Radiation Detection and Measurement Flashcards

Vocabulary-style flashcards covering radiation detection principles, gas-filled detector regions, and units of exposure and absorbed dose based on Dr Nanette Schleich's 2026 lecture notes.

Transmission ionisation chambers

The most common dose monitors in linacs, permanently embedded in clinical X-ray and electron beams to act as monitor chambers.

Linac Monitor Chamber Location

Positioned between the flattening filter (for X-ray beams) or scattering foil (for electron beams) and the secondary collimator.

Exposure ($X$)

The amount of electrical charge ($Q$) produced per mass of air ($m_{air}$) when all electrons and positrons liberated by photons in air are completely stopped; expressed as $X = \frac{Q}{m_{air}}$.

SI Unit of Exposure

Coulomb per kilogram ($C\,kg^{-1}$).

Roentgen ($R$)

A non-SI unit of exposure defined as $1\,R = 2.58 \times 10^{-4}\,C\,kg^{-1}$.

Absorbed Dose ($D$)

The energy ($E$) deposited in an absorber per mass ($m$); expressed as $D = \frac{E}{m}$.

Gray ($Gy$)

The SI unit of absorbed dose, defined as $1\,Gy = 1\,J\,kg^{-1}$.

rad

A non-SI unit of absorbed dose where $1\,rad = 0.01\,J\,kg^{-1} = 1\,cGy$.

Recombination region (Region 1)

Region where voltage is not high enough to collect all electrons and ions before they recombine; no useful detectors operate here.

Ion chamber region (Region 2)

Region where all ion pairs are collected with little to no increase in current as voltage increases; used for ionisation chambers.

Proportional counter region (Region 3)

Region where electrons gain enough energy to knock out more electrons, resulting in an amplified current that is still proportional to radiation energy.

Geiger-Müller (GM) region (Region 4)

Region where the entire gas volume is ionised, resulting in a large amplification factor of approximately $10^8$.

Electrical breakdown (Region 5)

A region where sparking or breakdown occurs, which destroys the radiation counter.

Thimble ionisation chamber

A practical detector used for radiation oncology measurement, consisting of an air cavity and a central electrode made of aluminium or graphite.

Farmer chamber

A specific thimble chamber used for absolute dose measurements by Radiation Oncology Medical Physicists (ROMPs).

$$W_{air}/e$$

The mean energy required to cause an ionisation in air, which is a constant valued at $33.97\,J/C$.

Ion Chamber Survey Meter

A portable instrument with a volume of $0.5\text{ to }1\,dm^3$ used for measuring stray radiation levels and leakage through shielding.

GM Counter Dead Time

The period, typically $0.2\text{ to }0.4\,ms$, during which the counter cannot detect a second particle until the previous ionisation is removed.

Pancake probe

A type of Geiger-Müller probe recommended for contamination monitoring.

Scintillation detectors

A method of radiation measurement involving the excitation of solids to produce light pulses.

TLD

Thermoluminescent dosemeter; a method used for personal monitoring by recording dose indications.