Answer to Question #13810 Submitted to "Ask the Experts"

There seems to be some sort of radiation source directed at my apartment. I checked the readings on my Geiger-Mueller (GM) detector and noticed that I'm getting a dose ranging from 20–35 counts per minute (cpm) at a rate of 10–20 microSieverts per hour (µSv hr^-1). This seems high and I wonder what could be creating these levels and if they are safe. I have not seen levels like this at other places that I have tested with the GM (outside and indoors). Is long-term exposure to this a health concern?

This is sort of neat—luckily, it's not worrying—just neat! So, let's see what we can figure out here. Since I work with radiation for a living, I have a few more radiation detectors in my apartment than the average person. So, give me a minute to pull some of them out and let's see what they read. And if you're curious, I share my apartment with:

• An ionization chamber.
• A few Geiger counters (one reads in dose rate and both read in count rate).
• A few sodium iodide detectors (here, too, one is a dose rate meter and the rest are count rate devices).
• And a three-foot tall stuffed penguin wearing a COVID mask (a present from my daughter) that is unlikely to be much help.

Okay, I dig out the various meters and turn them all on. I usually wait a minute or so for any meter to settle down—like me, they are sometimes cranky when they first wake up. While they're doing that, let me tell you a little about each of them.

The ionization chamber is my go-to instrument when a dose rate measurement has to be as accurate as possible. This is because radiation dose is a measure of the amount of energy deposited in an object—the ion chamber can measure that directly, making it very accurate.

GM counters, on the other hand, can't measure the energy deposition. All they can measure is the number of alpha particles, beta particles, or gamma rays that cause a count in the meter. So, they're great at giving an accurate count rate, but they have some limitations when it comes to dose rate. The reason for this is that every count looks the same to a GM counter. It can't tell the difference between a high-energy count and a wimpy one that's just barely able to register. That's fine when you're measuring, say, a cesium-137 (¹³⁷Cs) source and you know the exact energy of every count that hits your meter. But nature isn't like that.

I guess one analogy would be trying to guess the weight of a bunch of objects. If every single object is a glass marble, then you can make a good guess just by counting the marbles how much the collection weighs. Just multiply the weight of a single marble and multiply that by the number of them you're holding. But what if you've got a few marbles, a bunch of sand grains, a few little pebbles, and a rock? If you use the "marble approach" then you'll be counting every single grain of sand and assuming it weighs as much as a marble, and you'll be assuming that the rocks weigh the same as well. It's easy to see how your guess would be off. In nature we also see a wide range of energies, but to a Geiger counter they all look the same. Thus, GM counters aren't very good at measuring radiation dose rates most of the time.

And the sodium iodide detectors are used mostly to measure and analyze gamma radiation. Most of these are not set up to measure dose rate accurately (same story as with the GM counters), but they're very good at picking up gamma radiation and can even analyze it (if you pay the extra money for the software to do this) to figure out what kinds of radiation you've got.

Now it looks like my detectors are all eagerly awaiting some attention. Let's see what they read (I live in an apartment in Brooklyn. I've got one granite countertop and the building is made of brick).

My ionization chamber is showing about the same dose rate everywhere. It's reading…too low to really get an accurate reading. Somewhere less than about 0.5 µSv hr^-1, but I can't guess any better than that. There is one "hot spot" in front of my TV—that's where I keep my radioactive minerals and one of them gives a dose rate of about 300 µSv hr^-1 when I put the ion chamber right on top of it, but it falls off to less than 1 µSv hr^-1 at arm's length.

My GM is sort of interesting, though. Except for the area in front of the TV, I'm getting a count rate that's around 40–60 cpm, although it's a little higher near the countertop and the exterior walls (likely because of natural radioactivity in the granite and in the bricks). The dose rates (measured with the GM) are all over the place—from about 1 µSv hr^-1 all the way to about 1 mSv hr^-1 close to the rocks. That's a lot higher than what the ionization chamber was showing, and I don't really trust…well, any of the dose rate readings.

The sodium iodide detector shows dose rates similar to the GM, but the count rate is off the charts by comparison. My small detector is giving me a count rate of about 600 cpm, the intermediate-size detector is reading over 1,000 cpm, and my large detector is showing a whopping 5,000 cpm.

And finally, I decided to also break out what's called a PRD—that's a detector that police use when they're out and about to see if they find any radioactivity and it's designed to measure very low dose rates. Mostly the PRD will find nuclear medicine patients and granite walls, but sometimes other neat stuff pops up. My PRD right now is reading about 0.1 µSv hr^-1 by and large—a little higher near the countertop and exterior walls, much higher near my collection of hot rocks. It's got a background count rate of about 720–750 cpm.

Okay, so I know that there's a lot to take in here and I suspect your eyes might be glazing over at this point. Let me try to summarize all of this into a few pithy bullet points!

• Different radiation detectors respond differently, even when they're in exactly the same radiation field.
• Not all radiation detectors are very good at measuring radiation dose rate accurately—it's just not what they're designed to do.
• Even detectors that are designed to measure dose rate accurately might not be very accurate at low levels of exposure.
• And, depending on the materials your home is made of, as well as things you collect, you can have a fair amount of variability in dose rate within your home.

To wrap up….

• The count rate you mention seems to be in the ballpark of what I'm used to seeing from GM detectors,
• The dose rate seems higher than I'd expect to see—but we have to temper that with the realization that you don’t get a highly accurate dose rate from Geiger counters.
  • Especially at dose rates near the bottom of their indicating range.
  • And GM detectors are far more likely to read high than low when exposed to natural radiation.
• And it could be that some of the materials with which your home is made or decorated might be influencing the dose rates you're seeing.
• It's also not uncommon for even experienced radiation safety folks to read their meter incorrectly—especially if there's a manual switch to go from one scale to another, or if the meter face has multiple scales on it (e.g., cpm, µSv hr^-1, and a high-range dose rate scale). Without knowing the sort of meter you have, I don't know if this might be the case.

I apologize for the length of this, but I wanted you to understand the difference that the choice of detector and the materials that are being surveyed can make. I hope this helps—and let me know if you have any further questions or concerns.

P. Andrew Karam, PhD, CHP