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90 Cards in this Set
- Front
- Back
characteristic radiation occurs when...?
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e- interact with inner orbital e- instead of nucleus
incident e- knock out inner orbital e- atom left in ionized state |
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incident e- leaves with energy (equation)
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Ef=Eo-Et
Et=energy imparted to orbital e- |
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orbital e- leavew ith energy (equation)
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Ee=Ef-Ek
Ek=binding energy |
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ionized atom then has lower level e- which ________and creates a __________
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loses energy
creates a photon |
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photon created from lower level e- moving up and losing energy is called
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characteristic radiation
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equation for characteristic radiation (photon equation)
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hv= Ek-El
Ek=binding energy of inner orbit El=binding energy of outer orbit |
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characteristic radiation is .....
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characteristic of where it takes place
L radiation = L shell knocked out K radiation = K shell knocked out |
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as energy increases, characteristic radiation
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decreases
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relationship between energy and characteristic radiation is
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indirectly proportional
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relationship between characteristic radiation and megavoltabe x-ray machines?
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none, characteristic radiation is not seen in megavoltage x-ray machines just bremsstralung
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3 methods photons and e- interact
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photoelectric interaction
compton interaction pair production |
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as energy increases teh % of initial energy retained in photoelectric interaction__________
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decreases at all angels except 0 degrees
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what provides diagnostic contrast
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photoelectric interaction
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what is a photoelectron
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an ejected e- from the inner shell knocked out by a photon
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characteristic radiation aka
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florescent radiation
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electromagnetic radiation that causes 2nd e- called ______ to be ejected
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auger e-
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energy given to photoelectric e- (equation)
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Ek= hv-Eb
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k shell binding energy
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.5 kev or 500 ev photon/electron=low
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characteristic radiation is emitted w/an energy of
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.5kev with little to no Bremstralung
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with increasing Z materials
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e- is held tighter
= increasing binding energy |
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greatest angle e- can be scattered
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90 degrees
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characteristic radiation
adv dis |
adv= mono energetic
dis= bremstrullungs are greater in # than characteristic radiation |
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absorbtion edges are associated with
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photoelectric attenuation
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at photon increases
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photoelectric attenuation decreases
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photoelectric attenuation coefficient increases abruptly when
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energy of photon equals binding energy of shell
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when photon energy =binding energy
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produces discontinuities in attenuation curve= absorption edges
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discontinuities in attenuation curve
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absorption edges
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photoelectric effects predominate in
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H20 < 50kev (max 200kvp or 66kev)
Pb <500kev |
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photoelectric attenuation varies as
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1/E3 of the photon
Z3 of the absorber- more probable in bone than tissue |
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Our friend
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compton interaction
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ther are more ___interaction than______
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photoelectric interactions than compton interactiosn
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interaction of photon w/unbound e- (outer e- with low binding energy)
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compton effect
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unbound e-
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binding energy is low so considered free e-
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type of interacition in compton effect
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elastic
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______composed of low atomic #
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free electrons (soft tissue)
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in _____ photon ejects e- and e- receives part of energy, photon receds from collison with reduced energy
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compton effect
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equation for compton effect
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hv-hv1 + Ee
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photon direct hit with e-
what angle? |
e-travels straight forward
photon straight back e= max energy photon =minimum degree angle= 180 |
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photon grazes e-
what angle |
photon straight forward
photon maintains all energy e- receives non form photon angle 90 |
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Compton wavelength
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change in wavelength of photon
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Compton
low energy photon |
e aquires little energy phon gets most energy
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Compton
high energy photon |
recoil e- gets most energy
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how does Z affect comtpton effect?
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it doesn't because Compton is independent of Z (atomic number)
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as energy ____ probablity of compton interaction______
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increases probability of compton decreases (indirectly related)
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in soft tissue the range 60kev-20mev, ________ is more important than _____or ____________
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compton is more important than pair production or photoelectic
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soft tissue ranges for compton
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60kev-20mev
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does pair production occur in diagnostic range?
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no! starts at 1.02 Mev
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what is pair production
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when a photon passes near the nucleus of atom and is subjected to the field of the nucleus. photon may disappear and become e- or e+
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pair production relationship to E=mc2
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energy of photon turned into mass (e- and e+
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probability of pair production ___as photon energy ____
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increases
increases (directly related) |
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pair production relationship with Z
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vaires as Z2
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What is triplet production
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same as pair production except interactons occurs with electron fields
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what results from triplet production
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e- e+ and initial e-
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threshold for triplet production
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2.04Mev
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probability of triplet production
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1-10% as often as pair production
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above 20Mev-
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pair production is the most important process by which energy is absorbed
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fem heads get great dose with?
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pair production and there is shadowing of the prostate
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T/F
Can a 10Mev electron have a compton interaction in tissue |
FALSE because its an e- and not a photon
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types of interactions with e- and photons
e- = photon= |
e= collisions an bremstrulung
photon= photoelectic, compton, pair production |
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bone absorbs 6 times as much energy gm for gm as tissue
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photoelectric
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bone and tissue absorb the same amount gm for gm
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compton
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bone absorbs 2times as much energy gm for gm
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pair production
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why we treat with photons
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skin sparing
goes deeper carry energy deeper because they have no charge or mass when they interact they produce electrons= dose=biological changes |
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T/F photons don't give does, they carry energy which is released by electrons (production of an interaction)
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true
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compton
as energy of photon____ scattering angle___ for photon and e- |
increases, decreases
(indirectly proportional) |
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compton
as energy of photon___, more energy is given to e- |
increases
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if photon is scattered at 90 degrees
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max energy it can have is .511mev
e- scattered directly forward |
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if photon is scattered at 180 degrees
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e- given max amount of energy and
photon gets minimum |
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if photon is scattered at 0 degrees
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photon gets max amount
e is deftected at 90 degree angle e gets no energy from incident photon |
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form of Co60
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disk, cylinder, pellet, encapsulated in stainless steel capsule
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type of capsule
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dbl stainless steel to prevent leakage
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use waffers to
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fill inner capsule for varying amount of co60
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housing around capsule
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lead for shielding purposes
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source of "on" "off" mechanism
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1 rotation wheel
2. horizontally driven drawer 3. mercury switch |
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how are mechanisms "fail safe"
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must have power for motor to keep open, when power goes off it closes
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2 sets of collimators for co60
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primary
secondary |
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primary is
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stationary
close to the source defined larges filed size possible |
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secondary is
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variable collimators
fld size used to treat multiplane, made of depleted uranium inner surface always parallel to beam , diverge with source/beam |
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how do you avoid e- contamination in co60
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have secondary collimators at least 15-20 cm away from patient
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source diameter for co60
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1-2cm in diameter
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increase diameter of source=
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stronger source
BUT larger source diameter=large pneumbra |
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pneumbra equation
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w=s ((SSD-SCD)/
SCD) pneumbra= source diameter ((skin to source distance- source to collimator distance) / source to collimator distance) |
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other isotopes used for teletherapy in teh past
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Ra Cs
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Co60 developed in
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1952
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standard source strength
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4000-10,000 RHM
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1RHM=
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1Ci
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# of photons=
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dose rate created
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probability of making x-rays is
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1%, the rest is heat
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probability of making x-rays for linear accelerator
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50%
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type of x-ray used in both diagnostic and radiation therapy
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bremsstrulung
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