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133 Cards in this Set
- Front
- Back
path of x-ray beam from
e- to patient |
electron beam strikes target
goes through primary collimators -this creates a forward beaked beam therefore, we use a flattening filter housed on a carousel to flatten the beam next the photons pass through the ion chamber then through secondary collimators past slot for wedges, block compensators then to the patient :) |
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path of e- beam to patient
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electron beam passes by the target but does NOT hit it
goes through primary collimators then hits the scattering foil housed on a carousel which spreads out the e- beam next it goes through secondary collimators then through the accessory mount for electron applicator then to the patient :) |
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is the operations center for the linac
it supplies the timing pulse that initiates each pulse of radiation |
control console
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gantry rotates around it
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treatment couch
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major parts of the modulator cabinet
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DC power supply, pulse forming network (PFN), thyratron
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mayor parts of the stand include
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magnetron or klystron, circulator, part of the waveguide
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what defines the dose rate?
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the PRF-pulse repetition requency
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what does that thyratron control?
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controls the HV pulses that are supplied by the DC power supply
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what does electron gun do?
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produces electrons for acceleration
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magnetron does___
klystron does____ |
magnetron PRODUCES RF waves
klystron AMPLIFIES OR MAGNIFIES RF waves |
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job of waveguide
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transports the wave to the accelerating structure
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circulators does what
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is interposed to prevent reflected power from the accelerator reaching themagnetron or klystron and this reflected power is absorbed by a load connected to the circulator
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accelerator structure aka
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accelerating waveguide
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what happens in the accelerator structure/waveguide?
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electrons derive energy from microwaves in the accelerator
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the AFC (automatic frequency control) does what?
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drives the magnetron or klystron tuner at the resonant frequency of the accelerating structure
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bending magnet does what
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bends electron beam 270 degrees
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electron beam passes through
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scattering foil, dual ionization chamber, and finally collimation system
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photon beam passes through
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target creating photons through bremsstrahlung, flattening filter, ionization chamber, collimation system
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treatment head contains:
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bending magnet (sometimes)
target scattering foil/flattening filter ion chambers collimators ODI |
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monitor unit is what?
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the amount of charge collected by the ion chamber through which the linac beam passes.
it represents an amount of dose delivered to a certain reference point and is independent of dose rate |
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attenuation is?
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removal of photons from radiation beam (what blocks do)
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absorption is?
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conversion of photon energy to electron motion (in body)
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____delivers the dose not the ____
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electrons deliver dose
not photon |
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electron motion causes
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biological damage
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a photon is ___ ionizing
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indirectly ionizing
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if thickness of absorber increase then the number of photons removed form beam will
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INCREASE
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linear attenuation coefficient definition
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equal to the fraction of photons removed from eh beam per cm of absorber
dimensions= per cm |
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def. of HVL
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thickenss of absorber required to attenuate half of the incident radiation
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def. of TVL
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thickness of material to reduce transmission to 1/10 or 10%
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___determines the # of e- available
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density
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as density INC, ___ # of tragets available= ___ photoelectric and pair production
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Increase, Increase
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linear attenuation coefficient of a material depends on
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density of material
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how is mass attenuation coefficient different from linear attenuaton coefficient
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mass att. coef. eliminates the effects of density variations
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mass attenuation coefficient definition
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gm/cm^2
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coeff determined by (for MAC)
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dividing mass attenuation by the number of electrons per gram
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atomic attenuation coefficient=
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atoms/cm^2
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mass energy absorption coefficient def.
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reflects the amountof energy transferred to charged particles
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the only portion of energy from teh beam of photons which is not abosrbed is
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bremsstrahlung (however bones might cause this to occur)
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if attenuation coefficient is 2.4x10-1/m
transmission of 10m thick is ?? |
9%
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if attenuation coefficient is 2.4x10-1/m
trnasmission of 20m thick is ?? |
.8%
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if attenuation coefficient is 2.4x10-1/m
attenuation is?? of 30m thick? |
attenuation is 99.9%
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if attenuation coefficient is 2.4x10-1/m
trnamision of 5m thick is? |
30%
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if attenuation coefficient is 2.4x10-1/m
HVL of this material is??? |
2.8875m
formula= .693/u |
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if attenuation coefficient is 2.4x10-1/m
TVL is? |
9.583m
formula= 2.3/u |
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if attenuation coefficient is 2.4x10-1/m
transmission of 4 HVL |
6.25
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if attenuation coefficient is 2.4x10-1/m
transmission of 2TVL? |
.1
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how do you quantify radiation
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measure number of photons= fluence
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fluence=
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numer of photons to cross a given area
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fluence rate aka
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flux density
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fluence rate def.
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number of photons to cross a given area IN A GIVEN TIME
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energy per unit area=
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energy fluence
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energy flow=
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energy fluence rate
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energy fluence rate aka
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energy flux density
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Exposure is
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measure quanity through air
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Exposure formula
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Exposure =(change in electrical charges)/ (change in mass)
=C/kg |
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what happens when a photon beam passes through air?
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primary e- are produced
-they produce secondary e- if left to themselves , ions would recombine |
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what do you need to measure exposure?
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isolate charged particles produced
determine the total charge, (change A), produced by ionization as particles lose their energy through air need electronic equilibrium |
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divice to directly measure exposure
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free air ion chamber
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how is electronic equilibrium achieved?
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achieved when the # of electrons set in motion inside a volume is equal to the # of electrons come to rest in thevolume or
those that sneak in =those that sneak out |
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saturation voltage is?
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when voltage across electrodes is large enough for all ion pairs to be collected before they recombine
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exposure measures
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amount of ionization produced in air.....NOT ABSORBED DOSE IN MEDIUM
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exposure is defined only for___ not ___
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electromagnet radiation NOT charged or uncharged particles
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free air chamber exposure limit is
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3MeV
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density of air=
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.001293gm/cm^3 at STP
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Q=charge=
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1.6x10^ -19
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to measure Exposure with free air ion chamber use this equation
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Exposure= (NxQ)/(change in Lxp)
N=ions collected Q=charge=1.6x10^ -10 p= density of air= .001293 change in L=volume of air change |
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Exposure SI=
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C/kg
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t/F:
C/kg is larger than R? |
true
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1R=? C/kg
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2.58x10^-4 C/kg
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?R=1C/kg
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3759R
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kerma stands for?
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kinetic energy released in matter
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Kerma unit is?
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J/kg
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kerma definition
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sum of all the initial kinetic energy transferred from photons or neutrons to charged particles per unit mass of absorbing material
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absorbed dose=
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energy retained in the medium
this creates biological effects by ionizing our water molecules |
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absobed dose equation
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Dose= ED/M
ED=mean energy imparted to the volume element of M=mass |
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absorbed dose SI unit=
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Gy
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1Gy=?J/kg
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1Gy=1J/kg
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1rad=?Gy
1rad=?cGy 1Gy=?rad |
1rad=.01Gy
1rad=1cGy 1Gy=100rad |
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average energy to create one ion pair in air=
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33.97eV/ion pair
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1 R= ? ion pairs/gm of air
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1.61x10^12
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1R=?rad
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1R=.876rads
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1Sv=?rem
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1Sv=100rem
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dose equivalent equation
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H=DxQxN
D is absobed dose Q is quanity factor N=1 |
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convert exposure(C/kg) to dose (J/kg)
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(1ip/1.6x10^-19)x (33.97/ip) x (1.6x10^-19J/eV)=33.97J/C
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the air equivalent wall does what?
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attenuates photons at the same rate as air
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ionization will be a maximum when
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wall thickness equals range of e-'s scattered into the cavity
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mast maximum thickness the wall acts as
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an attenuator
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electrometer measures
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current, charge , resistance
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what do yo do o measure mass of air in chamber?
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send off to be calibrated
must be calibrated ever 2 years |
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wall thickness is= ___ for a ___beam
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1mm for a 300kvp beam
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what do you do to a chamber when used for cobalt?
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add a cobalt cap
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Temperature pressure corrections EQUATION
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TPcf= (760/PmmHg) x (273+t degrees C/295)
P=pressure |
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is an ion chamber sealed
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no...never!
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electrometer measures
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current, dose rate, accumulated charge, integrated dose over the irradiated time
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Calibration factors units=
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R/C or R/uC
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central electrode is charged to ? V?
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300V by electrometer
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examples of condenser chambers
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victoree Rmeter, pocket dosimeters
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write an equation which relates, u, the linear absorption and, (en u), the mass energy absorption coefficient
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Uen=(u/p)
p=density |
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two marameters which are necessary to define the Exposure from a beam of ionizing radiation
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density of air
below 3Mev Mass of collection volume # or charges of one sign in a collection volume |
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attenuation after 3HVL
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87.5% attenuated
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attenuation after 6HVL
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98.4375% attenuated
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attenuation after 2TVL
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99%attenuated
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attenuation after 5TVL
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99.999% attenuated
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thermoluminescent is
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the emission of light by the application of heat
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glow curve
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signal received from TLD material may be a function of either heating time or temp
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glow curve is most stable at
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200 degrees C
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humps on the glow curve graph represent
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dosimetry peaks at low number then 100 then 200 degrees C
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atomic # of LiF
of soft tissue of air |
8.18
soft tissue 7.4 air 7.65 |
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primary instruments to measure
Exposure dose |
Exposure= free air ion chamber
dose=calorimeter |
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secondary instruments
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ionization chamber
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tertiary methods
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TLDs, chemical dosimetry, diodes, film
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what happens to the response of LiF at 1000R?
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supralinear region
= response is independent of exposure rate until very high rates are reached |
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what is used for personnel monitoring, patient dose confirmation, monitoring environmental levels of radiation?
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TLD's
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density of a film equation
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D=log10 (Bo/B)
Bo is with out film=high reading B is transmitted with film=low reading |
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characteristic curve aka
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H&D curve
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gamma of film=
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slope of characteristic curve
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gamma determines
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minimum difference in exposure which will be detectable
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latitude of film=
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straight line portion of H&D curve
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latitude def.
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range of exposures over which densities will like on the linear portion of the curve
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speed of sensitivity=
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reciprocal of exposure required to produce density of 1
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speed of diagnostic
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333 or 1/30mGy or 1/.003Gy
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speed fo therapy film
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33 or 1/30cGy or 1/.03Gy
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why is diagnostic faster?
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it has a smaller exposure to create a density of 1
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why at lower energy is the speed higher
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photoelectric effect
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T/F
the absorbed radiation may produce a chemical change in the absorbing medium. the amount of this change may be used to measure dose |
True
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Fricke dosimetry is?
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relatively insensitive doses of 5,000-50,000rads needed before effect is measurable
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T/F
almost all energy absorbed from radiation turns into heat |
true
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what is a calorimeter
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device which measures teh rise in temp. of an irradiated medium
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only method to directly measure dose
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calorimeter
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p-type is what
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has 1 open spot in valence band
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n type is what
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has a donor e-
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diodes flow from __to__
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n to p
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types of diodes
type we use |
silicon=Si
Germanium=Ge we use Si cause it operates at room temp. |