(RadCon for preppers post 2)- How to use a Geiger counter
April 19, 2017 at 7:48 pm #82650xsquidgatorParticipant
This post is intended to supplement various other information out there on radiation safety for the preparedness-minded person. Reading this information will hopefully give you a better understanding of what radiation is, how it can affect/hurts you, how to detect and measure it, and how to mitigate it’s effects if radiation is present. Understanding this information, should help you better understand all the “nuke war survival stuff” that’s out there in various publications. We’re just hitting some highlights of what radiation is, how much is enough to worry about, how to measure it, etc.
This post will explain how to use a Geiger counter to measure radiation levels. There is more to it than simply turn it on and read the dial!
Radiation exposure can be external and internal. External radiation exposure is where you’re getting beta/gamma rays coming at you from something nearby you, but outside of you. Internal exposure occurs when radioactive material (fallout) you’ve eaten or breathed is stuck inside of you, and the alpha/beta/gamma rays are irradiating you from the inside.
So, how do you tell if radiation is present, and how much? Ionizing radiation can’t be seen, felt, tasted, heard, or smelled, it can only be detected by using instruments that can detect ionization caused by the radiation. Radiation detection is a yuuge topic and even a quick course in the theory of it requires a 15 week upper-division engineering course. Let’s pare it down and get rid of the exotic laboratory instruments, sticking to the radiation detection equipment that a prepper might acquire or encounter. These would include survey meters and possibly dosimeters.
Radiation Survey Meters
A survey meter is a handheld radiation detector that use to detect radiation, and measure the rate at which you’re accumulating it. Some survey meters, when properly calibrated, will tell you how many R/hr, rad/hr, or rem/hr you’re experiencing at the detector part of the survey meter. As Robert posted in here, it’s like the speedometer in a car telling you how fast you’re going. In a car, distance = distance rate (speed) x time. For radiation, dose = doserate x time.
Simple example: Your radiation meter tells you that the radiation level is 10 mrad/hr. How long will it take you to receive 100 mrad, which is the amount the US government says you can receive in a year?
(1) Dose (mrad) = Doserate (mrad/hr) x time (hours).
Solving for time yields :
(2) Time (hours) = Dose (mrad) / Doserate (mrad/hr) = 100 mrad / (10 mrad/hr) = 10 hours
What your “radiation dose limit” is, is a different question which we partially addressed in our earlier post “How much radiation is a lot?” The US Government limit of 100 mrem is extremely conservative (safe), relative to the amounts of radiation it would take to actually injure you in the short-term.
Radiation survey meters can be one of several types of detectors that convert ionizing radiation events into an electrical signal that you read on the meter. These types of detectors include Geiger-Mueller detectors (also known as Geiger counters), ion chambers, scintillation detectors, electronic dosimeters, and more. There are reasons why you might want or need to have each one of these detectors for different sources of radiation and situations, if you’re a radiation worker in industry or medicine. In my opinion though, most preppers wanting to buy a radiation survey meter should just buy a Geiger counter survey meter. Geiger counters are simple, robust, inexpensive compared to other types of detectors, and very versatile. A health physicist or radiation professional won’t use a Geiger counter for every radiation situation, but we can probably make it work for most anything we’re going to need.
Survey Meters – The Geiger Counter
A Geiger-Mueller or GM detector aka “Geiger counter” is a gas-filled radiation detector. <We’ll use the terms “Geiger counter”, “GM detector”, and “GM tube” interchangeably here> A voltage is applied across the gas, or rather, is applied between the walls of the chamber and an electrode. When ionizing radiation interacts with the (metal) walls of the gas-filled chamber and creates an ion pair (a positive charged ion and a negatively charged electron which is effectively knocked off of its parent atom), the electron (-) is attracted to the positively-charged electrode. The voltage applied across the GM tube is so strong that the electron rushes towards the positive electrode very vigorously, enough so that the electron itself ionizes gas molecules that get in the way. These ionized gas molecules give up electrons, which themselves are accelerated towards the positive electrode, creating more and more ions and electrons in a cascade or avalanche.
This electrical activity quickly ionizes all of the gas near the electrode, which creates a strong and easily detectable electrical signal. Geiger counters have circuitry set up to convert that electrical pulse into a “click” or a “chirp” which is played over an audio speaker, and which is also used to deflect the needle in a meter display. The higher the radiation level at the GM tube, the more clicks per minute or per second there will be. A fundamental concept here is that the Geiger counter is simply counting clicks, it’s not actually tallying the radiation energy deposited in the detector. If you recall, radiation absorbed dose (rads) is actually the absorbed dose per mass of material. The Geiger counter indirectly tells you the radiation level by the click-counting, which is fine so long as the conditions are similar to how they were when you calibrated it. If you send your GM counter off to be calibrated, the calibration lab exposes the detector to a known strength of gamma ray radiation and tells you what your instrument read compared to what it really was.
There’s something related that you should understand about Geiger counters and their click-counting. That concept is known as “detector dead time” and is the length of time it takes the gas in the GM tube to recover from the electrical pulse of an ionizing event. During this dead time following a “click”, radiation can interact with the detector and release ions and electrons, without causing another avalanche “click”. In weak radiation fields (low radiation levels) this doesn’t matter very much, but it could be crucial in a high level radiation field. Just because the GM counter isn’t registering very many clicks per second isn’t proof that the radiation level is low! The GM counter is less accurate in measuring the radiation rate (R/hr or rads/hr) in higher levels of radiation. You can think of it as simply the GM detector gets swamped by high levels of radiation, that’s not exactly true but the general idea is correct. This is one of the reasons radiation workers have additional types of radiation detectors that don’t have this limitation.
There are a number of Geiger counters available or “out there” which perform the same function. A Ludlum Model 14C is shown in the examples here, but the models below are so similar that they can be considered interchangeable for our prepper radiation safety purposes. Other equivalent models of Geiger counter include the
-Eberline Model 3
-Victoreen Model CDV-700
-(US Government) AN/PDR-27, and the AN/PDR-66
There are two common types of probes (detectors) that can be attached to a Geiger counter, “cylinder” probes, and “pancake” probes. See Figure 1.
Either type of probe should work for your survivalist purposes. The pancake probe is optimized for surveying the radiation coming off of a surface, whereas the cylinder probe is optimal for measuring general area radiation levels. Both probes can be used for both functions.
You can get a closer look at a pancake probe in Figure 2. The active volume of the pancake probe is a flat area in between the “pancake” housing of the probe. The metal housing of the probe helps shield the probe from extraneous background radiation. The pancake probe is most sensitive to radiation entering via the entrance window (yellow arrow).
A plastic cover protects the thin mylar window, when the probe is not in use. Remove the cover before taking a reading, so that you don’t shield the probe from what you’re trying to measure.
In Figure 2 (bottom), a cylindrical probe is shown. It’s a gas-filled detector just like the pancake probe, but in a different arrangement. The entrance window of the cylinder probe is on the end, covered by a red plastic protective cap.
Using a Geiger counter
If you purchase a Geiger counter, it will probably look similar to the one in the figure, which is one I use at work. The Ludlum Model 3 and Ludlum Model 14C are classic Geiger counters whose design has been used since probably just after WWII. They use D-cell batteries (have extras on hand!), and also you’ll want to acquire a “check source” of radiation to use to verify the thing is actually working and properly detecting radiation. You can buy a check source such as an “Exempt Quantity” of Cs-137, or you can acquire something that’s naturally radioactive, too. A Coleman lamp wick, taped to the side of the detector, works fine (these contain enough naturally-occurring Thorium and other stuff that you get a nice signal off of them).
*Each time* you pick up a Geiger counter or any radiation detector, you need to run a couple of checks on it to make sure it’s working, before you rely on it:
1) Give it a quick once-over physical inspection to see if it’s been dropped, damaged, etc on the outside.
2) Turn it on, and use the battery check function to make sure there’s enough battery power to run it properly.
3) Place the probe against the check source and make sure you get the meter reading you expect. Also turn on the audio indication and verify that you get rapid clicking/chirping with the probe placed up against the check source.
4) Then, place the detector on its most sensitive scale (that isn’t pegged high), and being taking readings.
Most or all Geiger counters have multiple scales, which are usually labeled something like x1, x10, x100, x1000, etc. You’ll have to experiment with the check source and the meter selection switch to see which one is the most sensitive, some manufacturers label their switches backwards from other ones. “x1000” may be the most sensitive scale, or the least sensitive, you’ll have to see for your detector and just know that.
Most Geiger counters have a single meter needle, but two sets of scales printed on the meter face. One is usually labeled “mR/hr”, and the other is labeled “counts per minute”. For general area radiation level readings, use the “mR/hr” printed scale and multiply/divide appropriately for the selector switch position you’re using, e.g. x1, x10 etc.
See Figures 3 and 4.
I recommend you use the following technique when taking general area radiation level readings. In one hand, hold the electronics/counter part of the Geiger counter against your hip. Hold the probe in your other hand, and only move the probe around while keeping the counter/electronics indexed into your hip. You are more likely to bang the probe into something if you are moving the probe and the counter-electronics at the same time, which could damage the probe or rub radioactive contamination off a nearby surface onto the detector probe. You also will only have to look down at one consistent place to read the meter.
When you record a meter reading, select the most sensitive meter scale that you can use without pegging out the displayed reading. This will give you the most accurate reading possible.
One nice thing about Geiger counters is that you can turn on the audio and listen to the count rate (clicks or chirps), as well as use the meter indication. This allows you to survey an area and hear if it’s getting “hotter” or “colder” as you move, without having to stop and read the meter. That’s a big deal when you’re surveying a large space and trying to find if there are any spots that are over the allowable limits.
You might just want to use the meter to get a single measurement of radiation levels somewhere, and that’s fine. A more typical use of a survey meter is to perform radiation surveys, that is, to record radiation level measurements over an area. Whatever you do, as Robert indicated, you need to RECORD (WRITE) DOWN YOUR READINGS including where and when a reading was taken. The radiation level itself is something you want to know, but you also want to know the rate or trend of increase/decrease as well, and you must have written records for that.
So far, we’ve talked a little bit about how Geiger counters operate, how to verify yours is working, and how to use it to record a general area radiation level. That’s pretty useful, but the Geiger counter can also make other kinds of useful measurements for you, such as how much loose radioactive contamination is on a surface, or what the concentration of airborne or waterborne radioactivity is.
Using a Geiger counter for surface contamination surveys, and some rules-of-thumb
There are two useful techniques for surveying a surface for loose radioactive contamination, “frisking” and “swiping”. “Frisking” means moving the GM probe near the surface you are checking for contamination, and monitoring the reading to see if there is any contamination. “Swiping” means that you take a bit of paper towel and wipe off the surface you want to check, and then bring that piece of towel or filter paper over to the GM counter to measure how much is on the paper. Swiping and frisking are both useful, depending on the situation. “Frisking” is convenient when background radiation levels are low, and you want to quickly check something, perhaps your own body head to toe after you’ve come in from outside, to see if there’s radioactive contamination on it. “Swiping” is appropriate if the background radiation level is high. (We’ll explain in a bit)
An example of “frisking” a surface to detect surface contamination is shown in Figure 5. Bottom line- slowly move the probe 1-2 inches off of the surface, no more than 1-2 inches per second, and monitor the GM counter reading while you do this.
Using a Geiger counter for swipe/frisk contamination surveys– how much of a reading is too much?
We’re going to give you some simple answers here for some situations, based on what we use in the nuclear power industry and in the radiation medical field. However, even for these simple cases of “is it more than this number or less than that number?, understand that your SHTF situation isn’t the same as the people who use this for their job. The more you know about this stuff, the better the judgments and decisions you’ll be able to make based on the radiation survey readings you take yourself.
Swipe survey for loose radioactive contamination – procedure
1) Perform your pre-use inspection and operational check of your Geiger counter.
2) Take a background radiation reading, in the most heavily protected (shielded) area you can. That is, find the area around you with the lowest radiation readings. Background radiation will make it more difficult to accurately measure the number of counts per minute above background you are reading from your sample. Take your time to get a background radiation level, especially if background radiation is now elevated from what it usually is. Log your background count rate “b” in counts per minute and also write the time this background level was taken in your log. Take a background level at least once per day that the GM counter is being used.
3) Put on rubber gloves.
4) Take a swipe of the area to be checked using a piece of filter paper or paper towel. Put the swipe in an envelope if you are going to count the swipe somewhere else. Mark the envelope with the location of the swipe and the time it was taken.
Optimal swipe technique: make an “S” shaped wipe pattern covering an area about the size of your hand, or about 100 square centimeters. (Figure 6)
Alternately, you can swipe a larger area if you want, but, you are concentrating possible material from a larger area onto your swipe- the thumbrules we’ll give you don’t apply if it’s not collected over 100 cm^2.
5) Remove the swipe from the envelope, and using gloves, hold it about an inch away from the entrance window to your GM probe. (Figure 7)
6) Record the number of counts per minute from your sample- this is “g” or your gross count rate on the sample.
7) Obtain the net count rate “n” on your sample by subtracting the background count rate from the gross count rate.
In equation form, n = g – b.
8) Apply this thumbrule: 100 COUNTS PER MINUTE ABOVE BACKGROUND IS ABOVE THE LIMIT. (see footnote 2)
Note about the “max allowed” limit. The number above, 100 cpm > background, is the maximum allowed limit for nuclear power plants, shipping containers used to ship radioactive material to hospitals, etc. right now (2017). I suggest that you consider this a guideline. We’ll talk later in another post about why and how you might want to modify ‘peacetime’ levels like these for your post-SHTF situation. The point for now is more along the lines of being able to use your radiation detection gear properly, first.
There are some things for you to consider here which are buried in various technical manuals etc but which are not obvious from reading them – these are from experience.
-This limit above is actually a somewhat strict limit- in normal industry and medical settings, we keep things very clean and so even a small amount of radioactive contamination will exceed this limit.
In a post-SHTF situation such as nuclear attack or a nuclear power station accident, levels around you may be high, perhaps much higher than this. But, simply being above the limit that applies “in peacetime” doesn’t mean it’s necessarily dangerous.
-If you happen to survey radioactive contamination that is juuust at the limit, you’ll need to be very careful in counting your sample in order to not miss the elevated count rate.
Example of how to use your Geiger counter post-SHTF
I can think of one very likely use for a Geiger counter after a radiological event in your AO: using the Geiger counter to keep your safe space, safe and uncontaminated. In very broad strokes (hardly any numbers), let’s go through a scenario. After reading this post and other posts we’ve done on this, you should have the background information and basic idea of how to use your GM counter to help you through it.
Suppose you are concerned that either WW III is happening soon, and you might be affected by radioactive fallout from a target area. Or perhaps you have prepared against the possibility that a nuclear power plant somewhere might have a serious accident, and again you may receive fallout.
The simple version of what you do if that happens, is to seal up your house and stay inside. You’d shut windows, turn off HVAC that would draw air from outside, tape up doorseals and any other holes from outside into your house, that kind of thing. If you can create a shielded space, to protect you from gamma rays being emitted from particles in the fallout plume, you’d do that and get inside it. Listen to the news and see what they say about how bad it is, where it’s going, what the authorities say you should do where you live.
With your Geiger counter, though, you have your own ability to measure radiation levels near you, and so you’ll know that background radiation levels around you are going up. You can seal up the house, and hunker down inside there. You can use your GM counter to find the lowest radiation levels in the house or in your basement, and stay there. If it rains and you get a puddle of water in your basement, you have a Geiger counter to check it and see if it is radioactive. You could even (wearing gloves and as much rubber protective gear as you have) clean up the radioactive puddle, and then swipe-survey the area after to make sure you got all or most of it.
At some point, you’ll need to go outside to check on conditions or for some other reason. When you return, you don’t want to contaminate your safe place inside by tracking in contamination. Set up a “control point” at the doorway, and on your return, don’t step past there until your entire body gets frisk-surveyed head-to-toe. If you are contaminated, you remove the contaminated clothing and put it in a garbage bag to put somewhere else. If you’re skin shows more than 100 cpm over background, gently wash it with lukewarm water and soap until you’re below the limit, or as low as you can get it. If you need to, you can shave the hair off an affected area to help decontaminate it, and you can also use duct tape to help pull radioactive dust off of a surface that isn’t easily washable. The rinse water, tape, everything used to decontaminate the area needs to go into a plastic trashbag and kept elsewhere.
If say your stomach area continues to show some counts per minute above background despite repeated washing, you may have ingested or inhaled some radioactive material (ie- internal contamination). Without medical help, you can still decontaminate an internally-contaminated person by having them drink lots of water, take laxatives, etc to move material through the body and out of it.
Eventually, even if you never hear from another person again, you’d be able to tell if it’s reasonably safe outside again. Fallout from a nuclear accident or nuclear attack should drop off quickly after an event (but still be prepared to spend a number of days plus inside!), especially if the winds change and blow it towards somewhere else. With your own radiation detection instrument, you’ll know for yourself as soon as possible.
If you’ve read and understand the sections on how to use a Geiger counter, hopefully you feel more comfortable about using your own device to take the radiation measurements mentioned above, and to help keep yourself as safe as possible from radiation post-event. A Geiger counter is to radiation, what night vision is to the dark.
Additional things to think about …
-How do you determine your radiation background level, b? The meter reading can be quite erratic, especially if the “FAST” meter response is selected (some meters have a FAST/SLOW response selector, some don’t).
-What do you do if your background radiation level is elevated? (ie- if background has shot up to 2000 cpm, can you reliably discern that what you’re surveying is 2100 counts per minute vs 2000?
-What are the limits of your Geiger counter, i.e. detector dead time? Do you need another, less-sensitive radiation detector for measuring higher levels of radiation?
-When do you use the meter to take a reading, and when do you use the audible speaker to note a reading?
-When do I use the “CPM” scale on the Geiger counter, and when do I use the “mR/hr” scale?
We’ll probably do a short post later involving simplified “counting statistics” to address these issues, so that you get an accurate reading and can interpret it correctly.
-Also to discuss in a later post: now that you know how to measure radiation levels, and radioactive contamination levels, how much radiation exposure would you receive from a contaminated surface (perhaps the outside wall of your bugout location, say)?
There are some simple rules of thumb that will let you make an estimate of this exposure, if you know how to measure the contamination levels. (Footnote <2>)
-How do I use a Geiger counter to measure the amount of airborne particulate radiation (fallout)? The concentration of radioactivity in a drinking water sample?
<1> For contamination surveys, swipe or frisk, 100 cpm above background corresponds to the (US Navy) limit of 450 uuCi/100 cm^2.
<2> The US Navy has put out an excellent technical education document, NAVSEA document “APPLIED ENGINEERING PRINCIPLES MANUAL” which covers a substantial portion of the US Naval Nuclear Power School’s academic curriculum. It’s heavy on math, physics, electrical, mechanical, and chemical engineering, but section 7.3 “Radiological Controls” is relevant to our discussion. See this section for rules of thumb, that you can use to estimate your radiation dose due to exposure to varying levels of radioactive contamination.
Available at http://www.navsea.navy.mil/Portals/103/Documents/NNPTC/Electrical Eng/applied_ee_v1.pdf
April 19, 2017 at 7:55 pm #82651xsquidgatorParticipant
Please stand by while I figure out how to get the Figures to show up…
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