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2 Types of Radiation and Their Properties
radiation effects (e.g. skin burns) do not occur until a threshold dose has been received
while the risk of developing other radiation effects (e.g. cancer) is proportional to
the dose received. Because of this, it is important to have a good understanding of
what radiation dose is and how the dose (and dose rate) change with changes in
distance, shielding, and the amount of radioactivity present. It is equally important
to understand the units in which dose and dose rate are measured and the different
sources of exposure.
2.2.1 Definitions and Units of Dose and Dose Rate
One of the single most important concepts in radiation safety is dose or exposure.
Dose rates are used as the basis for posting regulatory boundaries, for calculating
stay times in a radiological area, for determining what actions to take in the presence
of radioactivity, for reconstructing a radiation dose, and more. And the total dose that
a person received is not only used to demonstrate compliance with regulatory limits,
but is also the single most important factor in trying to predict—or to attribute—the
health impact of radiation exposure. There are a number of concepts associated with
radiation dose and dose rate, each of which will be discussed below.
Absorbed dose is simply a measure of the amount of energy deposited per unit
of mass. The Roentgen, a measure of absorbed dose which measures the creation
of electrical charge in dry air, is considered to be an obsolete unit and it not widely
used. Units still in common use are the rad (primarily in the United States) and the
Gray (the SI unit). It is important to note that the rad and the Gray are both measures
of energy deposition in any absorber; it is appropriate to speak of absorbed dose in
air, in water, or in tissue.
The Gray is defined as the deposition of 1 J of energy per kilogram of absorber.
The rad is defined as the deposition of 100 ergs of energy per gram of absorber.
Doing the appropriate unit conversions shows that 1 Gy = 100 rads.
2.2.2 Effective and Equivalent Dose
Measuring or calculating energy deposition is a good start, but we are primarily
interested in how radiation exposure will affect the health of the person(s) exposed.
Different types of radiation are more or less effective at causing genetic damage; beta
and gamma radiation for example tend to cause point mutations and single-strand
DNA breaks while alpha radiation can snap chromosomes or can cause multiple
sites of damage as they traverse a cell’s nucleus. Because of this, alpha radiation is
as much as 20 times as damaging to cells as are beta or gamma radiation.
For this reason, weighting factors are assigned to the different types of radiation to
calculate the amount of biological damage caused by exposure to the radiation—these
are referred to as the Quality Factor (QF) or the Relative Biological Effectiveness
2 Types of Radiation and Their Properties
radiation effects (e.g. skin burns) do not occur until a threshold dose has been received
while the risk of developing other radiation effects (e.g. cancer) is proportional to
the dose received. Because of this, it is important to have a good understanding of
what radiation dose is and how the dose (and dose rate) change with changes in
distance, shielding, and the amount of radioactivity present. It is equally important
to understand the units in which dose and dose rate are measured and the different
sources of exposure.
2.2.1 Definitions and Units of Dose and Dose Rate
One of the single most important concepts in radiation safety is dose or exposure.
Dose rates are used as the basis for posting regulatory boundaries, for calculating
stay times in a radiological area, for determining what actions to take in the presence
of radioactivity, for reconstructing a radiation dose, and more. And the total dose that
a person received is not only used to demonstrate compliance with regulatory limits,
but is also the single most important factor in trying to predict—or to attribute—the
health impact of radiation exposure. There are a number of concepts associated with
radiation dose and dose rate, each of which will be discussed below.
Absorbed dose is simply a measure of the amount of energy deposited per unit
of mass. The Roentgen, a measure of absorbed dose which measures the creation
of electrical charge in dry air, is considered to be an obsolete unit and it not widely
used. Units still in common use are the rad (primarily in the United States) and the
Gray (the SI unit). It is important to note that the rad and the Gray are both measures
of energy deposition in any absorber; it is appropriate to speak of absorbed dose in
air, in water, or in tissue.
The Gray is defined as the deposition of 1 J of energy per kilogram of absorber.
The rad is defined as the deposition of 100 ergs of energy per gram of absorber.
Doing the appropriate unit conversions shows that 1 Gy = 100 rads.
2.2.2 Effective and Equivalent Dose
Measuring or calculating energy deposition is a good start, but we are primarily
interested in how radiation exposure will affect the health of the person(s) exposed.
Different types of radiation are more or less effective at causing genetic damage; beta
and gamma radiation for example tend to cause point mutations and single-strand
DNA breaks while alpha radiation can snap chromosomes or can cause multiple
sites of damage as they traverse a cell’s nucleus. Because of this, alpha radiation is
as much as 20 times as damaging to cells as are beta or gamma radiation.
For this reason, weighting factors are assigned to the different types of radiation to
calculate the amount of biological damage caused by exposure to the radiation—these
are referred to as the Quality Factor (QF) or the Relative Biological Effectiveness