INTERACTION OF XRAYS AND GAMMA RAYS WITH MATTER -II

Sneha George

Interaction of photons with matter occur by 4 mechanisms:-

1) Coherent scattering

2) Photoelectric effect

3) Compton scattering

4) Pair production

PHOTOELECTRIC EFFECT Photon interacts with a ‘bound’ electron

Photon disappears Bound electron leaves completely the atom

Photon energy Binding energy + Kinetic energy

Ejection of an electron

– atom becomes ionised and highly unstable

– vacancy filled by electron from outer shell

– original neutrality and electron balance restored by an electron from outside being attracted into the ionised atom

THE PHOTOELECTRIC CO-EFFICIENT(Γ)

The mass photo-electric attenuation co-efficient (γ/ρ) = k Z3/E3

- directly proportional to the cube of the atomic number of the attenuator (Z3)

- inversely proportional to the cube of the radiation energy(E3)

Over a wide range of energy, the value of γ/ρ for lead is about 250 times as that of aluminium ( the ratio of the cube of their atomic numbers is 251:1)

There is a rapid general decrease in attenuation co-efficient as radiation energy is increased , the rate of fall being in accordance with the stated dependance on the inverse of the cube of the radiation energy

Sudden breaks occur and produce sudden departures of the smooth variation so far implied

ABSORPTION EDGES Sudden changes in the attenuation of the

radiation occur at photon energies equal to the binding energies of the different electronic shells

The likelihood or probability of an electron interacting with a photon increases the nearer the energy of the photon is to the binding energy of that particular electron

Absorption edges have two important consequences

In their neighbourhood, lower energy photons are less attenuated and therefore more penetrating than higher energy photons which is in direct contrast to the general situation

Any substance is relatively transparent to its own characterisitic radiation, the energies of which are always atleast a little less than the corresponding binding energies

PAIR PRODUCTION The photon interacts with the

electromagnetic field of the nucleus and gives up all its energy in the process of creating an electron and a positron

Since the rest mass energy of each particle is 0.51 MeV, the photon energy should be atleast 1.02 MeV for this to happen

The total kinetic energy carried by the pair is (hυ- 1.02 )MeV

ANNIHILATION The positron loses its energy as it traverses

through the medium

Near the end of the track with almost no energy left it combines with an electron and the total mass of these two particles is converted into two photons each with 0.51 MeV ejected in opposite directions

PAIR PRODUCTION CO-EFFICIENT Pair production occurrence likelihood increases

with field magnitude-> nuclear charge or the atomic number of the irradiated material

Pair production increases with radiation energy

PHOTONUCLEAR REACTIONS If a photon has an energy greater than the

binding energy that holds the neutrons and protons together in the nucleus , it can enter the nucleus and eject a particle from it , which is mostly a neutron

The photon disappears completely

Any energy in excess of that needed to remove the particle would be the kinetic energy of the particle

TRANSMISSION Some photons do not pass through the

material and are transmitted

Their energy and penetrating power are unaltered

Reduction in number but the survivors are not affected

The photo-electric effect dominates the attenuation scene at low photon energies especially in the higher atomic number materials

Pair production takes command for very high energies and especially for higher atomic number elements

For medium photon energies and especially for elements of low atomic number, the Compton scattering process is the main method of attenuation.

The main general features of the attenuation picture are

That the photo-electric effect falls off rapidly with increasing energy

That scattering decreases as photon energy increases

That scattering is the predominant process over the medium energy range

That the penetrating power of an X-ray beam increases with increasing energy until energies > 1 MeV are reached

The smaller the attenuation co-efficient the more penetrating the beam

Increasing pair production reverses the trend

Very high energy radiations are somewhat less penetrating than lower energy radiations

ABSORPTION

Taking up of energy from the beam by the irradiated material

Unmodified scattering involves no absorption

In Compton scattering part of the energy removed from the beam is absorbed and less and less scattered

The photo-electric process is not one of complete absorption since part of the energy of the photon originally removed is re-radiated as characteristic radiation

For pair production , all but 1.02 MeV of the abstracted energy is absorbed

At low energies where the photo-electric effect predominates the two co-efficients are almost identical as they are in the very high-energy range where most of the attenuation is by pair production.

Marked difference between the co-efficients in the range 40keV to 4MeV because here Compton process is the main cause of attenuation

Especially in the lower part of this range the scattered photons retain much of the original energy so that a relatively small part of the energy removed from the beam is absorbed by air. For eg. Only 15 percent for 100 keV photons

A noteworthy point about the absorption curve for air is its relative independence of photon energy over a wide range.

For a hundred fold range of energy the co-efficient varies by little more than a factor of two

ABSORPTION VARIATION WITH PHOTON ENERGY

The low absorption value for low photon energies shows the effect of the scattered photon retaining most of the available energy while the falling value at higher energies is due to the steady reduction of σ/ρ with energy

Over a wide range of intermediate energies the mass absorption co-efficients are practically identical.

This is the energy range of Compton process predominance which does not depend on atomic number but only on electron density

All substances which don’t contain hydrogen will have nearly the same mass absorption and attenuation co-efficients in the energy range in which the Compton effect predominates

SPATIAL DISTRIBUTION OF SECONDARY RADIATION

Scattered radiation Characteristic radiation

SECONDARY Annihilation radiation RADIATION

Due to interaction of radiation with matter electrons are freed from their parent atoms and set in motion.

X-rays are found travelling in any direction

Recoil electrons travel forward never making an angle of more than 90 degrees with the initial photon and generally at a much smaller angle

Photo-electrons and electron pairs though more randomly emitted also tend to travel forward especially for high energy radiation

Characteristic and annihilation radiations are given out in all directions , therefore their distribution is isotropic and are the scattered photons for low energy beams

In the megavoltage range (1-10 MeV) principally used for radiotherapy, the vast majority of the secondary radiation will be Compton scattered photons and at these energies will travel in a forward direction having suffered completely small angle scattering.

Very little of this radiation suffers 180 degrees scatter. i.e. there is very little back scatter.

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