Unit IV

 UNIT IV INTERACTION OF RADIATION WITH MATTER (Regulation 2013)

Interaction of charged particles with matter –Specific ionization, Linear energy transfer range,Bremsstrahlung, Annihilation,Interaction of X and Gamma radiation with matter- Photoelectric effect,Compton Scattering , Pair production, Attenuation of Gamma Radiation ,Interaction of neutron with matter and their clinical significance.

INTERACTION OF RADIATION WITH MATTER

Interaction of charged particles with matter – Specific ionization , linear energy Transfer Range, Bremsstrahlung , Annihilation Interaction of Gamma radiations with matter – Photoelectric effect, Compton Scattering , Pair Production, Attenuation of Gamma Radiation, Interaction of neutron with matter

Introduction

When radiation strikes matter, both the nature of the radiation and the composition of the matter affects. The process begins with the transfer of radiation energy to the atoms and molecules, heating the matter or even modifying its structure.

If all the energy of a bombarding particle or photon is transferred, the radiation will appear to have been stopped within the irradiated matter. Conversely, if the energy is not completely deposited in the matter, the remaining energy will emerge as though the matter were transparent or at least translucent.

Interaction of charged particles with matter

Specific ionization 

 linear energy Transfer Range

 Bremsstrahlung

 Annihilation 

Interaction of Gamma radiations with matter 

As the Gamma rays pass through matter, photons interact with atoms. The type of interaction is a function of the energy of the photons and the atomic number (Z) of elements composing the matter.

They are

            Compton Scattering

            Photo electric effect

            Pair Production

  

Photoelectric effect

A gamma ray of low energy, or one that has lost most of its energy through Compton interactions, may transfer its remaining energy to an orbital electron. This process is called the photoelectric effect and the ejected electron is called a photoelectron. This electron leaves the atom with energy equal to the energy of the incident gamma ray diminished by the binding energy of the electron.

An outer-shell electron then fills the inner-shell vacancy and the excess energy is emitted as an x-ray.

E_{photoelectron} = E_photon − E_binding

Compton Scattering

          In Compton scattering the incident photon transfers part of its energy to an outer shell or “free” electron, ejecting it from the atom. Upon ejection this electron is called a Compton electron. The photon is scattered at an angle that depends on the amount of energy transferred from the photon to the electron. The scattering angle can range from nearly 0◦ to 180◦.

Pair Production

Pair production is a direct conversion of radiant energy to matter. When a gamma ray photon with an energy of 1.022 Mev or greater, is passed through near a heavy atomic nucleus it can result the production of one electron and one positron.

Attenuation of Gamma Radiation

Interaction of neutron with matter