1.
RADIATION AND DOSE MEASUREMENTS
In radiation protection we need to be able to determine the potential hazard
from radiation. To do this we need to measure the quantities which describe
the radiation field (known as radiometric quantities) and we also need to
measure the effects produced by the radiation dose (known as dosimetric
quantities).
1.1.1 Energy
The energy of ionizing radiation is measured in terms of electron volts (eV),
where one electron volt is the amount of energy gained by an electron when
it is accelerated through a potential difference of one volt. In terms of joules:
1 eV = 1.6 x 10-19 J
The electronvolt is a very small unit of energy, even in atomic terms, so in
practice the energy of radiation is usually given in kiloelectronvolts (keV) or
megaelectronvolts (MeV).
1.1.2 Exposure (X)
Historically, x-rays and gamma rays were quantified by the amount of
ionization they produced in air (i.e. the exposure).
The special unit of
exposure (symbol X) was originally called the roentgen (R), named after the
discoverer of x-rays, Wilhelm Roentgen.
The SI unit of exposure is the
coulomb per kilogram (C kg-1) and this is related to the roentgen as
follows:
1 R = 2.58 x 10-4 C kg-1 of air
The roentgen is no longer used in radiation protection but there are many
instruments which give readings in roentgen (R) or roentgen per hour (R h -1).
1.1.3 Kerma (K)
The term kerma refers to the kinetic energy released per unit mass of
absorber and is basically a measure of the kinetic energy of charged
particles produced in an absorbing medium by uncharged radiations (i.e.
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photons and neutrons). When the absorbing medium is air, the term air
kerma is used.
The unit of kerma is the joule per kilogram and it is given the special name
gray (Gy):
1 Gy = 1 J kg-1
For x- and gamma-ray energies up to 1 MeV, the interactions which take
place between the radiation and the particles of air are nearly the same for
both exposure measurement and air kerma measurement. At x- and gamma
ray energies above 1 MeV interactions occur which lead to different
measurements of air kerma compared with exposure. For example, cobalt60 has two gamma radiations with energies 1.173 MeV and 1.333 MeV and
the measurement of air kerma produced by these radiations will be about
0.3% higher than if only exposure is considered.
1.1.4 Summary of radiometric quantities
Table 1 summarises the radiometric quantities.
Table 1
Summary of Radiometric Quantities
Quantity
Symbol
Energy
SI Unit
SI Unit
Name
Joule
J
electronvolt
(eV)
Exposure
X
Coulomb per kilogram
C kg-1
Kerma
K
Joule per kilogram
J kg-1
1.2
Conversion
1 eV = 1.6 x 10-19 J
1 R= 2.58 x 10-4 C kg-1
gray
(Gy)
1 Gy = 1 J kg-1
Dosimetric Quantities
The term dose is a very general term which applies to the amount of energy
deposited when radiation passes through a material. It is often used very
loosely and, depending on the context, may actually mean absorbed dose,
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equivalent dose, effective dose or even exposure or kerma. The following
sections explain the basic dosimetric quantities used in radiation protection.
1.2.1 Absorbed dose (D)
Absorbed dose is a measure of the energy deposited in any medium by any
type of radiation. It is given the symbol D.
The SI unit of absorbed dose, like that of kerma, is the joule per kilogram (J
kg-1) and is given the name of gray (Gy). However, when we are talking
about absorbed dose, it is very important to specify the type of material in
which the energy is being deposited, for example 1.3 mGy absorbed dose to
water.
The original unit of absorbed dose was the radiation absorbed dose (rad)
and one gray is equal to 100 rad.
1 Gy = 100 rad
or 1 mrad = 10 Gy
In radiation protection, we often need to convert between old (non-SI) and
new (SI) units for dose and dose rates. The following example shows how
this conversion is performed for absorbed dose.
EXAMPLE
Question
Convert the following absorbed doses and dose rates between old and new
units:
a) 0.4 mrad to Gy.
b) 7.5 Gy h-1 to rad h-1.
Answer
a) 1 mrad = 10 Gy
So 0.4 mrad = 0.4 x 10 Gy = 4 Gy
Hence 0.4 mrad is equal to 4 Gy or 4 x 10-6 Gy
b) 1 Gy h-1 = 100 rad h-1
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or 1 Gy h-1 = 0.1 mrad h-1
So 7.5 Gy h-1 = 7.5 x 0.1 mrad h-1 = 7.5 x 10-4 rad h-1 = 0.75 mrad h-1
Hence 7.5 Gy h-1 is equal to 0.75 mrad h-1 or 7.5 x 10-4 rad h-1
1.2.2 Equivalent Dose (H)
Absorbed dose tells us how much energy is deposited in an absorbing
material but it does not tell us how much damage may be done to tissue, nor
does it indicate the level of the potential hazard. For example, the level of
damage produced by an absorbed dose to tissue of 0.5 Gy would be very
much greater if the energy were deposited by alpha radiation or by neutrons
than it would be if the energy were deposited by gamma radiation. Hence, a
quantity known as equivalent dose is used as a measure of the biological
effect of a particular type of radiation on organs or tissues. It is calculated by
multiplying the absorbed dose to an organ or tissue (measured in gray) by a
dimensionless factor called the radiation weighting factor (w R). Radiation
weighting factors as recommended in ICRP 60 are given in Table 2.
Table 2
Radiation Weighting Factors
Type and Energy Range
Radiation Weighting
Factor (wR)
Alpha particles, all energies
20
Beta particles, all energies
1
Gamma and x-rays, all energies
1
Neutrons:
<10 keV
5
10 keV to 100 keV
10
> 100 keV to 2 MeV
20
> 2 MeV to 20 MeV
10
> 20 MeV
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Equivalent dose (symbol H) is defined as shown in Equation 1 for a
particular type of radiation interacting with a particular organ or tissue:
HT R = DT R x w R
[1]
4
where
HT R is the equivalent dose to an organ or tissue T delivered by
radiation type R
DT R is the absorbed dose to an organ or tissue T delivered by
radiation type R
w R is the radiation weighing factor for radiation type R
The SI unit for equivalent dose is also the joule per kilogram but it is given
the special name of sievert (Sv) to distinguish it from absorbed dose.
1 Sv = 1 J kg-1
Example 2 shows how the equivalent dose may be determined from
absorbed dose.
If an organ is irradiated by more than one type of radiation, it is necessary to
sum the equivalent doses to account for the different types of radiation.
Equation 2 shows how the total dose to a particular organ from a variety of
types of radiation may be determined:
HT =

(D T
R
x wR )
[2]
R
where HT is the total equivalent dose to an organ or tissue T delivered by
all radiation types

indicates the sum for each radiation type R
R
DT R is the absorbed dose to an organ or tissue T delivered by
radiation type R
w R is the radiation weighing factor for radiation type R
The original unit of equivalent dose was the rem where one sievert is equal
to 100 rem.
1 Sv = 100 rem
or 1 mrem = 10 Sv
Example 4 shows how we can convert between these old and new units:
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1.2.3 Effective Dose (E)
Some tissues and body organs are more sensitive to radiation than others
and an equivalent dose in one organ may be more hazardous than the same
equivalent dose in another organ. The ICRP recommends tissue weighting
factors (wT) which are applied to specific body organs.
These
dimensionless factors take into consideration the different radiosensitivities
of the different organs and tissues. Table 3 lists the tissue weighting factors
as recommended in ICRP 60.
Table 3
Tissue Weighting Factors
Tissue Weighting Factor
(wT)
Tissue
Gonads
0.20
Bone marrow (red)
0.12
Colon
0.12
Lung
0.12
Stomach
0.12
Bladder
0.05
Breast
0.05
Liver
0.05
Oesophagus
0.05
Thyroid
0.05
Skin
0.01
Bone Surface
0.01
Remainder
0.05
The effective dose (symbol E) is the total effective dose for all exposed
organs or tissues is the sum of the tissue equivalent doses multiplied by the
appropriate tissue weighting factor for those organs and tissues (see
Equation 3):
E =  (H T x w T )
[3]
T
where E is the total effective dose to all exposed organs and tissues
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
indicates the sum for each organ or tissue type T
T
H T is the equivalent dose to an organ or tissue T
w T is the tissue weighing factor for an organ or tissue type T
Note that the effective dose to the whole body is generally the most useful
quantity in radiation protection. This is because, in most work environments,
the whole body effective dose is more likely to approach dose limits than
individual organ or tissue doses are. Assuming uniform irradiation to the
whole body, the total effective dose can be calculated either by summing the
effective doses to each individual organ or tissue, or more simply we can just
assume a tissue weighting factor of 1 for the whole body.
4.2.4 Committed Dose
The quantities discussed so far have come from radiation sources outside
the human body. However, as you have learnt in Module 2.3 Protection from
Internal Radiation Hazards, if an intake of radioactive material occurs, it will
continue to irradiate the body until either the radioactivity has decayed away
or the body has excreted the substance. You will also remember that once
the radioactive material is inside the body, it may be distributed throughout
the body or it may concentrate in particular organs. We therefore also need
to be able to indicate doses which arise from intakes of radioactive materials
and these doses are known as committed doses.
Committed dose is defined as the dose accumulated by the body over 50
years following the intake (except in the case of intakes by children where it
is defined as the dose accumulated until the age of 70). Committed dose
can be committed absorbed dose, committed equivalent dose or
committed effective dose. It is given the symbols D(50), H(50), or E(50) where
50 represents the number of years over which the dose is being calculated.
4.2.5 Summary of dosimetric quantities
Figure 1 and Table 4 summarise the dosimetric quantities and their units.
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Equivalent dose
to a particular organ
HT =
Source Activity
(Bq)
 (D
x wR )
T
R
(Sv)
Effective dose
to the whole body
E=
 (H
T
x wT )
T
(Sv)
Absorbed dose (D)
to any medium
(Gy)
Figure 1
A Summary of Dosimetric Quantities
Table 4
Summary of Dosimetric Quantities and Their Units
Quantity
Symbol
SI Unit
SI Unit Name
Conversion
Absorbed
Dose
D
Joule per kilogram
J kg-1
gray
(Gy)
1 Gy = 100 rad
Equivalent
Dose
H
Joule per kilogram
J kg-1
sievert
(Sv)
1 Sv = 100 rem
Effective
Dose
E
Joule per kilogram
J kg-1
sievert
(Sv)
1 Sv = 100 rem
Figure 2 compares the average levels of occupational and medical
exposures with the average radiation levels from natural sources (see
section 4.2 for the definitions of dose).
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Average Annual
Occupational Exposure
(Nuclear Industry)
1.8 mSv*
Average Annual
Exposure from Natural
Background Radiation
Average Annual
Medical Exposure
(Diagnostic Procedures)
2.4 mSv*
0.4 mSv*
* From UNSCEAR Publication 2000
Figure 2
Comparison of Levels of Exposure
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