3.2 - X-ray Production Flashcards
When are x-rays produced?
When fast moving electrons are rapidly decelerated (let out energy and heat)
What are electrons?
Negatively charged particles that sit in shells around positive nucleus and are held in place by electrostatic force
What is the X-ray tube head composed of?
Filament - cathode
transformer
target - anode
target surround
evacuated glass envelope
shielding
filtration
collimator
spacer cone
What is the cathode?
Negative terminal of the tube that has filament which is the source of electrons (cathode) in x-ray tubes.
What is the filament made of?
Small coiled wire made from tungsten
What does the tungsten filament do?
Provide electrons for acceleration to the anode which is the target
What is tungsten
tungsten is an element with a very high melting point so won’t be degraded
How does filament work?
Low voltage current passes through the filament circuit which heats up to emit light and then electrons form a cloud around the filament
What is a transformer?
This is what we use in an x-ray machine - we use two transformers
step up transformer used to supply high voltage to anode
low voltage to supply low voltage to the filament
they allow the voltage of AC to be changed to DC
What is there between cathode and anode?
There is a huge attraction of negative electrons from cathode to positive anode
What keV should x-ray machines operate at?
60-70
normal domestic input is 240eV
What is the target?
This is the anode which is the positive terminal of the X-ray tube
What are the functions of the anode? (3)
provides complete circuit for accelerating electrons
houses the target material
helps to cool the tube as heat is produced
What does -xray production also provide?
Heat - 99% is heat and if ignored it would destroy target material in anode
What is the target made of ?
Tungsten or copper
Why is target made of tungsten or copper?
Good thermal properties to dissipate heat
What happens to incoming electrons from the cathode?
They are deflected by the outer shell electrons in tungsten or they collide with outer shell electrons displacing them which results in a small loss o energy in the form of heat which is removed by a copper block oil then air
What is the target surround made of?
copper - high melting point to get rid of excess head produced as its an effective heat conductor
What are target interactions?
They are a continuous spectrum of interactions that result in x-ray production
What happens when electrons come close to target atom nucleus?
The electron is rapidly decelerated and deflected and the amount of this is proportional to the electron loss
What is electron loss in the form of?
EM radiation as a continuous spectrum of energies
What happens to low energy photons?
useless to us and we need to get rid of them
What do incoming electrons collide with?
Inner shell target electron and when it collides with an electron it will either dispose it to an outer shell or knock electron completely out the atom
When an electron collides with another electron what does it do?
Either displaces it to outer shell
knocks electron out the atom
What happens when an electron has been displaced to another outer shell or has been knocked out the atom?
atom becomes unstable and must rearrange atoms to get back to stability (atoms want right number of electrons in each shell)
What happens when electrons rearrange in atom?
Electrons end up in shells that have different binding energy values and this energy is released and is known as characteristic radiation
What is characteristic radiation?
Characteristic X-rays are emitted when outer-shell electrons fill a vacancy in the inner shell of an atom, releasing X-rays in a pattern that is “characteristic” to each element.
What does the evacuated glass envelope do?
It prevents risk of interaction of electrons with air atoms prior to meeting the target
Why do we have led surrounding the x-ray machine?
To ensure the x-rays are only directed at pt in direction we choose - the led surrounding absorbs any x-rays that try to escape from the tube head any where else
Why is lead good at shielding?
High atomic number
What is filtration?
This is what is used to get rid of low energy photons that we dont want
How is filtration done?
It is done by using a metal with a low atomic number
Why is a metal with low atomic number used for filtration?
As its good at interacting with low energy x-rays and allows high energy x-rays to get through it
What metal is used for filtration?
Aluminium - low atomic number
How thick must the aluminium be if x-ray machine is 70kv and below ?
1.5mm
How thick must the Aluminium be if x-ray machine is >70kv?
2.5mm
What x-ray machine uses higher x-rays?
Panoramic
What is a collimator?
This costs the shape of x-ray beam and depending on how far it is from the pt also controls the size of the beam
What do most machines being by producing?
Circular beams
What beams do we want?
Rectangular
What is the law of maximum beam diameter at pt end of spacer cone?
60mm
how can we reduce dose to pt?
Rectangular collimation - it reduces the area by 30% as it is 40x50mm
What does the spacer cone do?
Helps us line up X-ray tube correctly
What does spacer cone control?
the target –> skin distance
What must the target to skin distance be?
20cm
