RADIATION ABSORPTION

MADe By: DR. ISHA JAISWAL

MODeRATOR: DR. S.P MISHRA

DATe : 25 NOv. 2013

eNeRgy ABSORPTION IN RADIOBIOLOgy

The enrgy is absorbed in form of small discrete packets: PHOTONS

The energy deposition is uneven, discrete & nonuniform.

Lead to chemical & biological changes.

Absorption of radiation may lead to ionization or excitation of atoms

IONIzATION vS. excITATION

EXCITATION: raising of electron to higher energy level without ejection.

IONIZATION:ejection of orbital eletron from atom resulting in production of ion pair

IONIzATION vS. excITATION

TyPeS Of RADIATION

• Radiation is classified into:

–Ionizing radiation–Non-ionizing radiation

Ionizing Versus Non-ionizing Ionizing Versus Non-ionizing RadiationRadiation

Ionizing RadiationIonizing Radiation

– Higher energy electromagnetic waves (gamma) or Higher energy electromagnetic waves (gamma) or heavy particles (beta and alpha).heavy particles (beta and alpha).

– High enough energy to pull electron from orbit.High enough energy to pull electron from orbit.

Non-ionizing RadiationNon-ionizing Radiation– Lower energy electromagnetic waves.Lower energy electromagnetic waves.

– Not enough energy to pull electron from orbit, but Not enough energy to pull electron from orbit, but can excite the electron.can excite the electron.

Primary Types of Ionizing Primary Types of Ionizing RadiationRadiation

PARTICULATE RADIATIONPARTICULATE RADIATION

Alpha particlesAlpha particles

Beta particlesBeta particles

NeutronsNeutrons

ELECTROMAGNETIC RADIATIONELECTROMAGNETIC RADIATIONGamma rays (or photons)Gamma rays (or photons)

X-Rays (or photons)X-Rays (or photons)

PARTIcuLATe vS. eLecTROMAgNeTIc RADIATIONS

Particulate Radiations are sub-atomic particles with mass.

(e.g., alpha and Beta particles, electrons, neutrons).

EM Radiations have no mass and no charge.

(eg. X-rays and gamma rays)

Types of Ionizing RadiationTypes of Ionizing RadiationTypes of Ionizing RadiationTypes of Ionizing Radiation

β

α

γ or X-

ray neutron

PRODucTION Of ALPHA & BeTA RAyS

Production of gamma rays

absorPtion mechanismsRadiation can be classified as

directly or indirectly absorption.Direct absorption -

charged particlesIndirect absorption -

photons, neutrons

directly vs indirectly ionizing radiation

Charged particles can directly distrupt the atomic structure of the atom through which they pass & produce chemical and biological damage.

• EM radiation & neutron when absorbed in matter give up their energy to produce fast moving charged particle which in turn produce effect.

directly vs indirectly ionizing radiation

sPecific ionization

• No of ion pairs produced per unit length of charged particle is called as specific ionization.

• Depend on charge & mass of particle.

Ionization by alpha particle, Xray & neutron

linear energy transfer

Energy absorbed in medium per unit length of particle is called LINEAR ENERGY TRANSFER

Expressed in kev/umLET determines rate of energy

absorption in medium

low let radiations

Properties:From x or gamma rays

Sparsely ionizingLow specific ionizationThey do not give up their energy quickly:longer range in air

More penentrationIndirectly ionizing

high let radiation

Properties:From particles that do possess mass

and chargeHigh specfic ionizationTheir energy is exhausted sooner

than that of an x or gamma rayLess penentrationDirectly ionizing

low vs high linear energy transfer

Absorption of EM rADiAtion

Indirect ionization depends onenergy of incident photonChemical composition of absorber.

Three ways of energy absorptionPHOTOELECTRIC EFFECTCOMPTON EFFECTPAIR PRODUCTION

photoElEctric EffEctInteraction between photon & bound

electron all energy transferred. More likely for low-energy photons Photon absorption proportional to Z3 (z is

atomic no of absorber)

Inner-shell electron is ejectedVacancy filled with outer orbital electronCharacterisic x ray & auger electron

coMpton scAttEr interaction betweenphoton & free

electronphoton strikes a free or loosely-bound

orbital electron (outer shell) transfers portion of kinetic energyMore likely at intermediate photon

energiesPhoton absorption is independent of Z

of absorber, thus energy absorption by this mechanism is similar for soft tissue, muscle and bone

pAir proDuction

Interaction between photon & nucleushigh-energy photon produces

positron/electron pairIncreases with square of atomic no. of

mediumThreshold energy of this process: 1.02MeVUltimately, the positron annihilates and

produces two 0.511 MeV photons

Absorption of chArgEDpArticlEs

Directly ionizingLarger mass than electronsGreater KE deposited over a

shorter path length: short rangeGreater specific ionization.greater LET than photons

Absorption of nEutron EnErgy

Uncharged, Indirect ionizingGenerate fast

recoil protons, alphas, and heavy fragments

DNA and Radiation

Ionizing Radiation at the Cellular Level

• Causes breaks in one or both DNA strands or;

• Causes Free Radical formation

Direct Ionization Caused By:

ProtonsAlpha

ParticlesBeta

ParticlesPositron

Particles

DIRECT IONIZATION

Indirect Ionization Caused By:

• Neutrons• Gamma Rays• X-Rays

INDIRECT IONIZATION