Monthly Archives: May 2016

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.

Decommissioning – Meeting “Free Release” Criteria

Dear Dr. Z – I have a room and some equipment that was used for isotopic work that I want to close out. My regulators said something about decommissioning and meeting “free release” criteria. What in the world does this mean? And can you tell me how I can reach this exalted state? Thanks!

Boy – you’re about to have a lot of fun. Or maybe not…but let’s be optimistic! The big picture is that you have to be able to demonstrate to your regulators that you’ve moved all of the radioactive materials out of the room you’re decommissioning – including any radioactive contamination that might be present – and that you’ve cleaned up all of the equipment that was used to work with radioactive materials. That’s the “decommissioning” side of things; “free release” is another way of saying “release for unrestricted use,” which means that it’s been cleaned up to the point where you can do whatever you want with it. “Free release” means that you can turn the room into a lunch room without worrying that they’ll be harmed from the radioactivity, or that you can throw the equipment into the trash or give to your dog to play with – again, without risk to Fido. So that’s the big picture – now let’s get a little into the nitty-gritty.

Let’s start with the decommissioning process – what do your regulators expect you to do to show that you’ve properly closed out a room?

First, you’ve got to be able to show that you’ve moved all of your radioactive materials out of the room. If this is the only room where you use or store radioactive materials then you have to be able to show that you’ve got rid of the stuff – the only acceptable ways to do this are by transferring it to a licensed radioactive waste disposal facility or by transferring it to another radioactive materials licensee. Whichever of these you choose, you’ve got to be able to document the transfer or disposal – you will have copies of shipping papers and the manifest if you ship materials for disposal, or a letter (and possibly shipping papers) from whomever you transfer your radioactive materials to. But be careful! First, you can only transfer your radioactive materials to someone whose radioactive materials license will permit them to possess the source(s) you’re transferring to them – and you need to see a copy of the license before you ship the materials to them (whether the recipient is a waste disposal site or another licensed facility). And second, you have to be able to document the transfer – this is what the shipping papers and (if possible) a letter or email from the recipient confirming what they received from you.

Second, you’ll also need to show that you’ve cleaned up any and all contamination in the room so that it can be released for unrestricted use – this gets into the same ground as releasing equipment for unrestricted use. How to do the decontamination is beyond the scope of this posting – although soap and water or most commercial cleaning products will normally do the trick unless contamination has soaked into a porous surface or has chemically attached itself to whatever it is that you’re trying to clean up. But what we’ll go over here are the cleanup limits, regardless as to what it is that you’re cleaning up. Here, the fundamental document you’ll be referring to (unless your regulators have other requirements) is the Nuclear Regulatory Commission’s Regulatory Guide 1.86 (Termination of Operating Licenses for Nuclear Reactors). And the key part of this document is the table on Page 5. Here’s the table, along with a discussion of how to use it.

Nuclide (a) Average

(b, c)

Maximum (b, d) Removable (b, e)
DPM/100 cm2
Uranium (natural), U-235, U-238, and associated decay products 5000 (α) 15,000 (α) 1000 (α)
Transuranics, Ra-226, Ra-228, Th-230, Th-228, Pa-231, Ac-227, I-125, I-129 100 300 20
Thorium (natural), Th-232, Sr-90, Ra-223, Ra-224, U-232, I-126, I-131, I-133 1000 3000 200
Beta-gamma emitters (nuclides with decay modes other than alpha emission or spontaneous fission) other than Sr-90 and others noted above) 5000 15,000 1000
  • (a) Where surface contamination by both alpha- and beta-gamma-emitting nuclides exists, the limits established for alpha- and beta-gamma-emitting nuclides should apply independently
  • (b) As used in this table, dpm (disintegrations per minute) means the rate of emission by radioactive material as determined by correcting the counts per minute observed by an appropriate detector for background, efficiency, and geometric factors associated with the instrumentation
  • (c) Measurements of average contaminant should not be averaged over more than 1 square meter. For objects of less surface area, the average should be derived for each such object.
  • (d) The maximum contamination level applies to an area of not more than 100 cm2

  • (e) The amount of removable radioactive material per 100 cm2 of surface area should be determined by wiping that area with dry filter or soft absorbent paper, applying moderate pressure, and assessing the amount of radioactive material on the wipe with an appropriate instrument of known efficiency. When removable contamination on objects of less surface area is determined, the pertinent levels should be reduced proportionally and the entire surface should be wiped.

Here’s how to interpret Reg Guide 1.86:

  • Notice that there are two types of contamination – removable and fixed. And with the fixed contamination, there is both the maximum and average levels. Removable contamination (according to Note e) is contamination that will come off when you wipe the surface with a piece of dry filter paper; fixed contamination is whatever can’t be wiped off.
  • We look at contamination in terms of disintegrations per 100 square centimeters. So what we do is to survey (or wipe) an area of 100 square cm (about 4”x4”) to see how much contamination we is there.
  • What you’re looking for is the amount of contamination, but you have to remember that there’s background radiation also and you don’t want to find yourself cleaning up contamination that isn’t actually there. Before you start to survey (or count a wipe) you’ll want to turn on your meter when you’re a distance away from any contamination to see what it reads. With a 1”x1” sodium detector, for example, background is likely to be several hundred cpm. So if you’re reading, say, 1900 cpm with your detector and the background count rate is 500 cpm then the net counts (the amount of contamination) is 1900-500=1400 cpm.
  • Here, we have to convert from the counts per minute (cpm) shown on your instrument to disintegrations per minute (dpm) required by regulations. The way to do this is by knowing the counting efficiency of your meter for the radionuclide you’re surveying for. For example, say you’re surveying for Cs-137 with a 1”x1” sodium iodide detector. When you send it for calibration you can ask the calibration laboratory determine your counting efficiency, which will probably be in the neighborhood of 5-10%. A 10% counting efficiency means that, for every 10 gammas that are given off by the Cs-137, only one is counted. So to turn cpm into dpm, you have to divide the meter reading (cpm) by the counting efficiency. In other words, dpm = cpm/efficiency. So if you’re reading, say, 1400 cpm with your detector, you have 1400/10% = 1400/0.1 = 14,000 dpm.
  • OK – now that all of that’s out of the way, here’s how you use the table.
    1. There are different limits for a variety of nuclides. If you have, for example, I-131 then you’re only allowed to have 200 dpm/100 square cm of removable contamination, average contamination levels of up to 1000 dpm/100 square cm (averaged over 1 square meter), and no single location can be higher than 3000 dpm/100 square cm.
    2. The first three rows list the limits for a number of specific radionuclides. And if what you’ve got isn’t one of the ones that’s specifically listed then you go to the final row. This is the most commonly used set of cleanup standards and you’ll frequently hear health physicists talk about 1000 dpm/100 square cm or removable (or smearable) contamination.
    3. So what you have to do is to first know what radionuclide(s) you’ve got, then determine the appropriate cleanup limits for each. After this, you survey with a survey meter to measure total contamination over each square meter and, finally, take a number of smear wipes to see how much of this contamination is removable versus fixed. Finally, if you exceed these limits, you’ll need to decontaminate until the limits are met.

Third, there will need to be a formal closeout survey. This is where your regulators or (more likely) a contractor will come in to check to make sure that you’ve done everything properly, including your decontamination. This will include a contamination (and likely a radiation) survey to confirm your own readings. Once the survey confirms that your lab is cleared up (and cleared out) then you can go on to the next step.

Finally, we need to talk about documentation. I already mentioned the need to show what you did with your radioactive materials – the shipping papers and whatever transfer documents you might have. You’ll also need to keep copies of your own survey maps, showing the radiation and contamination levels you measured while you were closing out and cleaning up. Or if you have a consultant or contractor do this work, make sure that they document it properly. Incidentally, there’s also a requirement for you to keep records of any spills or areas of contamination with long-lived nuclides – your regulators might want to check these locations to make sure that you got them properly cleaned up.

When you’ve done all of your work, you’ll need to write to your regulators, requesting to terminate your license. You’ll have to include all of your documentation so that your regulators will know that you’ve properly disposed of your radioactive materials and cleaned up your lab, then they might want to pay you a visit to verify your work. When they’re satisfied that everything is OK they’ll go ahead and let you close out your lab and terminate your license.