Basic Interactions Between X Rays and MatterBy Dr. Sofiya I. A. Modak

X-ray Interactions with MatterFor understanding the interactions of ionizing radiation and matter we must review a few points about atomic physics. The atom consists of a central nucleus and orbital

electrons. The positively charged nucleus exerts an electric

force of attraction and holds the negatively charged electrons in specific orbits or shells. The innermost shell is called the K shell and more the peripheral shells are named consecutively L, M, N, and so forth.

They have a limited electron capacity. The K shell can hold 2 electrons and L shell has

capacity of 8 electrons.

Basic structure of an ATOM : PROTON ( +ve charge )

An atom is made up of NUCLEUS NEUTRON ( neutral ) ORBITAL ELECTRONS ( -ve charge ) ORBITS / SHELLS ( K, L, M, N etc. )

Each shell has specific binding energy. The closer the shell is to nucleus, the tighter the electrons are ‘bound’ to the nucleus.

The electrons in the outer most shell are loosely bound. They are essentially free and are called ‘free’ or ‘valence’ electrons.

There are 5 basic ways that an x-ray photon can interact with matter. These are:1. Photoelectric effect 2. Coherent or unmodified scattering 3. Compton interaction with modified scattering4. Pair production5. Photodisintegration

1. Photoelectric effect The photoelectric effect always yields three end products:1) Characteristic radiation2) A negative ion ( the photoelectron)3) A positive ion ( an atom deficient with one

electron)

Characteristic radiation generated by the photoelectric effect is exactly the same as production of x-rays

The only difference in the method used to eject the inner shell electron.

In x ray tube a high speed electron ejects the bound electron.

While in photoelectric effect an X ray photon does the trick.

In both cases the atom is left with an excess of energy = the binding energy of an ejected electron.

It is usually referred to as ‘Secondary Radiation’ and is to be differentiated from scatter radiation.

CHARACTERISTIC RADIATION

How does this happen ?

After the electron has been ejected, the atom is left with a void in the K shell & an excess of energy equivalent to the binding energy.This state of the atom is highly unstable & to achieve a low energy stable state ( as all physical systems seek the lowest possible energy state ) an electron immediately drops in to fill the void. As the electron drops into the K shell, it gives up its excess energy in the form of an x-ray photon. The amount of energy released is characteristic of each element & hence the radiation produced is called Characteristic radiation.

Photoelectric Effect This type of interaction is most likely to occur when the

energy (hv) of an incident/ incoming photon with slightly greater energy than the binding energy of the electrons in one of the inner shells.

The incident photon looses all its energy on entering an atom being absorbed in this process.

Immediately, the atom responds by ejecting an electron, usually from K or L shell, leaving a hole in that shell.

Now the atom is ionized positively and in an excited state.

Electron from higher energy level fills the hole in the K shell, a ‘characteristic x-ray photon’ is being emitted.

Note: that the energy of the incident photon ultimately went to free the electron from its shell and set it motion as ‘photoelectron’

Summary: The energy of the incoming photon in the photoelectric interaction involving the K shell has the following fate:a) The photon enters atom and completely disappears. b) A K-shell electron is ejected, leaving a hole.c) Atom has excess energy – is in a excited state.d) A part of photon’s energy was used to liberate electron

and the rest to give it kinetic energy; ejected electron is a photoelectron.

e) Hole in K shell is filled by electron transition from a shell father out, accompanied by emission of a characteristic x-ray photon.

f) Holes in successive shells are filled by electron transitions from shells still farther out, each transition accompanied by a corresponding characteristic x-ray photon.

g) Sum of the energies of all the characteristic photons equals to binding energy of shell from which the photoelectron originated, in this case, the K shell.

Probability of occurrence:3 simple rules govern the probability of occurrence1) The incident photon must have sufficient energy to overcome

the binding energy of the electron.2) A photoelectric reaction is most likely to occur when the

photon energy and electron binding energy are nearly the same.

3) The tighter an electron is bound in its orbit, the most likely it is to be involved in the photoelectric reaction.

Application to diagnostic radiology:Advantage: It produces radiological images of excellent quality. Does not produce scatter radiation. It enhances natural tissue contrast (as some tissues absorb

more x-rays than other tissues.Disadvantage: Patients receive more radiation. All the energy of incident photon is absorbed by the patient.

Coherent or unmodified scattering

Radiation undergoes a change in direction without change in wavelength, thus sometimes it is called as “ unmodified scattering”

There are two types of coherent scattering:Both the types are described in terms of wave-

particle Interaction and therefore also called as ‘Classical scattering’

i. Thomson scattering: Single electron involved in the interaction.

ii. Rayleigh scattering: there is Co-operative interaction of all the electrons.

What happens in coherent scattering ? Low energy radiation encounters electrons

Electrons are set into vibration Vibrating electron, emits radiation. Atom returns to its undisturbed state

• No energy is transferred and no ionization occurs.

• Its only effect is to change direction of incident radiation.

• It occurs less than 5% and is not important in diagnostic radiology. It produces scattered radiation but of negligible quantity.

Compton interaction The Compton effect occurs when an incident x-ray

photon with relatively high energy strikes a free outer shell electron, ejecting it from its orbit.

The photon is deflected by the electron so that it travels in new direction as scatter radiation.

The reaction produces an ion pair A +ve atom A –ve electron ( recoil electron )

Almost all the scatter radiation that we encounter in diagnostic radiology comes from Compton Scattering.

Energy of photon distributed in two ways: Part of it goes to recoil electron as Kinetic energy. And the rest is retained by the deflected photon. Two factors determine the amount of energy the photon transmits: The initial energy of the photon. Its angle of deflection.

1.Initial energy :- Higher the energy more difficult to deflect.High energy : Travel straight retaining most of the energy. Low energy : Most scatter back at angle of 180º

2. Angle of deflection :- Greater the angle, lesser the energy transmitted. With a direct hit, maximum energy is transferred to the recoil electron. The photon retains some energy & deflects back along its original path at an angle of 180º.

The formula for calculating the change in wavelength of a scattered photon is : Δλ = 0.024 ( 1 – cos θ ) ,where Δλ = change in wavelength

θ = angle of photon deflection

Disadvantages of Compton reaction :Scatter radiation : Almost all the scatter radiation that

we encounter in diagnostic Radiology comes from Compton scattering. In the diagnostic energy range, the photon retains most of its original energy. This creates a serious problem, because photons that are scattered at narrow angles have an excellent chance of reaching an x-ray film & producing fog.

Exceedingly difficult to remove –• cannot be removed by filters; because they are too

energetic• cannot be removed by grids; because of narrow angles of

deflection.

It is also a major safety hazard. Even after 90˚ deflection most of its original energy is retained. Scatter radiation as energetic as the primary radiation. Safety hazard for the radiologist, personnel and the patient.

Pair ProductionWhat happens in Pair production ?A high energy photon interacts with the nucleus of an

atom.

The photon disappears & its energy is converted into matter in the form of two particles

An electron A positron (particle with same mass as electron, but

with +ve charge.)

Mass of one electron is 0.51 MeV.2 electron masses are produced.So the interaction cannot take place with photon energy

less than 1.02 MeV.It has no importance in diagnostic radiology.

Annihilation reaction:The Positron, as it comes to rest, combines with a negative electron-it disappears giving rise to two photons with an energy of 0.51 Mev, moving in opposite direction.This is annihilation reaction.

PhotodisintegrationA photon with extremely high energy ( 7-15 MeV), interacts directly with the nucleus of an atom. It does not occur with energies less than 7 Mev,which may eject a neutron, proton or on rare occasions even an alpha particle.

What happens in Photodisintegration ? A high energy photon encounters the nucleus of an

atom. Part of the nucleus which may be a neutron, a proton, an

alpha particle or a cluster of particles, is ejected.

It has no diagnostic importance. As we rarely use radiation>150 KeV in diagnostic radiology.

To Note:The photoelectric effect accounts for 75% of interaction.Compton scattering for 20%, andCoherent scattering for 5%,the total 100%.

Questions

1. what are the products of photoelectric effect?

Ans. The photoelectric effect always yields three end products:Characteristic radiationA negative ion ( the photoelectron)A positive ion ( an atom deficient with one electron)

2. . What is annihilation reaction?

Ans. Annihilation reaction:The Positron, as it comes to rest, combines with a negative electron- it disappears giving rise to two photons with an energy of 0.51 Mev, moving in opposite direction.This is annihilation reaction.

Thank you Have A nice day