Grad class notes Flashcards
lower energy limit for ionizing radiation
10 keV
wavelenght of soft vs hard xrays
soft: 10 nm
hard: 200 fm
EM radiation
oscillations are 90 degrees to direction of propagation
-wave-like and particle-like properties
-E = hv = hc/lambda
relativistic kinetic energy
T = mc^2 * {1/(root (1-v^2/c^2) - 1}
formula for cross section
p = a N x
a= area of each target
N= # of targets per volume
x= thickness
unit of barn
10^ - 24 cm^2
classical radius of the electron
2.8 * 10^-15 m
r= k * e^2/(mc^2)
solid angle equation
2pi sin(theta) dtheta
scatter cross section
difference between interaction cross-section and energy transfer cross section
total kinetic energy released in pair production
hv - 2mc^2
what is mass collisional stopping power proportional to?
-electron density (decreases slightly as Z increases)
-z^2, z is charge on heavy paticle
-ln(1/I), I is mean excitation energy of the atom to which the electrons are attached; I increases as Z increases, which means collisional stopping power decreases for higher Z
-1/velocity^2
-mass of particle
-density correction term accounts for “screening” effect of electrons in close proximity with each other
what is radiative mass stopping power proportional to?
Z^2
T (kinetic energy)
Na
radiative stopping power/collisional stopping power is proportional to what?
TZ/800
CSDA range
1/total stopping power
what is total kinetic energy lost to the medium (by the electrons in slowing down) equal to?
to the kinetic energy given to them
N/Z ratio
-must increase for heavier elements to minimize proton-proton repulsion
for Z< 20, N~Z for stability
for Z>20, N>Z
when does alpha decay occur?
-when ratio of neutrons to protons too low
-have to get to lower Z state where you don’y need as many neutrons for stability..
treshold energy for beta plus decay
1.02 MeV
when does electron capture occur?
too many protons in the nucleus of an atom and not enough energy to emit a positron (< 1.02 MeV)
1.02 MeV required for positron emission
isomeric transition
transition from metastable state to ground state
law of radioactive decay
dN/dt = -lambda N
N = No * e^(-lambda t)
A= - lambda N
mean half life
1/lambda
1.44 * half life
air kerma strenght
S = exposure rate * (W/e) * d^2
well chamber
-4 pi geometry
-response depends on position of source in well, length of source, because of absorption and scattering of photons and secondary electrons in chamber wall and gas
Co-60
1.25 MeV
half life = 5.3 years
-beta and gamma emission
corrections for the effects of attenuation and scattering in brachy are more important for…
low energy emitters as attenuation effects are substantial
can activity within a brachy source be verified directly?
No
2 main assumptions of bragg-gray cavity theory
- cavity is small enough not to perturb the charged particle field
- cavity dose is deposited entirely by crossing charged particles
what is pt of cavity theory?
-to measure dose, have to insert a dosimeter into the medium
-this pertubs the medium
-cavity theory attempts to correct for the pertubation
cons of calculating absorbed dose from exposure instead of using cavity theory
-exposure is only defined for x and gamma rays
-free air chambers that measure exposure cannot be built for E > 3 MeV
why is D> Kcol?
dose comes from upstream cavity
case 1: cavity very large but not large enough to perturb photon fluence
-ratio of uen/p
-only photons deposit dose
-have CPE because range of electrons set in motion in cavity medium is smaller than dimensions of the cavity
case 1b: cavity very large, and large enough to perturb photon fluence
-ratio of uen/p
-also exp(-ux)/exp(-ucavx)
case 2: small cavity
-range of electrons set in motion in medium is bigger than cavity
-photon interactions are negligible
-electrons come from surrounding medium and are slowed down by cavity material
–use stopping power ratios
case 3: intermediate sized cavity
-range of electrons set in motion about same size as cavity
-consider both photon and electron transport
assumptions for bragg gray cavity theory
-incident photon fluence is nearly constant over entire cavity
-no photon interactions occur inside cavity
-electron spectrum seen by cavity is same as that seen by medium
-No Bremstrhalung occurs
-electrons deposit their energy continuously per CSDA
assumptions of spencer attix cavity theory
-CPE or TCPE exists
-no Brems photons
-radiation fluence is produced by a homogeneous source of monoenergetic electrons of initial energy To, which emits N electrons per unit mass of medium
-delta ray production can occur in the medium but not in the cavity
delta
(spencer-attix)
-mean energy of electron that can just cross the cavity
i.e. range of electron with energy delta is cavity size
-mean energy necessary for a delta ray generated in the cavity to escape from it
restricted stopping power
-delta rays with energy greater than delta alter the electron spectrum, but do not contribute to the dose at the pt where they are created
-restricted stopping power is smaller than unrestricted stopping power
why were there failures in BG-theory for high Z materials?
-delta ray production enhances the low energy end of the electron spectrum
-at low E, electron interaction cross sections are much higher in high Z materials, making energy deposition in high Z materials more sensitive to changes in the low energy end of the equilibrium electron spectrum
S-A: so delta rays of energy T< delta contribute to the electron spectrum?
No. they deposit their energy at pt of creation
-they have no range and don’t contribute to the electron spectrum
Aion vs Pion
Aion is at the standards lab
Pion is reciprocal of Aion for a measurement made in the clinic
in air photon beam calibration in terms of absorbed dose (in free space)
Kc,air = X (W/e)
Kc,med= Kc,air * ratio of uen/p
K’c,med= A Kc,med
A is electronic equlibrium thickness attenuation correction
If TCPE,
D= beta * K’c,med
Kair = Kcair/ (1-g)
What does Prepl account for?
-in scatter effect: increases fluence in cavity since electron scattering out of the cavity is less than that in the medium
-obliquity effect: decreases fluence in the cavity since electrons take relatively straight line paths in the cavity as opposed to more oblique paths in the medium
-displacement in effective pt of measurement
where does TG51 apply?
-photon beams with E between 60Co and 50 MV
-electron beams with E between 4 and 50 MeV
why TG51?
conceptually simpler
-avoids in-air quantities
-can compare clinical protocols
-improved accuracy
-accounts for Al electrode
-uses up-to-date stopping power ratios
formula for kq
kq = Ndw,q/Ndw,60Co
kq = ration from Q to 60Co of : (L/P)water to air * Pgr * Pfl * Pwall * Pcel
Pfluence
corrects for pertubation of electron fluence due to scattering differences between air cavity and medium
TG51: why is reference dosimetry done for SSD 100 cm?
%dd(10) and R50 are functions of SSD while absorbed dose calibration factors are not
Zeff
=(a1Z1^2.94 + a2Z2^2.94 +…)^1/2.94
aj = Avogrado*Zj8wj/(Aw)j/(sum of this factor for all the included elements)
dosimeter for electron beam
-make wall as thin as possible to minimize pertubation to electron spectrum
well chamber
-response depends on energy of photon emissions, and position of source in well
-energy dependence arises from absorption and scatterinf of photons and secondary electrons in the chamber walls and the gas
why is dose measured at a reference depth?
-dose at dmax may be influenced by electrons originating outside the phantom whereas the reference depth is selected to go beyond the range of contaminant electrons
-additionally, depth of dose max varies with field size at high energies
pros and cons of OSLDs
PROS
-fast
-multiple readouts
-stable
-dose-rate and energy independent
-linear with dose
-no directional dependence
-sensitive, measures large range of dose
CONS
-sensitive to light
-Al2O3 not tissue equivalent
Pros and cons of radiochromic film
PROS
-self-developing
-grainless
-insensitive to ambient conditions
-may be handled with visible light
-no significant energy dependence
-linear up to 150 cGy
-dose-rate independent
-tissue equivalent
-high spatial resolution, 2 D
CONS
-sensitive to humidity, UV, temperature
-issues with thermal history, wavelength dependence, and local sensitivity of film
-expensive
-non-uniformity in emulsion
