1.1/1.2 ionizing radiation Flashcards
what are the classifications of radiation?
- electromagnetic radiation
- particulate radiation
- ionizing and non-ionizing radiation
electromagnetic waves, like all waves, can be characterized by their
- amplitude
- wavelength (λ)
- frequency (f)
- speed (v)
how is speed of a wave calculated?
c = λ x f
c = wavelength x frequency
what are photons commonly referred to as?
γ-rays
what unit is commonly used to denote photon energy?
the electron-volt (eV)
1 Joule is equal to
6.24 x 10^18 eV
in diagnostic radiology, the only particulate radiation that needs to be considered is the
electron
an electron has a rest mass of _____ and a rest energy of _____
9.1 x 10^-31
511keV
two ways radiation is classified
- ionizing - can ionize matter
- non-ionizing - cannot ionize matter
what frequency of electromagnetic radiation makes it ionizing?
frequency higher than the near-ultraviolet region of the electromagnetic spectrum
what types of electromagnetic radiation is non-ionizing?
visible light, infrared, radiofrequency
what are 3 ways photons can be absorbed?
- photoelectric absorption
- compton scattering
- pair production
describe photoelectric absorption
- a photon can be absorbed by transferring all of its energy to an inner orbital electron in an atom of the absorber
- the electron is ejected from the atom and the photon disappears as it has lost all of its energy and never had mass
- the atom is left with a vacant inner electron orbit which it will fill with one of the outer electrons
- when it does this it releases a small amount of energy in the form of a characteristic X-ray photon
why is the X-ray called a characteristic photon?
because its energy is characteristic of the absorbing material
why does an X-ray photon have fixed energy?
because orbital electron have fixed energies which correspond to the orbit which they occupy
in what case is photoelectric absorption the most likely form of absorption?
when the incident photon has a relatively low energy
(energies below 25 eV)
the odds of a proton being absorbed by photoelectric absorption depend on what factors?
- the energy of a photon
- the chemical elements in the absorbing material
- the number of protons in the atom (atomic number)
what is the relationship between the likelihood of photoelectric absorption and the atomic number of atoms?
the likelihood of photoelectric absorption increases as the cube of the atomic number, Z^3
(photoelectric absorption is more likely to occur in materials that have many atoms with relatively large values of Z because of this relationship)
an example of a metal that is a good absorber of X-ray photons
lead (Z = 82)
describe the attenuation of an X-ray beam in human tissues
- air: negligible
- bone: significant due to relatively high density (atom mass of Ca)
- soft tissue (ex: muscle) : similar to water
- fat tissue: less important than water
- lungs: weak due to density
what does a higher kVp value result in?
it will make the X-ray beam more penetrating, cause less difference in attenuation, and lowers contrast
(of x ray image)
how can bone and lung structures be simultaneously visualized?
higher kVp reduces photoelectric effect
what is the Compton Effect?
- an interaction with the OUTER electrons which are not tightly bound to an atom
- the photon collides with an electron and gives some of its energy to it
what are the outcomes of the different types of collisions of photons in the Compton Effect?
- if collision is head on - the photon has its direction of travel reversed and so it loses the maximum amount of energy
- if collision is only a glancing one - the energy given to the recoil electron will be much less
- if a single photon undergoes several collisions - it loses some energy each occasion and eventually is absorbed by the photoelectric effect
what does the actual loss of energy as a result of the Compton Effect depend on?
the angle through which the photon is scattered
the Compton Effect is the dominant effect for which photon energies?
above 200 keV and up to 2 MeV
between what energies can both the photoelectric and Compton Effect occur?
60 keV and 90 keV
explain why only the thickness of an absorber and its density are important for photon absorption at higher energies by the Compton Effect?
- photons in the Compton effect interact with electrons as though they were not bound to an atom
- meaning only the total number of electrons in a block of material matters, regardless of arrangement (thickness)
- the Compton Effect distinguishes between materials through their different densities
why is the Compton Effect relatively insensitive to variations in anatomy compared to the photoelectric effect?
because most soft tissues have very similar densities, which makes the Compton Effect invalid in this case