Transcript Document

Quantities and Measurements - 3
Radiation Protection Quantities
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Day 3 – Lecture 1
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Objective
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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Content
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Equivalent dose and dose rate
Radiation weighting factors
Effective dose
Tissue weighting factors
Weakly and strongly penetrating radiation
Ambient dose equivalent
Expanded and aligned radiation fields
Directional dose equivalent
Personal dose equivalent
Intake
Committed equivalent dose
Committed effective dose
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Equivalent Dose
The equivalent dose in tissue T is given by
the expression:
HT = ∑r WR DT,R
where DT,R is the absorbed dose averaged
over the tissue or organ T, due to radiation R.
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Equivalent Dose
• In radiological protection, it is the
absorbed dose averaged over a tissue or
organ
• It is weighted for the radiation quality of
interest
• The weighting factor is called the
radiation weighting factor, WR
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Equivalent Dose
• WR is selected for the type and energy of
the radiation incident on the body
• This weighted absorbed dose, called the
equivalent dose, is strictly a dose
• The unit of equivalent dose is the joule
per kilogram with the special name of
Sievert (Sv)
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Radiation Weighting Factors
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
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wR
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1
5
10
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Radiation Weighting Factors
Type and Energy Range
wR
Neutrons: > 2 MeV to 20 MeV
10
Neutrons: > 20 MeV
5
Protons: > 2 MeV
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Alpha particles, fission fragments,
heavy nuclei
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Equivalent Dose Rate
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The equivalent dose rate, HT, is the quotient
of dHT by dt, where dHT is the increment of
equivalent dose in the time interval dt, thus:
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HT =
dHT
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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Effective Dose
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 wT HT
where HT is the equivalent dose in tissue or
organ T and wT is the weighting factor for
tissue T.
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Tissue Weighting Factors
• Account for fact that the probability of
stochastic effects depends on the organ
or tissue irradiated
• 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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Tissue Weighting Factors
• Desirable that a uniform equivalent dose
over the whole body should give an
effective dose numerically equal to that
uniform equivalent dose
• Achieved by normalizing the sum of the
tissue weighting factors to one
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Tissue Weighting Factors
Tissue or Organ
Gonads
Bone marrow (red)
Colon
Lung
Stomach
Bladder
Breast
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WT
0.20
0.12
0.12
0.12
0.12
0.05
0.05
Tissue or Organ
Liver
Oesophagus
Thyroid
Skin
Bone surface
Remainder
WT
0.05
0.05
0.05
0.01
0.01
0.05
Tissue Weighting Factors
Remainder Organs
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Adrenals
Upper large Intestine
Small Intestine
Kidney
Pancreas
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Brain
Spleen
Thymus
Uterus
Muscle
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Concept of Effective Dose
• 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.
• 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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Operational Quantities
• For radiation measurement purposes, the
following operational quantities are
defined:
• Ambient dose equivalent
• Directional dose equivalent
• Personal dose equivalent
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Operational Quantities
• Where doses are estimated from area
monitoring results, the relevant
operational quantities are ambient dose
equivalent and directional dose equivalent
• For individual monitoring, the use of the
personal dose equivalent is
recommended
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Area Monitoring
• The quantities recommended for area
monitoring refer to a phantom termed the
ICRU sphere.
• 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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ICRU Reference Sphere
Radiation field
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Ambient Dose Equivalent
• 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.
• For measurement of strongly penetrating radiations, the
reference depth is 10 mm and the quantity denoted as
H*(10).
• The unit is J kg-1
• The special name for the unit of ambient dose equivalent
is Sievert (Sv)
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Expanded Field
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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Directional Dose Equivalent
• 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 .
• Directional dose equivalent is of particular use in the
assessment of dose to the skin or eye lens
• The unit is J kg-1
• The special name for the unit of directional dose
equivalent is Sievert (Sv)
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Personal Dose Equivalent
• The personal dose equivalent, Hp(d), is
the dose equivalent in soft tissue, at an
appropriate depth d, below a specified
point on the body,
• Hp(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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Personal Dose Equivalent
• The unit is J kg-1
• The special name for the unit of personal
dose equivalent is sievert (Sv)
• Hp(10), measured at a depth of 10 mm in
soft tissue, is the operational surrogate for
the effective dose, E
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Intakes of Radioactive Material
• When radioactive material (RAM) is
inhaled or ingested, the result is an intake
into the body
• Intakes of RAM are usually expressed in
units of Bq
• An intake should be contrasted with an
uptake of RAM into a specific organ or
tissue
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Intakes of Radioactive Material
• ICRP 60 defines the annual limit on intake
(ALI) for each radionuclide
• The ALI is based on an average effective
dose limit of 20 mSv per year
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Committed Equivalent Dose
• 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
• The time integral of the equivalent-dose
rate is called the committed equivalent
dose
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Committed Effective Dose
The committed effective dose E(50) for
workers is defined as:
E(50) = sum wT HT (50)
where HT (50) is the committed equivalent
dose and wT is the specific weighting factor
for the tissues and organs
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Summary
• Radiation protection quantities and
associated terminology were discussed
• 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
• Knoll, G.T., Radiation Detection and Measurement, 3rd
Edition, Wiley, New York (2000)
• 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, 2nd Edition,
Technical Reports Series No. 277, IAEA, Vienna (1997)
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Where to Get More Information
• International Commission on Radiation Units and
Measurements, Quantities and Units in Radiation
Protection Dosimetry, Report No. 51, ICRU,
Bethesda (1993)
• International Commission on Radiation Units and
Measurements, Fundamental Quantities and Units for
Ionizing Radiation, Report No. 60, ICRU, Bethesda
(1998)
• 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)
• 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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