Linacs - Photons Flashcards
What are the main components of the body of the linac?
Electron gun
Accelerating waveguide
Microwave generator
Modulator
What is the role of the modulator?
To supply ~3us pulses to cathodes of the electron gun and microwave generator
What does the modulator contain?
A pulse forming network and thyratron with a PRF of ~200-300Hz
How does a Magnetron produce microwaves of suitable energy?
Electrons are thermionically emitted for a cathode.
These are accelerated across the evacuated space between the cathode and anode.
They follow a curved path in the permanent magnetic field, creating oscillating electric fields as they cross cavity openings.
Electrical field feed-back bunches the electrons.
Circulating bunches amplify the electric field.
The resulting microwaves are injected into the accelerating waveguide.
What is the peak microwave power of a Magnetron?
~3MV, so only allows acceleration up to ~6MV.
How does a Klystron produce microwaves of suitable energy?
Low power microwaves from a solid state pilot oscillator and thermionic emission electrons accelerated using a poitential difference of ~10keV are injected into the first cavity.
The microwaves accelerate or decelerate the electrons depending on the phase at injection, leading to bunch at the input frequency.
The electrons supply power to the microwaves by the electric field amplifying the input.
there is an increased electron density.
The catcher cavity is tuned to resonate at the bunch frequency so the electrons create an electric field in the cavity.
Microwaves are then injected into the waveguide as they have been amplified.
What is the peak microwave power of a Klystron?
~6MV, depending on the number of cavities. There are usually 4 cavities. This enables acceleration greater than 10MV.
How much energy of the electron bunches is wasted in a Klystron?
~40%
How does the electron gun work?
Electrons are emitted by thermionic emission, heater ~1000celcius.
A concave cathode focuses the emissions. ~20keV
Theere is a constant cloud of electrons around the cathode.
A grid has a high switching of potential to permit a phase shift between the electron and microwave pulses. The grid is held at a high negative potential to stop electrons returning to the anode as it is held at ground potential.
All in a high vacuum.
How does the accelerating waveguide work?
The field of microwaves accelerate electrons.
Initial buncher sections, first 1.5-2.5 cavities, produce differential acceleration of the electrons to create bunching by varying the microwave phase. This accounts for the majority of the acceleration.
After this the cavities lengthen to a point, then energy is gained through increasing the mass when v~c.
What is the accelerating waveguide made of?
copper due to good electrical conductivity.
What maintain the electrons in a focused beam?
Focussing coils.
What are the differences between standing waveguides and travelling waveguides?
Standing: Short, complex, microwaves injected anywhere, side coupled cavities. The electrons are accelerated along standing waves.
Travelling waveguide: long, simple, microwaves injected at the start. Electrons are accelerated along travelling waves.
What controls the microwave velocity in the waveguide?
Discs with holes in between cavities.
What is the purpose of the bending magnet?
To deflect the electrons to pass through the isocentre
What angles can they be?
90 - not used as hard to focus electrons
112.5
270
What are the qualities needed of a transmission traget?
High Z, density, melting point. Can be made of alloy or composite
What type of cooling is used for a transmission target?
Water only due to high efficiency
What is the trade-off between thick and thin targets?
Thin targets get higher energy but more contaminant electrons
What is the role of the flattening filter?
To make the beam flat at 10cm deep
What effect does the flattening filter have on the beam?
hardens it, attenuates it, reduces the dose rate
What are the differences between flattening filters of different energies?
The height/thickness
What are the roles of the monitor chambers?
Chamber 1 - dose/dose rate
Chamber 2 - Dose/dose rate (redundancy)
Flatness/Symmetry
What type of ionisation chambers are used for the monitor chambers?
Transmission sealed chambers. Have thick walls as it won’t affect the beam
What is the role of the secondary collimators?
A pair of thick, high density jaws to define the edge of the field and reduce interleaf leakage
What is the role of the MLCs
Shape the beam
What is special about the shape of the MLCs?
Shaped and angled to minimise inter-leaf leakage
How are the MLCs made up?
Many leaf pairs (28-80) with leaf widths ~0.5-1mm at the isocentre
How does a physical wedge work?
High Z material, placed in the beam for a certain amount of time depending on the wedge angle.
Changes the spectrum of the beam and dose rate.
How does a virtual wedge work?
Varian Dynamic wedge: Sweep jaws across, constant dose rate, change speed of jaws
Siemens Virtual wedge: Sweep jaws across, constant speed of jaws, change dose rate
How is induced radioactivity produced in the linac head?
Neutron activation
Why is induced radioactivity a problem?
Contribute to patient dose bath, needs to be <1% of x-ray dose.
Engineers need to wait for it to dissipate before working on the head
When is neutron activation most likely?
At high energies and IMRT deliveries due to long exposures
What are the limits of leakage allowed from the head?
<0.5% of beam air KERMA at 1m from target
max <0.2% and mean <0.1% of air KERMA rat at 0.2m from the isocentre for a sampling area of 100cm^2
Which organisations perform audits?
IAEA, ESTRO, NPL, MD Anderson, regional groups, local departments
How is the MD Anderson audit performed?
Receive a TLD at Dmax in a lucite block. Irradiate with 3Gy with a 10x10 field at 100cm SSD and send back, stating you reference conditions.
How are the IAEA and ESTRO audits performed?
Receive a TLD rod in a jig. Place in a water phantom at the reference point.
How is the NPL audit performed?
Measurements made in a phantom with the auditor’s chamber and electrometer. NPL use a water phantom, others would use perspex or solid water.
Can use semi-anthropomorphic phantoms for different beams so they can be compared to the TPS.
