Quantities and Measurements - 2
Dosimetric Quantities
Kerma, Dose, LET and more
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Day 2 – Lecture 8
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Objective
• To know about dosimetric quantities,
associated terminology and underlying
concepts
• We will learn about kerma (rate),
exposure (rate), absorbed dose (rate),
linear energy transfer (LET), lineal
energy transfer, and organ dose
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Content
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Kerma (rate)
Mass energy absorption coefficient
Air Kerma
Exposure (rate)
Absorbed dose (rate)
Energy imparted
Linear energy transfer (LET)
Lineal energy transfer
Organ dose
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Kerma
Kerma (Kinetic Energy Released per unit Mass)
Kerma is defined as:
dEtr
K =
dm
where
dEtr is the sum of the initial kinetic energies of all
the charged particles liberated by uncharged
particles in a mass dm
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Kerma
• The unit of kerma is the J kg-1
• The special name for the unit of kerma is
gray (Gy)
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Kerma Rate
.
The kerma rate, K, is the quotient of dK by dt,
where dK is the increment of kerma in the time
interval dt, thus:
.
dK
K=
dt
The unit is J kg-1 s-1 and the special name for the
unit of kerma rate is gray per second
(Gy s-1)
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Exposure
Exposure is:
A quantity used to indicate the amount of
ionization in air produced by x- or gammaray radiation
The SI unit of exposure is the coulomb per
kilogram (C/kg)
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Exposure
The exposure, X, in units of C kg-1, is related
to the air kerma as follows:
X =
Ka (1 – g) e
W
where “W” is the average energy spent by an electron to produce an ion
pair and “e” is the electronic charge
where g is the fraction of initial secondary electron energy that is
radiated as bremsstrahlung
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Exposure
• Exposure is measured under conditions of
electronic equilibrium
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Exposure Rate
.
The exposure rate, X, is the quotient of dX by dt,
where dX is the increment of exposure in the time
interval dt, thus:
.
X=
dX
dt
The unit is C kg-1 s-1
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Absorbed Dose
The absorbed dose, D, is given by:
D = de/dm
_
Where de is the mean energy imparted to
matter of mass dm
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Absorbed Dose
• The unit of absorbed dose is J kg-1
• The special name for the unit of absorbed
dose is gray (Gy)
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Energy Imparted
Energy imparted is the energy incident minus
the energy leaving the mass (excluding the
energy released in nuclear transformations
to keep the dose from becoming negative
when the mass contains a radioactive
source)
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Absorbed Dose Rate
.
The absorbed dose rate, D, is the quotient of
dD by dt, where dD is the increment of
absorbed dose in the time interval dt, thus:
.
D=
dD
dt
The unit is J kg-1 s-1 and the special name for
the unit of absorbed dose rate is gray per
second (Gy s-1)
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Lineal Energy Transfer
Lineal energy transfer is the energy
transferred from a particle to the medium
traversed per unit length
The magnitude is expressed in kilo-electron
volts per micrometer (keV/µm)
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Lineal Energy Transfer
• Expresses the level of energy transferred at
a microscopic scale
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Linear Energy Transfer
Linear energy transfer (LET), is defined
generally as:
L =[
dE
]
dl
where dE is the energy lost in traversing
distance dl
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Linear Energy Transfer
• A measure of how, as a function of
distance, energy is transferred from
radiation to the exposed matter
• A high value of LET indicates that energy
is deposited within a small distance
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Organ Dose
• Organ doses can arise from both external and
internal radiation (i.e. intakes of radioactive
material)
• Measurement/calculation of organ dose from
external radiation is usually more straightforward
than for intakes of radioactive material
• Therefore, the next slides focus on organ doses
from internal radiation
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Organ Dose
• Following an intake into the body of a
radioactive material, there is a period
during which the material gives rise to
equivalent doses delivered 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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Specific Organs for Which
Doses Are Calculated
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Gonads
Bone marrow (red)
Bladder
Breast
Thyroid
Skin
Remainder
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Colon
Lung
Stomach
Liver
Oesophagus
Bone surface
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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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Phantom for Organ Dose Calculation
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Organ Dose
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Summary
• Dosimetric quantities and associated
terminology were discussed
• Students learned about kerma,
exposure, absorbed dose, linear energy
transfer, lineal energy transfer and organ
dose
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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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