is a measure of a radionuclides activity, i.e., by the number of
atoms that disintegrate per unit time.
The conventional unit for activity is the curie (Ci).
A curie is defined by the activity in one gram of naturally
occurring Radium-226 and equals 37-billion disintegrations per
second. The Systeme
International d'Unites (SI) is the recognized international
standard for describing measurable quantities and their units.
The standard SI unit for activity is the becquerel (Bq).
A becquerel is equal to one disintegration per second.
Radiation Exposure and Dose
primary concern of radioactivity is the amount of energy deposited by
particles or gamma radiation to the surrounding environment.
Of particular concern is the amount of damage this energy can
cause living tissue. When
radiation interacts with a substance, it interacts with electrons of
atoms which can alter the number of electrons associated with those
atoms (ionization). The
term “exposure” is used to express the amount of ionization produced
in air by electromagnetic (gamma and X-ray) radiation.
The unit of exposure is roentgen (R).
The average exposure rate from natural radioactivity in southeast
Idaho is about 0.130 R per year. Exposure
applies only to electromagnetic radiation in air.
Absorbed dose describes the amount of energy from ionizing
radiation absorbed by any kind of matter.
When absorbed dose is adjusted to account for the amount of
biological damage a particular type of radiation causes, it is known as
dose equivalent. The unit
for dose equivalent is called the rem (“roentgen-equivalent-man”).
The SI unit for dose equivalent is called the seivert (Sv).
One seivert is equivalent to 100 rem.
range of numbers experienced in many scientific fields, like that
of environmental monitoring for radioactivity, is huge and units
for very small and very large numbers are commonly expressed by
scientists as a prefix that modifies the unit of measure.
One example is the prefix kilo, abbreviated k, which
means 1,000 of a given unit.
A kilometer is therefore equal to 1,000 meters.
Prefixes used in this report include:
Scientific notation is used to express numbers which are very small or very large. A very small number will be expressed with a negative exponent, e.g., 1.2 x 10-6. To convert this number to the more commonly used form, the decimal point must be moved left by the number of places equal to the exponent (in this case, six). Thus the number 1.2 x 10-6 is equal to 0.0000012. A large number will be expressed with a positive exponent, e.g. 1.2 x 106. To convert this number, the decimal point must be moved right by the number of places equal to the exponent. For example, number 1.2 x 106 is equal to 1,200,000.
The amount of radioactivity in a substance of interest is described by its concentration which is described as the amount of radioactivity per unit volume or weight of that substance. Air, milk, and atmospheric moisture samples are expressed as activity per milliliter (mL). Concentrations in surface and drinking water and precipitation samples are expressed as activity per liter (L). Radioactivity in foodstuff and soil are expressed as activity per gram (g). Exposure, as measured by environmental dosimeters, is expressed in units of milliRoentgens (mR). This is sometimes expressed in terms of dose as millirem (mrem) or microseiverts (:Sv).