Radioactivity Measurements

Radioactivity 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

The 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.

Sources of Radiation

Unit Prefixes

The 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 ESER reports include:

Prefix   Abbreviation   Meaning  
Mega   M   1,000,000 (= 1 x 106)  
milli   m   0.001 (= 1 x 10-3)  
micro   µ  0.000001 (1 x 10-6)  
pico   p   0.000000000001 (= 1 x 10-12)  

Scientific Notation

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.

Concentrations of Radioactivity

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 (µ Sv