x ray production

how are x rays produced?

1. electrons are emitted from a filament by thermionic emission in an evacuated tube (lined with lead) and are accelerated through a high p.d towards the tungsten anode

2. once they collide with the tungsten anode, they will decelerate and emit part of their energy as x ray photons

3. some electrons collide with the orbital tungsten electrons and ionise the atoms causing higher shell electrons to move down and occupy gaps left by the ionised lectrons which releases energy in the form of x rays

what is braking radiation (bremstrahlung)? this is the continuous spectrum of radiation caused by the deceleration of electrons that have collided with anode

what is the characteristic spectrum? this is the energy of the x ray photons released by the orbital tungsten electrons moving down to lower energy levels. this only occurs at specific energy levels and they are represented by lines on a graph of intensity against energy of the x ray photons

why is the tube lined with lead? this is to block any x rays that may escape by absorbing them

why does the anode rotate? to avoid the target area from overheating

what is thermionic emission? a process where a metal is heated until the free electrons on its surface gain enough energy and are emitted

what percentage of the kinetic energy of the electrons is converted into x ray photons and where does most of the energy go?

• 1%

• most of the energy goes to the internal energy of the anode which increases its temperature

what properties should the material of the anode have (3)?

• good thermal conductor

• high specific heat capacity

• high melting point

why is tungsten used for the anode (4)?

• it is a good thermal conductor

• has a high shc

• high melting point

• has a large nuclear mass leading to an increased probability of x ray emission when an electron collides with it

how can overheating be prevented (2)?

• the anode is rotated at high speeds (3000rpm) so that the electron beam heats different areas of the target

• the anode has a bevelled edge which allows electrons to be focused on a large area while the source of the x rays remains small, which produces a sharper image

how can the intensity of the x rays be controlled (2)?

• the anode voltage can be increased which increases beam intensity so the electrons gain more kinetic energy therefore the x ray photons have high energies. the higher energy electrons can ionise electrons deep within the tungsten atoms too

• increasing the current in the filament causes more electrons to be released per second so more x rays are produced per second- the range of energy stays the same but there are just more x ray photons

how can the sharpness of an image be increased (3)?

• putting the detection plate as close as possible to the patient while moving the x ray source far away

• making sure the patient holds very still

• using a lead grid between the patient and film to stop scattered x rays from reducing the contrast of the final image