Regional Basic Professional Training Course in Korea

❙ 15 ❙
1. Nuclear Reactor Principles
in the nearby atmosphere. This measure of the radiation field is especially useful in the
concept of radiation protectionsince the biological effects of gamma-rays is a function of
the ionization they produce in a body, and thus the biological effects arising from a given
exposure can be expected to increase more or less with exposure.
When ionizing radiation interacts with a material, it can cause ionizations. These
ionizations can be measured, and the effects of the radiation can be estimated. The
commonly used units for radiation exposure and dose measurements in most of the world
are the SI units of the gray (Gy) and the sievert (Sv). In the U.S., the conventional
systems of the rad and rem are still used. *
The gray equals 100 rad and is a measure of
the absorbed dose, D, (or energy deposited) in a material. The sievert equals 100 rem
and is based on the biological damage caused by ionization in human body tissue. It is a
term for dose equivalence and equals the biological damage that would be caused by one
gray (100 rad) of dose. The Sv and the rem account for the fact that not all types of
radiation are equally effective in producing biological change or damage. In other words,
the damage from 0.01 Gy (1 rad) deposited by beta radiation is less than that caused by
0.01 Gy (1 rad) of alpha radiation. The Sv (or rem) is numerically equal to the dose in
Gy (or rad) multiplied by a quality factor, QF, which accounts for the difference in the
amount of biological damage caused by the different types of radiation. The quality factor
converts the dose in Gy (or rad) to the dose equivalent, Hin Sv (or rem). As shown below,
the quality factors are highest for the alpha radiation, which may deposit its energy within
the smallest volume.
*
Although the roentgen (R) has been used as a conventional term as a measure of
exposure to gamma or x-ray radiation, the concept of exposure and the unit of roentgen
are being phased out as an official record of dose. Quite typically, radiation survey
instruments faces will still read out in R or multiples of R until they can be replaced with
instruments reading out in rem or millirem.