Radiation can be measured in different ways using different units of measure. Those commonly used are:
- Curies
- Roentgens
- Rads
- Rems
Curie
Curies are used to measure the quantity of radioactivity in a material. In radioactive elements, the rate at which atoms "disintegrate" or emit radiation is measured in "disintegrations per second" (d/s). 1 curie is equal to 37 billion disintegrations per second, the rate of decay for 1 gram of radium. The quantity of radioactivity in materials such as spent fuel rods might be defined in curies.
The quantity of radioactive material in a household smoke detector is measured in millicuries (one thousandth of a curie), while very small amounts of radioactivity are measured in picocuries (one millionth of a millionth of a curie).
Example: The radioactive elements generally present in the human body total an average of 250,000 picocuries, while a mug of beer averages 390 picocuries.
Since curies or picocuries are only measures of radioactive decay activity - one also must consider the:
- Volume of material
- Radioactive elements present
- Types of radiation they emit
- Dose rate and length of exposure to determine biological impact, if any
Roentgen
Roentgen is a unit that measures radiation exposure in the air. This measurement tells you how much radiation, strictly "X" or gamma radiation, one might be exposed to in a particular location or environment (not what might be absorbed).
Example: Radiation levels in a containment structure might be measured in roentgens per hour.
Rad
Rad stands for "radiation absorbed dose" and measures the amount of energy deposited in a material: alpha, beta, neutron, gamma or "X." Over time, the particular type of radiation absorbed (deposited) in a nuclear plant component, such as a metal pipe, might be expressed in rads.
Rem
Rem stands for "roentgen equivalent man" and is the standard unit used to measure dose from radiation energy absorbed in human tissue. In this case, the term "dose" includes an allowance or factor for the biological effect of the particular type of radiation absorbed. For "X" and gamma radiation, the roentgen and rem value are essentially the same and are often used interchangeably. Exposure limits for nuclear power plant workers are typically expressed in rem.
Standard radiation
Standard radiation measures are relatively large numbers, so fractional units often are used in actual measurements of radiation. For example, the following may be used when measuring biological effects (dose):
- 1 millirem = one-thousandth of 1 rem
- 1 microrem = one-millionth of 1 rem
Comparison
By way of comparison, radiation effects in humans generally are not detectable below a dose of 50 rem. A dose of 500 rem would cause death in a few weeks to about 50 percent of those exposed. It should be recognized, however, that this exposure must be almost immediate (acute exposure). The body can replace damaged or dead cells caused by higher dosages delivered in small amounts over long periods of time (chronic exposure).
Common dose rates are:
- Millirem per hour
- Millirem per week
- Rem per quarter
- Rem per year
Average exposure
The average person in the U.S. is exposed to about 360 millirem of radiation annually. Natural (or background) radiation accounts for more than three-quarters of this yearly exposure. The accompanying chart shows the levels of radiation associated with some common sources.
