Lecture 8 Flashcards
What are the 4 types of quantities for radiation?
Define exposure
The number of ion pairs in air that carry one coulomb of charge per kilogram (generated by a quantity of radiation). It is a measurable radiometric unit so is known as an ‘exposure unit’.
What is 1 exposure unit in SI units?
1 C/kg in air at STP (standard temperature and pressure)
What devices can be used to measure exposure?
- Air-wall chambers (personal exposure detectors)
- Ionisation chambers
- Geiger counters
How do air-wall chambers work?
They are personal exposure detectors to record exposure over a day made of a capacitor that is charged to a known voltage; the decrease in voltage determines how much radiation has passed through the chamber.
What is the equation for change in charge (in terms of capacitance)?
∆Q = change in charge
C = capacitance
∆V = change in voltage
How do ionisation chambers work?
Charge from generated ion pairs (caused by incident particles of radiation) is measured in a volume of air which gives the exposure. This can be converted into dose. They are not very sensitive.
What voltage do ionisation chambers operate at?
200-400 V
How do Geiger counters work?
Every ionisation event is counted. It is a counter of particles or photons rather than a dosimeter. It is very sensitive. They are light weight, portable and give a general alert of radiation source.
What voltage do Geiger counters operate at?
Above 700 V
Describe the graph of ions collected in air against voltage (for different types of detectors)
How can the exposure rate be calculated for an exposure detector?
Calculate the current (C/s) produced in the ionisation cell and divide this value by the mass of air in the detector.
Define absorbed dose
The amount of energy deposited per unit mass in any target material from radiation. This is measured (Gy).
What is 1 gray in SI units?
1 J/kg
Absorbed dose is a ____________ quantity so it applies to a whole mass of tissue rather than at a cellular level.
Macroscopic
What is the conversion factor between exposure units and gray in air (the conversion of exposure to absorbed dose)?
1 C/kg = 34 gray
What is KERMA?
Kinetic energy released in matter. This is the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionising radiation in a sample of matter divided by the sample mass.
When does KERMA differ from the absorbed dose?
For high-energy (gamma) photons. They are similar for low-energy photons.
Why is KERMA higher than the absorbed dose for higher energy photons?
The higher energy photons escape the region of interest before depositing their radiation dose so the kinetic energy is considered but the radiation is not.
