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Quantities and Measurements - 2

Dosimetric Quantities

Kerma, Dose, LET and more

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

• 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:

K =

where

dEtr is the sum of the initial kinetic energies of all the charged particles liberated by uncharged particles in a mass dm

dEtr

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:

K =

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)

.

.

dK

dt

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Exposure

Exposure is:

A quantity used to indicate the amount of ionization in air produced by x- or gamma-ray 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 =

where “W” is the average energy spent by an electron to produce an ion pair and “e” is the electronic charge

W

Ka (1 – g) e

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 =

The unit is C kg-1 s-1

.

.

dXdt

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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 =

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)

.dDdt

.

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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 = [ ]

where dE is the energy lost in traversing distance dl

dEdl

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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 WhichDoses Are Calculated

• Gonads• Bone marrow

(red)• Bladder• Breast• Thyroid• Skin• Remainder

• Colon• Lung• Stomach• Liver• Oesophagus• Bone surface

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Remainder Organs

• Adrenals• Upper large

intestine• Small intestine• Kidney• pancreas

• 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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• 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)

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)

• 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)

Where to Get More Information

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• 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)

Where to Get More Information