X-rays

what are x-rays?

• electromagnetic waves

• can be polarised and diffracted

• have low wavelength (10^-11) and high frequency

• their photons have 10-10000 times more energy than visible light

• ionising radiation (can harm and kill living cells)

how are x-rays produced (part 1)

• in an evacuated tube

• a low voltage supply heats up the metal cathode, causing it to produce electrons by thermionic emission

• these electrons are accelerated towards the anode through a high p.d (30-100kV)

• the anode is made from a target metal (eg tungsten) with a high melting point

how are x-rays produced? (part 2)

• when the electrons hit the anode, they decelerate and x-ray photons are produced

• less than 1% of electron energy transferred to photons (the rest is transformed to thermal energy)

• water or oil is used to cool the anode, or it is rotated

• the x rays are emitted in the desired direction using a window

• the tube is lead-lined to shield from x-rays in other directions

how do you find the wavelength of the x-ray photon from just the p.d?

• due to conservation of energy, the maximum energy of photon=maximum kinetic energy of one electron

• therefore hf=QV

• hc/lamda=eV

• lamda=hc/eV

• wavelength is inversely proportional to accelerating p.d

what are the spikes in intensity on an x-ray spectrum?

• when a bombarding electron removes an electron that was inside an atom, a gap is created

• higher energy electrons drop down to that electron’s level to fill the gaps

• this releases a photon of a specific energy and wavelength

• the increase in energy causes a spike in intensity

what does an x-ray spectrum look like?

• the range of decelerations of the electrons produces the broad background of bremsstrahlung (braking radiation)

what is attenuation?

• the decrease in the intensity of electromagnetic radiation as it passes through matter

• as x-rays travel through flesh, some of their photons are scattered or absorbed so the intensity decreases

what is simple scatter?

• happens for photons with energy of 1-20keV

• photon interacts with electron in atom

• photon doesn’t have enough energy to remove the electron, so it just bounces off (is scattered)

• photon’s direction changes but energy doesn’t

• insignificant because X-ray machines have p.ds greater than 20kV

what is the photoelectric effect?

• photon energy < 100keV

• photon is completely absorbed

• electron uses energy to escape atom so electron emitted

• main mechanism for soft (lower energy) X-ray attenuation

what is compton scattering?

• photon energy is 0.5-5.0MeV

• photon interacts with electron inside atom

• electron is emitted

• photon is scattered with reduced energy

• used in hard X-rays to kill cancerous cells?

what is pair production?

• photon energy>1.02MeV

• x-ray photon interacts with nucleus

• photon disappears

• electron and positron pair produced

what is the equation for final intensity of X-ray?

• I = I₀e^{-mu * x}

• where x=thickness of material, I₀ is initial intensity and mu is the attenuation coefficient (like how much the material absorbs x rays)

what and why are contrast media?

• they are materials with high absorption coefficients which are injected or consumed by the person getting x-rayed

• as soft tissues have low absorption coefficients, a constrast medium improves the visibility of their internal structures by contrasting with the soft tissues

• attenuation coefficient is proportional to atomic number cubed (Z³)

• main two= barium and iodine (high atomic numbers)

• barium is swallowed to do digestive system stuff

• iodine is injected for blood vessels and stuff

how are x-rays used in therapy?

• high energy x-ray photons kill off cancerous cells

• via compton scattering and pair production