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dispersion of nuclear reactor waste or radiotherapy sources (e.g., cesium- 137 and
cobalt-60).
Gamma rays and x rays, which are emitted from radionuclides as well as
produced by machines, are the most penetrating forms of ionizing radiation and
consist of electromagnetic energy. While randomly colliding with electrons in the
body along a scattered path length, gamma rays may give up all or part of their
energy in tissue or, although it is unlikely, they may pass all the way through the
body without interacting. Therefore, exposure to gamma or x rays from sources
outside the body may cause ionizations in tissues at any location in their path.
Gamma rays are characteristic of a wide variety of radioactive contaminants
associated with nuclear weapons and nuclear waste and also with radioactive
sources used in medicine and industry, whereas x rays are most commonly
encountered in the use of radiation-producing equipment used in medical
applications (including those in combat medical facilities).

1.2 Radiation units and measurements
1.2.1 Radiation units
Ionizing radiation is measured in terms of:
• the strength or radioactivity of the radiation source;
• the energy of the radiation;
• the level of radiation in the environment;
• the radiation dose or the amount of radiation energy absorbed by the
human body.
Radioactivity or the strength of radioactive source is measured in units of
becquerel (Bq):1 Bq = 1 event of radiation emission per second.
One becquerel is an extremely small amount of radioactivity. Commonly

used multiples of the Bq unit are kBq (kilobecquerel), MBq (megabecquerel), and
GBq (gigabecquerel): 1 kBq = 1000 Bq, 1 MBq = 1000 kBq, 1 GBq = 1000 MBq.
An old and still popular unit of measuring radioactivity is the curie (Ci):
1 Ci= 37 GBq = 37000 MBq.
One curie is a large amount of radioactivity. Commonly used subunits are
mCi (millicurie), µCi (microcurie), nCi (nanocurie), and pCi (picocurie):
1 Ci = 1000 mCi; 1 mCi = 1000 µCi; 1 µCi = 1000 nCi; 1 nCi = 1000 pCi.
Another useful conversion formula is: 1 Bq = 27 pCi.
Becquerel (Bq) or Curie (Ci) is a measure of the rate (not energy) of
radiation emission from a source.
Radiation intensity from a radioactive source diminishes with time as more
and more radioactive atoms decay and become stable atoms. Half-life is the time
after which the radiation intensity is reduced by half. This happens because half of
the radioactive atoms will have decayed in one half-life period. Half-lives widely
differ from one radioactive material to another and range from a fraction of a
second to millions of years.

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