PHYSICS 160

EM spectrum

radio

IR

visible

UV

x

γ

particle radiation with

electron

positron

neutron

proton

what radiation is used for an MRI?

radio waves

what radiation is used for diagnostic radiation?

far UV and close x

what radiation is used for therapeutic radiation?

far x

γ rays

ionizing radiation can be

neutron

γ

x

sometimes UV

indirect ionization

an x-ray hits an electron that knocks another electron out of an atom

what is more mutagenic than radiation?

chemicals

ionization potential

minimum energy needed to ionize an atom

Z

atomic number

A

number of nucleons

for a stable isotope

Z = A/[1.98+0.0155*A^(2/3)]

Ar

atomic weight

atomic gram-atom

number of grams in Avagadro's number worth of a substance

N sub am

number of atoms

N sub A

avagadros number

ZN sub am =

Z * N sub A / Ar

Auger Electron

electron ejected w/ energy from another electron transition (to a previously vacated electron spot)

K alpha x-ray

L to K

K beta x-ray

M or N to K

probability of emitting a fluorescent photon

ω

probability of emitting an Auger electron

1 - ω

energy levels

K

L

M

N

Auger electrons are released mostly by

low Z atoms

fluorescent photons are released most by

high Z atoms

Bremsstrahlung comes from

a deflected electron

Bremsstrahlung means

"brake radiation"

what energy can Bremsstrahlung have?

any energy up to the energy of the incoming particle

For Tungsten (W), what percentage of incoming radiation will cause Bremsstrahlung?

1%

high energy electrons usually cause Bremsstrahlung in what direction?

forward

low energy electrons usually cause Bremsstrahlung in what direction?

not forward

"cross-section" refers to

how likely an event is to happen

I(x) =

Io*e^(-μx)

probability of a single interaction

σ/A

σ is target cross-section

number of interactions =

number of photons number of targets (σ/A)

sigma units:

barn (b)

Pauli Exclusion Principle

no two electrons can occupy the same state

stochastic

obeying the rules of chance

x-ray interactions

photoelectric effect

coherent scattering

incoherent scattering

(we only need to know Compton scattering)

pair & triplet production

photoelectric cross-section

τ

coherent scattering cross-section

σ-coh

incoherent scattering cross-section

σ-incoh

pair and triplet production cross-section

κ

How is Compton Scattering different from Rayleigh scattering?

energy is transfered to the electron (it's incoherent)

Compton Scattering takes into account

relativity and quantum mechanics

In Compton scattering, what must be considered a particle?

the photon and electron

Alpha Partical

helium nucleus

typical alpha particle energy

3-10 MeV

alpha particles can travel ________ in air

a few cm

beta particles are

fast electrons or positrons

β- decay happens with

neutron rich

β+ decay happens with

proton rich

β- decay process

n -> p + e- + v'

v' is an antineutrino

β+ decay process

p -> n + e+ + v

v is a neutrino

what energy does a β-particle have?

β-particles have continuous energy because it is shared with the neutrino/antineutrino

neutron rich means

more neutrons than the stable isotope

proton rich means

less neutrons than the stable isotope

most β-decays result in

excited daughter products that emit γ-rays

γ-decay

electrons dropping down (usually after β-decay)

are neutrons generally emitted directly?

no

how do we usually emit neutrons

alpha source and beryllium (Be)

Neutron Howitzer Reaction

Be-9 + He-4 -> n + C-12 + 5.76 MeV

fluence is represented by

Φ

fluence units

m^(-2)

fluence

number of rays passing through per unit area

attenuation coefficient

µ

Beer's Law

Φ = Φo *e^(-µx)

Beer's Law describes only

primary photons (photons that have not interacted yet)

Beer's Law does not describe _________

scatter

attenuation coefficient (µ) is dependent on _______

density

mass attenuation coefficient

µ/ρ

mass attenuation coefficient (µ/ρ) is independent of

density

exposure

ability of a photon to ionize air

(Q/kg)

Kerma (K)

energy transferred from uncharged particles to matter

"Kerma" stands for

kinetic energy released per unit mass

µtr/ρ

mass energy transfer coefficient

Kerma equation

K = Φhv(µtr/ρ)

µen/ρ

mass energy absorption coefficient

why does µen/ρ exist

some transferred energy escapes through radiative processes (mostly Bremsstrahlung)

µen/ρ =

µen/ρ = (µtr/ρ)(1-g)

what is g?

the energy fraction lost to radiative processes

what is g for diagnostic radiology?

g ≅ 0

total attenuation coefficient (µ) =

µ = τ + σ-coh +σ-inc + κ

Energy Fluence (Ψ)

Ψ = EΦ

E: energy of each partical

Energy Fluence (Ψ) units

J/m^2

Stopping Power:

S = dT/dx

Total Stopping Power:

S-tot = S-ion + S-rad

Ionization Stopping Power

S-ion

stopping power due to ionization

Radiative Stopping Power

S-rad

stopping power due to radiation (mostly Bemsstrahlung)

Mass Stopping Power

S/ρ

Mass Stopping power has a tendency to ______________ dose

overestimate

What do radiobiologists use to not overestimate dose?

restricted stopping power (S-res)

kerma is best used for

calibration

absorbed dose is best used for

effects of radiation

pair production

high energy photon passes near to an atomic nucleus it may turn into an electron-positron pair

threshold for pair production

1022 keV

triplet production

high energy photon passes near an orbital electron ejecting the electron and forming an additional electron and positron

annihilation of positrons with electrons does what?

creates more photons

Kerma units

gray (Gy)

is kerma energy spent in the volume it is created?

not necessarily