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
is contrast more affected by chemical composition or differences in density? explain
since the photoelectric absorption effect plays the dominant role, chemical composition affects contrast in imaging
what are some differences between the photoelectric effect and the Compton effect?
- the photoelectric effect is a low-energy phenomena, and the photons that interact with electrons vanish as soon as they give their energy to them
- the Compton effect, on the other hand, is a mid-energy phenomenon in which photons contact electrons and are scattered
- photoelectric is mainly responsible for image contrast, Compton contributes to artifacts in the images
what is the pair production effect?
another method of absorption, less important than the Compton and photoelectric effects, that only applies to very high-energy photons
what happens during pair production?
if the photon has sufficient energy, it can be absorbed by an atomic nucleus in the absorber, which results in the production of a positron and an electron
how much energy is needed to produce the pair of particles (positron and electron) in pair production? what happens if energy is higher than needed?
- 1.02 MeV
- (if photon has more than 1.02 MeV, then excess simply increases the velocity of the electron and positron)
what happens to the positron produced from pair production?
it will not live very long because if it meets an electron it will combine with it to produce two photons of 0.51 MeV
what are photons produced from the combining of a positron and electron known as?
annihilation radiation
pair production is the dominant effect for which photon energies?
energies above 5 MeV
what photon energies are each effect best suited for?
photoelectric effect - low energies
Compton Effect - mid energies
pair production - very high energies
which effect is a case of energy being converted into mass?
pair production effect
what is another way to reduce the intensity of radiation?
moving away from the source (point source) decreases the intensity of radiation
what happens if the radiation from a source can spread in all directions?
its intensity will fall off in inverse proportion to the distance squared
since ionizing radiation cannot be detected directly, how else can we detect it?
we rely on the radiation interacting with another material and producing an effect in which we can detect
who is the father of radiology and first discovered X-rays?
Wilhelm Conrad Röntgen
who first detected radiation and discovered radioactivity?
Henri Becquerel
what are the two methods used by Henri Becquerel to detect ionizing radiation?
film & electroscope
if a person is being exposed to radiation, why do we need to know the received dose rate ASAP?
- so that we can calculate the accumulated dose
- give a warning if the dose rate is very high
what is a dosimeter?
- an instrument that measures ionizing radiation
- comprises an electrometer (a measuring assembly) and one or more detector assemblies
how is monitoring equipment calibrated?
in terms of mrad/h or μGy/h
how are low levels of dose rate measured?
- using Geiger-Muller (G-M) tubes
- scintillation counters
what are other ways dose rate can be measured? what advantages do they hold?
- using an ionization chamber
advantages:
- more accurate
- less affected by radiation energy
- can measure high dose rates which would saturate other monitors
what are some instruments that measure dose/dose rate and can be worn on the body?
- pocket dosimeters
- G-M tubes with a dose rate alarm
- solid-state detector and scintillation counter systems (available but expensive and not suitable to routine dose measurements)
what is TLD?
- thermoluminescent dosimetry
- used by the cheapest and most common personal monitors
what are the advantages of film dosimeters?
a film badge as a personal monitoring device:
- is very simple, therefore not expensive
- provides a permanent record
- is very reliable
- is used to measure & record radiation exposure due to gamma rays, x-rays, and beta particles
what are the disadvantages of film dosimeters?
- usually cannot be read on site (need to be sent away for developing)
- for one-time use only
- less than 0.2mSv exposures of gamma radiation cannot be accurately measured
what are the advantages of TLD dosimeters?
- able to measure a greater range of doses in comparison with film badges
- doses from TLDs may be easily obtained
- can be read on site instead of being sent away
- easily reusable
what are the disadvantages of TLD dosimeters?
- each dose cannot be read out more than once
- the readout process effectively “zeroes” the TLD
what are electronic personal dosimeters?
- high-range, alarming, active dosimeters
- designed to be worn by occupational radiation workers in planned exposure situations
- to measure personal dose equivalence for regulatory compliance
- displays dose AND dose rate
- high level of radiation sensitivity
what are some qualities of electronic dosimeters?
- display dose AND dose rate
- sensitive to high levels of radiation
what is an electric current?
a flow of electrons or ions
how can an electric current flow in the air?
if some of the atoms in the air are ionized, then free electrons are produced and an electric current can flow
how does lightning occur?
the very high potential gradient between the cloud and the ground is sufficient to ionize the air and allow current to flow
what happens in an ionizing chamber? what allows current to flow through the chamber?
- when the chamber is exposed to ionizing radiation: (1) positive and negative ions are produced (2) electrons in the air are freed
- the electrons fill the chamber and allow a current to flow
- a potential is applied across metal plates
- the positively charged ions are attracted to the negative plate
- the negatively charged ions are attracted to the positive plate
- this allows current to flow through the chamber
what is the current measured by? why is that needed?
- a sensitive ammeter
- because the currents to be measured are often of the order of 10^-9 A
- which is equal to 6 x 10^9 electrons per second (quite difficult to measure)
what are the uses of an ionization chamber?
- used to measure the ionizing radiation output of therapeutic and diagnostic ionizing radiation generators
- used to make accurate measurements of patient radiation dose
what is a G-M tube?
- a very sensitive form of ionization chamber (so sensitive it can detect single ionizing particles which enter the tube)
- differs from an ionization chamber as it is filled with a gas (such as argon or neon) rather than air
what is the pressure of the gas inside a G-M tube?
about one-fifth of atmospheric pressure
what happens in a G-M tube?
- incident ionizing radiation will produce free electrons within the tube
- these electrons will be attracted towards the central electrode (held at a positive potential)
- the electrons are accelerated by the potential
- this allows them to gain sufficient energy to cause further ionization, causing a chain reaction
- when all hit the central anode, they can cause photons to be emitted
- these photons can cause more ionization in the gas of the chamber
what is the result of G-M tubes? how is it measured?
- ionization in the gas of the chamber results in the original incident ionizing radiation to produce about 10^5 electrons in the chamber
- it is measured as a pulse of current lasting about 1 μs
how is the G-M tube operation much simpler than an ionization chamber?
the G-M tube operation does not deal with the movement of the positive ions in the tube (since they travel much slower than electrons)
what is the “dead time” of the G-M tubes? why is it important?
- the time it takes the tube to recover from the recorded pulse
- it is important as it limits the number of events which can be recorded each second
which instrument that measures radiation can measure ALL types of radiation (gamma, beta, alpha)?
G-M tubes
the use of G-M tubes is not recommended in which case?
for diagnostic radiology
what are two main difficulties in diagnostic radiology for the use of G-M tubes/counters?
- response time of several seconds
- a strong energy dependence at low photon energies
