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Day 3 – Lecture 1
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To study the radiation protection quantities and associated terminology and to learn about equivalent dose, radiation weighting factors, effective dose, tissue weighting factors, intake, committed dose, committed effective dose, and various operational quantities
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Equivalent dose and dose rate
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Radiation weighting factors
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Effective dose
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Tissue weighting factors
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Weakly and strongly penetrating radiation
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Ambient dose equivalent
•
Expanded and aligned radiation fields
•
Directional dose equivalent
•
Personal dose equivalent
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Intake
•
Committed equivalent dose
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Committed effective dose
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The equivalent dose in tissue T is given by the expression:
H
T
= ∑r W
R
D
T,R where D
T,R is the absorbed dose averaged over the tissue or organ T, due to radiation R .
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In radiological protection, it is the absorbed dose averaged over a tissue or organ
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It is weighted for the radiation quality of interest
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The weighting factor is called the radiation weighting factor, W
R
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W
R is selected for the type and energy of the radiation incident on the body
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This weighted absorbed dose, called the equivalent dose, is strictly a dose
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The unit of equivalent dose is the joule per kilogram with the special name of
Sievert (Sv)
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Type and Energy Range
Photons: all energies
Electrons : all energies
Neutrons: energy < 10 keV
Neutrons: 10 keV to 100 keV
Neutrons: > 100 keV to 2 MeV w
R
1
1
5
10
20
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Type and Energy Range
Neutrons: > 2 MeV to 20 MeV
Neutrons: > 20 MeV
Protons: > 2 MeV
Alpha particles, fission fragments, heavy nuclei w
R
10
5
5
20
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.
The equivalent dose rate, H
T of dH
T by dt, where dH
T
, is the quotient is the increment of equivalent dose in the time interval dt, thus:
.
H
T
= dH
T dt
The unit is J kg -1 s -1 and the special name for the unit of equivalent dose rate is Sievert per second (Sv s -1 )
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The effective dose is the sum of the weighted equivalent doses in all the tissues and organs of the body. It is given by:
E = ∑t w
T
H
T where H
T tissue T.
is the equivalent dose in tissue or organ T and w
T is the weighting factor for
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Account for fact that the probability of stochastic effects depends on the organ or tissue irradiated
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Represent the relative contribution of irradiation of each organ or tissue to the total detriment due to the effects resulting from uniform irradiation of the whole body
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Desirable that a uniform equivalent dose over the whole body should give an effective dose numerically equal to that uniform equivalent dose
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Achieved by normalizing the sum of the tissue weighting factors to one
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Tissue or Organ
Gonads
W
T
0.20
Bone marrow (red) 0.12
Colon 0.12
Lung
Stomach
Bladder
Breast
0.12
0.12
0.05
0.05
Tissue or Organ
Liver
Oesophagus
Thyroid
Skin
Bone surface
Remainder
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W
T
0.05
0.05
0.05
0.01
0.01
0.05

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Adrenals
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Upper large Intestine
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Small Intestine
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Kidney
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Pancreas
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Brain
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Spleen
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Thymus
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Uterus
•
Muscle
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The relationship between the probability of stochastic effects (primarily cancer and genetic effects) and equivalent dose is found to depend on the organ or tissue irradiated.
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The effective dose combines the equivalent doses to the various body organs and tissues in a way which correlates well with the total of the stochastic effects
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For radiation measurement purposes, the following operational quantities are defined:
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Ambient dose equivalent
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Directional dose equivalent
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Personal dose equivalent
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Where doses are estimated from area monitoring results, the relevant operational quantities are ambient dose equivalent and directional dose equivalent
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For individual monitoring, the use of the personal dose equivalent is recommended
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The quantities recommended for area monitoring refer to a phantom termed the
ICRU sphere.
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The ICRU sphere (ICRU, 1980) is a 30 cm diameter, tissue-equivalent sphere with a density of 1 g cm -3 and a mass composition of 76.2% oxygen, 11.1% carbon, 10.1% hydrogen, and 2.6% nitrogen
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Radiation field
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The ambient dose equivalent, H*(d), at a point, is the dose equivalent that would be produced by the corresponding field , in the ICRU sphere at a depth d in millimeters on the radius opposing the direction of the field .
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For measurement of strongly penetrating radiations, the reference depth is 10 mm and the quantity denoted as
H*(10).
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The unit is J kg -1
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The special name for the unit of ambient dose equivalent is Sievert (Sv)
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An expanded radiation field is defined as a hypothetical radiation field in which the fluence, and its angular and energy distributions, have the same value throughout the volume of interest as the actual field at the point of reference
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The directional dose equivalent, H ‘(d,), at a point, is the dose equivalent that would be produced by the corresponding expanded field in the ICRU sphere at a depth d on a radius in a specified direction .
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Directional dose equivalent is of particular use in the assessment of dose to the skin or eye lens
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The unit is J kg -1
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The special name for the unit of directional dose equivalent is Sievert (Sv)
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The personal dose equivalent, H p
(d), is the dose equivalent in soft tissue, at an appropriate depth d, below a specified point on the body,
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H p
(d) can be measured with a dosimeter which is worn at the surface of the body and covered with an appropriate thickness of tissue-equivalent material
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The unit is J kg -1
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The special name for the unit of personal dose equivalent is sievert (Sv)
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H p
(10), measured at a depth of 10 mm in soft tissue, is the operational surrogate for the effective dose, E
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When radioactive material (RAM) is inhaled or ingested, the result is an intake into the body
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Intakes of RAM are usually expressed in units of Bq
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An intake should be contrasted with an uptake of RAM into a specific organ or tissue
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ICRP 60 defines the annual limit on intake
(ALI) for each radionuclide
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The ALI is based on an average effective dose limit of 20 mSv per year
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Following an intake into the body of a radioactive material, there is a period during which the material gives rise to equivalent doses in the organs or tissues of the body at varying rates
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The time integral of the equivalent-dose rate is called the committed equivalent dose
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The committed effective dose E(50) for workers is defined as:
E(50) = sum w
T
H
T
(50) where H
T
(50) is the committed equivalent dose and w
T is the specific weighting factor for the tissues and organs
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Radiation protection quantities and associated terminology were discussed
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Students learned about equivalent dose, radiation weighting factors, effective dose, tissue weighting factors, intake, committed dose, committed effective dose, and various operational quantities
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Where to Get More Information
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Knoll, G.T., Radiation Detection and Measurement , 3 rd
Edition, Wiley, New York (2000)
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Attix, F.H., Introduction to Radiological Physics and
Radiation Dosimetry , Wiley, New York (1986)
•
International Atomic Energy Agency, Determination of
Absorbed Dose in Photon and Electron Beams, 2 nd Edition,
Technical Reports Series No. 277, IAEA, Vienna (1997)
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Where to Get More Information
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International Commission on Radiation Units and
Measurements, Quantities and Units in Radiation
Protection Dosimetry, Report No. 51, ICRU,
Bethesda (1993)
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International Commission on Radiation Units and
Measurements, Fundamental Quantities and Units for
Ionizing Radiation, Report No. 60, ICRU, Bethesda
(1998)
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Hine, G. J. and Brownell, G. L., (Ed. ), Radiation
Dosimetry , Academic Press (New York, 1956)
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Where to Get More Information
•
Bevelacqua, Joseph J., Contemporary Health
Physics , John Wiley & Sons, Inc. (New York, 1995)
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International Commission on Radiological Protection,
Data for Protection Against Ionizing Radiation from
External Sources: Supplement to ICRP Publication
15. A Report of ICRP Committee 3, ICRP Publication
21, Pergamon Press (Oxford, 1973)
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