Equipment Flashcards
What does modulator consist of?
Pulse forming network
Thyratron
Circuit converting power supply to DC
What is the pulse repetition frequency and the length of pulses?
200-300Hz, pulse of ~3micro s
(3-5ms pulse intervals)
What first happens in modulator?
3 phase power supply converted to DC, supplies DC to PFN.
What is PFN origin of and how does it work?
Origin of high voltage pulse
Circuit of capacitors in parallel, when they discharge they produce a high voltage pulse as close to a square pulse as possible.
Capacitor provides power to charge up PFN until it reaches the voltage it wants and circuit closes
What does thyratron act as?
High voltage switch to rapidly discharge PFN and complete circuit to send pulse onwards
In magnetron what does spreading out of electrons correspond to?
Magnitude of induced EF
Size of EF determines amplitude of RF wave
In magnetron, what determines how tightly grouped electrons are?
Voltage between anode and cathode
(which is voltage of pulse from modulator)
How do you change frequency in magnetron and klystron?
Magnetron: determined by resonance of cavities and plunger can fine tune this
Klystron: input microwaves
Why is transmission waveguide filled with SF6 gas?
Reduce arching
Damage becomes antennae. Pit and peak.
What are the types of accelerating waveguide
Standing or travelling
What are MLC numbers and dimensions?
28-80 pairs
typically 0.5-1mm at isocentre
Shaped and angled to minimise inter-leaf leakage
How is induced radioactivity created?
Neutrons ejected from nucleii. Hazardous due to different shielding requirements. Cause induced radioactivity - mostly neutron activation of metals in head of linac
What is nuclear binding energy?
~9MeV
Result of neutron flux
Neutron flux for beams over 10MV, must be checked when using 15MV beams
More shielding in maze as neutrons scatter more than photons (baffles or neutron absorbing material in door)
Contribute to patient dose - RBE of 20
Wall shielding for photons is acceptable
Limits of leakage radiation
Max 5% of air kerma rate of useful beam at 1m from target
In patient, max 0.2% useful beam, mean 0.1%
What are we trying to achieve with ionisation chamber?
Measure something that allows calculation of dose - ionisation good thing to measure.
What do we need to decide when creating ionisation chamber?
Material for casing
Size of collecting volume
Polarising voltage
Polarity of voltage
Material for internal electrode
How to minimise extra-cameral current
What is job of casing of ionisation chamber?
Define the cavity
Conduct electricity
Same attenuation/absorption characteristics as medium that it’s in (air kV, water/medium MV)
What properties does casing of ionisation chamber need?
Conducting
Homogeneous
Water (medium) equivalent
Robust??
Waterproof?
How do you decide on size of collecting volume?
What is requirement of ionisation chamber? Reference dosimetry, SRS, PDD, area scatter chamber.
Small in relation to dose gradient
Does it behave like Bragg_gray?
Where does extra-cameral current come from?
Interactions in stem, cable, electronics
Leakage between currents (intrinsic leakage)
Charge accumulation in insulators (effects of irradiating plastic insulators is really complex)
Stressed cables
Why do you get intrinsic leakage in ionisation chamber?
Two adjacent conductors at different potentials
Insulator not perfect
Dampness or surface contamination can enable current
What is the job of the guard ring?
Ensure current only comes from defined volume
Thimble: avoid conduction through insulator
Plane-parallel: define volume for collection, exclude electrons scattered in from side walls
What is the job of the polarising voltage in ionisation chamber?
Pull ions apart
collect ions to make a current
Need specific voltage - minimise recombination and avoid charge multiplication
Why can we end up with proportionally more ions at higher voltage?
Electrons accelerate in electric field
E field strength increases close to central electrode
If V high enough, electrons can collide and generate more electrons
As voltage increases, electron multiplication increases
Electric field strength in ionisation chamber
Depends on voltage accross gap
Increases close to central electrode
What are requirements for central electrode of ionisation chamber?
Robust
Conducting
Same interactions as body
What are interactions in mediums that could be used for internal electrode?
Steel: too much PE
Aluminium: mostly compton
Conducting plastic: can be made to be mostly Compton
Graphite: compton
What are considerations of polarity of voltage in ionisation chamber?
Polarity can give different readings, although mostly negligible in high energy beams.
For relative measurements, use same polarity or calculate average or calculate kpol
For reference and x-calibrated, use same polarity as in chain
What is the difference between a virtual and dynamic wedge?
Virtual: constant velocity & variable dose rate
Dynamic: variable dose rate and jaw speed
What is role of cooling system?
Keep linac at optimum running temperature - this is 40 degrees. A lot of excess heat is generated which must be handled.
What is role of vacuum pump?
Keep waveguide, standing or travelling, under constant vacuum. Electrons need a clear path.
What is role of steering coils?
Change the path of electrons, angle, position and size of pulse.
How does magnetron operate?
Thermionic emission from heated cathode
Electrons accelerated across from cathode to anode
Curved path due to magnetic field - form spokes
Electrons initiate oscillating E field as they cross cavity openings
E field feed back bunches electrons
These bunches amplify existing E field
Oscillation of E field produces microwaves are channelled to accelerating waveguide
How does klystron operate?
Low power microwaves and electrons (from thermionic emission) injected into first cavity
Microwaves accelerate/decelerate electrons depending on phase at injection and bunches them
Get tightly bunched pulse of electrons travelling at same frequency as input microwaves
As electrons cross catcher cavity they induce electric field with same frequency as input MW but higher amplitude
What is temperature of heater in electron gun?
1000oC
What is potential of cathode, anode and grid in triode gun?
20kV cathode
Anode at ground
Grid has high speed switching of potential, held at higher -ve potential but pulsed to positive
How does electron bunching work?
When electrons meet wave, can be in a favourable phase or not. Roughly half of electrons are scattered to walls and eliminated.
Others gather in bunches
How does slalom bending magnet work?
Beam first bent 45 degrees, then another 45 degrees in opposite direction (to converge) then 112.5 degrees while converging so they exit bending magnet at same spot
What are the properties of the photon target, type of target?
Transmission target
High Z (W-74 or steel)
High density
High melting point
Forward peaked beam, brem. radiation, high efficiency wrt reflection target. No characteristic radiation
What are monitor chambers?
Transmission ionisation chambers which fully intercept treatment beam. More than one for redundancy.
What do two chambers monitor?
Chamber 1: dose/dose rate
Chamber 2: dose/dose rate (redundancy), flatness/symmetry
What are the two impacts of nuclear reactions form photons/electrons?
Neutron flux in beams greater than 10MV
Induced radioactivity
What order of magnitude difference should there be between current and leakage?
Should be two orders of magnitude between current and leakage
What is the difference between travelling and standing waveguides?
Travelling are longer and simpler
MW injected at end of travelling
Standing has side coupled cavities for nodes
How do we decide on polarising voltage?
Depends on size of cavity: field strength depends on voltage across gap V/mm, so size of gap determines strength
