Lecture 4

Question 1

What is a positron?

Answer 1

A positive electron, produced in the beta+ decay of O-15, Na-22, or F-18 nuclei.

Question 2

What is electron capture?

Answer 2

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.

Question 3

What happens when a positron collides with an electron?

Answer 3

Annihilation

Question 4

What is the by-product of annihilation?

Answer 4

Two 511 keV gamma rays

Question 5

What is annihilation used for in healthcare?

Answer 5

For diagnosis using PET (positron emission tomography) scanners, especially of metabolic functions like cancer staging.

Question 6

What is fluorodeoxyglucose (FDG)?

Answer 6

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.

Question 7

What is the average dose of FDG given to a patient?

Answer 7

200 - 400 MBq

Question 8

What is the effective half-life of FDG?

Answer 8

110 mins

Question 9

How is FDG produced?

Answer 9

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.

Question 10

What happens when the fluorine molecule in FDG decays?

Answer 10

It turns back into oxygen-18, a positron, and a neutrino and the FDG molecule becomes a regular glucose molecule.

Question 11

Define fission

Answer 11

A nuclear chain reaction in which neutrons bombard a target nucleus to produce 3 more neutrons and stable isotopes.

Question 12

Give an example of a product of fission

Answer 12

Uranium

Question 13

Give an example of a target nucleus used in fission

Answer 13

A light nucleus such as beryllium

Question 14

Give examples of a natural and a man-made source of neutrons to bombard a target in fission

Answer 14

Natural: mixture of radium and beryllium
Man-made: deuteron in a cyclotron

Question 15

What are cosmic rays?

Answer 15

A natural source of radiation, consisting mainly of protons. The rest of the cosmic rays consist of alpha, beta, and positron particles.

Question 16

What do the protons from cosmic rays become when they collide with the atmosphere?

Answer 16

Muons, kaons, and pions

Question 17

How do cosmic rays produce the carbon-14 isotope?

Answer 17

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.

Question 18

Cosmic rays contribute to a ____________ amount of a personal annual dose of radiation.

Answer 18

Significant

Question 19

What is the average annual radiation dose?

Answer 19

2.3 mSv

Question 20

What are x-ray photons?

Answer 20

A man-made photon that is produced by accelerating electrons or by electronic transitions.

Question 21

What are gamma photons?

Answer 21

A photon produced by natural sources.

Question 22

What is the difference between x-ray photons and gamma photons?

Answer 22

There is no physical difference, the terminology is just different due to their origin.

Question 23

What are low energy x-rays used for?

Answer 23

Imaging:
- Chest x-rays, broken bones, mammography
- Bone densitometry
- CT (computed tomography) scans

Question 24

What is the energy of a low energy x-ray?

Answer 24

30 - 120 keV

Question 25

What are high energy x-rays used for?

Answer 25

Therapy:
- Lin-acs

Question 26

What is the energy of a high energy x-ray?

Answer 26

6 MeV

Question 27

How does accelerating electrons produce x-rays?

Answer 27

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.

Question 27

What percentage of O-15, Na-22, and F-18 decay results in electron capture rather than beta+ decay?

Answer 27

10%

Question 28

What is the name for x-ray production by accelerating electrons?

Answer 28

Bremsstrahlung (German for breaking)

Question 28

How do electronic transitions produce x-rays?

Answer 28

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.

Question 29

What are x-ray tubes?

Answer 29

Tubes containing a cathode, anode and target material so that x-rays can be produced by both electron acceleration and electronic transitions.

Question 30

Why do x-ray tubes heat up?

Answer 30

In most cases, electron energy is converted into heat instead of an x-ray.

Question 31

What methods are used to prevent the heating of an x-ray tube?

Answer 31

• 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.