INTERACTIONS OF RADIATION WITH MATTER

DR VIJAY KUMARDNB PGTDEPT OF RADIATION ONCOLOGY

Basic Concepts Of Interaction

Attenuation

The intensity reduction of x-ray photons as they pass through matter

Primary radiation – attenuation = remnant or exit radiation

Photon attenuation is characterized by attenuation coefficient µ.

For a narrow mono energetic beam, attenuation coefficient is : I(x)=Io e

And hence HVL= 0.693/µ

-µx

Attenuation Of An X-Ray Photon

The Five Interactions Of X and Gamma Rays With Matter Photoelectric effect

Very important in diagnostic radiology Compton scatter

Very important in radiotherapy Coherent scatter

Not important in diagnostic or therapeutic radiology Pair production

important in diagnostic radiology Photodisintegration

Neutron contamination of therapy beams

Photoelectric Effect All of the energy of the

incoming photon is totally transferred to the atom Following interaction, the

photon ceases to exist The incoming photon

interacts with an orbital electron in an inner shell – usually K

The orbital electron is dislodged

To dislodge the electron, the energy of the incoming photon must be equal to, or greater than the electron’s energy

Photoelectric Effect The incoming photon gives up all its energy, and

ceases to exist The ejected electron is now a photoelectron This photoelectron energy = energy of the incoming photon- the binding

energy of the electron shell This photoelectron can interact with other atoms

until all its energy is spent These interactions result in increased patient

dose, contributing to biological damage

Photoelectric Effect A vacancy now exists in the inner shell To fill this gap, an electron from an outer shell

drops down to fill the gap Once the gap is filled, the electron releases its

energy in the form of a characteristic photon This process continues, with each electron

emitting characteristic photons, until the atom is stable

The characteristic photon produces relatively low energies and is generally absorbed in tissue

The Byproducts of the Photoelectric Effect Photoelectrons Characteristic photons

The Probability of Occurrence Depends on the following: Mass photoelectric coefficient is ª Z/E

It increases as the photon energy decreases, and the atomic number of the irradiated object increases

When the incident photon’s energy is more or close to the binding energy of the orbital electron

In water or soft tissue This type of interaction is prevalent in the diagnostic kVp range – 10-25keV(30-75kVp)

2 3

What Does This All Mean?

Bones are more likely to absorb radiation This is why they appear white on the film

Soft tissue allows more radiation to pass through than bone These structures will appear gray on the film

Air-containing structures allow more radiation to pass through These structures will appear black on the film

Compton Scattering An incoming photon is

partially absorbed in an outer shell electron

The electron absorbs enough energy to break the binding energy, and is ejected

The ejected electron is now a Compton electron

Not much energy is needed to eject an electron from an outer shell

The incoming photon, continues on a different path with less energy as scattered radiation

Byproducts Of Compton Scatter Compton scattered electron

causes projectile damage in the tissue. Possesses kinetic energy and is capable of ionizing

atoms. The atom becomes a free radical, causing biological

damage in the tissue Scattered x-ray photon with lower energy

Continues on its way, but in a different direction It can interact with other atoms, either by photoelectric

or Compton scattering It may emerge from the patient as scatter

Probability Of Compton Scatter Occurring Probability of a Compton interaction is inversely

proportional to energy of the incoming photon. In water More probable at kVp ranges of 10-150. and

decreases further with increase in energy. Most dominant interaction in tissues at treatment

energies(30keV-24MeV). It is independent of atomic number, so at treatment

energies, bone and soft-tissue interfaces are barely distinguishable (= poor contrast)

At diagnostic x-ray energies, Compton Scattering direction is fairly random; at treatment x-ray energies, it is forward-peaked

Coherent Scatter Only significant at lowest diagnostic x-ray energies

(<5% interactions) Incoming photon is deflected (absorbed and

immediately re-emitted), with minimal direction and energy change

May result in radiographic film fog

Pair Production

Occurs only at high photon energies (>1.02 MeV) and preferentially in high-Z tissues

Incoming photon (energy) is converted to mass (electron and positron) in the vicinity of atomic nucleus via E=mc2

Pair Production An incoming photon of

1.02 MeV or greater interacts with the nucleus of an atom

The incoming photon disappears

The transformation of energy results in the formation of two particles

Negatron Possesses negative

charge Positron

Possesses a positive charge

Positrons Will interact with the first electron they encounter An electron and the positron destroy each other

during interaction Known as the annihilation reaction

This converts matter back into energy Both the positron and electron disappear Two gamma photons are released with an energy

of .51 MeV and travel at an angle of 180º.  A simultaneous detection of gamma ray photons in two detectors places the source on a line between those detectors (PET SCAN: where radioisotopes used for positron emission).

Pair Production

Electron causes projectile damage in the tissue

Significant pair production can be seen in blocking of the oncoming beam, since blocks are high-Z materials (for lead, this is the main effect at energies >5 MeV)

Table 5.2 Relative Importance of Photoelectric ( ), τCompton ( ), and Pair Production ( ) Processes in Waterσ Π

 Photon Energy (MeV)Relative Number of Interactions (%)τ σ Π

 0.01 95 5 0 0.026 50 50 0 0.060 7 93 0 0.150 0 100 0 4.00 0 94 6 10.00 0 77 23 24.00 0 50 50 100.00 0 16 84 Data from Johns HE, Cunningham JR. The Phys ic s o f Ra d io lo g y . 3rd ed. Springfield, IL: Charles C Thomas; 1969.

Photodisintegration Occurs at above 10 MeV A high energy photon is

absorbed by the nucleus The nucleus becomes

excited and becomes radioactive

To become stable, the nucleus emits negatrons, protons, alpha particles, clusters of fragments, or gamma rays

Source of low-level neutron production

Interactions Of Particulate Radiation With Matter Electrons, protons, neutrons, alpha particles,

beta paticles are examples of particle radiation.

Charged particle interaction or collisions mediated by coulomb force between the electric field of travelling particle and electric fields of orbital electrons and nuclei of atoms of the material.

They interact primarily by ionization or excitation.

All particles exhibit Bragg peak near end except electrons due to excessive scattering.

Electrons

Two fundamental interactions: Radiation (Bremsstrahlung) - bending of electrons

around nucleus => shedding of energy as EM x-rays

Ionization (Characteristic X-rays) - impact with orbital electron => electron release => vacancy fill => shedding of energy as Characteristic x-rays

Protons Incoming protons also lose energy mainly by interacting with

orbital electrons; however, since they are much heavier (~1800x), they only lose very small fraction of their kinetic energy with each interaction, and thus scatter only minimally

The interactions (and thus energy loss) become more frequent at slower energies. Thus the slower the proton moves, the more energy it loses to the tissue electrons, in a feed-forward loop, until it abruptly loses all energy. This region of rapid energy loss (and its deposition into the tissue) is called the Bragg peak.

The distance at which Bragg peak occurs, and the energy is deposited, can be calculated very precisely (unlike electrons). The rapid drop-off in dose make it ideal for delivering dose precisely to the tumor, and not to the healty tissue beyond the tumor.

Incoming protons also rarely interact with the nucleus, and may enhance cell kill by ~10%

Neutrons

Interact by ejecting recoil protons from hydrogen and recoiling heavy nuclei from other elements or by producing nuclear disintegrations.

Lead is an efficient absorber of x-rays but not of neutrons.

The most efficient absorber of neutrons is a hydrogenous material such as water, paraffin wax, and polyethylene.

Heavy ions

Stopping power of ionization interactions is proportional to square of particle charge and inversly to square of its velocity

They interact with tissue similarly to protons, but since they are heavier still, they scatter less initially, and have a faster dose fall-off (Bragg peak) at the end.