Lecture 11

Question 1

What is the combination of scintillation crystals and photo-diodes used for?

Answer 1

• CT scanners (via a linear array of detectors)
• PET scanners

Question 2

What is a semiconductor?

Answer 2

A material halfway between an insulator and a conductor that has a small band gap between the condition and valence band.

Question 3

What are the two types of semiconductor?

Answer 3

N-type and P-type

Question 4

The intrinsic structure of semiconductor devices acts as a ___________ of photons.

Answer 4

Detector

Question 5

How do semiconductor devices work?

Answer 5

1) The n-type semiconductor acts as a detector of photons (like a gas ionisation detector but in solid state).
2) Incoming photons are prevented from recombining with the material by an imposed electric field, generating electron-holes pairs.
3) A current flows proportionally to the incident photon flux, showing the location of the beam.
4) The p-type semiconductor acts as a voltage multiplier.

Question 6

What is a benefit of semiconductor devices?

Answer 6

The p-type voltage multiplier is cheaper than a photomultiplier.

Question 7

What is neutron dosimetry?

Answer 7

A form of dosimetry that works due to thermal neutron capture (then gamma decay).

Question 8

How does neutron dosimetry work?

Answer 8

1) A nuclear reaction in the form of thermal neutron capture then gamma decay.
2) Gamma emission occurs which is detected by scintillation and photodiodes (or a gas-filled ionisation detector).
3) These reactions form long-lived isotopes that can be read later.

Question 9

What are the benefits of portable dosimeters?

Answer 9

• Portable (light and wearable)
• Cheap (cost effective)
• Easily readable
• Sensitivity
• Reusable (device not the film)

Question 10

What are the main professions that use personal dosimetry?

Answer 10

• Emergency services
• Recycling industry
• Security
• Nuclear
• Hospital workers