Video: Screening People for External Contamination: How to Use Hand-held Radiation Survey Equipment
Segment 2 of 4:
Alpha Scintillation
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Now, let’s take a little time to consider scintillation detectors, another major type of instrumentation. This is a zinc sulfide alpha scintillator. Alpha scintillation detectors work as follows — an alpha particle deposits energy in the scintillator, the scintillator then emits a flash of light, and the flash of light is converted into an electronic pulse by a device known as a photomultiplier tube. One alpha particle, one flash of light, one electronic pulse — and that’s how it works. These pulses are then registered on the meter as counts per minute, or cpm. Now, for some specifics about alpha scintillators. For an alpha particle to reach the zinc sulfide scintillator and cause a flash of light, it must penetrate a very thin window of illuminized Mylar. The light is then reflected to the photomultiplier tube, which is located in the handle of the probe. A major advantage of the zinc sulfide alpha scintillator is the fact that it only responds to alpha activity — it does not detect beta particles or gamma rays. And since there is relatively little alpha activity in the background, the alpha scintillator should read zero, 1, or maybe 2 counts per minute in an uncontaminated environment. If you have a higher count rate than that, you need to determine what is happening. Because of its low background count rate, the zinc sulfide scintillator is the instrument of choice for locating alpha contamination on people or other surfaces. As you might well discover, the major problem with alpha scintillators can be the false counts that occur when there are tiny holes in the aluminized Mylar window. These holes might be hard to locate because they can sometimes be too small to be seen. Fortunately, once you know where they are, the holes are easy to repair — all it takes is a tiny dab of black paint. So don’t forget to have a bottle of paint on hand, the sort you can get at any store that sells hobby supplies. Black nail polish is an even better choice because the bottle will come with an applicator. As always, you need to make sure that the instrument is working properly before you try to use it. By now, you should be reasonably familiar with how to do this. First, examine the device for visible signs of damage — dents in the probe or damage to the window, for example. Next, check the batteries. Again, your meter will either have a range switch that you turn or a button that you push to check the battery condition. If the needle on the meter does not move to that position that indicates the batteries are okay, go ahead and replace them and then repeat the battery check. Next, rotate the knob to the appropriate scale available and determine the instrument’s response to a check source. As I mentioned before, a thorium-containing gas-lantern mantle or a plastic button check source can be used here. When checking the alpha scintillator, be sure to remove the plastic cover from the probe, assuming that it has one, and if the check source is in a bag, remove it, because alpha particles cannot even get through a piece of paper. The next step is to check background. Turn the meter on in an area where elevated radiation levels are not expected and, using the most sensitive scale, record the reading on the meter. As mentioned before, the typical background for an alpha scintillator would be zero to 2 counts per minute. Finally, if you’re going to use the zinc sulfide alpha scintillator, you need to check for light leaks. With the audio on, point the face of the probe towards a source of light — an open window or an overhead light, for example. If there is a light leak, the meter will register many pulses. Then you need to locate the hole in the window and cover it with paint. In a pinch, a tiny piece of black electrical tape can be used. Once you have determined that the instrument seems to be working properly, you’re ready to survey.