Dosimetry: Measuring Radiation [Hackaday]

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Thanks to stints as an X-ray technician in my early 20s followed by work in various biology labs into my early 40s, I’ve been classified as an “occupationally exposed worker” with regard to ionizing radiation for a lot of my life. And while the jobs I’ve done under that umbrella have been vastly different, they’ve all had some common ground. One is the required annual radiation safety training classes. Since the physics never changed and the regulations rarely did, these sessions would inevitably bore everyone to tears, which was a pity because it always felt like something I should be paying very close attention to, like the safety briefings flight attendants give but everyone ignores.

The other thing in common was the need to keep track of how much radiation my colleagues and I were exposed to. Aside from the obvious health and safety implications for us personally, there were legal and regulatory considerations for the various institutions involved, which explained the ritual of finding your name on a printout and signing off on the dose measured by your dosimeter for the month.

Dosimetry has come a long way since I was actively considered occupationally exposed, and even further from the times when very little was known about the effects of radiation on living tissue. What the early pioneers of radiochemistry learned about the dangers of exposure was hard-won indeed, but gave us the insights needed to develop dosimetric methods and tools that make working with radiation far safer than it ever was.

Rads and Rems, Sieverts and Grays

While there are a lot of tools for measuring the dose of radiation a person receives, there needs to be some way to put that data into a meaningful biological context. To that end, a whole ecosystem of measurement systems exists, all of which boil down to some basic principles of physics and biology.

The first principle is that sources of radiation are all capable of imparting kinetic energy into tissues, either in the form of ionized particles (alpha and beta radiation) or electromagnetic waves (gamma radiation and X-rays). Different types of radiation have different impacts on tissue, and those differences need to be taken into account when calculating dose, through weighting factors that reflect the relative biological effectiveness (RBE) of the radiation. This is basically a measure of how much punch the radiation packs. For example, alpha particles, which are relatively massive helium nuclei, are weighted 20 times higher than beta, gamma, or X-rays.

The second principle behind dosimetry is biological in nature, and reflects the fact that in almost all cases, whatever deleterious effects of radiation experienced by an organism are caused by interactions with its DNA. There are certainly other effects, like ionization in the cytoplasm of cells and production of free radicals, but by and large, the big problems with radiation happen as a result of it crashing into DNA, particularly while it’s in the act of replicating itself. That’s why the rapidly dividing cells in the blood-forming organs (bone marrow mostly), the linings of the digestive system, and the gonads are particularly sensitive to radiation.

Bright yellow SPIDs bearing the familiar red, white, and blue Civil Defense logo became very popular during the Cold War in the United States. Millions of the devices were made, some calibrated with scales that would only be useful only in catastrophically high radiation environments. SPIDs are extremely robust devices and most of them still work after many decades, and even the chargers, with their very simple electronics, can still be found in working order.

Solid(er) State

Film badges and quartz-fiber SPIDs were handy, but technology marches on, and cheaper, better methods for dosimetry have largely supplanted them. Thermoluminescent Dosimetry (TLD) has become a very popular method for keeping track of exposure. It relies on the tendency of certain materials to “trap” electrons excited by high-energy photons passing through them. These trapped electrons, which accumulate in the crystal matrix in proportion to the amount of radiation that has passed through it, can be released to their ground state simply by applying some heat. The light released is picked up by a photodetector and used to calculate the dose received.

TLDs for most commercial dosimetric applications are based on crystals of lithium fluoride doped with a small amount of manganese or magnesium, which creates electron traps. TLD crystals can be small enough to build into a plastic ring, to monitor the dose received by the extremities while handling radioisotopes, for example. A related method, known as optically stimulated luminescence (OLS), uses a beryllium oxide ceramic as the trapping material; electrons are released from the trap using a laser tuned to a specific frequency and a photodetector reads the emitted light.