How Radiation is Measured
Radiation is measured in many related, but not quite the same, forms. This section is meant to be a resource for understanding what the main kinds of measurements mean, to better understand both natural and human-caused radiation exposure. To understand what you're exposed to naturally, from medical treatments or nuclear power plants, there are four different categories of measure: Radioactivity, Exposure, Absorbed dose, and Dose equivalent (READ). While all of these measures are important and interrelated, a dose equivalent is the most directly related to health effects.
Before contact with anything living, radioactivity is a measure of atoms disintegrating over time. Using Becquerel (1 disintegration per second) or Curie (37 billion disintegrations per second), the raw activity of a material is measured. The curie, so many times larger than a becquerel, is a benchmark equal to the activity of 1 gram of pure radium. This activity, however, doesn't take into account what is emitted; which could be an alpha particle, beta particle, gamma ray, x-ray or some combination(1). As each of these forms of radiation pass through any object, there is an ionizing effect which disrupts molecules and, ultimately cells and DNA. While all ionizing radiation causes damage in this way (measured in doses as the energy left behind), damage is not done equally by all forms.
Long term exposure to radiation can cause normally non-radioactive materials (for example, some items Marie Curie used in her research) to be similarly active.
Exposure, either acute (a large dose over a short period of time) or chronic (long term exposure to smaller or more constant levels of radiation) can be measured in roentgen (R) and coulomb/kilogram (C/kg). From the CDC "a measure of ionization in air caused by x-rays or gamma rays only."
Doses (absorbed and equivalent)
Two common units of radiation are Gray and Sievert. For most normal exposure levels measurements are done in the scale of centi- or milli-Sieverts--with typical, natural background radiation leading to 2.4 mSv per year (according to UNSCEAR data(2)). While the two can be compared to each other in a one to one ratio(3), they define different aspects of radiation exposure.
Gray (Gy) is defined as an absorbed dose of radiation. For one Gray, that is one joule of energy (in the form of ionizing radiation) absorbed by one kilogram of tissue. Because Gray doesn't distinguish between forms of radiation, just its total energy, it can misrepresent some of the more damaging forms of radiation (gamma). From the U.S. Department of Health and Human Services' Radiation Emergency Medical Management: "Gray can be used for any type of radiation (e.g., alpha, beta, neutron, gamma), but it does not describe the biological effects of different radiations."(4)
The Sievert (Sv) on the other hand, is defined as the amount of radiation necessary to produce the same effect of one Gray. It is a measured dose equivalent, which takes into account the kind of ionizing radiation that a person is exposed to (some are less damaging to health than others). Because not all radiation causes the same biological effect, even for the same energy of an absorbed dose, this is a useful translation of a measure like Gray. Measuring radiation, a Geiger Counter will typically display either a dose equivalent like Sievert or a count of ionizing events per second.
Other forms of doses include: Rem (a dose equivalent like Sievert) and Rad (a absorbed dose measure like Gray).
Some good sources on radiation measurement can be found below:
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