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Thursday, April 28, 2016

Measuring Radiation

Since Erin has explained what ionizing radiation is and the hazards it poses, I thought I'd add an article covering ways to measure it. Since you can't see, taste, or feel ionizing radiation, you need a tool to find out if it is present (qualitative analysis) and how much is there (quantitative analysis). Some detectors are designed for a specific type of radiation, while others have options or shields that will let you choose which kind is being measured.

Different meters also have different ranges of detection, which will require a decision on your part regarding what you expect to find before you get a meter. As Erin covered here, there are various standards for reporting an amount of ionizing radiation, and the meters that are on the market use most of them. Be prepared to do some conversion of units when looking at buying a meter.

Most of my experience with radioactive materials was many years ago while I was in the Army. I worked around nuclear materials, and we used a variety of meters to detect contamination. Technology has since replaced all of them with solid-state detectors and LCD displays -- no reason to carry a 10 pound meter when a 1 pound meter will do the same job! Unfortunately, costs have not come down with the size and weight, so expect to pay as much for a quality meter as you would a quality firearm. 

I do not currently work with radiation, and haven't used anything digital, so I won't be giving recommendations. I will however offer my thoughts on what I've seen on the market, and give a few caveats about what to look out for.

I'm going to break the meters down by how the detectors are used, and then further by how they work, since that determines type and range of radiation detection.

Dosimeters are carried on your body as you move through an area that may be radioactive. They will normally only indicate the totaldose you have received and aren't useful for finding out the intensity of radiation present. Most will not measure Alpha particles.

Film Badge
These have been around since the 1940s, and you may still see them in use by staff in hospitals or clinics that run X-Ray machines. They are basically a piece of black-and-white film that is sealed in a paper or plastic container (to keep out light) and is worn on the body with a clip or in a holder. Positioning of where it is worn varies by work environment, but it is normally at the hip or on the breast pocket. 

Sometimes they are placed in a plastic holder that has varying densities of blocking material to allow measurement of the energy of the radiation. After use, the film is removed from its container and developed. Any radiation (other than Alpha) will cause white space on the film, so by measuring the white space it is possible to deduce how much radiation passed through the badge.

Pen-style Dosimeter
My personal favorite, these use no batteries and the only moving part is a hair-thin wire. They are cheaper than many of the other reusable types, but do require a “charging” station. When I was in the Army, we wore them duct-taped to our left bicep so we could check them without using our hands.

These work by applying a high voltage electrical charge to a fine wire, causing it to be repelled from one side of the body of the meter. Ionizing radiation depletes the static charge by (duh) ionizing the air in the meter. As the charged wire relaxes, it moves along a scale etched onto a piece of glass that you look through. This makes them very easy to read as long as you have a source of light.

Thousand of surplus pen-style dosimeters and their chargers are available as surplus Civil Defense stock, but make sure you're looking at checked and calibrated items. There is usually a reason for them being surplussed, and it isn't always because they are being replaced with something newer. They are made in a variety of ranges from 0-500 mrem to 0-5 rem, so watch what you buy. A combination of a low range dosimeter and a higher range one would cover most exposures.

Digital Pocket Dosimeter
Fairly new technology, these can be as small as a keychain, but at $140 they aren't cheap. The NukAlert is made in the USA and has a good warranty, 10 year battery life, and a wide range of measurement (100 mR/hr to 5000 R/hr). There are other models that are pocket-sized, but there are too many versions for me to try to compare. I'd be wary of any of the imports that can't even find a native English speaker to write their ads and descriptions. Equipment made in Europe, including Russia and the Ukraine, seems to get better reviews than the stuff coming out of China. Quality control is vital when playing with radiation.

Indicator Cards
Using chemicals that change color when ionized (similar to pH indicators) on a credit card sized piece of plastic makes sense. (There is also a postage-stamp sized sticker that you can apply to the back of an ID card that works the same way.) It's similar to a film badge in that it is a one-time use indicator, but doesn't require a developing/analyzing lab. These have limited shelf-life and cost about $20 each for the cards, $25 for a five-pack of the stickers.

Survey Meters
Sometimes called Radiac meters from the military acronym Radio Activity Detection, Identification, and Computation. survey meters are used to check physical objects to see if they are radioactive. Normally hand-held units that can be “swept” over an object or person, they may also have removable detectors connected to a cable for remote sampling. 

Survey meters only measure what is in front of the detector, and the analog versions will not record total dose.

There are a lot of the yellow Civil Defense surplus meters available for anywhere from $20 to $1000, but they have some issues:
  • They require calibration every 3-5 years at about $100 each time, and
  • There is only one civilian company certified to calibrate them. 
  • Many of the ones you'll see for sale cheap on sites like eBay are in need of maintenance or are broken beyond repair. Just because it passed a battery check doesn't mean it is in working order.

Geiger-Mueller (G-M) Tubes
These are the source of the generic name of “Geiger counter”. A sealed chamber filled with an easily ionized gas and having a high (<400V) voltage applied to the shell and a wire inside, ionizing radiation creates a path for electrons to flow from the wire to the shell, creating a spark which is easy to detect. The spark is what causes the ticking or clicking noise associated with radiation detectors. 

They are most commonly used for Gamma/X-Ray detection because the material of the tube will block Alpha and Beta, but some will have a “window” that will allow Beta through. These are very sturdy (they are designed for field use after all) and have been around for decades with a good track record.

Ionization Chamber
Some detectors use a flow of easily ionized gas through a chamber instead of a sealed tube. The ones I'm familiar with used propane and worked the same as a G-M tube, but had a much larger “face” that allowed for checking a larger area at one time, which is handy when checking for Alpha particles with their short range.

Scintillation Detectors
There are certain chemicals that emit light when hit by ionizing radiation, and they make for a fairly simple detector. Light is easy to measure with a photodetector, so pairing the two inside the detector head is common. Used to detect and measure Alpha/Beta/Gamma radiation, these are sturdy and don't require the high voltage of a G-M tube. If you see a probe that has sides sloping up towards the handle, it is probably a scintillation detector.

Solid State
The market is full of detectors of questionable quality. There are $30-50 add-ons for a smart phone that will detect Gamma/X-Ray radiation, but have accuracy of plus or minus 30%, which is poor to say the least. 

Be sure that you are comparing apples to apples as well. There are a lot of Electromagnetic Radiation meters out there that use the words “dosimeter” and “radiation detector”. While these are useful for checking to see if your microwave oven is leaking or chasing ghosts through an abandoned mental hospital, they will not help you find or measure ionizing radiation; they are designed to pick up a wide band of radio-wave energy, not radioactivity. 

The odd-ball that doesn't fit either category, the Kearney Fallout Meter was developed for use in a fallout shelter to give the residents a way of checking for hot spots and leaks of radiation, as well as assuring that they were safe where they were. 

Designed to be built with common household items, the KFM is a larger version of a pen-style dosimeter. It uses an electrostatic charge on a folded piece of tin foil that has been suspended over a printed scale. Ionizing radiation reduces the charge on the folded foil much like it does the quartz fiber or wire in a pen-style dosimeter, and readings are taken in a similar manner.

Get a set of the plans -- they're freely available on the web -- and keep them in your shelter. Maybe even build a test model on some rainy weekend, just to play with it. Tear apart an old smoke detector to get a test source of Americium-241 if you want to make sure it works.

There are few things less well understood, and therefore more frightening to the general public, than radiation. Certain segments of society use this ignorance as a method of control or profit, and education is the only real cure. Knowledge is power and I believe we should all be willing to share it

Check reviews on trusted sites and do your research before relying on anything that can impact your health, radiation meters are no different. Quality comes with a higher price tag and you usually do get what you pay for.

The Fine Print

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