xray interaction with matter

1. INTERACTION OF XRAYS AND
GAMMA RAYS WITH MATTER -II
Sneha George

2. Interaction of photons with matter occur by 4 mechanisms:-
1) Coherent scattering
2) Photoelectric effect
3) Compton scattering
4) Pair production

3. PHOTOELECTRIC EFFECT
 Photon interacts with a ‘bound’ electron
Photon disappears Bound electron leaves
completely the atom
 Photon energy  Binding energy + Kinetic energy

5.  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

7. 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)

9.  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

10. 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

11.  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

12. 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

14. 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

15. 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

16. 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

17. 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

19.  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.

21. 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

22.  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

24. ABSORPTION
Taking up of energy from the beam by the
irradiated material

26.  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

27.  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

28.  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

29.  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

30. ABSORPTION VARIATION WITH PHOTON ENERGY

31.  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

32.  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

33. 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

34.  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

35.  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.