How are x-rays produced? Flashcards
What part of an x-ray machine produces the x-ray beam?
The glass xray tube
What is the purpose of the step-up transformer?
Step-up (increase) mains voltage of 240 volts to 60-70 kV (60,000-70,000 volts)
What is the purpose of the spacer cone?
Indicate direction of beam and set ideal distance from pt’s skin
What is the purpose of the surrounding oil?
To facilitate heat removal
What letters represent electron shells?
K, L, M, N, O
What is the atomic number?
Number of protons in the nucleus = greater density = bigger atom
What is the filament made out of?
Tungsten wire
What is the x-ray target made out of?
Tungsten
What happens in the x-ray tube when the exposure button is pressed?
An electric current is passed through the cathode and warms up the tungsten filament at the end of the cathode.
This then causes the release of electrons from the filament which then travels to the target.
For the electrons to be able to travel through the gap to the target it requires the 60-70 kV voltage so it has energy to reach target. Lots of heat is produced (& travels down copper anode) as well as x-rays.
Why is there leakage?
Because some photons can stray from target
Is the x-ray process considered efficient?
No, because only 1% produces x-rays, the rest is heat
How does the electron and tungsten produce x-rays?
- A negatively charged electron is accelerated towards the atoms of tungsten
- The e- passes near to the positively charged nucleus, and it is deflected
- This causes the electron to slow down enough to lose some of it’s energy (the more it slows = the more energy lost = the bigger the photon)
- This energy is then emitted as an x-ray
What is said to form by the electron-nucleus interaction?
Emitted x-rays produce a continuous spectrum
Which type of photons are more dangerous?
Low energy
Which type of photons are more diagnostically useful?
The high energy photons
What is photon energy mainly determined by?
It is determined by the energy of the incoming electron
What determines the energy of the incoming electron?
The size of the voltage applied (kV)
What is E.max?
It is the highest possible energy a photon can have
Why is a continuous spectrum produced?
Due to the incoming electrons slowing down and deflected to varying degrees
What happens to the continuous spectrum and Emax if you increase the votlage?
The Emax increases and more photons are produced in the continuous spectrum
What is another way x-rays can be produced with the tungsten atom?
The incoming electron can instead collide with an orbiting electron.
This causes it to displace.
The incoming electron is then deflected.
This destabilised the atom due to the electron vacancy. So an electron from adjacent shell/energy level drops to the inner shell and so on.
This movement of electrons causes energy to be released in the form of x-ray photons.
Eventually free electron is captured to stabilise atom
What spectrum is produced when electrons are rearranged resulting in the emission of x-rays?
Characteristic spectrum
(e.g. characteristic of tungsten)
The photons produced are only of specific size or energy
Why does the x-ray equipment have to operate at 70 kV minimum with tungsten?
Because any lower voltage would not have enough energy to displace the inner electrons of the tungsten atom
How are the lowest energy photons removed from the beam to prevent hitting the patient?
The beam is passed through a sheet of aluminium which absorbs and filters out lowest energy photons.
