The threshold energy (in MeV) for pair production, in the Coulombic field of the nucleus is
1.02 MeV.
The threshold energy in MeV for pair production in vicinity of a nucleus is 2.04 MeV.
Monday, February 11, 2008
Energy losses by electrons
Energy losses by electrons are dependent on Z.
Energy losses are dependent on Z in collision interactions.
Stopping power is divided into collision and radiative interactions.
For radiative collisions, energy losses when electrons are stopping in a medium vary as Z to the 3rd power
Energy losses are dependent on Z in collision interactions.
Stopping power is divided into collision and radiative interactions.
For radiative collisions, energy losses when electrons are stopping in a medium vary as Z to the 3rd power
Sunday, February 10, 2008
Probability of photoelectric effect
The probability of photoelectric effect in a medium roughly vaires as Z to the 4th (reported in some as Z to the 3rd power)
Pair Production
The probability of pair production (atomic cross section) in the interacting medium varies as Z squared (mnemonic Z squared , the two is a pair!)
In pair production after expending energy for the creation of the pair, the excess photon energy is shared by the electron and positron.
In pair production the positron at the end of it's range in the medium is annihilated resulting in two annihilation photons.
In pair production after expending energy for the creation of the pair, the excess photon energy is shared by the electron and positron.
In pair production the positron at the end of it's range in the medium is annihilated resulting in two annihilation photons.
Tuesday, February 5, 2008
Port films and simulator films and cassettes
Source Van Dyk
The cassettes and films used to generate port films are quite different from those found in the simulator. Rather than a high atomic number fluorescent screen, the x-ray detector consists of a front metal plate and a rear plate made of either plastic or metal. The front metal plate acts as a buildup layer that generates high energy electrons, while the rear metal plate acts as an electron backscattering material and as a compression plate to ensure good contact between the film and the surrounding materials.
Most commercially available portal film cassettes use a 1 mm thick copper plate with a curved cassette lid.
The cassettes and films used to generate port films are quite different from those found in the simulator. Rather than a high atomic number fluorescent screen, the x-ray detector consists of a front metal plate and a rear plate made of either plastic or metal. The front metal plate acts as a buildup layer that generates high energy electrons, while the rear metal plate acts as an electron backscattering material and as a compression plate to ensure good contact between the film and the surrounding materials.
Most commercially available portal film cassettes use a 1 mm thick copper plate with a curved cassette lid.
Friday, February 1, 2008
Photoelectric vs Compton
In a photoelectric interaction, the photon loses all the energy
In a Compton interaction, the photon loses part of it's energy.
In a Compton interaction, the photon loses part of it's energy.
Photo-electric vs Compton effect
To interact by photoelectric effect, the interacting photon must have an energy equal to or just greater than the binding energy of the electron.
To undergo a Compton effect interaction the energy of the interacting photon must be very much larger than the electron binding energy. Thus explaining why the freed electron acts as almost a free electron.
To undergo a Compton effect interaction the energy of the interacting photon must be very much larger than the electron binding energy. Thus explaining why the freed electron acts as almost a free electron.
Electron positron pairs (Pair production)
To produce an electron-positron pair in the vicinity of a nucleus, the interacting photon must have a minimum energy of 1.02 MeV.
High energy electrons in high Z materials
While high energy electrons scatter less than low energy due to (Z squared/ E squared) and the information in the previous post below this one in the blog, scattering of high energy electrons does increase in High Z materials.
High energy electron scattering < low energy electron scattering
High energy electron scattering increases in High Z materials!
High energy electron scattering < low energy electron scattering
High energy electron scattering increases in High Z materials!
Scatter and electrons
HIGH ENERGY ELECTRONS SCATTER LESS
High energy electrons are faster than low energy electrons and hence, interact for less time and hence scatter less than low energy electrons.
REMEMBER Scattering is approximately = Z squared/ E squared
High energy electrons are faster than low energy electrons and hence, interact for less time and hence scatter less than low energy electrons.
REMEMBER Scattering is approximately = Z squared/ E squared
Pencil beam of electrons
A pencil beam of electrons incident on a foil spreads into a beam of larger cross-section due to multiple Coulombic interactions within nuclei of atoms.
This is the principle behind the production of large field sizes for clinical treatment with electrons.
This is the principle behind the production of large field sizes for clinical treatment with electrons.
Compton interactions
Compton interactions are like billiard type collisions.
Because the binding energies are small compared to the photon energy, so the photon sees the electron as a "free" electron in a Compton interaction.
I always think of playing pool in Compton, California which is something i might not have the courage to do in real life!
Because the binding energies are small compared to the photon energy, so the photon sees the electron as a "free" electron in a Compton interaction.
I always think of playing pool in Compton, California which is something i might not have the courage to do in real life!
Electrons and bremmstrahlung
As electrons interact with matter, they produce x-rays due to bremmstrahlung (or braking radiation)
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