5/6 photon interaction w matter

electron beams treat… what depth of target

superficial

high energy photon treat (gamma/xray)… what depth of target

deep tagets (used for imaging)

protons treat… what depth of target

deep — minimal entrance/exit dose to healthy

describe coherent scatter

thompson vs rayleigh scatter

what energy does coherent scatter happen

• LOW photon energy !!

relationship btw Z and E in coherent scatter

coherent is more probable in high-Z and low E

describe photoelectric effect

when is photoelectric effect more likely to occur (energy)

<100kev (diagnositc energy range)

mass absorption coeff depends on…

• material type (Z)

  • ↑ Z = ↑ MAC

• photon energy (↑ E = ↓ probability)

• dominant energy range (<= 100kev range)

mass absorption coeff

proability of photoelectric interaction occuring

absorption edges

when incoming photon energy reaches/just exceeds exact BE of specific shell

• before thres = not enough liberate e

• at/after threshold = whole new shell available for e- to absorb photons (∴ ↑ mass absorption coeff)

describe compton scatter

when does compton scatter dominate

100kev-10Mev

3 sscatter scenerios of compton scatter (explain)

backscatter, side scatter, forward scatter of photon

probability of compton scatter

• low Z = ↑ #e/nucleon = ↑ compton (plastic, water, tissue)

• high Z = ↓ #e/nucleon = ↓ compton (bone, metal)

• dominant in highish energy (100kev-10mev)

probability of direction of scatter in compton

• ↑ high energy = ↑ likihood of forward scatter

pair production describe

pair production probability

• ↑ photon energy (>1.022mev) = ↑ likihood of pp

• ↑ High-Z materials = ↑ probability

photonuclear reaction/photodisintegration

extremely high energy photons (>10mev) interact with atomic nucleus and cause nucleus to eject neutrons

• threshold of >10Mev

coulombs law formula

as charged particle move thru material = lose energy via..

1. colliosnal energy loss

2. radiative stopping power

describe S_coll

charge particle loses energy bc it gets transferred to orbiting e- via coulomb

describe S_rad

lose energy bc of acceleration and curve and radiation as bremsstrahlung photons (

impact parameters vs canonical size

impact parameters (b) = distance from charged particle to center of atom

canonical size (a) = radius of atom

• radiative loss (b<a)

• ‘soft’ collision (b>a)

  • too far, too weak to even ionize)

• ‘hard’ collision (b~a)

  • incident particle pass close to e- oribital

  • will transfer large energy that ejects e- from atom w high energy (delta ray)

proportional eqn for radiative stopping power

and ↑ acceleration = ↑ energy = ↑ bremsstranglung = ↑ Srad

proportionality eqn of collisional stopping power

• mainly depends on electron density (#e/gram)

  • ↑ #e/gram = ↑ frequency of coulomb interaction = ↑ energy transfer = ↑ S_collision

• slight Z dependence

  • ↑ Z-material = ↓ #e/gram = ↓ less frequent = ↓ energy transfer = ↓ S_coll

• NOT strong energy dependance (radiiative stopping power only overtakes for very high energy e’s (>10mev) in high Z-matierals

is collisional or radiative energy loss applicable to energy loss in protons/heavy particles

radiative energy loss = NA

key dependencies of stopping power in heavy charged particles

• NOT depend on mass

• DOES depend on speed (energy)

• does depend on Z/A of material

• higher stopping power in proton/heavy than e-s

do photons or electrons deposit dose

only electrons (photons indirectly do..)

CSDA

continuous slowing down approximation (CSDA): avg total path length travelled by charged particle

• CSDA in water (in cm) = E (in Mev)/2 cm

how does energy change neutron behaviour

• low energy (thermal) neutrons

  • energy <0.025eV

  • interact via neutron capture

  • 1) 1H ⟹ 2H + gamma photon + KE

  • 2) 14N ⟹ 14C + free photon

  • 3) 10B ⟹ 7Li + alpha-particle → most effective neutron sheilding*

• high energy (fast) neutrons

  • energy >100ev

  • fast neutrons undergo elastic scatter

LET + unit

linear energy transfer: amt of enegry that ionizing particle transfers to material per unit distance (locally)

• controlls biological damage

• unit = keV/µm

low LET vs high-LET

low-LET: few keV/µm (xray, gamma, electrons)

high-LET (up to >100keV/µm) (p+, heavy ions, C, alpha-particles -

• driven by Z² of collisional stopping power ?? (high z = ↓ #e/gram = ↓ Scoll)

proportionality of LET

charge — 1/velocity — # of LET

which particles have highest RBE as LET

• photons/electrons

• proton

• neutrons fast

• carbon (heavy ion)

• alpha