Measuring units
The various measuring units described below can be confusing
at first sight. To help you understand them, let's compare a radioactive
source that emits ionizing radiation to an apple tree dropping apples onto
someone's head.
The number of apples that fall out of the tree is measured in a radioactivity
context using the Becquerel. The energy that the apples transfer
to the person below is measured for radioactivity using the Gray.
The bumps that the apples make on the person's head is measured for radioactivity
using the Sievert.
The
unit for activity of a radioactive material: Becquerel
The unit used for measuring radioactivity is the Becquerel,
or Bq for short. The Becquerel measures the quantity of radiations emitted
by radioactive materials. 1 Bq equals one disintegration per second. For
example, the water in the oceans has a radioactivity of approximately
12 Bq per litre and the human body has an average radioactivity of about
120 Bq per kilogram.
The
unit for absorbed dose: Gray
Ionizing radiation transfers energy to living tissue. The energy
given to a specific amount of tissue is called the absorbed dose, which
is expressed in Gray (Gy). 1 Gray equals 1 Joule/kilogram.
The
unit for dose equivalent: Sievert
Some types of radiation cause more ionization than others:
alpha radiation more so than beta and gamma radiation. To take this into
account, the absorbed dose is multiplied by a quality factor to give the
dose equivalent. The dose equivalent is expressed in Sievert (Sv).
The
unit for effective dose: milliSievert
Some tissues and organs are more sensitive to radiation than
others (genital glands much more than bones, for example). To take this
into account, the dose equivalent is weighted by a specific risk factor
for each tissue or organ to give the effective dose. This scheme has the
advantage that all kinds of human exposure to ionising radiation can be
put on a common scale of risk.
Effective
dose, commonly called dose, is expressed in Sievert (Sv). Because this
value is very low, it is often expressed in milliSievert (mSv).
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