Radiation - physics Flashcards
Photons
Photon is a ‘packet’ of energy, can be thought of as single particle of light or x-ray
An x-ray beam is made up of a great many photons
Atomic physics
-vely charged electrons orbit a +vely charged nucleus with each orbit having a fixed energy energy level
An electron in an orbit has an energy equal to that energy level
Energies dependent on particular atom
Transition of an electron to higher energy orbit will require input of energy, in this case a photon
Transition of electron to a lower energy orbit will cause a loss of energy, in this case a photon will be emitted
If an electron in orbit 1 (so has E=10), and a photon with at least E=20 hits it
It can go up to orbit 2 (as E=30)
If space becomes available in orbit 1
An electron from orbit 2 will drop into it and give off a photo of E=2-
If we give an electron enough energy
We can completley free it from pull of nucleus and we have a +ve ion (the atom minus 1 electron) and one free electron
This process is ionisation
What are first and second shells called
K-shell and L-shell
Electromagnetic radiation includes
Light, UV, microwave, radio waves, X-rays and gamma rays
Electromagnetic radiation
Not charged Beams consist of many single photons Travels at speed of light Energy determines frequency and wavelength; E=hv h= Planck constant v=frequency
Particle radiation includes
Alpha and beta (electrons) particles from radioactive decay
Particle radiation
Many particles are charged
Beams consist of many individual particles
Speed and energy are dependent on initial emission conditions
Why can electromagnetic radiation be bad
Free radicals produced
Alpha particles
Unstable nucleus can emit 2 neutrons and 2 protons to reduce proton number
Emitted particle identical to H nucleus, having charge of +2
Relatively high mass and charge make alpha particle highly ionising
Alpha particle is stopped by ~10mm air or less than a mm of matter
Beta particles
An electron. Name merely designates its origin
Unstable nucleus ejects an electron by internal conversion or when a neutron converts into a proton and an electron (n->p+e- +y)
Emitted beta particle has charge of -1
Also highly ionising
More penetrating than alpha particle but still stopped by few mm of aluminium
As frequency increases
Energy of individual photon gets higher
Wavelength get smaller
X-rays and gamma rays
Exactly the same, just denotes where they have come from
Gamma rays
Excited nucleus may emit y-ray in order to return to its ground state
Alpha and beta emitters also emit y-rays because the daughter (Y) nuclei produced are often in excited states as result of their decay
Emitted y-ray has no charge
y-ray less ionising than particulate radiations
y-ray can be very penetrating, tens of mm Pb may be required to reduce their intensity appreciably
y-rays identical to x-rays, except for manner of production
X-rays discovered in
Nov 1985 by Roentgen, medical X-ray departments running by 1896
Production of x-rays
produced by accelerating electrons towards metal target, in high-voltage, evacualted tubes
Two distinct processes by which this creates x-rays which in turn shapes the spectrum of photons emitted
1) Bremsstrahlung
2) Characteristic x-ray formation
Diagnostic X-ray tube
Needs to produce x-rays as efficiently as possible
Creating high x-ray intensities generates a lot of heat
Anode needs high Z to absorb electrons and high melting point of 3410degrees C and atomic number of 74
Glass envelope makes it vacuum
Copper block takes away heat
Lead shielding around whole tube to stop x-rays going all directions
Sloped target to direct electrons towards thin window
Cathode on filament side, anode on copper block side (Look at lecture)
Bremsstrahlung
Braking radiation
Continuous spectrum produced by rapid deceleration of electrons passing close to target nuclei (look at slides)
Characteristic X-ray formation
An electron is knocked out of K shell, leaving gap
L shell electron falls into gap and emits energy as x-ray photon
Energy of photon emitted = (energy of electron in L shell) - (energy electron now has in K shell)
Which is determined by material ionised, hence ‘characteristic’
(look at slides)
Difference between gamma and x-rays on graph
Line spectra for gamma, all same energies
Continuous spectra for x-rays, photons of different energies produced
Properties of alpha particles
Fluorescence on a phosphor Has an effect on photographic plate/ film 10^4 number ion pairs/mm in air Stopped by up to 10^-2 mm Al Stopped by up to 10mm air Deflection by E and B as +ve charge 10^7 m s^-1
Properties of beta particles
Less fluorescence on a phosphor than alpha
Has an effect on photographic plate/ film
10^2 number ion pairs/mm in air
Stopped by up to mm Al
Stopped by up to m air
Deflection by E and B as -ve charge
10^8 m s^-1 but variable
Properties of gamma particles
Very little fluorescence on a phosphor Has an effect on photographic plate/ film 1 number ion pairs/mm in air Stopped by up to 100 mm Pb No deflection by E and B 3 x 10^8 m s^-1
Properties of x-ray particles
Fluorescence on a phosphor Has an effect on photographic plate/ film <1 number ion pairs/mm in air Stopped by up to 1-2 mm Pb No deflection by E and B 3 x 10^8 m s^-1
Tube current mA
intensity I∞mA
Quality unaffected
Shape of spectrum unaltered
Attenuation (removing energy from beam)
For a monoenergetic beam of x-ray photons with intensity I0
μ = Linear Attenuation Coefficient (Higher for lower energy photons. More easily
attenuated)
t is usually measured in kg/m2
μ/ρ the Mass Attenuation Coefficient is measured in m2kg-1
X-rays beams are polyenergetic i.e. have a spectrum of energies. Therefore, the lower energy component is preferentially absorbed.
Attenuation (removing energy from beam)
I (intensity) = I0 (initial intensity) x e^μt (thickness)
For a monoenergetic beam of x-ray photons with intensity I0
μ = Linear Attenuation Coefficient (Higher for lower energy photons. More easily
attenuated)
t is usually measured in kg/m2
μ/ρ the Mass Attenuation Coefficient is measured in m2kg-1
X-rays beams are polyenergetic i.e. have a spectrum of energies. Therefore, the lower energy component is preferentially absorbed.
Tube voltage kVp
Intensity I∞kVp^2
Quality increases with kVp
Tube voltage kVp
Intensity I∞kVp^2
Quality increases with kVp
Compromise between contrast and dose
Most tissues are of similar density and Z; therefore need to choose low kV to optimise contrast
But transmission increases with kV (< px dose)
So, we need to compromise between contrast and dose
Highest absorption at low energies
Transmission increases with kV
Lower energies give greatest relative difference in attenuation between different materials
Effect of distance
Inverse square law
Absorption in tissue - photoelectric effect
Photon interacts with tightly bound ineer electron (80% K, 20% L)
The photons energy is absorbed by electron. If energy absorbed exceeds
electron binding energy electron emitted from atom, known as Photoelectron
The gap in shell filled by electron from higher shell and energy released is emitted as Characteristic Radiation
Absorption in tissue - Compton scatter
If binding energy is small it can be ignored (BE«_space;incident photon E. i.e. outer shell electrons)
Therefore probability of interaction remains same whatever nuclear size
As binding energy increases interaction is with whole atom The nucleus is massive wrt the electron and absorbs impact
No energy lost
Wavelength remains same
Target atomic number
Intensity I∞Z (Z is number of protons)
Quality unaffected
Rectification
Intensity reduced by ripple
Quality reduced by ripple
Constant potential gives maximum intensity and quality
Filtration
Intensity decreases with filtration
Quality increases with filtration
Filtration removes low energy radiation
Distance from focus
Intensity I∞ r^-2
Quality unaffected
Known as inverse square law
How far alpha particles into tissue
Stopped by air or dead layer of skin
How far beta particles into tissue
Go into tissue, scatter at some point
May be harmful, may not be
How far gamma or x-rays into tissue
Some will scatter in tissue
Some will be absorbed
Some will go all the way through
We need this ratio to detect an image
