Video: Screening People for External Contamination: How to Use Hand-held Radiation Survey Equipment
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Screening People for External Contamination: How to use Hand-held Radiation Survey Equipment
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Welcome to the CDC's training program on the use of hand-held radiation survey equipment. I'm Dr. Armin Ansari, Certified Health Physicist at the CDC. The purpose of this program is to provide training on how to use hand-held radiation detection equipment to screen people who may have radioactive contamination on their skin or clothing. After a large-scale radiation incident, individuals from a variety of professional backgrounds may be asked to help with such screening. This is often referred to as a radiological survey. Ideally, this role should be defined in your emergency response plan and the task should be assigned before any incident happens. Training in the use of the equipment should be conducted as part of your emergency preparedness planning. To use this training video, however, it's not required that you have any prior familiarity with radiation survey equipment. So in a pinch, this program can be used as a just-in-time training tool. If you have some familiarity with radiation survey equipment, you may use this program as refresher training. You may also want to review the supplemental material on this DVD and the use of alpha scintillation detectors and ion chambers. Now, please remember that individuals with specialized training, such as health physicists, medical physicists, or radiation safety officers should guide the overall radiological survey process and assume responsibility for meeting screening standards and making judgment decisions about triage. Before learning about the equipment, it helps to understand the difference between radioactive or radiological contamination and radiation exposure. If this basic radiation principle is unfamiliar to you, take a moment, please, to review these terms. You can do this by viewing a short two-minute animated film entitled "Radiological Contamination and Exposure” in the Supplemental Resources section of this DVD. Now, let's begin our introduction to radiation survey equipment.
Welcome, I'm Sarah Roberts, a Certified Health Physicist with the Oak Ridge Institute for Science and Education. It is my pleasure to introduce you to the radiation detection instrumentation typically used to survey people for contamination. These instruments are Geiger-Mueller, GM, or Geiger detectors, and within this class, there are several principal types. As you can see from this display of instruments, there are many companies that manufacture instruments, so they come in all sizes, shapes, and colors. Some of you may not have seen these instruments or be familiar with these terms I just used, therefore let's take a look at these detectors in a bit more detail. When monitoring people for radioactive contamination, very sensitive detection instruments are necessary. Survey meters, especially Geiger counters, serve this purpose well. Survey meters are the hand-held instruments most commonly used to measure radiation. They are frequently referred to as "radiation detectors,” "survey meters," or "friskers.” This kind of detector has several possible uses -- locating gamma-emitting sources of radiation, locating contamination on people and equipment, and establishing control boundaries at an incident scene. The best GM to use when looking for contamination on surfaces is the pancake GM, because it has a very thin window which allows even beta radiation to be detected. Geiger-Mueller detectors, like any other piece of equipment you might employ in your line of work, have their good points and bad points. GM detectors are easy to use, easy to maintain, and are generally pretty rugged. They are almost always going to be your instrument of choice when you want to look for radioactive contamination on people. They have other applications as well. On the negative side, the window is easily broken, so handle it carefully. Before we consider how GM detectors are actually used, there are four important steps I would like to outline. First, check the batteries, cable, and detector to make sure all of them are okay. Second, take a background reading in an area that you know is not contaminated. The term "background measurement" refers to radiation levels that are already present in our natural environment. Third, while you are surveying, scan slowly and as close to the object as possible without touching it, so as to prevent contamination on the detector. And lastly, always remember to record your survey readings accurately. Now, with these four overall steps in mind, let's walk through the entire procedure. We've provided a step-by-step handout of this procedure in the inside cover of the DVD case. You may want to follow along with me with the written procedure in front of you. When you look at this instrument set, what you'll notice first is a count-rate meter of some sort. You also see a cable with connectors on each end and a long handle with a small pancake on the end, again, with a very thin entrance window on one side. Now, examine all these items for obvious signs of damage -- a broken window or frayed or broken wires, for example. Next, assemble your equipment by connecting one end of the cable to the meter and the other end to the pancake probe. With the meter on, wiggle the cable near the connectors to see if this causes erratic behavior of sound or the display. If so, the cable was defective. Now, let's discuss the different features of this instrument. First, all instruments require periodic calibration and testing. Ideally, your institution will have a routine calibration schedule for your instruments, therefore there should be a sticker on the instrument with a current date of calibration. However, in the event of an emergency, if the sticker shows an out-of-date calibration, do not let that stop you from using the instrument. Instruments can still be used for emergency screening, even though they may be out of calibration. Go through the routine check of the instrument. If it's performing properly, you can use it for screening. But remember -- this should be done only in an emergency situation, as directed by the person in charge. Okay, the first step in using a radiation detection instrument is checking the batteries. Dead or dying batteries are the number one cause of problems for these instruments. For most instruments, checking the batteries will be done by either pressing a battery check indicator button or turning the control knob or range switch to battery, or "BAT". This instrument has a control knob, so rotate the knob in a clockwise direction from the "OFF" position. The first position to the right is usually the battery check. Turn the switch to that position. If the needle does not move to the position on the meter face that indicates the batteries are okay, you'll probably need to replace them. For this instrument, you can see that the needle is indicating that the batteries are okay. While we're talking about batteries, let me give you a few additional tips. It's not a good idea to leave the batteries in the meter for long periods of time -- months, for example -- if you aren't going to use them. They can corrode in the meter and ruin it. Also, look at the battery terminals to see if they are corroded. If so, you might be able to clean them with the eraser on the end of a pencil. Do not use batteries that have been rolling around in a drawer somewhere, that are unwrapped or have been removed from their original packaging. You should be sure to keep a fresh stock of batteries available for each use. Please remember that, even though you may have installed new batteries, you should not forget to turn the range switch to the battery check position and confirm that they are okay. Let's continue talking about the range switch. As you continue to move the range switch clockwise, you are on the scale settings. The scale controls the sensitivity of the instrument. At its smallest scale, times 1, the instrument is very sensitive and can measure small amounts of radiation. Likewise, at its highest scale, times 1000, the instrument is able to measure higher amounts of radiation. When taking a radiation reading with this type of instrument, the reading on the display must be multiplied by the scale setting. Now, let's look at the variety of displays that can be found on the instruments. They may display either in units of counts per minute, cpm; milliroentgen per hour, mR/hr, or milliR, microroentgen per hour, microR/hr or microR, or some combination. Instruments have at least one display and a variety of controls. Different instrument manufacturers will place the display and controls in different design configurations. The basic components will be the same, but they may be found in different locations on the instrument. Displays are either analog or digital. On this screen is an example of an analog display. The display has a needle that will move from left to right as the amount of radiation increases, similar to the way a speedometer needle moves from left to right as you increase the speed in your automobile. Different instruments may display different units, or the same instruments may display multiple units. When using a GM detector, always use the count per minute, cpm, scale. If your instrument does not have a cpm scale, then use whatever scale is displayed, but always remember to note the units when you record the results. When taking a radiation reading with this type of instrument, remember that you must multiply the reading on the display by the scale setting. For example, this instrument is reading 350 counts per minute. The instrument is set at times 1, so 350 times 1 is 350 counts per minute. Let's take a look at another reading. In this image, the needle is at approximately 280. You may think that the reading is 280 counts per minute, but remember to check the scale. Since the instrument is set at times 10, the reading is 280 times 10, or 2800 counts per minute. This is a digital display. In this image, you see a numerical readout in counts per minute. You should now be ready to use the instrument. For our final and possibly most important subject, let's talk about how to use this instrument to conduct a survey. Most GMs have an audible output. One particle of radiation going through the window of the pancake probe produces one electronic pulse. One pulse produces one count or click. Let's listen to a typical response to background radiation.
[ Clicking ]
Again, this background radiation refers to radiation levels already present in our natural environment. Following a large-scale incident, you may have a lot of noise, therefore you may want to consider having a set of headphones plugged into your survey instrument so that you can hear the clicks. Also, eliminating the audible clicking noise may lessen the anxiety of the person being screened. While surveying, in addition to listening to the clicks, you need to watch the meter face to see if the count rate is changing. Your instrument is likely to have a toggle switch to select "F" for fast response or "S" for slow response. make sure the toggle switch is set for "F," or fast response, when surveying for contamination. Next, we need to conduct a source check to make sure the meter is working properly. With the meter on, place the detector close to the radiation source, such as a thorium-containing gas-lantern mantle, an orange Fiestaware plate, or a plastic button check source. With the appropriate range selected, probably times 10 or times 100, verify that the meter responds.
[ Clicking rapidly, loudly ]
It's not a bad idea to indicate the reading on the label and stick the label on the side of the case. That way, the next time you check the GM, you will know if its response is changing. Some rate meters come supplied with a convenient check source on the side and a label indicating what the meter response should be. In a pinch, if you need to use the instrument, but you do not have a check source available, you can assume the GM is working properly if its response to background radiation is between 30 to 200 counts per minute. Depending on the environment and what type of building you're in, the background radiation level can vary. In the event of a radiation emergency, background radiation levels may be higher than usual because of environmental contamination. One of the first things to consider -- this isn't a requirement, but a suggestion -- is wrapping the pancake probe with a protective covering. This will prevent contamination of the detector. You can use a rubber glove or a plastic bag or Saran wrap, for example. Next, I'm going to take a background reading. Make sure your range switch is set to times 1. Note that the needle fluctuates at low readings, and this is normal. You can change the toggle switch to "S," or slow response, to make a more accurate reading. I have determined that the background count rate is 40 counts per minute, or cpm. Now, make sure you move the toggle switch back to "F,” or fast response, when surveying for contamination. Keep in mind that you should turn the detector on well before you get into a suspect area. If the count rate steadily increases to a very high level, the meter pegs at the maximum reading, and then the response drops off dramatically, the GM detector may have saturated and zeroed out. This can happen in high radiation fields, and you should leave that area immediately. Now, let's conduct a survey. When surveying a person, hold the probe approximately 1/2 to 1 inch from the person's skin and systematically survey the entire body from head to toe on all sides. Move the probe slowly, about 1 inch per second. Try not to let the probe touch anything, and pay particular attention to hands, face, and feet. If massive numbers of people require screening, to avoid delays, it may be necessary to perform only quick-look spot surveys of the head, face, shoulders, and hands -- the most likely locations for contamination. Under those circumstances, it may also be necessary to use a screening criterion higher than twice the background. This guidance will be provided to you by the appropriate radiation authority. Locate the point that produces the most clicks. Document those locations and the radiation measurements. In general, areas that register more than twice the previously determined background are considered contaminated. Now, you have seen the GM in use, so this concludes the demonstration of the GM pancake detector. Let's review the four important points about using GM detectors. Remember -- check the batteries, cable, and detector to make sure all of them are okay. Take a background reading in an area that you know is not contaminated. While you are surveying, scan slowly and as close to the object as possible without touching it to prevent contamination of the detector. And accurately record your survey readings. If you need additional information about this instrument, refer to the enclosed GM detector job aid.
- Page last reviewed March 20, 2009
- Page last updated March 20, 2009
- Content source: Radiation Studies Branch (RSB), Division of Environmental Hazards and Health Effects (EHHE), National Center for Environmental Health (NCEH), Coordinating Center for Environmental Health and Injury Prevention (CCEHIP)
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