Applied radiation science and technology Flashcards
Name of the process which causes electrons to be emitted from the filament in the cathode
Thermionic emission
Material which makes the coiled filament in the cathode and target in the anode
Tungsten
Name two ways in which the intensity of X-ray production can be increased
Energy of incident electrons need to be increased, can be achieved by two ways:
1. Increased current supplying the filament
2. increase voltage supplying the electrodes which creates larger electric field strength
Advantages of tungsten
Highly durable, high melting point, high atomic number
Two types of radiation produced
Bremstrallung and Characteristic
Bremstrallung radiation
Incident electron is fired towards the atom of the target material. Interacts with the electrostatic forces of attraction of the nucleus causing it to change its direction of motion. This change causes a loss in energy and attenuation of the incident electron. Loss in energy is given off as photons of X-rays. Degree of radiation emitted depends on level of attenuation of incident electrons. Resulting in a continuous spectrum of radiation.
Characteristic radiation
Incident electron collides with orbiting electron in the target atom. Orbiting electron is fired off and creates a vacancy in an inner electron shell. Outer electron fills this vacancy resulting in an excess of energy due to inner shells requiring less energy. Excess energy is given off as photons of radiation. This is a specific amount and depends on the target atoms binding energies. This creates a line spectrum of radiation.
Focussing hood purpose
Focussing hood has a negative potential difference across it which repels the electrons produced at the filament and causes them to form a focussed beam of electrons being fired towards the target.
Vacuum
prevents oxidation of the electrodes and electrons colliding with air molecules reducing their net energy.
Thermal cut out switch
Safety mechanism which is triggered when oil within the housing element expands due to overheating.
Absorbed dose unit
Gray (Gy)
what is 1 Gy equivalent to?
1 joule of energy per 1 kilogram of material
absorbed dose meaning
measure of energy absorbed per unit mass of material being irradiated
Dosimeter
A device used to measure the absorbed dose of ionising radiation.
Give 3 ways in which dosimeters work?
1) ionisation chamber (uses either air or a semi conductor)
2) thermoluminescence
3)photographic film
Summarise mechanism in which TLDs work:
Radiation causes electrons to gain energy and move from valence band to conduction band. electrons lose energy and fall into traps (impurities in the crystal). Heat is required to release trapped electrons. Produce light when released. light is an indication of radiation absorbed.
Common material used in TDLs
Lithium fluoride with metal impurities
what is Dose area product
DAP is total amount of radiation delivered to a patient by doing dose multiplied by area.
Diamentor
device used to measure dose area product
Dose reference levels
Average amounts of radiation delivered for different diagnostic procedures - a guide to what would be expected.
What is a linac ionisation chamber
Used to measure total dose delivered to a patient - stops the linac when desired dose has been reached.
Most commonly used semiconductor material
Silicon
Absorbed dose meaning
Measure of energy absorbed per unit mass of material irradiated
Unit of absorbed dose
Gray (Gy)
What is 1Gy equivalent to?
1 joule of energy per kilogram of material
What is the primary standard in terms of calibration
A measure of absorbed dose under carefully controlled conditions which local dosimeters can be calibrated against to ensure measurements are accurate and comparable
Exposure
A measure of ionisation of air ( quantity of charge on ions per unit mass of air)
Exposure units
Colombes per kilogram
Thimble chambers
Detection devices which use the ionisation of air to measure absorbed dose of ionising radiation
