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.

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