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Intro to Radiation

A Layman’s Intro to Radiation

Updated 8/9/11 – Article Source

Here is an article from a Senior Reactor Operator at the Reed Research Reactor, whose duties include being aware of radiation levels in the facility and adjusting behavior appropriately.

What is Radiation Dose?

When people talk about dose, they’re usually using sieverts (or rem, if in the US). Sieverts (Sv) are best used for external doses, like say the dose you receive from a giant hunk of radioactive cobalt across the room.

To understand sieverts, you should first understand grays (US equivalent: rad). 1 gray is equal to 1 Joule of energy per kilogram of matter. Energy/matter provides an idea of how much effect the radiation has, scaled for the size of whatever it’s hitting. Sieverts include a quality factor in order to take into account the biological effect of the radiation (1 Sv = 1 Gy*quality factor). Alpha particles have a quality factor of 20, because they deposit all their energy in a small area. Gamma rays and beta particles have a quality factor of 1, because their energy is more spread out, meaning the tissues struck by gamma rays are more likely to recover. So, while different types of radiation have different effects, 1 Sv of alphas is equivalent to 1 Sv of gammas.

Contamination is a little different from radiation. Radiation consists of tiny invisible particles, the largest of which is the size of a helium nucleus. Contamination consists of actual atoms or molecules, and is best understood as “radioactive stuff wherever we don’t want it”. It is usually expressed in terms of counts per minute (cpm) or decays per minute (dpm), and can be separated into “removable” and “fixed”, which are exactly what they sound like. Counts per minute are what you see on your geiger counter. Dpm is just cpm corrected for the efficiency of the detector (about 10% on your average geiger counter, should be calculated when the meter is calibrated). At the Reed Reactor, we consider something “contaminated” and in need of cleaning when it’s reading 1000 dpm per 100 square centimeters.

Inhalation and Ingestion

1000 dpm/100 cm^2 of radioactive material won’t create much of a radiation field. The concern with contamination is instead that it might get inside someone’s body. When radioactive material is inhaled or ingested, describing its effect gets a bit more complicated. One measure which seeks to provide an equivalence between inhalation/ingestion and external doses is the committed effective dose equivalent, or CEDE. Defined by the Nuclear Regulatory Commission, it describes the dose over the next 50 years to the affected organs, multiplied by a weighting factor based on how easily damaged the organ is. The 50 mSv annual dose limit on the chart below is the limit on both internal and external exposures. All internal exposures can be treated like this, providing an effective means of comparison between a hand x-ray and radioiodine thyroid treatments. If you’re interested in the math, I’ve explained it here.

Charts

First, here’s a chart laying out the scale of various doses. The top chart covers doses in one day, while the bottom covers doses over a whole year.

I separated these charts into doses received on a short timescale (in a day) and doses received on a longer timescale (a year) for a reason. A dose which is acute, that is, received over a short period of time, has a much greater biological effect than one received over a longer period of time. This is because radiation acts by depositing energy in tissue, damaging your cells. Very minor damage, repeated often, will be better repaired than a major assault. Doses can then cause either deterministic effects, where X dose causes Y problems, or stochastic effects, where X dose raises the probability of Y by Z amount. The most well-known deterministic effects are what’s commonly known as radiation poisoning, while stochastic effects mostly consist of cancer of one sort or another. Both the timing and absolute dose determine the effect — 1 Sv over the course of 20 years (a US radiation worker receiving his or her limit for the year 20 years in a row) will not cause radiation poisoning, but a dose of 1 Sv over 5 minutes certainly will.

With that in mind, here is a chart outlining the more extreme doses — where various effects set in, and what limits are in place in emergencies.

If you’d like to see and compare all these doses together, check out Randall Munroe’s chart, which covers a wider scale.

Radiation Protection Measures

The easiest way to avoid getting a dose is to not be in the field in the first place. This is made easier by the inverse square law, explained in the graphic below.

What this means is, getting farther away from the radiation source goes a long way towards limiting your exposure.

If you can’t get and stay far away from the radiation source, your second option to limit your dose is to just not spend that much time next to it. Radiation fields usually have units of dose rate, meaning the time you spend in them is just as important as the strength of the field.

Finally, if you have to spend a long time near a large radiation source, you can still limit your exposure by putting a lot of the appropriate shield between you and it. Lead is most commonly used, but if the source is giving off a lot of neutrons, you will want some water and boron instead. These three principles are often distilled down into just “time, distance, and shielding”, and motivate many radiation protection measures.

Most of the rest are motivated by controlling radioactive contamination in the form of dust or aerosols, because ingesting, inhaling, or otherwise absorbing radioactive material into your body is worse than just standing in a field. This is what all this business with iodine pills is about — iodine is retained in the thyroid, and iodine pills aim to flood the thyroid with regular stable iodine so the radioiodine can’t hang out there. That said, don’t all rush out to buy them — they’re pretty bad for you too, and shouldn’t be taken unless you know you’ve actually been exposed to radioactive iodine. Entire west coast of the US: you haven’t, stop panicking.

Hopefully, this information is helpful in providing some context about the events in Japan, as well as some general background on radiation protection. Comments, corrections, and complaints can be directed to emcmanis at reed.edu.

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