Production and properties of X rays Flashcards
atomic mass unit
1/12 mass of carbon atom
mass of proton, neutron and electron
proton and neutron= 1amu
electron= 1/1840
structure of an atom
nucleus of protons and neutrons
electrons in orbit around nucleus
what is Z
proton number
and electron number
what is A
atomic number. number of protons and neutrons in nucleus
define excitation
moving electron inner ā> outer shell to excite the atom (move to higher energy level)
define ionisation
complete removal (or addition) of an electron from an atom, causing it to gain or lose charge
what is binding energy
attraction between -ve e- and +ve protons (nucleus)
where is binding energy greatest
K shell (greater for electrons closer to nucleus)
what is a photon
packet of X ray energy, equivalent to a quantum of energy
what kind of radiation is x rays
electromagnetic radiation, ionising
high frequency, short wavlength
what is the biggest source of ionising radiation
radon (gas released from soil/rocks)
what % of ionising radiation is artificial
13%
name the other type of ionising radiation
gamma rays
wavelength and freq of x rays/ gamma rays
short wavelength
high freq
how fast do x rays travel in a vacuum
speed of light
range of x ray wavelengths
0.01-0.05nm
x rays obey the inverse square law. what is this
intensity of radiation reduces to 1/4 when distance from source is doubled
why must x ray production be in a vacuum
otherwise atoms of air would be in the way of electron movement
3 steps to produce x rays
- produce lots of electrons
- accelerate them to very high energy
- smash them in to target
main components of tube head and their function
- glass x ray tube (filament, copper block, target)
- step up transformer (high voltage to accelerate electrons)
- step down transformer (low voltage to heat filament
- lead casing (to stop x rays escaping)
- oil (conduct heat away from insert)
- aluminium filtration (removes unwanted low energy electrons)
- collimator
- beam indicating device
what direction do electrons flow?
negative (cathode) to positive (anode)
how are electrons produced
thermionic emission from heating the filament at the cathode
what is tube current?
the amount of electron flow
how to increase tube current
increase heating of filament
how are electrons accelerated to anode
high potential difference
what happens when the electrons reach the anode
hit a small target area, stop moving (lose energy)
energy lost as heat (99%) or x rays (1%)
how is the heat removed
copper block and surrounding oil
why is the tungsten used for the target
- high melting point (not affected by heat from electrons)
- high atomic number (74) so lots of interaction for electrons
what happens to produce a) heat-producing reaction
b) x ray producing reaction
a) heat-producing reaction: incoming electron hits an outer-shell tungsten electron
b) x ray producing reaction: incoming electron avoids outer shell tungsten electrons and comes very close to tungsten nucleus. the electron-nucleus attraction causes the electrons to lose energy as x rays. the further round the nucleus the electrons are deflected, the higher energy the photons
tube current value for dental radiographs
7-10mA
kV value for dental radiographs
60-70kVp
why is high kV used?
less dose to patient because electrons more accelerated, more likely to reach nucleus
function of kV
- determines quality of X ray beam ie energy of photons
- determines penetrating power of photons
- affects film contrast
function mA and time
determine quantity of x ray photon
affects degree of blackening of film (optical density)
name 2 methods of x ray production
Bremsstrahlung radiation
characteristic radiation
describe Bremsstrahlung radiation
incoming electron penetrates outer electron shells and passes close to nucleus of tungsten atom
this is slowed down and reflected by nucleus with large loss of energy emitted in the form of x rays
what is maximum potential energy
enough energy given to moving electron so that it fully stops at the target nucleus
what is filtration
removal of low-energy photons produced when bombarding electrons are only partly deflected. they have little penetrating power so are not useful
describe and explain spectrum produced by Bremsstrahlung radiation
continuous. incoming electrons slowed down/ deflected to varying degrees
describe characteristic radiation
incoming electron collides with inner shell tungsten electron, displacing it to outer shell (excitation) or displacing it from atom (ionisation). an outside electron jumps down to fill gap. this decrease in energy causes x ray production
describe pattern of x ray production in characteristic radiation
always the same amount of x ray produced (hence characteristic) because same amount of energy lost when electrons move rings Lā>K on the tungsten atom
what electrons are useful for diagnosis and why
K shell elctrons because they have the highest binding energy
what kV is needed to displace K shell electron to produce x ray?
69.5kV
what is
k alpha
k beta
k alpha: L to K electron movement
k beta: M to K electron movement
what type of radiography uses filtration?
both
