Review Exam 1 Flashcards
Define Exposure
of ionization produced in air
- Photons passes thru air and liberate electron through photon interactions.
- Electrons will then cause ionizations (anions and cations
- Ion collection plate with voltage applied to have anions go to the positive side and cations go to the negative side)
- This starts occurrent and a charge of either sign can be measured with an electrometer.
Ion chamber
Explain how an ion cahmber works
- Photons passes thru air and liberate electron through photon interactions.
- Electrons will then cause ionizations (anions and cations
- Ion collection plate with voltage applied to have anions go to the positive side and cations go to the negative side)
- This starts occurrent and a charge of either sign can be measured with an electrometer.
How does low air density affect ion chamber readings?
low reading
because air molecules are more spread out (less density) at higher temps
What air conditions would cause a high reading for an ion chamber (2)
high density, Low temp
List FAC corr factors: (3)
Which correction factor is only used if it is a secondary chambers calibrated against FAC chamber?
- Air density changes
- Ion recombination
- Air attenuation of photons
calibration factors
What has a collection volume .6cm^3
Farmer
What are the 3 electrodes of a farmer chamber?
Central, guard, thimblewall
What does the central electrode do in a farmer chamber?
Collector
What does the guard electrode do in a farmer chamber? (4)
Insulator
prevents leakage
defines collection volume
prevents contamination
What chamber(s) use(s) a build up cap?
Thimble chamber & farmer chamber
What is a solid air equivalent shell (like a foam material)
Buildup cap
What does a bulidup cap do?
Creates CPE in the buidup region to allow for accurate measurement in this region
Why is it hard to measure dose in the build up region?
Steep gradient that can cause very inaccurate measurements if only off by a mm
What measures dose in the build up region (2)
Parallel plane/plate/pancake & extrapolation chamber
Used for in vivo (2)
TLD & diode
When positive and negative ions recombine the result in a charge created that never reaches collecting electrode
Ion recombination
Bias voltage approx ___ V does what?
300
helps reduce ion recombination
Ideal characteristics: (8)
no energy dependence
sensitivity
no saturation limit
spatial resolution
effective water equivalent Z (tissue eq)
no directional dependence
min ion recombination
reusable
quick read-out
inexpensive
Fricke dosimeter uses what to measure change?
Spectrophotometer
TLD most commonly uses what material?
LiF
Electrons excited, become trapped in conduction band, when heated they are released and recombine in valence band, and release light describes what radiation measurement device?
TLDs
Why is LiF used? (2)
the atomic number is close to tissue equivalent
thermoluminescence
For TLDs light released is _________ to the radiation dose
proportional
How are residual affects from exposure and removed from TLDs?
Annealing
heating plus slow cooling explains the process of __? What is it for?
Annealing
Remove effects of previous exp
Which dosimeter has advantages of reusable, good tissue equivalence and small?
TLD
TLD advantages (3)
reusable
tissue equivalence
small
TLD disadvantages (3)
No immediate readout
low spatial resolution
fragile
Which dosimeter uses a P and N type junctions
diode
What dosimeter is similar to how an ion chamber works by inducing a current, but it uses a solid instead of air?
diode
where the P region and N region meet is called?
depletion region
Advantages to silicon diodes (4)
- immediate readout
- small
- sensitive
- durable
Which dosimeter has the following advantages?
immediate readout
small
sensitive
durable
diode
Which dosimeter is used for QA (depth dose and profile)
diode
Disadvantages of diodes: (3)
energy dependence in photons beams
directional dependence
sensitive to heat and radiation damage
A film that is not based on silver halide but a radiosensitive monomer
Radiochromic film
Advantages of MOSFETs:
immediate read-out
reusable
small and lightweight
MOSFETs stands for
Metal oxide semiconductor-field effect transistor
What would be used for QA and light radiation field.
Radiochromic film
What is the type of filtration? Filtration that would remove all low energy x-rays
Ideal filtration
What is being measured under “good geometry”?
HVL
Thoreaus filter order (4 things)
tin, copper, aluminum, patient
Filter that must be arranged in proper order of highest atomic # closest to the target
Thoreaus filter
Collisions between photons and charge particles results and electrons which dissipate energy vehicle collisions leading to ionization and excitation
Coll KERMA
Energy transfer via radiative photons (bremsstrahlung )
Radiative KERMA
energy fluence formula
(#Ofphotons x energy of photons) + (# Of photons x energy of photons)
What are limits for exposure (R): (3)
Only valid for photons( xray & gamma ray)
3 mev limit
Only for air ionization
What is CPE used to measure?
Exposure in ion chamber
Limits to exp: (3)
Only valid for photons (xray & gamma ray)
3 mev limit
Only for air ionization
Correction factors for FAC: (3)
Air density changes
Ion recombination
Air attenuation of photons
Stem effect varies w/?
energy
What chamber has a collection volume .6cm^3
Farmer
What chamber is well guarded and has 3 electrodes
Farmer
What are the 3 electrodes of a farmer chamber
Central electrode - collector
Thimblewall - tissue equivalent ( pure graphite)
Gaurd electrode
Which of the following would be a primary standard for ion chamber
FAC
What uses temperature rise as a dosimeter
Calorimetry
TG 21 was based what machine
Co
Which TG is based on exposure to air kerma
TG 21
Which TG is based on absorbed dose in water
TG51
If you put collimator or shield closer to the skin it does what? What affect does it have on penumbra?
Increases SDD
Decreases penumbra
Fricke dosimeter uses what to determin change
Spectrophotometer
What measurememnt device is this? Electrons excited, become trapped in conduction band, when heated they are released and recombin in valence band, and release light.
TLD
Which dosimeter has advantages of reuable, good tissue equivalence and small?
TLD
What are limitations to TLD? (3)
No immediate readout, low spatial resolution and fragile
Which dosimeter has immediate readout
Silicon Diodes
Adv diodes (4)
immediate readout
small
sensitive
durable
What measurement device is energy dependent in photons?
diode
What is insensitive to radiation and requires 10-50Gy
Radiochromic film
MOSFETs stands for
Metal oxide semiconductor-field effect transistor
Why are new couches all carbon fiber?
Low Z material and therefore easier to treat thru
Magnetron or kylstron:
Used for high energy and more expensive
Klystron
Magnetron or klystron: require low energy microwave source (RF driver)
Klystron
Magnetron or klystron: electrons get exposed to a magnetic field which converts them into microwaves
magnetron
What type of gas is in the accelerator tube
SF6
What type of target is in a linac?
transmisssion
What type of target is in an xray tube
Reflection target/rotating anode
What are the conditions at which a MU is defined
Amount of time to deliver 1 cGy at 10x10 cm FS at DMax at 100cm awa
Why are secondary collimators move divergently with the beam?
transmission penumbra
What penumbra is due to the finite dimension of the source/ focal spot
geometric
Which penumbra is the spread of dose distriubtion near field borders
physical
Does ion chamber go before the flattening filter?
no
Which of the following linacs uses a Physical source?
Co
Average energy of cobalt machine?
1.25 MeV
How many modes of decay does cobalt 60 have & what are they?
2 modes: 1.17 mev / 1.33 mev
Cobalt 60 is housed in what?
Stainless steel capsule
What is the size of cobalt source?
1-2 cm in diameter
Cobalt loses __% activity per month?
1
Betatron is also called
donut
Xray output is sensitive to the ___ ___.
filament current
Xray output increase____________ with tube current, and________of the to tube voltage
Proportionally / squared
The higher the energy of the electron bombarding the target the more__________ the direction of the xray emission
forwardss
What are the 3 primary dosimeters?
FAC
Calorimetry
Fricke
