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 (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.