Author Archives: Dr. Zoomie

What is Instrument Calibration?

Dear Dr. Zoomie –

I keep hearing about instrument calibration and don’t quite know what that means. Can you tell me what it is, when we have to do it, and whether it’s something I can do myself or should hire a contractor?

Let’s start off with why we have radiation instruments at all – it’s because we have no way of sensing radiation ourselves. No matter how high the rad levels are, you won’t feel your skin tingle, you won’t get a strange taste in your mouth, and you certainly won’t see or hear anything. That’s why we have instruments – to make up for this deficit in our senses. So if we’re using instruments to tell us whether or not we’re at risk – or even just to meet regulatory requirements – it behooves us to make sure that the instruments are working properly and to make sure we can trust their readings. That’s what calibration is for.

So calibration makes sense from a practical perspective, but it’s also a regulatory requirement – you’re required to perform a yearly calibration on all radiation instruments that are used for health and safety purposes or to perform surveys used to meet regulatory requirements. So every one of your radiation meters that’s used for routine radiation or calibration surveys has got to be calibrated every year. You should also have your instruments calibrated anytime they undergo extensive repairs – changing the batteries or cables is OK, but replacing probes or repairing the inner workings calls for calibration to make sure it’s still working properly.

If you’ve got a lot of radiation meters you might want to calibrate them yourself, but you’ll need to amend your radioactive materials license to let you do so, you’ll need to purchase the appropriate equipment, and you’ll need to get trained up so you can do it properly. Unless you have a few hundred instruments or more it probably doesn’t make sense to go through this process – but if you have a lot of instruments, calibrating them in-house can save you enough money to make it worth considering.

If you only have a few instruments you should send them out for calibration. Here you need to be careful to send them only to a facility that’s licensed to do the calibrations – most of the major instrument manufacturers can do this, but there are a lot of other facilities as well who can help you out. Doing an on-line search for “instrument calibration services” will give you a bunch of options; you can also look through the companies listed under Instrument Calibration Services in the Affiliates section of the Health Physics Society’s website (http://hps.org/aboutthesociety/affiliates/services.html).

One last thing – in addition to the annual calibrations there are quick and simple checks you should be performing every day that you use your instruments. Check to make sure the batteries are charged (many meters have a “Bat Test” button or switch position), make sure the meter’s physical condition is OK, and (for contamination detectors) check the meter for proper response against a check source to make sure your meter reads within 20% of the expected reading. For this one you can purchase a “button source” that can be mounted on the side of the meter; when your instrument is calibrated the calibration certificate will include information on the expected count rate. So, for example, if the expected count rate is 1000 cpm, you should get a reading that’s between 800-1200 cpm when you do the response check.

How Do You Read a Radiation Instrument?

Dear Dr. Zoomie – I bought a radiation meter and I’ve been making some measurements but I have to admit I’m not sure what they mean. The numbers are always going up and down a little bit and I’m not sure why.  When should I be worried? And shouldn’t my meter always read zero unless there’s radiation around somewhere?

You know, you’ve put your finger on one of the most important aspects of using radiation instruments – unless you know what the readings mean you’re just looking at numbers. Sort of like looking at your speedometer and not knowing if it’s reading in miles per hour, kilometers per hour, feet per second, or what.

One thing to remember is that there is always going to be natural background radiation that’s registering on your detector. So you should always get something registering on your meter. If you’re reading radiation dose rate then natural background readings should be anywhere up to about 100 microR/hr (µR/hr) or up to about 0.1 mR/hr – a µR is one millionth of a rad and one thousandth of a milliR (mR). If you’ve got a contamination meter then you’ll be making readings in counts per minute (CPM) or counts per second (CPS). Background count rate can vary a lot depending on what sort of radiation detector you’re using. With a GM it can be as low as just a few tens of CPM (1-2 CPS) or as high as a few hundred CPM (2-3 CPS); with a scintillation detector background count rate can be from several hundred to several thousand CPM (10-100 CPS).

A general rule of thumb is that background radiation levels can vary by up to a factor of 2 or 3 from moment to moment so if you see your count rate or dose rate spike up momentarily it doesn’t necessarily mean anything. Think of when you’re driving with your car on cruise control – if you look carefully at the speedometer (don’t do this unless somebody else is behind the wheel, by the way!) you’ll see that the speed will drift slightly up or down from time to time, but it never drifts very far from the set speed. You don’t worry that your cruise control is broken unless the speed changes considerably or changes for an extended period of time. So if your GM pancake probe has a normal background reading of 50-60 CPM and is fluctuating between, say, 40 and 70 CPM then there’s nothing to be concerned about. But if it goes up to 150 CPM and stays there then you might have found something radioactive. By the same thinking, if you normally see radiation dose rates of, say, 25 µR /hr or so, it’s not surprising to see your readings fluctuate between, say, 10-50 µR /hr. If they go up to 75 or 80 µR R/hr and stay there steadily then, again, you might have found some radioactivity.

Just because you find elevated readings, though, doesn’t put you at risk. In fact, any dose rate that’s in the µR/hr range is going to be fairly harmless, and there are a number of places on Earth where natural radiation levels are hundreds of µR /hr. If dose rates rise into the mR/hr range (remember that 1000 µR = 1 mR) then there’s still very little (if any) risk, but regulations start to come into play – if dose rates reach 2 mR/hr then there has to be some sort of restrictions (barriers, for example) to keep the public out of the area. But radiation dose rates don’t become potentially dangerous until they rise into the R/hr range (1000 mR = 1 R).

Count rate readings can also vary considerably, and they can be fairly high without posing a risk to you. As one example, after the Fukushima reactor accident the Japanese government didn’t require that people be decontaminated until they had over 100,000 CPM of skin contamination. So even if you get a reading of a few hundred CPM, while it might mean that you’ve found some radioactivity, it doesn’t mean that it’s dangerous. But that being said, any count rate that’s more than three times as high as normal – and that remains elevated rather than just spiking and dropping back down again – should be looked into to see if there’s a problem.

What do I do if I see that my readings have changed?

There have been some videos posted online that show people making radiation measurements and commenting on how the radiation levels seem to be increasing. It’s not uncommon for these people to be worried about the increases that they see; even to think that they’re seeing evidence of dangerous levels of radioactivity from the Fukushima reactor plant. In reality, it’s not nearly that dire. For example, some kinds of rock contain higher levels of radiation than others – if you’re walking through a city and walk past a granite building (or if you’re outdoors and walk past a big granite rock) you can see your radiation levels and count rates increase as long as you’re in range of the building or rock formation. And some types of clay contain more radioactivity than others – if the soil you’re walking over has a change in its composition then you can also see your levels increase. And for that matter, since bricks and concrete contain clay, brick walls or buildings – even brick sidewalks – can cause your readings to increase as well. The bottom line is that there are a lot of very innocent reasons for your radiation dose rates and count rates to increase and you don’t necessarily have to worry just because your readings go up a bit.

The thing to do is to try to figure out why they’ve changed. Say, for example, you notice your readings have gone up as you’re walking along outside. Stop and take a careful look at your detector and see if they stay elevated or if they fluctuate up and down. If they stay elevated, take a look around to see if something has changed – maybe you walked from an asphalt road to a concrete stretch of pavement, perhaps you walked next to a granite wall, or maybe you’re near a hospital (many hospitals have nuclear medicine, radiation oncology, and x-ray departments). Walk back the way you came to see about where the rates started to increase, then continue walking the way you were going to see if they start to go down again – by doing this you can figure out where the higher readings are coming from. If you can see a reason for the readings to be higher (different types of buildings, paving materials, etc.) then chances are that you’ve found the reason your readings have gone up. Another reason to see elevated readings would be a nuclear medicine patient close to you – these factors (changes in soil, rock types, buildings, and nuclear medicine patients) account for virtually every change in radiation levels you’re likely to come across.

Finally, remember that having a radiation survey instrument doesn’t mean that you’re a radiation professional. Figuring out exactly what your readings mean can be tricky, and sometimes even experienced professionals can be stumped. If you find elevated readings that you can’t figure out it doesn’t necessarily mean that you’re at risk or that you’ve found evidence of a radiation accident. If you’re confused by your readings you should write down what your readings are and exactly where you got them and then try to get in touch with a radiation safety professional. If you’re near a large teaching hospital or a large university you can contact the radiation safety office; otherwise you can contact your state radiation regulators to let them know what you found. And under no circumstances should you go into an area where the dose rates are higher than 2 mR/hr, nor should you try to recover radioactive materials yourself – these are jobs for radiation safety professionals.

How is Radiation Detected and Measured?

Dear Dr. Zoomie, how is radiation detected and measured?

Great question – and very important. Since we can’t sense radiation ourselves, our instruments are the only things that make it possible for us to know if we’ve got potential problems. In fact, you can be in a dangerously high radiation area and still have no way of telling you’re at risk unless you have radiation instruments.

OK – so we need our instruments, but you can’t just go out and buy the first radiation detector you see. There are a bunch of different types of radiation instruments – some measure radiation, some measure contamination, some are specialized to detect only one type of radiation, and some are designed to identify exactly what kind of radioactive materials you’ve found. So let’s talk a little bit about each of these.

Measuring radiation dose rate

If you’re trying to keep yourself safe – to keep from getting radiation sickness for example – you’ll be interested in measuring radiation dose rates. These are measured in units of mR/hr (in the US) or mGy/hr (in the rest of the world).

Although most people think of Geiger counters when they think about measuring radiation, Geiger counters aren’t always the best instruments to use for measuring radiation dose rate. In fact, the run-of-the-mill Geiger counter is going to give you a reading that’s too high most of the time because it will over-respond to the low-energy gamma rays that make up most of the natural background radiation. On the other hand, if you’re trying to measure radiation from high-energy gamma rays (like those given off by radioactive cobalt-60) a Geiger counter will read too low. There is one specific type of Geiger counter that’s good for measuring radiation dose rates – it’s called an “energy-compensated” Geiger counter. Unless you have one of these you should avoid measuring radiation dose rates with a Geiger counter.

The best way to measure radiation dose rates is by using an instrument called an ion chamber (or a pressurized version designed to measure very low radiation dose rates). These instruments are made up of a chamber filled with gas – when radiation passes through the chamber it changes the electrical properties of the gas. We can measure these changes and they tell us how much radiation the instrument is being exposed to.

Measuring radioactive contamination

Even if the radiation dose rates are low you might still have radioactive contamination. As one example, when I traveled to Fukushima shortly after the reactor accident I was in some areas in which radiation dose rates were nothing to worry about, but I could still measure contamination from radioactive cesium and iodine. The areas where I was traveling were not inherently radioactive, but radioactivity had settled there from the accident. Contamination can be cleaned up, just as we can clean up a dirty surface or we can clean grease from our hands after we work on an engine. Geiger counters are great for measuring contamination – in fact, a type of Geiger counter called a “pancake GM” is one of the best things around to measure it. So Geiger counters might not be the best things to measure radiation dose rate, but they’re just the things to have for measuring contamination. Oh – we measure contamination in units of “counts per minute” that are abbreviated CPM.

Measuring different types of radiation

Once we get past the different types of measurements (dose rate and contamination levels – or count rate) we have to contend with the fact that there are four different kinds of radiation; alpha, beta, gamma, and neutron. Each one of these has different properties and each of them is measured somewhat differently. This isn’t the place to get into each of the types of radiation, although that might be a good topic for a later posting. Rather than going into a long discussion of each type of radiation detector it might be easier to summarize them in a table.

Radiation Instruments Table

Radiation Instruments Table

Measuring neutrons is not easy and most neutron detectors are expensive. Luckily there are very few times we have to make neutron measurements – mostly around nuclear reactors – so we won’t talk about neutron detectors here.

sodium_iodide_probe

Sodium Iodide probe

Sodium iodide (NaI) probe for gamma contamination and radiation surveys.  This should be used for contamination surveys unless it is attached to a meter that has been calibrated to measure in radiation levels (this information should be noted on the instrument calibration records.  Record results in CPM.

Geiger-Mueller (GM) Pancake Probe

Geiger-Mueller (GM) Pancake Probe

Geiger-Mueller (GM) “pancake” probe for beta and gamma contamination surveys.  Record results in CPM.

Geiger Mueller (GM) Hot Dog Probe

Geiger Mueller (GM) Hot Dog Probe

Geiger-Mueller (GM) “hot dog” probe for beta and gamma contamination surveys.  This may be used for measuring radiation levels only if the meter was calibrated for the isotope (e.g. Cs-137) present.  Record results in cpm.

Zinc Sulfide Alpha Scintillation Probe

Zinc Sulfide Alpha Scintillation Probe

Zinc sulfide (ZnS) alpha scintillation probe.  The window on this probe is exceptionally fragile and must be protected from accidental puncture.  Record results in cpm.

Ion Chamber

Ion Chamber

Ion chamber.  This detector is used to measure radiation levels from beta (with bottom window open) or gamma (with bottom window closed) radiation sources.  Record results in mr/hr.

What does it mean when a nuclear reactor goes critical?

Dear Dr. Zoomie – I was watching a movie the other day and everyone was upset that the reactor was going critical. What’s this mean? How does a reactor go critical, and how dangerous is it really?

The word “critical” certainly sounds bad, but it’s not as dangerous as you might think. Physicists use the term “critical” all the time – as one example, if you shine a light at a piece of glass it will pass through it up to a point – once you reach a certain angle the light will reflect off the glass. This angle is called the critical angle and, among other things, it helps light to reflect endlessly along fiber optic lines. But I digress…in the case of nuclear reactors, “criticality” simply means that the reactor is in a configuration that will let it operate at a steady power level. It’s a term developed by physicists during the Second World War and it just stuck around as reactors went from being scientific demonstrations to a mainstream source of power. Here’s how it works.

When a uranium atom is hit by a neutron (a slow-moving neutron) it can split apart – this is the fission process. It splits into two smaller atoms (called fission products) and it also emits a few neutrons. A neutron might escape the reactor entirely, it might be absorbed by metal in the reactor machinery (or by anything that’s not another uranium atom), it can be absorbed by a control rod (more on these later) or it might be absorbed by another uranium atom, causing another fission. To keep reactor power steady we need to have one of these neutrons to cause another fission. When that happens – when the reactor is in a configuration such that one neutron from each atom fissioned causes another fission – the reactor is said to be “critical.” So to a reactor operator, “criticality” is what they’re shooting for and it’s certainly nothing to be feared. A military reactor I was assigned to was once critical for low-power physics testing and was making about enough energy to heat up a cup of coffee.

Atomic Chain Reaction

The image above is a diagram of an atomic chain reaction. In an atomic chain reaction a neutron (in blue) hits a uranium atom and causes it to split in two parts, and also release two neutrons.  The neutrons cause other atoms to split (fission), releasing even more neutrons; which create more fissions.  Energy is released with each fission.  In a nuclear bomb, this chain reaction is very fast and uncontrolled, causing a huge explosion.  In a nuclear reactor, the chain reaction is slow and controlled and is used to produce energy.

I mentioned earlier that slow neutrons are most effective at causing fissions. So say (to pick a number) a neutron travels 5 inches before it can cause another fission – the fuel has to be at least 10 inches in diameter to be large enough that most of the neutrons are likely to run into another uranium atom before they escape from the fuel (it’s actually a little larger, but you get the idea). With a density that’s nearly 20 times that of water, a ball of uranium that’s 10 inches in diameter weighs about 380 pounds – this is the mass of uranium required to achieve criticality – the critical mass. Incidentally, this is NOT the actual number – just made up to illustrate the point. The real number will vary depending on the enrichment of the uranium (e.g. natural uranium, reactor-grade, or weapons-grade) and a number of other factors.

But it’s not enough to have a critical mass of uranium – it has to be in the right configuration as well. Picture a thin sheet of uranium and one of the atoms fissions. The only way that a neutron will be able to find another uranium atom to cause a fission is if it’s emitted in the plane of the sheet of paper – this means that the overwhelming majority of neutrons are probably going to escape the sheet of uranium without encountering a uranium atom at all. So you can have a critical mass of uranium that can’t achieve criticality because it’s in the wrong configuration – it doesn’t have a critical geometry. Now, if you crumple up the sheet of uranium it will eventually be in a shape that where neutrons are likely to cause fissions – this is the critical geometry. Unless you have a critical mass of uranium AND it’s in a critical geometry you will not be able to make a reactor operate.

There’s another factor – neutrons from fission move very quickly and they have to be slowed down; they slow down by bouncing off of hydrogen atoms in the water that keeps the reactor cool. So this means that our reactor core has to space out the uranium with water in between the rods of uranium to give the neutrons water to pass through (this process is called “moderation”). This adds even more to the size of the reactor core, as well as calling for even more uranium since the core has grown.

Diagram of How Nuclear Power is Generated

Simple diagram of how nuclear power is generated. Click the image to see an enlarged view.

Yet another thing to consider is that we want our reactor to be able to operate for awhile before we have to add more uranium to make up for what has fissioned – any reactor that has exactly the minimum critical mass will be able to operate only once (just as we build a fire with more than just a single match). So it’s necessary to have still more uranium to keep the reactor running for months or years.

OK – so now we have a reactor core that has, say, 2 years’ worth of uranium in it. This is great from the standpoint of longevity, but it will have too much uranium to be stable unless we have a way to control it – this is where the control rods come in. Control rods are made of material that are great at absorbing neutrons – when control rods are inserted between the fuel rods they absorb neutrons and keep them from causing new fissions; when control rods are pulled out the fuel is exposed to neutrons and fission can proceed. So when the control rods are fully inserted into the core the reactor is shut down; to start up a reactor we pull the control rods out – at some point there will be enough fuel exposed to the neutrons that there will be a chain reaction; at this point the reactor will be critical.

What in the World Does “Health Physics” Mean?

Dear Dr. Zoomie – what in the world does “health physics” mean? Why not use something easier to understand?

Funny you should ask – I’m traveling this week and when the guy sitting next to me on the plane asked what I do I told him I’m a health physicist. He started telling me about his back problems, his prostate, and his family history of insomnia. When he finally paused to take a breath, I explained that I’m a PhD and not an MD, and I was unable to help him out.

When the Manhattan Project got rolling, the higher-ups realized they needed to know more about the health effects of radiation and they needed to learn how to use it safely. At the same time, security was everything. – since we knew that Nazi Germany was researching nuclear weapons, we didn’t want to give away any information that could let them know we were working on them as well. Hence the term “health physics” instead of “radiation safety” was used describe helping protect people from the damaging aspects of working with radiation and radioactivity. Paul Frame, the historian of the Health Physics Society, quotes one of the first health physicists in one possible explanation of the term: “The coinage at first merely denoted the physics section of the Health Division… the name also served security: ‘radiation protection’ might arouse unwelcome interest; ‘health physics’ conveyed nothing.”

So this is why nobody knows what a health physicist is – it was intended from the start to be obscure. The best indication of its success as a code term? After 70 years nobody still knows what it means, and I still get beset with unwanted health information when I tell people that I’m a health physicist. Maybe I should start telling people I’m an industrial hygienist…but then they’ll probably think I mop floors and take out trash. Sigh….

What does a Health Physicist do?

Dr. Zoomie – I know that health physicists are radiation safety specialists, but what do they do?

We spend a lot of time bemoaning the fact that nobody knows what a health physicist is or what we do. Unfortunately, there aren’t many health physicists so it’s hard to assemble a support group unless you live close to a nuclear power plant or a national lab so we generally suffer in silence and try not to develop drinking problems. But now I’m thinking that maybe you were thinking about what we do on the job…. So let me try to answer that!

Cartoon - I Hear You're a Health Physicist

Radiation safety is a lot more than doing radiation surveys and decontaminating things. In a hospital, for example, health physicists help to make sure that x-ray machines are working properly, that radiation therapy sources are stored and used safely, and that nuclear medicine “hot labs” can account for their radioactive materials (among other things). At a university, health physicists will check to make sure that scientists follow regulations and use their radioactive materials safely, they organize and run radioactive waste programs, respond to radioactive spills, and help review research plans, and work with researchers to keep radiation exposure to their staff as low as possible. And as regulators, health physicists have a huge role in making sure that licensees use radiation and radioactivity safely as well as making sure they follow all of the applicable laws.

And there’s more to it than that – a friend of mine is a health physicist for NASA, which uses radioactive sources to power many of their deep space missions. Another friend of mine works on making plans to help keep the public safe if there’s a radiological or nuclear accident. After the Fukushima accident, I traveled to Japan with many of my colleagues. Some helped to map the extent of contamination, others helped assess radiation dose to the public, and still others helped to provide information to physicians, emergency responders, and governmental officials. There’s more than that, too, but space is running out. So let it suffice to say that, if it involves radiation (medical, research, industrial, nuclear power), health physicists will be involved to help make sure that people and the environment are protected.

How do I become a Radiation Safety Officer?

Dear Dr. Zoomie – our Radiation Safety Officer just left and my boss tells me I’m the new RSO. I’m not sure if I’m qualified – how can I find out?

For starters, no matter what your boss says, you’re not the RSO until your regulators say that you are. So you’re going to have to write a letter to them requesting that they amend your license to name you as the RSO. And in order to do that you’re going to have to provide them with your credentials to show that you’re qualified to be an RSO. But then that gets into your second question.

The exact skill set you’re going to need to be an RSO is going to depend on how big and how complex your radioactive materials program happens to be. If all you have is a few small (i.e. low-activity) sources, then you won’t need nearly as much education, training, and experience as you will if you work for a major university or hospital. But in general, what your regulators will be looking for is to see whether or not your education, experience, and training are adequate to run your company’s radiation safety program.

Although there aren’t any specific regulatory requirements to be an RSO for specific types of radiation safety programs , there are guidelines – federal guidelines are found in a document called NUREG 1556, and many states have their own requirements (most of which are very similar to NUREG 1556). I’ve worked with some companies that had a very small radiation safety program – their RSOs only had to have a high school degree and no radiation safety experience at all. To be a RSO for a major university or hospital, you’ll probably have to have at least an undergraduate degree in a scientific or technical field along with 6 years of radiation safety experience. In most cases, you will need to attend a 40-hour RSO short course. And make sure you keep the course completion certificate so you can prove to your regulators that you did actually finish the class! If you need a little help figuring out what kind of training you’ll need please contact us.

So what you’re going to have to do to be named RSO is to send a letter to your regulators asking that they amend your license naming you as RSO. You’ll need to attach copies of your qualifications (course completion certificates, resume, diplomas, etc.) that should either meet or exceed the requirements your state asks for. Then you wait for the regulators to get back to you – within a month or so you should receive an amended license that names you as the RSO for your organization. Until then, no matter what your boss might think, you’re not officially the RSO.

You might not meet all of the requirements to be an RSO. In some cases, you can work with the regulators to get past this snag. For example, if you haven’t yet had a chance to attend the RSO class you might be provisionally approved to be an RSO, provided you send a course completion certificate within a few months to show that you’ve completed the training. Or if you lack whatever amount of experience they’re looking for it might be possible to contract with a consultant who can visit your facility once a month or so to check on things and to act as a mentor for a year or so. And it’s also possible that your regulators will simply insist on an RSO who meets all of their requirements – if they do (and you don’t meet the requirements) then your boss is going to have to find a different person to be RSO.

Once you’re named as RSO the fun begins…

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