Environmental Engineering Reference
In-Depth Information
A Note on Units of Measurement and Concepts of
Radiation Dose
Now it is time for an apology. The units and definitions used by radiation scientists can be
quite confusing (to me anyway). I am going to offer the standard definitions mainly to help
if you do some further reading. Do not be intimidated by this terminology. I will do my best
to avoid it as much as possible in this chapter, but the definitions do help explain some of
the concepts of radioecology and radioprotection.
The becquerel (Bq): Thisisthemostcommonunitofionizingradiation.Onebecquerel
(Bq) is equal to one disintegration or nuclear transformation per second (1 Bq = 1/s or 1 s -1 ) .
It replaced the older curie (Ci) unit.
The gray (Gy): This is used to quantify the amount of radiation energy that is absorbed
per gram of biological tissue. This is known as the absorbed dose .
The sievert (Sv): The gray (absorbed dose) does not tell us all we need to know about
dose because the same dose of α radiation is more biologically damaging than one of β or
γ radiation. Therefore, a weighted dose has been devised - known as the dose equivalent -
that is measured in sieverts. It is also equal to 1 J/kg. In addition to being used as a measure
of dose equivalence, the sievert is also used to measure another important concept of dose
known as the effective dose equivalent . This introduces a weighting factor (W T ) to account
for differences in radiation sensitivity of different tissues and organs. The stomach is more
radiosensitive than the oesophagus. If the stomach (W T = 0.12) and bladder (W T = 0.05) are
exposed separately to radiation and the equivalent doses to the tissues are 100 and 70 mSv,
respectively, the effective dose equivalent is (100 mSv × 0.12) + (70 mSv × 0.05) = 15.5
mSv. In other words, the risk of harmful effects from this radiation is equal to 15.5 mSv re-
ceived uniformly through the whole body.
For radiological protection purposes, 0.001 sievert (1 millisievert, or mSv) is used as
a yearly dose limit of exposure for members of the public to all anthropogenic radiation. It
amounts to an increased risk of fatal cancer of 0.005%.
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