Lecture 4 Flashcards
What is a positron?
A positive electron, produced in the beta+ decay of O-15, Na-22, or F-18 nuclei.
What is electron capture?
A similar process to beta decay where there are too many protons in the nucleus so one must be changed to a neutron, however, it occurs when there isn’t enough energy to emit a positron/electron so an electron is absorbed and a neutrino is emitted instead.
What happens when a positron collides with an electron?
Annihilation
What is the by-product of annihilation?
Two 511 keV gamma rays
What is annihilation used for in healthcare?
For diagnosis using PET (positron emission tomography) scanners, especially of metabolic functions like cancer staging.
What is fluorodeoxyglucose (FDG)?
A useful tracer that behaves identically to a glucose molecule and has the exact same function but with a fluorine-18 attachment. This attachment undergoes beta+ decay to emit a positron which can be detected by scanners. The glucose part of this molecule acts as would be expected for a regular glucose molecule so FDG can be absorbed into tissues in the body.
What is the average dose of FDG given to a patient?
200 - 400 MBq
What is the effective half-life of FDG?
110 mins
How is FDG produced?
Fluorine-18 is produced using a cyclotron to accelerate a proton to several MeV before directing it at its target: oxygen-18. This can then be combined with glucose to make FDG.
What happens when the fluorine molecule in FDG decays?
It turns back into oxygen-18, a positron, and a neutrino and the FDG molecule becomes a regular glucose molecule.
Define fission
A nuclear chain reaction in which neutrons bombard a target nucleus to produce 3 more neutrons and stable isotopes.
Give an example of a product of fission
Uranium
Give an example of a target nucleus used in fission
A light nucleus such as beryllium
Give examples of a natural and a man-made source of neutrons to bombard a target in fission
Natural: mixture of radium and beryllium
Man-made: deuteron in a cyclotron
What are cosmic rays?
A natural source of radiation, consisting mainly of protons. The rest of the cosmic rays consist of alpha, beta, and positron particles.
What do the protons from cosmic rays become when they collide with the atmosphere?
Muons, kaons, and pions
How do cosmic rays produce the carbon-14 isotope?
1) Protons interact with other particles in the atmosphere.
2) A range of particles (muons, pions, neutrons, etc.) are released.
3) A neutron collides with a nitrogen nucleus to produce a stable carbon-14 isotope.
Cosmic rays contribute to a ____________ amount of a personal annual dose of radiation.
Significant
What is the average annual radiation dose?
2.3 mSv
What are x-ray photons?
A man-made photon that is produced by accelerating electrons or by electronic transitions.
What are gamma photons?
A photon produced by natural sources.
What is the difference between x-ray photons and gamma photons?
There is no physical difference, the terminology is just different due to their origin.
What are low energy x-rays used for?
Imaging:
- Chest x-rays, broken bones, mammography
- Bone densitometry
- CT (computed tomography) scans
What is the energy of a low energy x-ray?
30 - 120 keV
What are high energy x-rays used for?
Therapy:
- Lin-acs
What is the energy of a high energy x-ray?
6 MeV
How does accelerating electrons produce x-rays?
Electrons are released from a heated cathode by thermionic emission and accelerated towards an anode by a potential difference. The electron decelerates when it hits a target (usually a metal plate), releasing an x-ray photon with energy equal to the kinetic energy lost.
What percentage of O-15, Na-22, and F-18 decay results in electron capture rather than beta+ decay?
10%
What is the name for x-ray production by accelerating electrons?
Bremsstrahlung (German for breaking)
How do electronic transitions produce x-rays?
Electrons are bombarded at a target (usually a metal plate) and eject an inner-shell electron from the target atom. This electron is then replaced by an outer shell electron and an x-ray is emitted due to the energy change.
What are x-ray tubes?
Tubes containing a cathode, anode and target material so that x-rays can be produced by both electron acceleration and electronic transitions.
Why do x-ray tubes heat up?
In most cases, electron energy is converted into heat instead of an x-ray.
What methods are used to prevent the heating of an x-ray tube?
- Tubes are cooled.
- The anode has a higher atomic number than the cathode so more of the energy is transferred to x-ray generation.
- The anode is embedded in a copper block so that heat transfer is minimised due to the high thermal conductivity.
- The anode rotates so it isn’t heated up too much in one position.
