Interactions with matter Flashcards
Name the 3 Key Interactions that occur in matter
Transmitted
Absorbed
Scattered
What 4 things does attenuation depend on?
The beam energy; the energy of the X-ray photons from the tube
The density of the tissue
The atomic number
The thickness of the tissue
Place these is order of energy - lowest first: A: Megavoltage B: Mammography C: Kilovoltage radiotherapy D: Diagnostic x-rays
Mammography - lowest energy
Diagnostic x-rays
Kilovoltage radiotherapy
Megavoltage
List the 4 types of interactions which occur in matter?
Elastic Scattering
Photoelectric Absorption
Comptom Effect
Pair Production
Describe Elastic scattering
Photon energy is significantly less than electron binding energy i.e. it occurs with low energy radiation
Photon is deflected as it passes close to electron
There is no loss of energy
Describe Photoelectric Absorption
Photon collides inelastically with an electron in an inner shell
If the photon has an energy greater than the binding energy of the electron it will escape
Electron is ejected from the atom as a photoelectron
Vacancy is filled by electron cascade releasing characteristic radiation as they move down the fill the vacancy.
How is an Auger electron produced?
In materials with a higher atomic number it is possible an additional interaction may occur.
Incoming X-ray photon ionises a K-shell electron
The K-shell vacancy is filled by an electron from a higher shell
The energy emitted in this transition impacts on another electron, if the energy exceeds the binding energy it too is ejected from the atom
This second ejected electron is called the Auger electron
What energy level does the Photoelectric absorption occur between?
Occurs between 40 – 80 kVp
What relationship does Photoelectric absorption have to the Beam energy?
Photoelectric absorption is inversely proportional to energy cubed
What relationship does Photoelectric absorption have to the Atomic number?
Photoelectric absorption is proportional to Atomic number cubed
Describe Compton Scattering
Photon energy is significantly higher than electron binding energy
Photon gives up proportion of its energy to electron and electron is ejected
Photon continues in different direction
Recoil electron ejected at between 1 - 90° relative to the incident photon
Scattered photon can be scattered in any direction
Higher photon energies result in lower scattering angles
What relationship does Compton Scattering have to the Beam energy?
Compton scattering is inversely proptional to Beam energy
What relationship does Compton Scattering have to the Atomic number?
Compton scattering is independant of Atomic number
What is Compton scattering proptional to?
Compton scattering is proptional to the electron density i.e. density of the attenuator
Describe Pair Production
Occurs when photon energy is > 1.02MeV
Photon interacts with electrical field of nucleus
Photon is turned into Positron and Negatron
Negatron is slowed down and Positron will eventually
collide with an electron
This results in annihilation radiation i.e. Mass is converted back to energy
What relationship does Pair Production have to the Beam energy?
Pair Production is proptional to the Beam energy
What relationship does Pair Production have to the Atomic number?
Pair Production is proptional to the Atomic number
Which interaction is the most prevalent at the following energies?
• Kilovoltage Diagnostic x-rays
• Kilovoltage Radiotherapy treatment
• Megavoltage Radiotherapy treatment
Kilovoltage Diagnostic x-rays:
- Low energy: i.e. mammography: 75% interactions in soft tissue will be that of photoelectric with Compton scatter playing a relatively minor role
- Beam energy 30-40keV: Photoelectric=Compton scatter in soft tissues
- High energy: i.e. radionuclide imaging: 10-15% interactions in soft tissue will be that of
photoelectric with Compton scatter playing a dominant role
Kilovoltage Radiotherapy treatment:
- High energy kilovoltage: Compton scatter playing a dominant role
Megavoltage Radiotherapy treatment:
- Compton effect and pair production are the dominant attenuation processes.
True Or False:
Elastic scattering causes no change to the energy of the incoming photon
True
What conditions are required for the exponential relationship of an electromagnetic beam to attenuator thickness?
Homogenous beam
Homogenous attenuator material
Radiation beam must be parallel
What Greek letter represents total linear attenuation coeffiecient?
µ
What is Half-value thickness/layer (HVL/HVT)?
The half-value thickness/ layer is that thickness of a substance which will transmit one half of the intensity of radiation incident upon it
What are Homogenous and Monochromatic beams?
An electromagnetic beam with the photons having the exact same energy
What is a Polychromatic beam?
An electromagnetic beam with a spectrum of photon energies
What 2 things increase Beam quality?
A larger HVL/HVT
A larger filter thickness
Why is half value thickness not a constant value in a polychromatic beam?
Due to beam hardened, causing the quality of the
beam to improve, therefore the beam becomes more
penetrating, therefore requiring greater thickness of
HVL
Suggest an appropriate metal that can be utilised as an attenuator for DR & kV Superficial?
Aluminium
Suggest an appropriate metal that can be utilised as an attenuator for kV Orthovoltage?
Copper
What is the Law of Radioactive Decay?
The rate of decay of a nuclide is proportional to the
number of nuclei left in the sample
What is Half life?
The time required for the activity of the radioactive sample to decay to one half of it’s original value
What 2 things can you work out from Half life?
What the activity was at a previous time
What the activity will be in the future
What is the mass and charge of an Alpha particle?
6.645 x 10^-27kg
+2 charge
What is the penetration in air of an Alpha particle?
0.5cm
What is the mass and charge of an Beta minus and plus particle?
- 11 x 10-31 kg
- 1 or +1 charge
What is the penetration in air of an Beta particle?
3.5m
What is the mass and charge of an Gamma particle?
No Mass
Neutral
What is the penetration in air of an Gamma particle?
8,200m *
