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72 Cards in this Set
- Front
- Back
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Explain the role of the Modulator?
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Power supply supplies to the modulator, it takes that power and breaks it up into pluses, then shoots pluses at the machine.
*Supplies a pulsed DC power to microseconds in duration to the magnetron/klystron and at the same time signals e-gun into the accelerator portion of the waveguide. |
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Where is the Magnetron located?
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To the right of the Modulator
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Explain the role of the Magnetron(Klystron)?
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These devices take the energy and convert the pulsed DC power into microwaves radiation that is supplied to the accelerator tube.
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Explain the role of the e-gun?
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This device inject e- into accelerator tube on command from the modulator
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Explain the wave guide system?
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Copper tubes that are used to efficiently transport microwave energy from the magnetron to the accelerator tube.
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What is the reason for copper tubing?
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Because at these high Frequencies, it doesn't travel at the center of the wire, so take wire, hollow it out and make it into copper tubes, this makes for very efficient transfer of microwave radiation
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Explain the accelerator tube of the waveguide?
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Copper tube with washers placed on quarter of a wavelength apart of incoming microwave frequencies
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In the waveguide, microwave energy is transfered to the e- in the form of kinetic energy T/F?
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True
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What are the 2 types of waveguides?
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Traveling Waveguide
Standing Waveguide |
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Explain the equipment found in the TX head?
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- x-ray target
- primary fixed collimators - carousel - ionization chambers - secondary adjustable collimators - and then comes the Pt. |
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State where primary collimators are found and their function?
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Come after the x-ray target
*They are used to determine the maximum extent of the radiation field. |
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Explain he TX head of the Linear Accelerator?
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Either straight on or at right angles to e- flow
- Contains equipment for a e- beam=scattering foil, and photon beam=tungsten target And other necessary equipment ion chambers and collimators |
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Where is the Carousel found and what is it's function?
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Comes after the primary collimators
- houses the e- beam scattering foil, and x-ray beam flattening filter - Rotates/spins so that directly under appropriate filter/foil for what ever it's set up for |
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Explain where Ionization chambers are found and state their function?
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Come after the carousel
- There are 2 ion chambers, one backup in case the other fails. - it is the charge collected by the chamber that is used to shut down the machine. |
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What are some functions of Ionization Chambers in TX head?
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- Tells machine when to shut down
- can be used as a way to make sure beam is centered/symmetrical. |
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State where the secondary collimators are and their function?
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Come after the ionization chambers
- There are used to set up the appropriate field size for the radiation TX fields |
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Explain how the beam profile is as it emerges past the fixed collimators?
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One would find VERY HIGH INTENSITY at the center of the beam
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What is done to compensate for higher intensity at center of the beam?
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Add a beam flattening filter in the path of the x-ray beam, which is thicker at the center, then the edges, this will give a flat beam profile across the field of the pt.
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Explain how a beam flattening filter works?
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Works # of photons to be the same across the field, so it attenuates the center of the beam a whole lot more than the lateral edges of the beam so that on the other side of he flattening filter the beam is flat.
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Each e- energy and photon energy has it's own flattening filter and scattering foil T/F?
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True
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Explain how the e- scattering foil works?
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it produces a pencil beam, so want to take that beam and scatter it across so it covers the whole field.
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What are Bending Magnets used for in the Linacs?
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Used to bend e- beam as it emerges from the accelerator tube
** e- can be bent either 90º or 270º |
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Explain how e- energy varies in a 90º bend?
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Before e- come out or right before they strike the exit window they pretty much all have the same energy, due to variations in energy as it emerges from and because of interaction with the window the accelerator tube waveguide, e- will start to separate out into different paths as they enter and interact with magnetic field of the magnet.
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Nature says that as a charged particle moves thought a magnetic field it will bend or rotate T/F?
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True
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Explain what happens to high, low, and mid energy e- in a 90º bending magnet?
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- High energy e- will be less effected by the magnetic field and will have the largest radius curve
- Low energy e- will be most effected by the magnetic field and will have the Sharpest radius curve (bend sooner) - Mid energy e- will range between these two extremes |
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Explain why we don't use a 90º bend in bending magnet?
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Cause of undesirable characteristics such as larger target, more penumbra(unsharpness)
- as e- enter the magnetic field they will spread out due to energy vibrations - as they strike the target they will generate x-rays but over a larger target size and this will lead to a larger penumbra. |
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What eliminated the problem of e- spread?
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Using a 270º bending magnet
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Explain how 270º bending magnet works?
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As e- traverse the magnetic field they will separate out due to energy vibrations but because of the 270º bend they will come together and merge at one point on the target
- because of this focusing of e- on the target, the penumbra characteristics will improve |
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Explain what the Magnetron is composed of?
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Made of a solid copper metal disk, in which resonant cavities have been cut out
- at the center of the disk is the cathode(-) and the outer rim is the anode (+) - Usually used in low energy machines. |
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Explain how a Magnetron works?
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The cathode in the middle produces an e- which follows a complicated path to the anode, as it passes by it'll set up a magnetic field in there, resonant cavity will generate an oscillating vibration in the cavity.
- - this oscillation is in the frequency range of microwave radiation and will produce microwave energy depending on the dimensions of the cavity. |
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When using a Magnetron what needs to be adjusted if you know what frequency you want?
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Just need to adjust the size of the cavity.
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Explain the components of a Klystron?
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Essentially 2 tin cans with a pipe in the middle
1st cavity BUNCHER is connected to an incoming waveguide and a drift tube that is connected to 2nd cavity CATCHER which is attached to a outgoing waveguide and an e- beam collector. |
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What is a Buncher energized by?
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Low power microwaves
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Explain the function of the Buncher?
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Low energy microwaves enter the cavity at the same time.
- e- stream which is accelerated by negative pulsed voltage also enters the cavity - it bunches e- together so that they all go into the collector at the same time. |
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Explain Velocity Modulation?
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as e- travel into the buncher, which is energized by low power microwaves, the microwaves interact with the cavity and set up an electrical field which will interact with the e-
- depending on when e- enter the cavity they will either be speed up, slowed down, or no change at all, so as they exit there will be a variation in speed. |
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Explain the function of the catcher?
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as e- arrive here they suffer deceleration as the kinetic energy of e- is converted into high power microwaves
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What is the function of the e- beam collector?
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Residual energy of the e- beam is dissipated here
- Usually low energy microwaves are taken from the output of the 2nd cavity and fed back into the 1st |
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Explain the components of a traveling waveguide?
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Accelerator waveguide consists of hollow copper tube with metal washers placed inside.
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Explain the spacing of the washers on traveling waveguide?
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Initially spaced closer together in the beginning but become uniformly spaced as they get further along
- This is because as e- enters it is traveling slowly so want spaces closer together but, as e- speed increases want spaces further apart so e- can stay in step with the microwave energy. |
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Why do spaces remain the same at certain point?
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Because e- can never go faster than the speed of light, so in order to gain energy won't be traveling faster but becoming more massive.
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How does traveling waveguide work?
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1) Microwaves coming from magnetron or klystron enter the accelerator waveguide tube
2) Then the electric field builds up in tube and travels down the waveguide. 3) At the right moment e- are injected into the waveguide. 4) e- will ride he electrical field of the microwaves down the tube gaining energy along the way. |
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What is a microtron?
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Single waveguide with microwave power coming from either a magnetron or Klystron.
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How are the group of e- (bunches) accelerated in a microtron?
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With the oscillating electric field of a single resonant microwave cavity
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Explain how a microtron works?
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With each revolution e- bunch gains energy, e- keeps getting a boost every time pass through the resonant cavity, as e- gains energy it will increase it's orbital path length.
- when it reaches the appropriate energy it will be extracted from appropriate orbit with the small moveable narrow steel deflection tube. |
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What is good about using a microtron?
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Can get any energy you want out of this, and can get extremely high energy
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What are the components of a cyclotron?
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electro magnets (all you need are 2 magnetic with space between)
+Hollow copper DC are connect to an alternating accelerating voltage. |
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Explain how Cyclotron works?
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Electro Magnets set up perpendicular magnetic field that will cause CHARGED particles (NOT E-) to spiral in a circular motion so that they will encounter the accelerating voltage many times
+ As goes around magnetic field keeps getting a kick, with each kick picks up energy and speed. |
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With a Cyclotron what kind of charged particles do we mean?
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Proton or Deuteron to produce a neutron beam
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Explain Deuteron?
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Basically a heavy hydrogen
+ normal hydrogen has 1 proton and e- + Deuteron has a neutron associated with it, striped of its e- |
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Explain the Deuteron Beryllium interaction?
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As Deuteron strikes the beryllium target, the beryllium target will go from beryllium to boron with the ejected e- of a neutron
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Why are Cyclotrons used?
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To accelerate heavy atomic particles (protons and nuclei of atoms)
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Proton beams for pts TX are given in what energy range?
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150-250 MeV
+ protons can also be produced in a linac |
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Heavy charged particles exhibit what is called a?
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Bragg Peak
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Neutrons can also be produced in a D-T generator T/F?
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True, a 100-300 KeV low energy deuteron beam is bombarded a tritum target to yield an essentially monoenergetic, isomorphic neutron beam of 14 MeV
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Explain how neutrons don't operate at a distance
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Don't ionize at a distance, have to literally hit something, no charge, no electric field therefore in order to see "particle" have to come exceptionally close
+ Max energy transfer will happen between 2 particles of the same mass. |
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When do you see the max energy transfer?
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Between neutrons and protons because have essentially the same mass, so have max energy transfer and it will be the proton that is liberated and deposits all its energy.
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In neutron interaction with matter, the greater the diff in masses between the particles-->
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-->the less energy is transferred thus, neutron will retain most of its energy after this type of collision.
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Explain nuclear disintegration?
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If neutron is going slow enough, once neutron has lost enough energy, neutron will actually get absorbed and this will cause an instability in the nucleus and it will ball apart and get alpha particles.
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The dose deposited in tissue from a high energy neutron beam is manly contributed by?
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+Recoil protons
+Neutrons strikes proton and frees it form H2O molecule, once it hits the proton have a whole bunch of ionizing events coming off that proton +Even though treating with neutrons, the way dose gets into the pt is by protons that are liberated. |
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How much dose nuclear disintegration contribute to tissue dose?
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30%
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What machine was truly the first high energy machine that came out?
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Van de Graaff
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How does the Van de Graaff generator work?
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+Stores charge on a metal dome
+Uses belt to transfer e- onto the dome +At some point dome is fully charged and e- are accelerated down to strike the target and produce a short photon beam |
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Van de Graaff can only operate in the e- mode T/F?
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False, can operate in e- or photon mode
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Explain the Betatron components?
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Looks like a doughnut that's been hollowed out
+ Filament supplies e- +Injector tells e- where to go, which direction + Deflector-after attain desired energy, deflected to the target. |
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Explain how a Betatron Works?
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e- goes into a doughnut shaped device and because of the magnetic field that's been put around this device what basically happens around in a circle and get energy added to them.
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Why is Co60 double sealed?
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Place in 2 steal containers that are welded shut, as to prevent any leakage of radioactive material
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Co60 has two energies of?
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1.17 & 1.33 MeV
+it decays with the emission of a beta particle |
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Explain what the Beta particles interact with cobalt material and stainless steal container and both contribute to beams x-rays and some characteristic rad?
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These x-rays are of low energy and therefore, do not contribute appreciably to pt dose
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Explain what the Photon beam interacts with?
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Source material, steal container, housing and collimator system which provides scatter radiation that contributes approximately 10% to the total intensity of the beam
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List some Co60 Facts?
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1) HVL 1.1 cm(11 mm) of lead
2) Depth at Dmax is .5 cm (5 mm) below the skin 3) Typical source diameter ranges from 1.0 to 2.0 cm |
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Explain the leakage radiation with beam in ON position?
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Leakage from house shall not exceed .1% of the useful beam dose rate measured at 1m from the source
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Explain leakage radiation with beam in OFF position?
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Leakage from the house shall not exceed 2mrad/h on average 10 mrad/h maximum, in any direction measured 1 meter from the source
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