PRM-7000 / 8000 / 9000 Users Guide | 33
15. Interpreting Measurements
15.1 Units
There are four different but interrelated quantities related to the measurement of radia-
tion. Each quantity has associated units of measurement that are represented in both
the English System of Measurements as well as the International System of Units, also
known as SI.
The four quantities are radioactivity, exposure, absorbed dose, and dose equivalent. In
the order presented, these quantities start by indicating characteristics of the radiation
source and end by indicating biological impact on human beings.
Radioactivity refers to the amount of ionizing radiation released by a material. Whether it
emits alpha or beta particles, gamma rays, x-rays, or neutrons, a quantity of radioactive
material is expressed in terms of its radioactivity (or simply its activity), which represents
how many atoms in the material decay in a given time period. The units of measure for
radioactivity are the curie (Ci) and in SI becquerel (Bq).
Exposure describes the amount of radiation traveling through the air. The units for expo-
sure are the roentgen (R) and in SI coulomb/kilogram (C/kg).
Absorbed dose describes the amount of radiation absorbed by an object or person. That
is, the amount of energy that radioactive sources deposit in materials through which they
pass. The units for absorbed dose are the radiation absorbed dose (rad) and in SI the
gray (Gy).
Dose equivalent (or effective dose) combines the amount of radiation absorbed and
the medical effects of that type of radiation on a human being. For beta and gamma
radiation, the dose equivalent is the same as the absorbed dose. By contrast, the dose
equivalent is larger than the absorbed dose for alpha and neutron radiation, because
these types of radiation are more damaging to the human body.
NOTE: The detection and measurement of neutrons requires specialized instrumen-
tation, Geiger counters do not detect neutrons.
The units for dose equivalent are the roentgen equivalent man (rem) and in SI Sievert
(Sv). These biological dose equivalents are commonly measured in one thousandths of a
rem and is known as a millirem (mrem) or 1 millionth of a Sievert known as microSievert
(uSv).
For further clarifi cation, note that a measure given in Bq indicates the radioactivity of a
substance, while a measure in mrem or uSv indicates the amount of energy that a radio-
active source deposits in living tissue.
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