12.5 Units of Radiation Dose
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the gray (Gy) defined as 1 J/kg. The two units are thus related by 1 Gy = 100 rad.
These units are used to specify the maximum permissible level a person can receive,
which may not cause alarming damage to the human body.
12.5.3 Rem
Another unit of radiation is rem, which is used for biological system. The need for
a separate unit for the biological system arises because the amount of damage made
to the tissue of the biological system depends upon the extent of absorption of the
radiation during its path in the biological system. For example, α-particles of same
energy as that of γ -radiation will do much extensive damage to the biological tissues.
We have seen earlier that when radiations pass through the biological tissues, they
create chemical alteration to the biological molecules. The severity and permanence
of these changes are directly related to the local rate of energy deposition along the
particle track, known as linear energy transfer L. Obviously, α-particles will have
a larger value for “L” as compared to γ -rays, as the latter cannot transfer energy to
the biological molecule as effectively as the former.
Thus, the magnitude of damage to the biological tissues would depend upon the
value of “L”. Hence, for biological molecules, we have to consider two aspects of the
radiation, its magnitude of radiation being received by the tissue (i.e., “D” in terms
of rad/h, as defined earlier) and a quality factor (i.e., “Q” a dimensionless factor)
which is related to the magnitude of “L” and is related to the nature of radiation being
considered. The product of these two quantities i.e., “D” and “Q” can, therefore, be
taken as the real measure to express the unit of radiation for the biological system.
The product of these two items is defined in terms of rem. In other words, for a
biological system, the effective quantity of radiation which will damage the system
has to be multiplied by a factor arising of the fact that radiations with high specific
ionization properties (like α-particles) will cause damage to the large number of
biological tissues of the body as compared to radiation of less specific ionization
properties (like γ -rays), though both radiations may have the same energy.
12.5.4 Maximum Permissible Level
Our body is exposed not only to radiations due to radioactive isotopes present in
the laboratory, but is also continuously exposed to small intensities of ionization
radiations from cosmic radiation or from radioactive substances present on the earth
and in the air. The scientific and medical investigations of these radiations on the
body and the effect of the higher dose rate on the animals has shown that there is a
tolerance level of radiation dose, due to either external exposure or internal ingestion.
The former level is called maximum permissible level of external radiation, and is
defined as the dose rate or level of radiation below which no permanent physiological
changes are thought to be likely. The level is not a fixed value and is liable to be
189
the gray (Gy) defined as 1 J/kg. The two units are thus related by 1 Gy = 100 rad.
These units are used to specify the maximum permissible level a person can receive,
which may not cause alarming damage to the human body.
12.5.3 Rem
Another unit of radiation is rem, which is used for biological system. The need for
a separate unit for the biological system arises because the amount of damage made
to the tissue of the biological system depends upon the extent of absorption of the
radiation during its path in the biological system. For example, α-particles of same
energy as that of γ -radiation will do much extensive damage to the biological tissues.
We have seen earlier that when radiations pass through the biological tissues, they
create chemical alteration to the biological molecules. The severity and permanence
of these changes are directly related to the local rate of energy deposition along the
particle track, known as linear energy transfer L. Obviously, α-particles will have
a larger value for “L” as compared to γ -rays, as the latter cannot transfer energy to
the biological molecule as effectively as the former.
Thus, the magnitude of damage to the biological tissues would depend upon the
value of “L”. Hence, for biological molecules, we have to consider two aspects of the
radiation, its magnitude of radiation being received by the tissue (i.e., “D” in terms
of rad/h, as defined earlier) and a quality factor (i.e., “Q” a dimensionless factor)
which is related to the magnitude of “L” and is related to the nature of radiation being
considered. The product of these two quantities i.e., “D” and “Q” can, therefore, be
taken as the real measure to express the unit of radiation for the biological system.
The product of these two items is defined in terms of rem. In other words, for a
biological system, the effective quantity of radiation which will damage the system
has to be multiplied by a factor arising of the fact that radiations with high specific
ionization properties (like α-particles) will cause damage to the large number of
biological tissues of the body as compared to radiation of less specific ionization
properties (like γ -rays), though both radiations may have the same energy.
12.5.4 Maximum Permissible Level
Our body is exposed not only to radiations due to radioactive isotopes present in
the laboratory, but is also continuously exposed to small intensities of ionization
radiations from cosmic radiation or from radioactive substances present on the earth
and in the air. The scientific and medical investigations of these radiations on the
body and the effect of the higher dose rate on the animals has shown that there is a
tolerance level of radiation dose, due to either external exposure or internal ingestion.
The former level is called maximum permissible level of external radiation, and is
defined as the dose rate or level of radiation below which no permanent physiological
changes are thought to be likely. The level is not a fixed value and is liable to be