Calibration of Dose Output/ Radiation measurements Flashcards
Exposure is a measurement in ____
air
What are the stipulations for exposure?
Only under 3MeV
Only measured in air
The quantity of radiation for all types of ionizing radiation, (charged and uncharged), all materials and all energies
Absorbed dose
SI unit for absorbed dose
Gray
Traditional unit for absorbed dose
Rads
the biological effect of radiation depends on the dose and also type of radiation
Dose equivalent (H)
Used to assess risk from radiation and not radiation prescriptions?
Dose equivalent (H)
Traditional unit for (H)
rem
SI unit for (H)
Sievert (SV)
Quality Factor (Q)
accounts for how different radiation types affect cells differently
Unit of Q
Unitless
Q is similar to using___
RBE
Quality factor for xrays, gamma rays and electrons
1
Quality factor for neutrons, energy < 10keV
5
Quality factor for protons
2 or 1.1
Quality factor for alpha particles
20
the ratio of the dose of 250 kVp xrays to produce a specified biological effect to the dose of the given radiation to produce the same effect
RBE
when a volume is irradiated by photons, energy is given to charged particles by such photons
Collisional Kerma
The energy given to irradiation volume is ______ to energy given to charged particles
equal
???Kerma is _____ in the buildup region compared to absorbed dose
higher
After dmax, what happens to absorbed dose compared to kerma
it is higher than kerma
Deeper the buildup region
the greater the skin sparing
Look up redmaxing effect
F med (F factor) converts?
Roentgen to Rad (exposure to dose)
What is the mean energy required to produce an ion pair in dry air (constant for all electron energies)
W= 33.97 eV/ionpair
D=X * W/e
33.97 J/C * 2.58 x 10^-4 c/kgR = 0.876 rad/R
Transmission factor in air
AQE
Factor to take dose in air to dose in other media
Dair = 0.876 * X * A
The energy loss by electrons per unit path length of a material
stopping power
Two types of stopping power
Collisional and radiative
Limitations in calculations from absorbed dose from exposure
derived from exposure in air
only valid for photons <3MeV
Ratio for the number of photons or particles passing through a cross sectional area of a sphere
Fluence
Units of fluence
1/cm^2 or cm^-2
the ratio of radiant energy passing through a sphere of cross-sectional area
fluence energy
Photons in a fluence can be
monoenergetic or polyenergetic
How to find energy fluence
Take the number of photons x the keV of each and add together
i.e (photons/cm^2 * keV) + (photons/cm^2 * keV)
KERMA
Kinetic energy released in Matter per unit mass
energy transfer via radiative photons (bremsstrahlung (g))
Radiative KERMA
collisions between photons and charged particles results in electrons which dissipate energy via collisions leading to ionization and excitation
Collisional KERMA
KERMA units
Gy
Kerma is defined
in any medium and indirectly ionizing radiation (Photons and neutrons)
occurs when the number and energy of charged particles (electrons) are equal entering and exiting the measurement volume
Charged particle equilibrium (CPE)
CPE is used to measure
Exposure (ion chambers)
measure the charged produced in air volum by the electrons liberated by a photon beam
ion chamber (plus electrometer)
Electonmeter has what type of voltage
bias
pros of bias voltage
-keeps ions from recombining and not being read by ion chamber
what is bias voltage
no ground and it isn’t held at a neutral voltage
what are free air ion chambers used for
primary devices that check clinical devices
A free air ion chamber is
an open system to the outside air
combined before can be read by collecting device/ charge created by never collected
Ion Recombination
Desirable ion chamber characteristics
- minimal variation in sensitivity or exposure calibration factor over wide energy range
- minimal dependence with direction of incident radiation
-minimal stem leakage
-minimal ion recombination
-should be calibrated against a primary standare
Cons of free-ion chambers
-air density changes, temp, pressure and humdity
-Ion recombination
-air attenuation of photons
-secondary chamber chambers calibrated against free air chamber use expousre calibraion factors
Thimble chamber has what surrounding it
solid air shell; material made with equivalent Z as air
Wall on thimble chamber has to be as thick as what?
as thick as max electron range, may need buildup cap for higher energies
What must a thimble chamber have to measure exposure
-Have a defined cavity volume
-be air equivalent
-wall thickness to provide electronic equilibrium
Thimble chamber must be calibrated at
a national lab that uses a FAC
Not really in practice now, example is victoreen R meter
Condenser chamber
Farmer chamber measures
photon exposure in the therapeutic energy range
Farmer chamber wall is
pure graphite (tissue equivalent)
Central electrode of farmer chamber
Pure aluminum
Farmer chamber insulator is
polytricholorfluoroethylene
Collection volume of farmer chamber
0.6cm^3
Farmer chamber has ____ electrodes
Three
Bias voltage volts
300
