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Ford > Grad class notes > Flashcards

Grad class notes Flashcards

1
Q

lower energy limit for ionizing radiation

A

10 keV

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2
Q

wavelenght of soft vs hard xrays

A

soft: 10 nm
hard: 200 fm

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3
Q

EM radiation

A

oscillations are 90 degrees to direction of propagation
-wave-like and particle-like properties
-E = hv = hc/lambda

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4
Q

relativistic kinetic energy

A

T = mc^2 * {1/(root (1-v^2/c^2) - 1}

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5
Q

formula for cross section

A

p = a N x
a= area of each target
N= # of targets per volume
x= thickness

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6
Q

unit of barn

A

10^ - 24 cm^2

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7
Q

classical radius of the electron

A

2.8 * 10^-15 m

r= k * e^2/(mc^2)

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8
Q

solid angle equation

A

2pi sin(theta) dtheta

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9
Q

scatter cross section

A

difference between interaction cross-section and energy transfer cross section

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10
Q

total kinetic energy released in pair production

A

hv - 2mc^2

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11
Q

what is mass collisional stopping power proportional to?

A

-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

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12
Q

what is radiative mass stopping power proportional to?

A

Z^2
T (kinetic energy)
Na

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13
Q

radiative stopping power/collisional stopping power is proportional to what?

A

TZ/800

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14
Q

CSDA range

A

1/total stopping power

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15
Q

what is total kinetic energy lost to the medium (by the electrons in slowing down) equal to?

A

to the kinetic energy given to them

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16
Q

N/Z ratio

A

-must increase for heavier elements to minimize proton-proton repulsion
for Z< 20, N~Z for stability
for Z>20, N>Z

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17
Q

when does alpha decay occur?

A

-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..

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18
Q

treshold energy for beta plus decay

A

1.02 MeV

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19
Q

when does electron capture occur?

A

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

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20
Q

isomeric transition

A

transition from metastable state to ground state

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21
Q

law of radioactive decay

A

dN/dt = -lambda N

N = No * e^(-lambda t)

A= - lambda N

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22
Q

mean half life

A

1/lambda
1.44 * half life

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23
Q

air kerma strenght

A

S = exposure rate * (W/e) * d^2

24
Q

well chamber

A

-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

25
Q

Co-60

A

1.25 MeV
half life = 5.3 years
-beta and gamma emission

26
Q

corrections for the effects of attenuation and scattering in brachy are more important for…

A

low energy emitters as attenuation effects are substantial

27
Q

can activity within a brachy source be verified directly?

A

No

28
Q

2 main assumptions of bragg-gray cavity theory

A
  1. cavity is small enough not to perturb the charged particle field
  2. cavity dose is deposited entirely by crossing charged particles
29
Q

what is pt of cavity theory?

A

-to measure dose, have to insert a dosimeter into the medium
-this pertubs the medium
-cavity theory attempts to correct for the pertubation

30
Q

cons of calculating absorbed dose from exposure instead of using cavity theory

A

-exposure is only defined for x and gamma rays
-free air chambers that measure exposure cannot be built for E > 3 MeV

31
Q

why is D> Kcol?

A

dose comes from upstream cavity

31
Q

case 1: cavity very large but not large enough to perturb photon fluence

A

-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

32
Q

case 1b: cavity very large, and large enough to perturb photon fluence

A

-ratio of uen/p
-also exp(-ux)/exp(-ucavx)

33
Q

case 2: small cavity

A

-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

34
Q

case 3: intermediate sized cavity

A

-range of electrons set in motion about same size as cavity
-consider both photon and electron transport

35
Q

assumptions for bragg gray cavity theory

A

-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

36
Q

assumptions of spencer attix cavity theory

A

-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

37
Q

delta
(spencer-attix)

A

-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

38
Q

restricted stopping power

A

-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

39
Q

why were there failures in BG-theory for high Z materials?

A

-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

40
Q

S-A: so delta rays of energy T< delta contribute to the electron spectrum?

A

No. they deposit their energy at pt of creation
-they have no range and don’t contribute to the electron spectrum

41
Q

Aion vs Pion

A

Aion is at the standards lab
Pion is reciprocal of Aion for a measurement made in the clinic

42
Q

in air photon beam calibration in terms of absorbed dose (in free space)

A

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)

43
Q

What does Prepl account for?

A

-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

44
Q

where does TG51 apply?

A

-photon beams with E between 60Co and 50 MV
-electron beams with E between 4 and 50 MeV

45
Q

why TG51?

A

conceptually simpler
-avoids in-air quantities
-can compare clinical protocols
-improved accuracy
-accounts for Al electrode
-uses up-to-date stopping power ratios

46
Q

formula for kq

A

kq = Ndw,q/Ndw,60Co
kq = ration from Q to 60Co of : (L/P)water to air * Pgr * Pfl * Pwall * Pcel

47
Q

Pfluence

A

corrects for pertubation of electron fluence due to scattering differences between air cavity and medium

48
Q

TG51: why is reference dosimetry done for SSD 100 cm?

A

%dd(10) and R50 are functions of SSD while absorbed dose calibration factors are not

49
Q

Zeff

A

=(a1Z1^2.94 + a2Z2^2.94 +…)^1/2.94

aj = Avogrado*Zj8wj/(Aw)j/(sum of this factor for all the included elements)

50
Q

dosimeter for electron beam

A

-make wall as thin as possible to minimize pertubation to electron spectrum

51
Q

well chamber

A

-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

52
Q

why is dose measured at a reference depth?

A

-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

53
Q

pros and cons of OSLDs

A

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

54
Q

Pros and cons of radiochromic film

A

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

Ford (3 decks)

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