to high levels of radiation – high enough to cause these short-term health risks – we’re way beyond the free radical-scavenging stage. At this point, there is so much radiation that’s causing so much DNA damage that a couple of shots of vodka just won’t help out at all. Sorry.

Vodka won't help with radiation

Vodka won’t help to protect against radiation, sorry.

OK – so that’s the specific question. But what about the more general one? And I guess I haven’t stated this yet – the general question is “Are there any drugs or treatments that can help reduce the effects of radiation exposure?” And here the answer is a qualified “yes.”

Remember – the first physical symptom that shows up is a drop in blood cell counts. This is because the blood-forming organs (for reasons too involved to get into here) are among the most sensitive in the body to the effects of radiation. So as radiation dose increases, we see faster and more precipitous drops in red and white blood cell counts. This can leave a person weak and susceptible to disease. So if we can build up – or find a way to stabilize – the blood-forming organs then we can help to avert this problem. Interestingly, cancer cells have many of the same characteristics of the cells in our blood-forming organs – in fact, cancer therapy takes advantage of these vulnerabilities, which is one reason that so many cancer therapies make us feel so bad. In order to help mitigate these effects, a number of drugs have been developed, and they also show promise in helping to protect the blood-forming organs from radiation as well as from the effects of radiation. So administering these drugs might well help to protect us from the effects of radiation. That being said, this is still an area of research and clinical trials – the drugs are approved for cancer patients but not yet (to the best of my knowledge) for radiation victims. Also, these drugs are only useful to help protect against short-term effects from blood cell loss – they don’t mitigate radiation sickness (the nausea and vomiting), skin burns, and so forth. But it’s a good start!

One other thing we worry about is that inhaling or ingesting radioactivity might cause harm from internal exposure. The worry here is that the internalized radioactivity will continue irradiating us from the inside out for days, weeks – up to decades if the radioactivity lodges in our bones or in an organ where it might reside for long periods of time. Americium, for example, will go into the bone and will remain there for the rest of your life, irradiating your bone, bone marrow, and surrounding tissues. But even shorter-lived radionuclides (I-131 for example) can still cause problems if they come to rest in sufficient quantities in a sensitive organ. This is why we worry about iodine – it’s easily absorbed by the thyroid, a radio-sensitive organ. Luckily, protections here are somewhat more advanced.

First, there’s iodine. What we can do here is to saturate the thyroid with non-radioactive (stable) iodine so there’s no place for the radioactive I-131 to find a home. So we can take potassium iodide (KI). There’s iodine in iodized salt, in seafood, and in some forms of water treatment tablets also, but the quantity is variable and uncertain. Plus, simply chugging salt (or salt water) can have dangerous health effects so we don’t want to do that. But if (and only if) somebody’s about to be exposed to radioactive iodine then taking stable iodine can certainly help to protect you. Taking iodine after the fact can help too, but it’s got to be within a few hours of exposure.

Another protective agent in the news is Prussian blue – an industrial dye that also happens to scavenge cesium from the body. So if a terrorist sets off a “dirty bomb” that uses cesium-137 (Cs-137), taking Prussian blue can help reduce your exposure by speeding the excretion of the radionuclide. A word of caution – it turns your stools blue, so you’ll have Smurf-poo while you’re taking it. But that’s a small price to pay, I would think!

An infographic detailing how the medical treatment prussian blue works. Provided by the Centers for Disease Control and Prevention. More information can be found at https://emergency.cdc.gov/radiation

An infographic detailing how the medical treatment prussian blue works. Provided by the U.S. Centers for Disease Control and Prevention.

And then there are a bevy of other of what are called “decorporation agents” for other radionuclides. If you’re exposed to americium, for example, a doctor might infuse you with a compound called DTPA; and there are a number of compounds that help to reduce the absorption of strontium by the bone (aluminum hydroxide, for example, or barium sulfate). But many of these drugs have never been approved for use in humans and some, in fact, might not have even been administered to humans so their effectiveness and safety have not yet been demonstrated. They’re certainly not FDA-approved! So – yes – there are drugs that, in theory, can help reduce the health effects of exposure to radioactive materials. But in practice, while we’re developing and testing new ones all the time, it’s more hit-or-miss in this category. The reason for this is that every element behaves differently, chemically and when it’s in the boy, so each one requires a different protective compound. We simply haven’t had the time, or the money, to develop protective drugs against every element out there so we’ve started with the most important and the easiest.

OK – so – getting back to your original question! We do have a number of treatments for exposure to radiation or for the ingestion of inhalation of radioactivity. Sadly, vodka is not one of them – it might make you feel better, but only until the hangover hits.

What Should Be in a Response Plan for Skin Contamination?

Dear Dr. Zoomie – I’ve got a hot lab and I was told I need to develop a response plan in case someone gets skin contamination. I was thinking soap and water; is there anything else I should include?

Skin contamination doesn’t happen often but if you work with enough radioactivity – especially in liquid form (as in a hot lab) it’s going to happen from time to time. And it doesn’t mean that people are being careless and sloppy (well…sometimes it does, but not always); sometimes we just make mistakes. I’ve had skin contamination myself at least a couple dozen times – mostly just random splatters here and there – and every time it’s cleaned up fairly quickly with soap and water. But let’s back up a little bit and talk about what goes into dealing with skin contamination. Specifically, you need to have a procedure to follow, you need to know how to clean up the contamination (and how to know when you’re done cleaning up), and you have to know how to tell if any follow-up is required. Let’s take these one at a time.

Skin Contamination

Commonly missed areas of the hand during decontamination

Your skin contamination procedure doesn’t have to be very complicated; in fact, it only needs to have a couple of parts. It should:

  1. Define what’s meant by skin contamination. For example, you might define skin contamination as the presence of any contamination above background or you might decide that contamination levels have to exceed a certain limit (100 cpm above background, for example).
  2. Describe the steps to be taken when a person is contaminated. For example:
    1. Contact the Radiation Safety Officer at the earliest opportunity.
    2. Perform a count rate survey over the contaminated area and write down the number of CPM.
    3. Start to clean the contaminated skin in the nearest sink or at the nearest decontamination station.
  3. Discuss cleanup techniques (more on this in a moment).
  4. Determine when to call for outside assistance and/or follow-up (more on this later as well)
  5. Document what’shappened.

Cleaning up contaminate skin isn’t always simple, but it can be. For example, most cases of minor skin contamination can be cleaned up with soap and warm water. In fact, every time I’ve had skin contamination, soap and water has worked for me. It’s also possible to wipe down the contaminated skin with baby wipes or other cleaning-type wipes. Wiping down with a damp rag or sponge will often do the trick as well, and other specific circumstances (or specific compounds that you might be using) might call for more specialized products. No matter how you choose to decontaminate a person there are a few rules of thumb to keep in mind.

  • Don’t do anything painful or uncomfortable. For example,
    • Don’t use hot or cold water – keep it cool to warm.
    • Don’t scrub with harsh substances (e.g. steel wool, scrub pads, wire brushes, etc. – and don’t laugh; I’ve seen all of these used).
    • And above all, don’t do anything that will draw blood. Your skin acts as a pretty good barrier, keeping contamination out of your bloodstream – if you do something that breaks the skin then you’re simply scrubbing contamination into the blood, which is never a good idea.
  • Count for contamination every several washes or wipes. As long as contamination levels are dropping then whatever you’re doing is working and you should keep doing it. If contamination levels stop dropping then what you’re doing is no longer working and it might be time to try something else.

You also have to understand when it’s time to follow-up or call for help. And “call for help” is not necessarily as dramatic as it might sound – that can simply mean calling a consultant. There are a number of possibilities in this category – here are a few of the more common.

  • For example, you might consider performing a thyroid count for every case of skin contamination with radioactive iodine, or performing urine bioassay if skin contamination exceeds, say, 10,000 cpm (you’ll have to determine what these “trigger” levels are for the nuclides you’re using).
  • You might also consider contacting a consultant to determine the possibility of uptake and/or to calculate skin dose if skin contamination exceeds a given count rate.
  • You (or, more likely, your consultant) might have to calculate radiation dose to the skin if contamination levels are sufficiently high. Duke University has an online calculator to help you with this. For more complicated cases, you can also use a program called Varskin, which is quite possibly the best software available for this purpose.
  • You’ll also have to contact your regulators if you (or a consultant) determine that a person has exceeded 50 rem to the skin.

Finally, you’re going to have to document what happened and how you responded to it. Start with a short description of the circumstances causing the skin contamination (e.g. liquid splashed on the skin) and write down the number of counts you measured as well as the instrument used for the measurement (e.g. 45,000 cpm measured with a GM pancake probe). You should also briefly describe the decontamination procedure (e.g. washed with soap and water for five minutes) and the results (e.g. contamination reduced to less than 100 cpm above background). And note any follow-up measures or samples that were taken (e.g. performed urinalysis to check for uptake) and the results.


Finally, let’s put all of this in perspective. There are times that skin contamination can be damaging – the skin can be harmed from a sufficiently high radiation dose. But most of the time skin contamination is more of a nuisance than a risk. You have to take it seriously; you need to decontaminate the affected area, you need to try to get an accurate count rate to see how bad it might be, you need to document everything, and you need to know when to call for someone to give you a hand. But you don’t have to panic! Take a deep breath, break out your procedures and your supplies, and work methodically and you (and the person who was contaminated) should be OK!

My Wife Is Pregnant – Will an X-ray Hurt the Baby?

Dear Dr. Zoomie – my wife is pregnant and she needs to have an x-ray. Is this going to hurt the baby? Or should she wait until after she delivers? How does radiation affect pregnancy?

My kids look perfectly normal – in my humble opinion maybe even a tad better than normal. This became an issue, actually, in the months following the 2002 arrest of Jose Padilla on charges he was plotting to set off a “dirty bomb.” How it became an issue is that I was interviewed by a reporter interested in the reproductive effects of radiation – she was wondering if we could expect to see legions of children born with birth defects in the aftermath of a radiological attack. I spent a fair amount of time helping her to understand the basic science behind why this was unlikely to happen and then, to lighten the conversation a tad, threw in the line “let’s face it – if parents have strange-looking kids they should probably blame the in-laws and not the radiation.” Guess what quote she used. For a few weeks I was getting e-mails from colleagues around the world asking to see photos of my kids. And I’m happy to say that in spite of my years working around radiation, my kids look perfectly normal. At least as close to normal as we can expect from teens….

The point here is that the reproductive effects of radiation are exaggerated to the point of irrationality – more so than most other reproductive hazards. True – radiation can cause birth defects and it has been shown to induce mutations in animals. But the amount of radiation required to cause birth defects in humans is substantial (at least 5 rem or 50 mSv to the fetus) and the medical literature has not noted a single instance in which pre-conception radiation exposure to humans has caused birth defects when the woman eventually conceives. And if more people – physicians included – really understood these points there would be far fewer worries.

Consider – the BBC documentary Nuclear Nightmares (which was about radiation phobia) stated that the Soviet government performed a few hundred thousand abortions on women exposed to radiation after the accident and others have stated that there were at least 100,000 abortions conducted in Europe due to fears about the reproductive effects of radiation exposure. It is almost certain that few – if any – of these abortions could have been justified by the radiation exposure alone. I understand that the numbers cited are not from the peer-reviewed literature and that they might be high. But the 2006 report by the World Health Organization concluded that after 20 years there had been fewer than 100 deaths attributable to radiation exposure from the accident (including radiation-induced cancers) and projected that as many as 10,000 people might eventually develop cancer from the accident – even if the WHO’s worst-case estimates come to pass and even if the abortion numbers are over-stated by a factor of 10 we will still find that fear, ignorance, and misinformation was deadlier than the accident itself. This is tragic.

As a radiation safety officer I calculated nearly 100 fetal dose estimates, usually when a pregnant woman was involved in a car crash and, while unconscious, received the “trauma series” of x-rays from head to foot, possibly followed by CT or even fluoroscopy. Sometimes when the woman woke up she told the doctor she was pregnant, sometimes she didn’t know this herself for another few weeks. In either case, our policy was that I was to be informed so that I could perform fetal radiation dose calculations and write a letter explaining the results to the woman’s OB/GYN. There was not a single case in which the fetal dose estimate was high enough to warrant taking any actions at all, even though some of the women had been advised they might need to terminate their pregnancies. And I was not alone in this – the Health Physics Society runs a wonderful feature on their website (Ask the Experts) that has a section for radiation and pregnancy. Over the last decade or so they have accumulated hundreds of inquiries on this topic and almost none of them warranted any concerns at all. Sadly, many physicians in the US are taught that radiation can cause problems with pregnancy, some of them might vaguely remember a dose of 5 or 10 rem (50-100 mSv) but don’t know the fetal radiation dose from the radiation they might prescribe, and are then told little more. Is it any wonder they sometimes give bad advice?

For the record, the Centers for Disease Control and Prevention maintains a web page that includes information on the impact of prenatal radiation exposure aimed at parents and at physicians. CDC includes a table that summarizes the impact of prenatal radiation exposure based on the post-conception age and the fetal radiation dose – they conclude that for any radiation exposure that occurs less than 2 weeks into the pregnancy and for any fetal radiation exposure of less than 5 rem (50 mSv) there is no need to take any actions at all. To put this number in perspective, it can take tens of x-rays or a few CT scans that image the uterus (the exact number depends on the x-ray machine being used, the amount of tissue between the x-ray beam and the fetus, and a number of other factors) to reach this level of fetal exposure. And for x-ray exposures that do not image the uterus – a chest or head x-ray for example – the dose is even smaller. But believe it or not, I even took a call from a woman who had dental x-rays wondering if she should take her physician’s advice to have a therapeutic abortion.

Having said all of this I don’t want to make it sound as though I’m advocating throwing caution to the winds – according to the ALARA principle (to keep radiation exposure As Low As Reasonably Achievable) we should not simply run up the dose through unnecessary medical imaging – I agree with the goals of the Image Gently initiative to help reduce pediatric (and prenatal) radiation exposure. But I would suggest that if the mother’s health or life are at stake then physicians should avail themselves of the tools they have without letting unwarranted fears deny them access to valuable diagnostic information. And the physicians need to remember that – before giving any medical advice about the pregnancy – fetal radiation dose should be calculated by a qualified and competent health physicist or medical physicist. Radiation health effects depend on the radiation dose – absent a solid radiation dose estimate it simply is not possible to give good, informed advice to the prospective parents.

The sad fact is that the programs that train our physicians – not just in the US by the way, remember the numbers from Europe – are not doing a good job of teaching their students about the impact of radiation on their patients. I discussed this in an earlier blog, where you can find references on this point. This is ironic given that, according to the National Council on Radiation Protection and Measurements, our exposure to medical radiation had increased dramatically in the last few decades. Given our society’s heavy reliance on radiation in industry, medicine, research as well as our dependence on nuclear power I would like to think that our physicians can be better prepared to give good advice to their patients about the effects of the radiation to which they are unavoidably exposed, just as I would like to think that the public can be provided with solid information so that they can participate more fully in the process of making decisions about radiation exposure.

To wrap this all up – and bring us back to your original question – a single x-ray, even a single CT scan, doesn’t give enough radiation dose to the developing baby to cause birth defects. Remember, too, that there’s a reason the doctor is prescribing an x-ray for your wife – he needs diagnostic information to help keep her healthy. There is a risk to your wife – and your developing child – from not having this information. Since your baby’s health depends on that of your wife, there’s a risk in NOT getting this diagnostic information. Chances are that the risk from not having an x-ray or CT scan (due to lack of diagnostic information) is higher than the risk from the x-ray images. In other words, if the doctor feels that an x-ray is needed then it makes sense to follow the doctor’s advice.

Mixed Oxide (MOX) Explained

Dear Dr. Zoomie – every now and again there’s something in the news about something called MOX. Some people seem to like it and it seems to make others unhappy. Can you tell me what the story is?

This is one of those issues that, like you, most people don’t know much about – in fact, most of the population probably doesn’t know anything about it at all. Those who do follow the controversy tend to come down heavily in favor or vehemently against – not many are on the fence. With that as a bit of a prelude, here’s what the facts are – I’ll let you decide how you feel about the issue.

To start, MOX stands for Mixed Oxide. What this refers to is the fact that MOX fuel contains both uranium and plutonium mixed together with each metal in a chemical form called an oxide (rust, for example, is iron oxide). Since the uranium and plutonium are both fissionable, the reactor generates power by fissioning both of these elements instead of just the uranium.


Mixed oxide, or MOX fuel, is a mix of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.

This plutonium can come from one of two sources. One possibility is that plutonium from former nuclear weapons – or from plutonium stockpiles – can be mixed in with standard uranium fuel. Alternately, plutonium can be extracted from spent reactor fuel (more on this in a moment) and mixed in with fissionable uranium to form reactor fuel. In either case, this is sort of like blending ethanol in with gasoline – they both burn so you can mix them together and still fuel your car.


As for where plutonium is created…well, this gets interesting. Whether the plutonium in the MOX fuel comes from weapons or weapons stockpiles, or from spent reactor fuel it was produced in the same way – inside a nuclear reactor. In fact, every operating reactor produces plutonium during its operation. Every nuclear reactor generates power by nuclear fission and the atoms most likely to cause these fissions are the lighter isotope of uranium, U-235. But if reactor fuel is, say, 6% U-235 (at the upper range for commercial reactor fuel) then the other 94% is U-238. The core of a reactor has a lot of neutrons flying around (the neutrons come from fission) and many of those neutrons will be captured by U-238 atoms to form U-239. U-239, in turn, is radioactive and it emits beta radiation to become fissionable plutonium (Pu-239). So normal fission results in the formation of Pu-239 in every single nuclear reactor on the planet. The question is what happens to the Pu-239 after it’s formed.

Some of the Pu-239 fissions as the fuel continues to produce energy. Some of it will capture another neutron to become Pu-240 (which also fissions) or can even capture more neutrons to form heavier forms of plutonium. But not all of the Pu-239 will fission or capture neutrons – a lot of it simply remains in the fuel until it’s removed from the reactor as spent fuel. And this is where some of the controversy starts to arise – since all spent reactor fuel contains Pu-239, a nation (or a sophisticated terrorist group) can chemically process the spent fuel to extract the plutonium. In truth, much of this will be marginally useable for nuclear weapons because of the Pu-240 and heavier isotopes. But the fact remains that it’s still there and it can be extracted by anyone with access to the right chemistry, handling equipment, and the rest of the necessary technology. This is, in fact, one reason that the US gave up reprocessing spent reactor fuel – since the plutonium presents a possible proliferation risk if the fuel is reprocessed, deciding not to reprocess the fuel means that the Pu-239 remains locked up in the fuel rods alongside the highly radioactive fission products, rather than being extracted for possible theft or diversion into nuclear weapons.

Another approach – no matter where the Pu-239 comes from (reactor fuel or nuclear weapons stockpiles) – is to deliberately mix the Pu-239 in with uranium fuel and to use it to produce energy. The thinking here is that, if the Pu-239 is sequestered away inside of spent fuel, it still exists and can be extracted at some time in the future; or if it’s locked in a secure bunker it still exists and can be stolen or formed into weapons by a nation that has decided it’s time to make new weapons again. On the other hand, according to this line of thought, if the Pu-239 is used to make reactor fuel, the actual atoms of Pu-239 are split and can never be fissioned again – if the plutonium is fissioned then it no longer exists; no more than carbon dioxide and water vapor can be reconstituted into the gasoline that they once were. When the Pu-239 is fissioned it is forever gone from this world and it will never again be a proliferation risk.

On the other hand, groups that oppose MOX fuel point out that, until the fuel is inserted into a reactor and fissioned, it poses even more of a security risk. The thinking here is that, normally, Pu-239 is either locked up inside of secure storage vaults or is locked up within dangerously radioactive spent fuel rods (which are, themselves, locked up behind multiple barriers). Either way, the plutonium is fairly secure.

By comparison, those who are opposed to the use of MOX fuel point out that it makes the plutonium much easier to steal. First, it’s already been separated from the dangerous fission products (if the source is spent reactor fuel). Second – and even more important – the fuel itself can be stolen when it’s in transit to the reactor in which it will be used. Typically, the most vulnerable time for any dangerous material is during shipping, when it is outside the normal security barriers and safety systems. No matter how much security is in place, materials in transit are never as secure as when they’re locked up behind multiple barriers in a hardened facility.

So this is the situation – we have (quite literally) tons of plutonium in various places around the world, and every nuclear reactor is producing more all the time. It doesn’t matter whether you are pro- or anti-nuclear power – the fact is that the world we live in has this material in it. That being the case, the question – and the controversy – is what to do with this material. Do we lock it up to achieve maximum security (but the plutonium continues to exist)? Or do we add it to uranium to form MOX so that it can be destroyed forever (but introducing potential vulnerabilities during transit)? There is logic to both positions – the question is whether the ability to gain useful energy, as well as removing the plutonium from the face of the Earth, is worth the added risk of theft.

How Do You Respond to a Radioactive Spill?

Dear Dr. Zoomie – I run a radiation safety program at a small laboratory and I’m working on our emergency response procedures. I was wondering if you can tell me what goes into responding to a radioactive spill.

How Do You Respond to a Radioactive Spill?

How Do You Respond to a Radioactive Spill?

Good question – and a very good topic! I ran an academic radiation safety program for several years and spills were most common type of incident we had; on average, about one spill a week. Most of these were fairly minor – a researcher with a leaky pipette for example, or a little spray from around the cap of a stock vial when you open it up. Of course you can have larger spills too – we had a researcher once drop 100 ml of radioactive solution on the floor, and (in our hospital) had an incontinent patient who urinated on the floor – nearly a liter of radioactive urine. Anyhow, what it means is that you are likely to have spills ranging from nearly invisible to fairly significant, and you have to know how to respond.

The acronym we learned in the Navy was SWIMS – it stands for:

•    Stop the spill
•    Warn other people
•    Isolate the spill area
•    Minimize radiation exposure
•    Stop ventilation if possible and if it will help

Here’s what that means….

Stop the spill doesn’t mean cleaning it up so much as stopping it from getting worse. So you want to pick up (wearing gloves so your hands don’t get contaminated) the bottle or vial or whatever you might have dropped or knocked over (if that’s what happened) and put some absorbent materials over whatever spilled to keep it from spreading further.

Warn other people about the spill. Let the RSO know about it so he or she can send help. Let people in the vicinity know about the spill so that they don’t walk into your spill area (also so that people who might be contaminated – anybody closer than about a meter away – stay put so they don’t spread contamination around). Even if it’s a minor spill you have nothing to lose by warning others – especially Radiation Safety.

Isolate the spill area to keep people from wandering in and getting contaminated. This means putting up physical barriers – rope, tape, even a table across a doorway – something that people have to physically move in order to cross. What you should do is to give yourself enough room to work – at least a meter past the furthest droplet you can see. Once a spill boundary is put up, you shouldn’t let anybody enter the spill area unless they have proper protective equipment (PPE) – at the least, gloves, shoe covers, and a lab coat. And once somebody is inside the spill area, only Radiation Safety should be permitted to survey them out of the area.

Minimizing radiation exposure is not so much a procedural step as a way to approach the incident. Remember – there is nothing life-endangering about a spill and you don’t have to rush in, unthinking, to save the day. Take a moment – give yourself the luxury of thinking about what’s happened and the best way to deal with it. Do you need respiratory protection? Do you have your gloves on? Do you have the right materials on-hand to clean it up? Or do you need to wait until you can get the right materials to clean it up safely? By doing this you’ll be minimizing everybody’s exposure.

Stop ventilation if possible and if desirable – but this isn’t something that needs to be done every time. First – running the ventilation can spread radioactivity through the ventilation system, which can cause problems. In addition, air blowing on a spill – especially if it’s volatile – can cause the activity to go into the air, turning it into an inhalation concern. So if you can turn off the ventilation then it will keep these things from happening. On the other hand, if you don’t know how to turn off the ventilation or if you have to stand in the middle of the spill to do so then you might want to leave it be for the moment. Oh – also, don’t forget to stop ALL the sources of ventilation if you make this decision. This includes, say, refrigerators and freezers (the compressor blows out air), pumps (the motors usually have vents), and even computers or projectors that have vents that might blow onto your spill.

Another quick comment about these actions – SWIMS is a mnemonic to help you remember these steps – they do not have to be done in that order. Just make sure you remember to do them!

Once you’ve got through these actions you’ve earned the right to take a short break – at this point things aren’t getting better, but they’re also not getting any worse. So take a minute to think about what you’ve got and the best way to clean it up and restore the area. Cleaning up is part of this – here’s a little on that.

•    First, work from the outside of the spill area towards the inside, and to work from the top to the bottom (if the spill is, say, on a table or countertop and has dripped onto the floor).

•    Most of the time, commercial cleaners will work just fine; although you might want to use a specialty cleanup product if you have radioactive metals (cobalt, cesium, etc.) in the spill – this is mostly at nuclear power plants, though, and not so much at universities.

•    As you clean, you should put the cleaning materials (paper towels, bench pads, or whatever you’re using) into a plastic bag as you use them. If someone is holding the bag they should be wearing gloves to keep from being contaminated. And every now and again, survey the cleaning materials to make sure your cleanup is having an effect – if you start finding that no contamination is coming off on the paper towels (or whatever you’re using) then either the area is fully decontaminated OR the contamination is fixed to the surface and you should move on to another location.

•    Finally, you’ll have to survey the entire spill area when you think cleanup is completed in order to show that contamination levels are acceptable. Usually this means less than 1000 dpm per 100 square cm, but these vary depending on the radionuclide and the type of radiation it emits. A good reference is a document called RegGuide 18.6 – it’s a Nuclear Regulatory Commission regulatory guidance document that, even after nearly 40 years, is still the standard reference on the issue.

There’s a lot more on this topic than what I’ve got here, but what I’ve got here will get you off to a good start. Hopefully you won’t have to use this often but, if you do, I hope this helps out.

What is Alpha and Gamma Spectroscopy?

Dear Dr. Zoomie – we’ve got to do some environmental sampling and the lab asked if we wanted to have alpha or gamma spectroscopy done. It sounds important, but I’m not sure what they mean or what they’re used for. Can you help me out?

Spectroscopy can be incredibly valuable, but only if you need to use it. We’ll get into that in a minute – first, let’s talk about what it is. So let’s start with the science part and then go to when you might need to use it.

First – radionuclides give off alpha, beta, or gamma radiation. For reasons that are too complicated to get into here, beta radiation isn’t amenable to spectroscopy – but alpha and gamma radiation are.

Second – we can analyze the alpha and gamma radiation to see how much energy the radiation has. Every nuclide gives off radiation with very specific energy. Cesium-137, for example, gives off a gamma ray that has exactly 662 thousand electron volts (662 keV) and cobalt-60 gives off two gamma rays with energies of 1170 and 1330 keV. These are as specific as fingerprints – anytime you see a gamma with an energy of 662 keV you can bet that it comes from Cs-137. One way to think about it is that every alpha- and gamma-emitting nuclide has a distinctive “fingerprint” of radiation where the identifying features are the energies of the emitted radiation. So if we can measure the radiation energies – alpha or gamma – then we can identify exactly what radionuclides are there.

Third – it’s relatively easy (and not terribly expensive – no more than a few tens of thousands of dollars) to perform gamma spectroscopy, but the cheap and easy way of doing it isn’t tremendously precise. To do gamma spectroscopy well costs over $100 K and is best done by a radiochemistry laboratory. Alpha spectroscopy is another kettle of fish entirely – there is (as of now) no quick, easy, or cheap way to do alpha spec – you pretty much have to have a trained chemist and a full-blown laboratory with a few hundred thousand dollars’ worth of equipment.

The graphics here are from some gamma spectra I made in early 2016 – the top one is Cs-137 and the bottom one is Co-60. Looking at these, it’s easy to see how the gamma spectrum can be used for radionuclide ID – you can do the same thing with alpha radiation; I just don’t have the equipment (or the training) to make these measurements in my lab.

Gamma Spectra - Cs-137

Gamma Spectra – Cs-137

Gamma Spectra - Co-60

Gamma Spectra – Co-60

OK – so this is how spectroscopy works; the next question is when you might need to use it. Here are some examples.

Several years ago a rail car filled with scrap metal set off a radiation detector at a steel mill. The gondola car was sent back to its point of origin, which happened to be just a few miles from where I was working at the time. The scrap metal dealer didn’t have the ability to figure out what was causing the alarm – it might have been natural radioactivity or it might have been something manmade; nobody knew how to handle the material until the radioactivity could be identified. Using gamma spectroscopy we were able to identify the source of radioactivity as radium (specifically Ra-226), which made it possible to find a disposal site.

Another example was with one of my consulting clients – they measured elevated levels of radioactivity at a site where they had used thorium in decades past. We had to find out if the elevated radiation levels were due to their operations with thorium or something else. By identifying the specific isotopes present (using both alpha and gamma spectroscopy) we were able to show that my client hadn’t contaminated the site with thorium; the extra radioactivity was naturally occurring and came from some coal-processing that had taken place on the adjacent site nearly a century earlier. This meant that it wasn’t due to any actions of my client and, not only that, wasn’t anything that required remedial action.

There are tons of other examples I could give but they all come down to pretty much the same thing – there are times that you need to know exactly what types of radioactivity you might (or might not) have and the best way to make this ID is through alpha or gamma spectroscopy. If you have to do a lot of it then you might want to do the work yourself, otherwise it makes more sense to send samples off to a commercial lab. One caution – gamma spectroscopy is fairly inexpensive, but alpha spec can cost a few hundred dollars per analysis and a thousand dollars or so to completely analyze a sample. But if the alternative could call for spending hundreds of thousands – or millions – of dollars in cleanup and/or waste disposal, this is money well-spent.

The New Radioactive Source Security Regulations in 10 CFR Part 37

Dear Dr. Z – I’ve been hearing about new source security regulations in 10 CFR part 37 but I’m not quite sure what they mean or how they might affect me – can you fill me in?

Without knowing exactly what sorts of sources you have I can’t give a precise answer to this. But let me tell what I’ve noticed in these regs and how they’ve affected me – hopefully this will help you out with your program.

I was Radiation Safety Officer at a major university and hospital from the late 1990s through the first few years of the oughts and one of my responsibilities was to help assure the safety and security of our radioactive materials. When I took the job (pre-September 11) my major concern was that a disgruntled grad student would try to dump low levels of radioactive materials into a colleagues lunch – this had happened at Brown University, NIH, and a few other places in the previous decades. But in the aftermath of the 9/11 attacks and the subsequent arrest of putative “dirty bomber” Jose Padilla in May, 2002 my worries changed dramatically – instead of theft of relatively small amounts of radioactivity by an amateur, I had to worry about trying to thwart an attack by terrorists or professional thieves; an entirely different kettle of fish.

Most of the time I am in favor of less-prescriptive regulatory guidance – as an experienced radiation safety professional I’d rather come up with my own solutions to, say, keeping radiation exposures as low as reasonably achievable (ALARA, the guiding philosophy of radiation safety in most of the world’s nations). But faced with so sudden and so dramatic a change in paradigm – and when faced with a problem that was outside my expertise in radiation safety – I found myself wishing for more guidance from my regulators. Nobody will be injured by swallowing minor amounts of radioactivity – the worst that had happened before 9/11 – but even if the radioactivity from a dirty bomb attack is not dangerous, the explosion and panic can be deadly (not to mention the cost of a cleanup). I’m a health physicist, not an expert in security or counter-terrorism – what I really wanted from my regulators was advice on how to secure my potentially dangerous sources from a threat that was outside all of my experience.

While the regulators didn’t exactly spring into prompt action, the Nuclear Regulatory Commission did issue a series of orders beginning in 2005 that helped clarify measures that could be taken to help safeguard high-risk radioactive materials. These are about to be collected into a single new regulation, 10 CFR 37, that will put much of this information in one place.

As a former Radiation Safety Officer (RSO) I think it’s great to collect all of the disparate NRC orders into a single new regulation, but I find myself wishing the NRC had gone further, keeping in mind that most radiation safety professionals aren’t security experts. For example, the upcoming rule has tons of verbiage devoted to telling licensees how to establish a program to ensure the trustworthiness and reliability of workers granted access to radioactive materials, but it doesn’t provide any guidance on how to actually secure specific radioactive materials (For example, are normal locks OK, or is keycard access needed to control access to a 1 Ci Cs-137 source?).

One of the keystones of these rules is the requirement that people must be deemed “trustworthy and reliable” before they are permitted to have unescorted access to high-activity radioactive sources, and this is one of the parts that I’m not sure I’d feel comfortable with as an RSO. Part of the T&R program is specified in the rules – requiring fingerprinting and a background check. But part of it is left to the judgment of the T&R officer. The T&R officer is required to have a background check and to be fingerprinted, and the NRC verifies their trustworthiness and reliability. But not much more is written – the T&R officer can be the RSO, the head of security, a manager in Human Resources, or virtually anyone else proposed by the licensee. The reason this gives me pause is that, as an RSO I was our organization’s most knowledgeable radiation safety professional, but security is not my game – I know how to select technically competent staff and how to find a technician who will give me a full day’s work, but I’m not trained in how to evaluate a person as a possible saboteur or terrorist. I’d rather have Security handle this task, but under the new regulation there’s no requirement that security evaluations be performed by security professionals.

The NRC will undoubtedly be developing guidance on how to implement the new regulations – there is a draft out (dated 2010) that I hope will be updated and issued to help licensees figure out how to comply with both the letter and the intent of the new regulations. Absent such guidance we are likely to end up with a hodge-podge of approaches to radioactive source security by RSOs who are professional health physicists, physicians or medical technologists, industrial radiographers, and so forth. It would have been helpful for the NRC to have required participation by a security professional – the head of institutional security for large organizations or a security consultant for smaller ones (there are a number of relatively small businesses that possess dangerously high-activity sources).

OK – so what? We’ve got a new law on the way that should help to consolidate a lot of the existing rules and orders on radioactive materials security, and (hopefully) some guidance on how to implement these new laws. It doesn’t seem to do any harm, and by putting so much in one place it can certainly make things easier for the licensee. What’s not to love?

The biggest thing is that this seems to be a rule written by administrators, for administrators. Don’t get me wrong – the administrative stuff is important! It’s nice to know the standards I should meet in order to decide (and demonstrate) that a person should be allowed to have unescorted access to high-activity radioactive sources. It’s also good to force licensees to have a security plan, to know when to notify law enforcement agencies that something has gone wrong, and so forth. But there’s more to security than getting the paperwork right, and that’s where licensees could use some more help – what would be great would be specific practical guidance.

Say I’m applying for a new radioactive materials license and I am to be RSO at a small facility with just enough radioactive materials to fall under this new rule, but we’re not large enough to have its own security force. I know that I need to secure the sources, but what constitutes adequate security? Is a padlock sufficient, or should I have a full-blown safe or vault? Should I put in motion detectors and, if so, what specifications should they meet? What about cameras (and if so, what kind and how many)? Do I need to have an alarm that automatically sounds at the local police precinct? And so forth and so on…. I need more than a bunch of file folders filled with plans and lists – what I need is a list of vendors and model numbers, or at least a list of minimum acceptable specifications for my cameras and locks. In short, I want a radioactive materials security program that will keep secure my sources against theft in addition to meeting paperwork requirements.

All in all, the new rule is a good start – at the least it makes it easier to figure out where all of the requirements are located. But from a practical standpoint it’s no more than a start – let’s hope that more practical assistance is on its way.

Why Do We Use Highly Enriched Uranium Research Reactors (HEUs)?

Hi, Dr. Zoomie – I’ve read some stories about research reactors that are fueled with weapons-grade uranium, which we now worry about as a proliferation risk. Why in the world would anybody do something like that? Didn’t they think this could be a problem?

Every so often we hear something in the news about nuclear reactors fueled with highly enriched uranium (HEU); usually with regards to nuclear weapons proliferation. Back in the good old days both the US and USSR constructed over a hundred small HEU-fueled reactors and shipped them all over the world – Uruguay had one, there were some in the Balkans, the Ohio State University reactor was fueled with weapons-grade uranium, and there were plenty more. Given today’s concerns about locking up and accounting for every gram of weapons-grade uranium it’s only natural to wonder “What were they thinking?”
Even today there are a number of reactors fueled with uranium that could be turned into nuclear weapons. One category is military reactors – the nuclear reactor on my submarine was fueled with HEU. But outside of the military, the biggest reason to use high enrichments is for research and to produce radioactive materials for research and medicine. Here’s why.

First a little bit of background. Two kinds of radionuclides are produced in nuclear reactors – in one, stable atoms that capture a neutron can become radioactive by a process called neutron activation and the products are called neutron activation products (activation products for short). Cobalt-60 is a neutron activation product, formed when stable cobalt-59 captures a neutron to become radioactive cobalt-60. In the other process, a uranium atoms splits (fissions) and the fission products are radioactive; these include the nuclides we saw in Fukushima (radioactive isotopes of iodine and cesium mainly) as well as molydebenum-99 (the parent nuclide of technetium-99 that is the workhorse of nuclear medicine) and others.

So – to create a neutron activation product you need two things – target atoms (such as cobalt-59) and neutrons; the neutrons come from uranium fission. All things being equal, a larger number of neutrons means a larger amount of radioactive product; a higher flux of neutrons means more rapid production. The way to get a lot of neutrons is to have a lot of fissions – the more fissionable atoms that are crammed into a volume, the more neutrons. A higher uranium enrichment is the best way to cram the highest number of fissionable atoms into a volume and, thus, to increase the neutron flux.

With fission products this rule works double – a larger number of fissionable atoms not only boosts the neutron flux but it also gives a larger number of target atoms to fission. So, again, using more highly enriched uranium produces a higher yield of the desired nuclides. And with nuclides that have a low probability of being produced the only way to make useful quantities is to use more-enriched uranium.

In both cases we end up with the same choice – do we choose the greater profits from HEU or the lower risks of LEU? An LEU-fueled reactor can do everything that an HEU-fueled one can – just more slowly.

At the moment it looks as though the nation is moving towards security rather than production. Unfortunately for the nuclear medicine industry this coincides with the shutdown of some Canadian reactors that produced medical isotopes, causing some shortages in our supplies of medical nuclides.

With medical science using radionuclides in ever-increasing amounts, this places a strain on our nuclear medicine system (with the exception of PET nuclides, which are produced on-site in a type of particle accelerator called a cyclotron). Our only real options are to cut back on nuclear medicine procedures or to build more isotope production reactors.
There is more to HEU-fueled reactors than producing medical nuclides – they’re also used to produce nuclides for industry, for basic research (bombarding rocks with neutrons, for example, can tell us what the rocks are made of), developing and testing nuclear instruments, and more. All of these things go more quickly with a higher neutron flux, but they can also be done in a less neutron-rich environment. When we put it all together we pretty much have to conclude that HEU-fueled reactors are nice, but they’re not essential. If our priority is to make the largest amounts of nuclides possible then we need the HEU-fueled reactors; if security is more important then we have to shut them all down and replace them with the slower (but more proliferation-resistant) LEU-fueled devices.

How Do You Do An Inventory And Leak Test Sources?

Hi, Dr. Zoomie – I just took over as RSO and found out that I have to do an inventory and leak testing of my sources. Can you tell me how often I have to do these and the best way to go about it? Thanks!

Wow – I can sense your excitement with the topic! And, I have to say, I can sympathize – inventory is one of those things that we all have to do, but that nobody really gets excited about. But it’s got to be done – and done correctly – even if only because they’re regulatory requirements. Not to mention the fact that leak testing your sources will let you know if one might be starting to give you problems. Let’s take them one at a time.


Most of the time you’re going to have to inventory your sources every six months – this is the default periodicity written into the regs. But you need to check this in your license; sometimes regulators will require a quarterly inventory. Your inventory has got to account for every source and it has got to be documented in writing, including your initials or signature confirming that you performed and vouch for the inventory.

There’s more to an inventory, though, than just accounting for the sources – you also have to account for the activity of radioactive materials in your possession. This means that, when you bring in new radioactive sources, you need to add them to your inventory and, as radioactive materials leave your facility, you again need to update your inventory accordingly.

Remember, too, that there are a number of ways that radioactive materials can leave your inventory, and you should account for all of these. For example:

  • Your inventory should include corrections due to radioactive decay so that it shows (at least every 6 months) the decayed activity rather than the strength of the source as it was when you first bought it.
  • If you’re using radioactive materials in experiments, medical procedures, industrial processes, and so forth remember to reflect what is used up and leaves your facility (for example, radiopharmaecuticals in your patients).
  • As you dispose of radioactive waste, your inventory also has to be updated.
  • Devices with radioactive sources (electron capture devices inside of gas chromatographs are one example) should not only be on your inventory, but should be updated if you sell or dispose of the devices.

These are only a few example – again, make sure that you’re updating your inventory so that it reflects not only the total number of sources (and tracks each individual source), but also the decay-corrected activity of each one.

Something else to keep in mind is that there are a number of ways you can perform an inventory.

  • Best, of course, is to visually sight each source – to lay eyes on them so you can say “I saw source #12345 on this date in this location.”
  • You might not be able to do this for sources that are installed inside of devices (the Ni-63 source inside of an electron capture device for example). One option in this case is to confirm that the device operates. If an electron capture device only operates when the source is installed, then showing that it operates properly will also show that the source is present.
  • Alternately, you can check radiation levels on the outside of a device to confirm the source is present. Say (for example) you have an irradiator that normally reads 2 mR/hr with the source inside. As long as you get a reading of about 2 mR/hr then you have a high degree of confidence that the source is still there. But if readings drop to, say, 20 µR/hr (0.02 mR/hr) then you’ve got to wonder what happened to the source.
  • If you have a source (or several sources) in storage then you’re allowed to inventory the sources by confirming that the storage location is sealed and has remained sealed since your last inventory.
  • And if you can’t figure out how to inventory your source(s), call the manufacturer and ask for a copy of their inventory procedure – they should be happy to send you a copy that you can use.

Finally, if you find out that you can’t account for a source, you have to tell your regulators. According to 10 CFR 20.2201, you have to make an immediate telephone report to your regulators if the source is more than 1000 times as active as the amount requiring licensing – but only if the circumstances were such that a member of the public might be exposed to radiation from the source. The next paragraph of the regs also notes that you have to make a written report to your regulators within 30 days of losing any source that’s more than 10 times the licensable quantity.

As an example – according to 10 CFR 30.71, Schedule B the limit for exempt sources of Cs-137 is 10 µCi (you’ll also find this in Appendix C to 10 CFR 20). This means that any Cs-137 source that’s smaller than 10 µCi is exempt from regulations. If you lose a source that’s less than 100 µCi (10 times the exemption limit) you don’t have to report anything to your regulators. If you lose a source that’s more than 100 µCi you need to make a written report within a month and, if you find that a source greater than 10 mCi has been lost, stolen, or gone missing then you need to call your regulators to inform them of this as soon as you realize what’s happened.

Leak testing

Something else you’ll have to do periodically is to check your sealed sources for leakage (you don’t have to check unsealed sources such as vials of stock solution used for experiments, or syringes filled with radiopharmaceuticals). In addition to unsealed sources there are other sources you don’t have to leak test:

  • Beta- and gamma-emitting radionuclides don’t need to be leak tested as long as the activity is less than 100 µCi
  • Alpha-emitting sources don’t need to be leak tested as long as the activity is less than 10 µCi
  • Sources filled with radioactive gases (such as Kr-85) need not be leak-tested

Specific leak testing requirements are found in a number of places in the regulations:

  • 10 CFR 32.59 (for companies that manufacture and sell items that contain sources)
  • 10 CFR 34.27 (industrial radiography) and 10 CFR 34.67 (for depleted uranium shielding)
  • 10 CFR 35.67 (medical sources)
  • 10 CFR 39.35 (for well logging sources)

There may be others; these are the ones that showed up in a search of the NRC website.

The purpose of leak testing is fairly obvious – to check to see if your radioactive sources are still sealed tightly or if they’re leaking. A leaking source can be a big deal – a single leaking source at a university I used to work at was responsible for contaminating an entire laboratory and adjacent rooms and a different leaking source contaminated an entire warehouse.

Performing leak testing on lower-activity sources can be fairly straight-forward; simply wipe the entire source with a piece of filter paper or a cotton-tipped swab. But you might not be able to do this – the source might be locked away inside a piece of equipment, for example. If this is the case, what you need to do is to wipe the outside of the source holder or an access panel. You’re not going to be permitted to disassemble a device to get at the source, so you should try to get as close as you can without doing so. One specific example would be a self-shielded irradiator (such as a blood bank irradiator) – this is a device with a drawer or a sample holder that’s exposed to the radiation, even though it’s heavily shielded to prevent exposing you to radiation. In this case, wiping the sample holder will suffice for a leak test since, if the source is leaking, the sample holder will be contaminated.

You’ll want to be careful when you’re leak testing high-activity sources. First, you should make a habit of doing all of your leak testing with a cotton-tipped applicator and not with a wipe you’re holding in your hands. And for any source that’s more than about 1 mCi in activity you should also be holding the source (if at all) with tweezers or tongs to avoid a high dose to your fingers. And with higher-activity sources even this might not be enough to ensure your safety. For example, industrial radiography sources often contain several tens – even up to a couple of hundred – curies of activity; forget about dose to your hands, this much activity could push you over an annual dose limit fairly quickly. For sources like this you want to survey the guide tube that the source travels through, or the shutter or exit port that the source is extended through – the thinking is to survey something that the source passes through or comes in contact with under the assumption that, if the source is leaking, these are the things that will become contaminated.

There are still more ways to check for leakage. I once had to perform leak tests on a 50,000 Ci source of Cs-137 that was stored in a large pool of water. First, the only way to wipe the source directly would have required scuba gear, which I’m not qualified to use. But more importantly, there’s no way in the world I’m going to go anywhere close to a source of that activity – such a source is an immediate danger to health and safety. On the other hand, cesium is soluble and any leakage would quickly dissolve into the water of the pool. Our approach was to take a liter of the water, evaporate it away, and count the residue in our gamma counter; if the source were leaking then this would show in the sample.

As with various methods for inventorying your sources, if you can’t figure out the best way to safely perform a leak test, ask the vendor. They’re certain to have developed a procedure as a part of their radiation safety program and they should be happy to share it with you.

Taking the sample is a start, but you also have to count the sample to show whether or not the source is leaking. A lot of licensees will send their wipes to a consultant or a laboratory for counting, but this isn’t always necessary as long as you can demonstrate that your counting methodology is capable of detecting leakage. Here’s how you can do that. And keep in mind – this is an example only – you’ll have to do this for the equipment you actually use at your facility!

  1. To start – the standard you have to be able to detect is 185 Bq (0.005 µCi). This is 185 radioactive decays per second, or 11,100 decays per minute.
  2. So – say you’re using a 1”x1” sodium iodide detector for the counting and the source contains Cs-137. The counting efficiency of this detector for Cs-137 is about 10% (you can look this up, but it’s better to have this determined when you send your meter out for calibration).
  3. In addition, normal background count rates with this detector will be about 500-1000 counts per minute (cpm).
  4. If you’re using a 1”x1” sodium iodide detector to count a wipe that holds 11,100 dpm of activity, you’ll get a count rate of 1110 cpm.
  5. For the final step, you can point out that the count rate you’d see from 185 Bq of contamination with your detector is at least double the normal background count rate you see with this same detector.
  6. This is “visible” to your detector; thus, your analysis methodology is capable of detecting a leaking source.

You might still want to have a consultant do your leak test counting for you – that’s your choice. But if you’d rather count your own leak testing, this is how you can get started.

Radioactive Decay: Half-life, Mass, & Activity Per Gram

Dear Dr. Zoomie – I am trying to brush up on some of my radiation knowledge and am having some trouble figuring out some of the calculations and concepts about radioactivity. For example, I know there’s some sort of relationship between a nuclide’s half-life, its mass, and the amount of activity per gram but I’m not quite sure how these all go together. Can you help explain how it all works?

You’re right – these factors are all tied together, and the relationship is fairly straightforward. That’s the good news – but it will take going through a bit of math to see how these all fit in with one another. Luckily it’s not too involved, so let’s walk through it a step at a time. First, let’s start with some equations!

Decay constant (λ)

The first concept is something called a decay constant, which is represented mathematically with the Greek letter lambda (λ). The decay constant is simply the probability that given atom will have a radioactive decay in a particular amount of time. Or, if you have a bunch of radioactive atoms, the decay constant tells us how many of those (what percentage of them) will decay away in a given amount of time. The equation is  . The natural logarithm of 2 (ln 2) is roughly equal to 0.693 and t1/2 is the half-life of the nuclide you’re calculating activity for. So for Co-60 (which has a half-life of 5.27 years) the decay constant is equal to 0.693/5.27 years = 0.1315 yr-1, which you would read as 0.1315 per year. What this tells us is that any particular Co-60 atom has a 13.15% chance of decaying during the course of a year, or that 13.15% of the atoms in a bunch of Co-60 will decay during a year.

The next part of this is to turn this into a measure of radioactivity. Radioactivity measures the number of radioactive decays an amount of radioactivity will undergo in a second. One curie of radioactive material will undergo 37 billion decays every second; one Becquerel of radioactivity will undergo 1 decay every second. The way to find out how many atoms will decay in a given amount of time is to count (or calculate) the number of radioactive atoms and to multiply this by the fraction of those atoms that will decay in a second. Mathematically, this is written as A= λN where A is the amount of radioactivity (decays per second) and N is the number of radioactive atoms.

OK – so say you have a billion atoms of Co-60 and you want to figure out how much radioactivity this represents. Using this equation, over the course of a year 13.15% of these atoms will decay, so one billion Co-60 atoms will undergo 131.5 million radioactive decays, giving a decay rate of 131.5 million decays per year. This is easier to calculate with if we use scientific notation – written this way we’ve got 1.315×108 decays per year. Unfortunately, our instruments don’t read out in counts per year so we have to convert this to a more useful measure. There are roughly 31.4 million (3.14×107 seconds in a year, so we divide 1.315×108 decays per year by 3.14×107 seconds per year to get a decay rate of about 4.2 decays per second (dps), which is about 4.2 Bq, measuring radioactivity in SI units. To convert to the US units of curies we have to remember that one Ci is 37 billion Bq, so 4.2 Bq = 1.13×10-7 mCi, or about 113 pCi (a pCi, or pico-curie, is a trillionth of a curie. So a billion atoms of Co-60 gives us 113 pCi of radioactivity.

Now, let’s see what happens when you have a nuclide with a different half-life. What if, instead of Co-60, you have a billion atoms of Cs-137 (which has a half-life of about 30 years)?

First, the decay constant for Cs-137 is going to be different;  so the same billion atoms will produce only 23.1 million decays per year, compared to the 131.5 million Co-60 decays. So Cs-137, which has a longer half-life, decays more slowly than the shorter-lived Co-60. And this is a good rule of thumb – for the same number of radioactive atoms, the ones with the longer half-lives will decay more slowly (will have lower levels of radioactivity). And as a corollary, nuclides with shorter half-lives will be more intensely radioactive for the same number of radioactive atoms.

Specific activity

OK – so this tells us how half-life and radioactivity go together. But we don’t count atoms (at least, not normally) – weight is how we normally measure things. So a more useful measure – a more useful calculation to perform – will tell us how much radioactivity we have per gram of material; the name for this is “specific activity.” And when we look at specific activity we have to take into account not only a nuclide’s half-life, but also how massive the atoms are. This gets a little more complicated, but there’s a bit of a shortcut at the end that makes things a bit easier.

Say we have one gram of Co-60. If we want to use the equation we already know then we have to figure out how many atoms of Co-60 there are in a gram. This is the somewhat complex part. Going back to high school chemistry, remember that the mass of one mole of anything (the number of grams that it weighs) is equal to the molecular (or atomic) mass. The mass part is easy – it’s just the numerical part of the nuclide. So one mole of Co-60 weighs 60 grams (just as one mole of Cs-137 will weigh 137 grams, and one mole of Ra-226 will weigh 226 grams). And one mole of Co-60 (or anything else, for that matter) has 6.022×1023 atoms, a number called Avogadro’s Number, after the 19th century scientist who first calculated it). So the number of atoms in one gram of Co-60 is equal to one sixtieth of Avogadro’s number, or  atoms of Co-60. And from here, we just multiply by the decay constant, which we know from earlier to be 0.1315 per year. Doing this tells us that one gram of Co-60 will undergo about 1.315×1021 decays per year, or about 4.2×1013 decays every second. Remember that one curie will undergo 37 billion (3.7×1010) decays per second:  So one gram of Co-60 gives us 1132 Ci – this is the specific activity of Co-60.

Using the same reasoning as before, we can guess that a nuclide with a longer half-life should have a lower specific activity since fewer atoms are decaying every second. And adding to that, a heavier radionuclide should also have a lower specific activity (fewer Ci or Bq per gram) because there are fewer atoms in a gram when each atom is heavier.

So here’s how it all goes together. All else being equal:

  • A shorter half-life means a more radioactive nuclide (more Ci or Bq for the same number of atoms)
  • A longer half-life means lower activity (fewer Ci or Bq per gram for the same number of atoms)
  • A heavier nuclide (the number part of the nuclide is larger) means fewer atoms in a gram, so there’s less radioactivity per gram
  • A lighter nuclide means more atoms per gram, so a higher specific activity
  • And if both half-life AND weight change then you have to figure out which is more significant before you can tell.

An easier way to do the math

OK – that’s the “pure” way to figure this out, but as I promised earlier, there’s an easier way to do it – just compare to a radionuclide with a known specific activity. And the one I compare against is the nuclide that gave us the definition of 1 Ci – Ra-226.

One gram of Ra-226 has an activity of 1 Ci (not exactly, but close enough for our purposes) and Ra-226 has a half-life of 1600 years. So a radionuclide with a similar mass and a shorter half-life will have a higher specific activity. Consider Am-241, with a half-life of about 432 years and a mass that’s fairly close to that of Ra-226. Roughly speaking, Am-241 has a half-life that’s about a quarter as long as Ra-226 so we’d expect to see four times as many atoms decay in the same amount of time. This means that 1 gram of Am-241 should have a little less than 4 Ci of activity. And if we look it up, we find that Am-241 has a specific activity of 3.4 Ci/gm – right in the ballpark of what we predicted.

Another one we can try is Co-60. In this case we have to correct for both the mass and the half-life:


Remember – a longer half-life means a lower decay constant (fewer atoms decay in the same amount of time) and a heavier atom means that there are fewer atoms in a gram of the pure nuclide. This is very close to what we calculated earlier, close enough to not worry about the relatively minor discrepancy that comes from the rounding off we’ve been doing.

Wrapping up

After having gone through all of that I have to admit that there aren’t times you’ll need to go through this, and there aren’t many radiation safety officers who need to make these calculations. Those who manufacture radioactive sources need to know this of course. If you’re working in a nuclear pharmacy or in radiation oncology then these calculations will also be useful. And if you’re a scientist using radionuclides for your research then these are calculations you need to be able to perform. And, I guess, if you hang out at geek bars, this particular skill set might win you a free drink (or, then again, maybe not…). But if you’re not in one of these groups (and if you’re not studying up for an exam of some sort) then you can probably get by with using a spreadsheet for all of this.