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
