# basic radiation interactions, definition of dosimetric ... ?· basic radiation interactions,...

Post on 04-Dec-2018

213 views

Embed Size (px)

TRANSCRIPT

Basic Radiation Interactions,Definition of Dosimetric Quantities,

and Data Sources

J.V. SiebersVirginia Commonwealth University

Richmond, Virginia USA

2009 AAPM Summer School2009 AAPM Summer School

Learning Objectivesg j

T i d d ib th b i f 1. To review and describe the basics of radiation interactions for understanding radiation dosimetry

2 To review definitions of quantities 2. To review definitions of quantities required for understanding radiation dosimetrydosimetry

JVS: 2009 AAPM SS

Constants Units Conversions

JVS: 2009 AAPM SS

JVS: 2009 AAPM SS

ScopeRadiation Types

IonizingIonizingInteractions can remove atomic orbital electrons Non-Ionizing

Particulate ElectromagneticParticulate-electron-positron

Electromagnetic

-proton-neutron- alpha

JVS: 2009 AAPM SS

p- etc.

Types of ionizing radiationyp g

Di tl i i i di ti Directly ionizing radiation Direct interactions via the Coulomb force along a

particles track Charged particles

electrons positrons

protons protons heavy charged particles

JVS: 2009 AAPM SS

Direct IonizationCoulombic Interaction

e-Coulombic Interaction

A charged particle exerts exerts electromagnetic forces on atomic Energy transfer can electrons result in the ejection

of an electron (ionization) (ionization)

JVS: 2009 AAPM SS

Indirectly Ionizing Radiationy g

Uncharged particles that must first transfer energy to a charged particle which can then further ionize matterT t Two step process

ExamplesEl t ti di ti Electromagnetic radiations: x- or -rays

Neutrons

JVS: 2009 AAPM SS

Indirectly Ionizing RadiationPh t l t i Eff tPhotoelectric Effect

e-

Ej t d

hEjected

electrons further ionize ionize matter

JVS: 2009 AAPM SS

Radiant Energy Rgy

R Total energy excluding rest mass R Total energy, excluding rest mass, carried by particles Photons: E = h = hc/ Electrons + other CPs: kinetic energy T

JVS: 2009 AAPM SS

Energy imparted

- Energy impartedR R Q - Energy imparted i l i

in outR R Q Q mass to energy conversion resultingfrom interactions or radioactive decayQ

if(mE), Q>0

inR outRhe-

he-

JVS: 2009 AAPM SS

if(Em), Q

Dose

GydD

Energy deposited per unit mass

ydm

Energy deposited per unit mass

1 Gy = 1 J/kg

Knowledge of D is the object of dosimetry

JVS: 2009 AAPM SS

Equilibrium Part 1: R di ti E ilib iRadiation Equilibrium

R Rh

e-

in outR Rhe- e-

hhe-

R R QQ d Qd RE in outR R Q Q d QdDdm dm

RE RE

JVS: 2009 AAPM SS

Radiation SourcesS

Radioactive decay Radioactive decay Alpha-decay Beta-decay Electron capture Electron capture Isomeric transitions

Accelerated charged particles Direct Direct X-ray generators

Atomic energy transitionsCharacteristic X rays Characteristic X-rays

Auger electrons Interaction products

JVS: 2009 AAPM SS

Radioactive Decayy

General balance equationsGeneral balance equationsR R

R R

A A AAZ Z Z ZP D R Q

P D RQ M M M

JVS: 2009 AAPM SS

Q

JVS: 2009 AAPM SS

Radioactive Decay

ActivitydNA Ndt

0t

tA A e

1ln 2t

JVS: 2009 AAPM SS

12

Radioactive Decay

4 42 2A AZ ZP D He Q

s have short range /

01 1A AZ ZP D Q

01 1A AZ ZP D Q

Neutrino ( , ) results in spectrum of energies( ) p g maxE and E are tabulated ( , ) are non-ionizing

Electron Capture 0A AP D Q 01 1A AZ ZP e D v Q

Can occur when energetically prohibited Followed by characteristic x-rays or Auger electron

Isomeric Transition so e c a s o * 00

A AZ ZP P Q

decay from meta-stable state Internal Conversion

* 0 0A AP P Q

JVS: 2009 AAPM SS

0 01 1A AZ ZP e P e Q

Competes with isomeric transition Results in ejection of atomic electron

JVS: 2009 AAPM SS

15 15 0 0 1 732O N M V + 15 15 0 08 7 1 0 1.732O N MeV

15 0 15 08 1 7 0 1.732O e N MeV Electron Capture

+

JVS: 2009 AAPM SS

8 1 7 0Capture

JVS: 2009 AAPM SS

Accelerated Charged Particlesg

Di t Direct use Electrons, protons,

Indirect via production of electromagnetic radiationradiation Synchrotron radiation Bremmstrahlung Bremmstrahlung

JVS: 2009 AAPM SS

Synchrotron Radiation

hRadiation

Magnetic Field

e-

JVS: 2009 AAPM SS

Synchrotron image courtesy of http://www-project.slac.stanford.edu/ssrltxrf/spear.htm

Bremmstrahlungbremsh

Bremmstrahlungbrems

e-

JVS: 2009 AAPM SS

Atomic Energy TransitiongyCharacteristic x-ray

xray h

JVS: 2009 AAPM SS

Atomic Energy TransitionAuger Electron

e-

JVS: 2009 AAPM SS

Quantifying Radiation FieldsQ y g

Th f Thus far R D

JVS: 2009 AAPM SS

Radiation Fluence

N is number of particles i h

dN particles crossing sphere surrounding P with cross-sectional area da

2

pda m

sectional area da

Integrated over all directions and energies

Single particle type

JVS: 2009 AAPM SS

Equivalent definition of fluenceq

l = particle track l l = particle track length through a volume

nTracksl

V

l need not be straight

Volume can be irregularU f l f M t Useful for Monte Carlo applications

JVS: 2009 AAPM SS

Energy Fluencegy Definition

dR J 2

dR Jda m

Poly-energetic Mono-energetic

da m

E Diff ti l fl

EE E dE E Differential energy fluence

E E d dE JVS: 2009 AAPM SS

E E d dE

Attenuation

td n dl tl 0e

l 1n JVS: 2009 AAPM SS

l 0 tn m

Attenuation coefficient l 0e

l

Attenuation coefficient

t th i t ti ( l) f represents the interaction (removal) of primaries from the beam

No consideration is given to what occurs as a result of the interaction Secondary particles Energy-to-mass conversion

To remove density dependence, tabulated as /[ 2/ ]

JVS: 2009 AAPM SS

[cm2/g]

TERMA Total Energy Release per unit MAss

Jkg

TERMA

*

Describes loss of radiant energy from uncharged

kg

primaries as they interact in material Energy lost can be absorbed locally or at a distance

JVS: 2009 AAPM SS J

kgEE

ETERMA E dE

For poly-energetic spectra*

Aside:Photon InteractionsPhoton Interactions

To understand what happens with the radiant energy removed, understand the interactions(e.g. interactions)

JVS: 2009 AAPM SS

Photon interactions contributing to Photon interactions contributing to

-1mRayleigh

= Rayleigh + Compton scattering = Rayleigh + Compton scattering = photo-electric = pair production = pair production = photo-nuclear

JVS: 2009 AAPM SS

Rayleigh Scatteringy g S g

Elastic coherent scattering of the photon by an atomy

Important for low energy photonsC t ib t < 20% t t t l tt ti Contributes < 20% to total attenuation coefficient

JVS: 2009 AAPM SS

Compton ScatteringCompton Scatteringe-

h

e

h

h 2cmAN Z

JVS: 2009 AAPM SS

ge A

Comptonp

JVS: 2009 AAPM SS

Photoelectric EffectPhotoelectric Effect

e-

h b AeT h E T beT h E

JVS: 2009 AAPM SS

Photo-electric

3 4

2 3Zh

Au

increases when increases when shell can participate in reactionreaction

JVS: 2009 AAPM SS

Pair ProductionPair Production

e-h

+

pairh

e+22 ee eavailT T T h m c ee eavail

2

diom c

JVS: 2009 AAPM SS

radianoT

Triplet ProductionTriplet Production

22avail eT h m c

e-

e- tripleth

e+22h

JVS: 2009 AAPM SS

223

eh m cT

Photo-nuclear interactions

( n) ( Xn) ( p) (,n), (,Xn), (,p),

BE (Binding Energies) result in thresholds >~ 10 MeV

Cross-section is small (

JVS: 2009 AAPM SS

Pb attenuation coefficient

JVS: 2009 AAPM SS

Relative importance of interactionsRelative importance of interactions

JVS: 2009 AAPM SS

Summary photon interactions

JVS: 2009 AAPM SS

Energy transferred to charged particlesi t ti

Energy transferred to charged particles per-interaction

general nonrtr in outu uR R Q photo

u u

= compton pair

==

Averagei

trn

JVS: 2009 AAPM SS

itr

in

RecallAttenuation coefficient

l 0el

Attenuation coefficient represents the interaction (removal) of represents the interaction (removal) of

primaries from the beam No consideration is given to what occurs as a No consideration is given to what occurs as a

result of the interaction Secondary particles Secondary particles Energy-to-mass conversion

To remove density dependence, tabulated as /[cm2/g]

JVS: 2009 AAPM SS

Mass-energy transfer coefficient

Recommended