Physics Corner #2: A Primer on Radiation Units and Quantities

Physics Corner was a series of articles I wrote for an incarnation of the department newsletter between 2000 - 2001. They're collected here for posterity and maybe to provoke me into starting it up again as an online column.

When it comes to measuring and counting radiation, there are more ways to measure radiation than you can count on your fingers. And to top it all off, there are the traditional Imperial units, and the more modern SI units to confuse things even more. In this month's column, I'll attempt to demystify the world of radiation units.

When you boil it all down, there are essentially two types of units used for quantifying radiation. There are physical quantities used to measure the amount of radiation or energy deposited, such as the roentgen or curie (for radioactive materials), and there are units to measure the biological effect of radiation (such as the rem). The table below summarizes the different types of units used.

Quantity measuredTraditional UnitSI UnitConversion
Exposureroentgen (R)X (Coloumb/kg)1 R = 2.58x10-4 C/kg
Absorbed Doserad (rad)gray (Gy)1 Gy = 100 rad
Absorbed Dose Equivalentrem (rem)sievert (Sv)1 Sv = 100 rem
Radioactivitycurie (Ci)becquerel (Bq)1 Bq = 2.7x10-11 Ci

Ionizing radiation produces charge pairs (electron and ionized atom) when it interacts in matter, so naturally this is the first thing we look at. Plus, it's also very easy to measure. The roentgen represents the amount of charge produced per unit mass of air and is the unit most commonly seen. In SI units, the unit of exposure is simply coulombs/kg (C/kg), which is sometimes referred to as X. The roentgen or C/kg represents a considerable amount of radiation. Exposures in the diagnostic radiology realm are generally measured in milli-roentgens (mR), or 1/1000th of a roentgen. For a little bit of perspective, the exposure to a patient receiving a chest X-ray at MUSC is typically 10-12 mR.

While knowing the exposure can be useful (in a "more is usually bad" kind of context), it doesn't really mean much when it comes to predicting biological effects of radiation exposure which is generally what most people are interested in. In order to assess biological effects of radiation, first we need to know how much energy is deposited in matter. This is generally referred to as the absorbed dose. In the old system, the unit for absorbed dose is the rad (radiation absorbed dose), while the gray (Gy) is used in the SI system. Both units measure the amount of energy in joules (J) deposited by radiation in a unit mass of material (J/kg). For most diagnostic x-ray procedures, the absorbed dose is generally measured in the milli-gray or milli-rad region.

When it comes to radiation risk and protection, a couple more units were introduced because different types of radiation have different effects. In the old system, the unit is the rem (radiation equivalent man) and the sievert (Sv) in the SI system. As with the rad and the gray, 1 Sv = 100 rem. Both the rem and sievert unfortunately have units of energy per unit mass, just like the rad and gray. However, the rem and sievert are related to the rad and gray by a numerical scaling factor known as RBE (relative biological effectiveness). RBE relates the effect of a particular type and amount of radiation to a reference amount of x-ray radiation. X-rays and gamma rays are assigned an RBE of 1, while alpha particles are given an RBE of 20. So, while an exposure of alpha particles might deposit say 1 rad of energy within an organ, the actual effect on the organ would be the same as that produced by 20 times as much x-ray radiation.

All of this might look and sound rather messy but in the world of diagnostic radiology where the conversion factor from roentgens to rads is close to 1 and x-rays have an RBE of 1, things are a little simpler: 1 R ~ 1 rad = 1 rem.

For radioactive materials, the amounts of radioactivity present are measured in curies (Ci, old unit) or becquerels (Bq, SI unit). Traditionally, the curie was defined as the amount of radioactivity contained in 1 gram of radium-222. Now, the curie is defined as 3.7x1010 disintegrations per second. The becquerel is defined to be 1 disintegration/sec, so that 1 Ci = 3.7x1010 Bq. The curie represents a tremendous amount of radioactivity, and is definitely not something to be carrying around in your front pocket. In nuclear medicine departments, the amounts of radioactivity used are generally measured in milli-curies (mCi) or micro-curies (μCi) (mega-becquerels (MBq) and kilo-becquerels (kBq) respectively).