Radiation Physics Flashcards
What is a photon?
Packet of energy - single particle of light or x-ray
What are x-ray beams made from?
Photons
What is relationship between electrons and photons?
Input of a photon causes an electron to transition to higher energy orbit
Transition of electron to lower energy orbit = loss of energy = photon emitted
What particles are released from radioactive decay?
Alpha and beta partciles
What are alpha particles?
Unstable nucleus which can emit 2 photos and 2 neutrons
The emitted particle identically helium nucleus has charge of +2
Why is alpha particles so ionising?
Relatively high mass and charge
What stops alpha particles?
10mm air or less mm matter
What are beta particles?
Unstable nucleus ejects electron by internal conversion
Emitted particle has charge of -1
What is more ionising alpha or beta?
Beta particles
Stopped by few mm Al
What is a gamma ray?
Excited nucleus can emit gamma ray to return to ground state
Emitted g-ray has no charge
How ionising are gamma rays?
Less ionising but very penetrating
Few mm Pb required
Difference between g-rays and x-rays?
Same - different manner of production
How are x-rays produced?
Accelerating e- towards metal target
Why are x-rays less damaging a/b particles?
No charge and they travel of the steep of light
How are diagnostic X-ray tubes efficient?
Creating x-ray = lot of heat
Anode absorbs electrons and has high melting point
What is Bremsstrahlung - breaking radiation?
Rapid deceleration of e- passing close to target nucleus = photons given off
Creates lots of low energy photons
What is attenuation?
Max photon energy is set by peak tube voltage, kVp
Interaction of orbital electrons = heat
X-ray beams are polyenergetic what does this mean?
Have a spectrum of energies - lower energy components are preferentially absorbed
What is filtration?
Low energy photons no use in imaging as weakly penetrating - but still contribute to patient dose
Remove these low energy photons w/ metal on output ports
What filtration is used?
Legal requirement in all diagnostic X-ray tubes - 1.5mm Al
What is Compton effect?
Scattering of x-ray causing fogging film
How to optimise contrast? - what is disadvantage of this?
Most tissues are of similar dentistry - low kV optimise contrast
Transmission increase w/ Kv = lower pt dose - need to compromise between contrast and dose
How can beam quality be increased?
Increasing kV or using more filtration
What dose doubling mA do?
Doubles dose
Number of photons directly proportional to tube current