Radiation Physics

what is alpha decay?

• the loss of a helium nucleus (2 protons, 2 neutrons)

• occurs in elements with high Z

• example: decay of radium to radon

What is B- decay?

• high neutron to proton ratio

• nuclei wants to reduce this ratio to be stable

• the average energy of B- particles is about 1/3 of the max energy

• ex: Cobalt-60 to Ni-60

What is B+ decay?

• low neutron to proton ratio

• nuclei wants to increase ratio to achieve stability

• ex: PET

Are beta particles emitted with a spectrum of energies?

• yes, ranging from 0 to a maximum

What is electron capture?

• unstable nuclei with neutron deficit captures an orbital electron to make a proton into a neutron

• ex: Na-22: 10% of atoms decay by electron capture

What is internal conversion?

• nucleus has extra energy after nuclear transformation

• transfers excess energy to an orbital electron, which is ejected from the atom

what is isomeric transition?

• nucleus stays excited for a period of time

• daughter nucleus loses energy in the form of gamma rays

• ex: Tc-99m produced by decay of Mo-99

What is photodisintegration?

• interaction of high energy photon with a nucleus

• neutrons are emitted by the nuclei

• ex: Cu bombarded with a photon beam

Half-lives

what factors determine the probability of a photon interacting with matter?

• photon energy

• atomic number and density of the material

what is coherent scattering?

• A photon interacts with the entire atom, causing the atom's electrons to oscillate together. The atom then re-emits a photon in a different direction.

• no energy change

• depends on high atomic number and photons of low energy

What is photoelectric effect?

• responsible for bony contrast in imaging

• photon is absorbed by an inner shell electron

• electron is ejected leaves vacancy → characteristic x-ray or Auger electron

• Probability: Z3/E3

What is the compton effect?

• independent of Z

• incident photon transfers energy to outer shell free electron

• photon loses energy, transfers to electron, changes direction

• useful n therapy, undesirable in diagnostic imaging

• main way MV photons deposit dose

what is pair production?

• photon interacts w/ electromagnetic field of nucleus, give sup all its energy in the process creating a negative electron & positive electron

• energy is at least 1.02 MeV (twice the rest mass of an electron, which is 0.511 MeV)

• probability increases with higher photon energies and high atomic number materials

How do charged particles interact?

• excitation

• ionization

what are the 2 modes of x-ray production?

• bremsstrahlung

• characteristic

what is Bremsstrahlung radiation?

• the result of radiative “collision” between a high-speed electron and a nucleus

• electrons coming toward the nucleus are deflected, losing part of their energy, which is emitted as radiation

in Bremsstrahlung radiation, what determines the direction of emission?

• depends on the energy of the incident electrons

• below 100 keV, x-rays are emitted equally in all directions

• as energy of incident electrons increase, direction of x-ray emission becomes increasingly forward

does Bremsstrahlung produce continuous or discrete energy?

• produces a spectrum of energies (continuous)

How are characteristic x-rays produced?

• when an incoming electron collides with an electron in a shell, knocking it out and leaving a “hole”

• an outer shell electron will fill the hole, with the energy difference released as characteristic radiation

• can eject Auger electron

Are characteristic x-rays discrete or continuous?

• energy is always equal to the difference between electron shell energies

• discrete

what is the critical absorption energy?

• threshold energy that an incident electron must possess to remove an electron from an atom

• for characteristic x-rays

What does each part of the LINAC do?

• magnetron or klystron: generate or amplify microwaves for the accelerator tube

• modulator: generates control pulses to electron gun and magnetron is that timing is maintained between electron pulses and electric accelerating field

• electron gun: source of electrons

what is the journey of a photon through the linac?

• target → primary collimator → flattening filter → ion chamber → secondary collimator → patient

what is the journey of an electron through the linac?

• no target → primary collimator → scattering foil → ion chamber → secondary collimator → accessory mount → electron applicator

What are the major differences between x-rays (photons, gamma rays) and particle beams (electrons, protons, alpha particles)?

• x-rays have no mass, no charge, no finite range, no Bragg peak

what is the difference between attenuation and stopping power?

what is the purpose of flattening filters?

• flatten a beam at 10cm depth

• w/ flattening filter, dose rate is lower, but average energy and penetration are higher

• causes horns in dose

What are examples of treatment machines historically used?

• Cobalt-60 unit: like a 4 MV linac, depth of dmax is 0.5cm, treated at 80cm SSD

what is the purpose of the ion chamber?

to monitor beam symmetry, NOT beam flatness