Gamma and X ray interactions NUCP 2371 Rad Pro III

EM Energy Energy of the EM radiation is determined by its frequency E= hf h= planks constant =6.6 x 10 -34 J sec Or 4.1 x 10 -15 eV sec F= frequency of the EM radiation Frequency is the inverse of wavelength

Photon Interactions Rayleigh Scattering Thompson scattering Photoelectric Effect Compton Scattering Pair production Nuclear transformations EM radiation interacts with matter in three main ways

Rayleigh Scattering Is the scattering of light or other low energy electromagnetic radiation by particles much smaller than the wavelength of the light, such as an atom or molecule The amount of scattering that occurs is dependent upon the size of the particles and the wavelength of the light. Usually the EM ray bounces off the electron with no change in energy the main reason why the sky is blue

Rayleigh scattering greater proportion of blue light scattered by the atmosphere relative to red light. (from Wikipedia)

Thompson Scattering Free charged particle absorbs the gamma ray The particle oscillates in an excited state Then radiates the gamma ray in a random direction The energy is the same but is going in a different direction Background microwave radiation

Photoelectric Effect Low energy gamma or x ray interactions Electron absorbs all of the energy of the incoming EM wave and has enough energy to break away from the atom to which it is associated First observed by Hertz in 1887 N Tesla received a patent for a photoelectric motor in 1901 Explained by Einstein in 1905

Photoelectric Effect Ejected Electron Incoming Photon The photoelectric effect is one of the discoveries that won Albert Einstein his noble prizes. Photoelectric Effect: Predominates with incident photons of low energy; complete energy transfer of the incoming photon to the ejected electron

Photoelectric Equation Ee- = E - B.E. Ee- = kinetic energy of the ejected electron E = energy of the X-ray or gamma ray B.E. = binding energy of the orbital electron Probability if PE effect occurring is proportional to the Z of the material and inversely proportional to the energy of the EM wave Energy of the ejected electron is equal to the energy of the incoming gamma ray minus the binding energy of that electron.

Compton Scattering Medium energy EM wave interaction in which the electron absorbs some of the energy of the EM wave and get ejected from the atom , but he secondary EM is created with a wavelength greater (less energy)than that of the incoming EM wave Discovered by A Compton in 1923

Scattered Photon Incoming Photon Ejected Electron Compton Scattering Scattered Photon Incoming Photon Ejected Electron Compton scattering creates a new but lower energy gamma ray than the original. The angle and energy of the new EM radiation can be calculated. Compton Scattering: Predominates with incident photons of medium energy; partial energy transfer of the incoming photon to the ejected electron and the scattered photon.

Compton Equation Δλ = λc(1 - cos θ), Δλ = the difference in wavelength of incomeing and scattered EM wave λc is the Compton wavelength of the electron (h/mc= 2.43 X 10-12 m) θ is the angle between the directions of incident and scattered radiation. Scattered EM wave has less energy hence a longer wavelength than the oncoming EM wave.

Difference in wavelength What is the new energy of a compton scattered ER ray if the ER is scattereded through an angle of 400

SHIELDING Backscatter Deflection of radiation by scattering process through angles greater than 90 degrees with respect to the original direction of motion What does this mean? What are the implications?

Pair Production High energy EM interaction, EM wave comes in very close to the nucleus and spontaneously disappears and two charged particles(Beta – and +) are created First observed by Patrick Blackett

Emitted Annihilation Photons Incident Photon Detector Material Pair Production Emitted Annihilation Photons + Incident Photon - Detector Material Even though it takes a minimum of 1.022 MeV to produce the pair of changed particles , this process does not become a major contributor to the EM interaction until the energy of the EM is about 6-7 MeV. Pair Production: Predominates with incident photons of high energy (at least 1.02 MeV); Positron and electron formed, then two 0.51 MeV annihilation photons are emitted

Pair production Can occur when the gamma energy is above 1.022 MeV but does not occur in any appreciable percentage until the gamma energy is >5MeV Electron will get captured Positron will find an electron and will annialate producing two 511 keV gamma rays

Photon Attenuation in Lead Graph shows the energy ranges of the different types of interactions. As mentioned before each interaction has its particular energy that is mostly occurs at.

Nuclear transformation Occurring at high energies >6 MeV EM ray gets absorbed by the nucleus A particle, usually a neutron, is emitted from the nucleus

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