In Part 1 of this special series, I shared with you what radiation is. In this installment, I hope to explain the mysterious ways that the lab coat guys identify and quantify it.
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If only radiation could be that simple. The fact is, there is no ‘bright line’ for determining how much radiation is too much. In the old days, there was a standard in the nuclear field that if you got enough radiation that your skin started turning red, you had enough for that day. Fortunately, with the advent of solid-state electronics, there are much better ways to determine what kind and how much radiation you are getting. Unfortunately, the question of ‘how much is too much?’ remains.
In order to understand the types of devices used to measure radiation, you have to first understand how radiation is measured. There are two separate, but interrelated benchmarks, exposure rate and dose.
Exposure rate is a term we will use to describe the amount of radiation activity at a certain location.
Your exposure rate is determined by a formula consisting of the strength of the radioactive source at a given distance. Strong sources (high activity) create more exposure at a given distance than do weak ones in exactly the same way that you can feel a roaring bonfire at a longer distance than you can a teeny campfire. You can also increase your exposure rate by coming closer to the source.
The device that measures exposure rate is very similar to the speedometer in your patrol car. The faster your car goes, the more it reads. Or, consider a film camera. The brighter the scene is, the faster the shutter snaps. The same concepts apply to the device used to detect radiation exposure. The greater the exposure rate, the more it reads.
The device used to measure exposure rates is called a ratemeter. It’s also called a frisker, Geiger counter, detector, radmeter, RADIAC, and other things. It’s the item you will most likely see on TV and the movies. The guy in the suit with the grim look waves this thing around, and as he advances, the clicks increase until they become a dull roar. Then, he melts into a puddle.
Ratemeters are important tools. In trained hands and properly configured, they are the tools that will tell you that you didn’t respond to a hoax. The problem is that they don’t make a single unit that detects all forms of radiation. And, the better units are fragile (spelled not-cop-or-trunk-safe).
To understand why one sensor doesn’t detect all radiation, lets’ go back to the first article. Remember that I said Alpha radiation hardly penetrates at all, and Beta doesn’t do much better. Probes that detect these types of radiation use a very large sensor and a very thin window-like area to allow them to enter. Any unit that was sensitive enough to read Alpha/Beta would be immediately overwhelmed by the more powerful forms of radiation. In fact, one common Alpha probe used in the nuclear industry can be damaged by static!
This means that the current pager-style and other ratemeters in use today may do a great job of detecting Gamma and X-ray radiation, and maybe some high-energy Beta, but no low-energy Beta or Alpha, and probably not Neutron. Even an Alpha-sensitive probe may miss Alpha contamination if it is held too far away, or moved too fast. And, I don’t think anything but an air sampler will pick up low levels of airborne contamination.
Dose on the other hand, is the cumulative total of how much exposure you received. It is very much like the odometer on the car, or the timer on the film camera. The longer you are exposed, the greater the dose.
Determining dose is a complicated task. It is based on your length of exposure, the exposure rate, and the type(s) of radiation involved. Your extremities can have significantly more dose than your trunk, because there are fewer blood-forming or radiosensitive organs there.
There are two common methods to determine dose rate. One is called dose reconstruction. After you get slimed, a tech with a ratemeter and other sampling gear goes where you went and determines exposure and product(s) involved. Then, another guy uses computer modeling and the amount of time you remained in the radiation field to determine how much you received, both whole-body and extremities. It is sort of accurate. If you are actually contaminated, they can also make some assumptions based on examining your, uh, ‘bodily output’.
The other way is by wearing a dosimeter. A dosimeter is like a radiation odometer. You clip it on you, preferably on your outer garment and near your chest. Then, as you are exposed to radiation, it starts counting. When you aren’t being exposed, it stops.
Occasionally, you read your dosimeter, and when you reach your limit, you leave the area. Since no one working in an emergency can remember to look at their dosimeter any more than they check their SCBA air gauge, newer dosimeters vibrate or chirp or blink to let you know you’ve been toasting long enough.
Like ratemeters, dosimeters also come in different flavors for the different types of radiation. I have seen training where one person may wear six dosimeters, two on the chest (one for beta/gamma, and one for neutrons), a ring-style on each hand, and one on each foot.
I think that’s overkill, but it underscores how much a short distance can change your exposure rate.
Now that you understand the types of measurements that are taken to quantify radiation, lets’ talk about the actual units of measurement.
The yardsticks that the nuclear industry uses to measure radiation are very complicated, at least to me. There are at least two standard systems to measure with (standard and metric), and at least two generations of measurements.
I am not going to slog you through the entire history and process. If you are that kind of masochist, I recommend you take a course, or hit some of my references at the end. Me personally, I tape the conversion charts onto the sides of my meters.
However, to understand how much is too much, you have to understand their yardsticks.
The first yardstick is called activity. A radioactive atom is (usually) large and unstable. Nature loves balance, so the atom flings parts of itself off in order to reach a balanced state. This flinging off of parts is radioactivity (ionizing radiation), and how rapidly and energetically an atom goes about its’ business is its’ activity level.
Activity is measured using the curie (Ci), or the becquerel (Bq) {metric}.
A curie is a HUGE yardstick. A more likely measurement would be a thousandth of a curie (a millicurie, or mCi). A becquerel, which came later to address how unwieldly curies are, is nowhere near the size of a curie. It takes about 37,000,000,000 Bq’s to equal one curie.
Activity is important to us, because we worry more about the higher-activity products.
The second yardstick has two prongs.
The first prong you don’t have to worry about too much. It uses the standard measurement called a Rad. Yes, it stands for something. No, you don’t need to worry about it. The SI (metric) version of the Rad is called the gray. They both refer to the amount of energy a radioactive product is depositing into nonbiological items. We don’t care about how much exposure a fire plug or our car is getting. I only include it for two reasons; the first is because some meters are calibrated in millirads (mR) or rads (R), and these terms are sometimes confused with the next set, and the second reason we’ll discuss later.
Recall that we discussed that in order to figure out the exposure to people, you have to take into consideration the type of radiation. Remember that Neutrons do more damage to people than, for instance, Beta.
Therefore, radiation protection specialists use a formula to determine the second prong, which they call the dose equivalent. (For you nerdy types, they multiply the rad or the gray times a quality factor which is determined by what type of radiation field is present.)
Well, multiplying on the fly is cumbersome. So, what the manufacturers do is engineer the multiplying into the sensor probe. If you pick up a ratemeter or dosimeter that reads in rem or sieverts per hour (r/hr or Sv/hr), it is already calculating the external exposure your body is receiving. Again, a rem is a massive yardstick, more likely your meter will read out in millirem.
The genius to these things are in the probes, which is why you have to be careful mixing and matching probes to meters. The correct probe is sensitive to a certain radiation energy band, and has a filter on it to simulate the correct density of human tissue.
For instance, while high-energy Beta radiation may just burn your forearm skin, it can significantly injure your unprotected eyes. So, a Beta probe is sensitive to the energy characteristic of Beta radiation, and is filtered to match the density of your corneas.
Whew! I think that’s enough for now. In the next installment, we’ll take these measurements, and see how they apply to you and me on scene.
In the final installment, I’ll share some do’s and don’ts when dealing with an incident involving radiation.
Until next time,
-Shawn
Part 1 (Secure- Law enforcement only)
Part 3 (Secure- Law enforcement only)
