Interactions of Charged Particles Flashcards
(18 cards)
What is collision Stopping Power?
S = -dE/dx
Average energy loss (dE) per unit distance (dx) along particle track
Mass collision stopping power
Very frequent but small magnitude
S(Z, A, I, v, Ze)
Decrease v, increase S - slow electrons spend more time close to the atom and interact
S increases with Z^2
What is total linear stopping power?
Sum of collisional and radiative losses.
S=-dE/dx = -[(dE/dx)col + (dE/dx)rad]
Linear: MeV/cm
Mass: MeVcm^2/g
What are radiative losses?
Bremsstrahlung
Electrons interact with the electric field of the atom, slowing down and changing direction and decelerate, generating a photon
As energy and Z increase, probability of radiative losses increase
Srad/density proportional to Z^2/A E B(E,Z)
What are collision losses?
Charged particles (M) transfer energy to electrons (m) via the Coulomb force
Both nuclei and electrons feel the Coulomb force due to the transient field but impulse exerte as the electron passes transfers momentum
M»_space; m
Energy transfer to electron is small
The more massive the charged particle, the less it is deflected
When colliding with atomic electrons the atomic lectons can either be excited of ionised whihc in turn collide with other atomic electrons - trail of excitation along ~straight path
delta E = E(4m/M)
Many collisions before the electron stops
What is relativistic rise?
Increase in stopping power (S) at relativistic energies due to the electric field of the moving particle undergoing Lorentz contraction
Field is weakened in the forward/backward direction
Field is strengthened in the transverse direction
Moving particle acts on the bound atomic electron for a shorter time, therefore the impact parameter (b) increases and S increases
What are the three mechanisms through which electrons interact with material?
Collision (inelastic scatter) with bound atomic electrons
Radiative (Bremsstrahlung)
Scatter (elastic scatter)
Explain tracks of alphas in a cloud chamber
Short, fat, straight
Alphas are heavy, +2 charge
They are slow and highly ionising
Explain the tracks of beta particles in a cloud chamber
Long, thin, curved
Betas are light, singly charged => more easily deflected and ‘bounce’ off atoms
They are faster and less ionising and hence travel further
What is meant by particle range?
The distance traversed by a particle before it comes to rest in the stopping material
Proportional to M/Z^2
For alphas: a few micrometers in tissue
What is the Bragg curve and the mechanisms leading to it’s shape?
A plot of the specific energy lost along the track of a charged particle
Energy of the initial particle determines how deep the particles will penetrate
At the start - speed is maximum
As particles penetrate, speed decreases as the particles lose energy
As they slow down, interaction with surrounding material increases (S prop 1/v^2)
Leads to a sharp peak where charged particles deposit most of their energy followed by a sharp drop off due to electron pickup (Z changes) as the particles slow down
What is the differences between proton and carbon ion therapy?
Carbon is heavier and more charged therefore higher stopping power, more ionisation, quicker deposition, sharper peak, more localised
What is a soft collision?
Electron collides at a large distance, much greater than the radius of the atom (b»R). The Coulomb force field affects the atom as a whole. The electron is either excited or ionised. It is the most common type of collision, with only a small amount of energy transfer.
What is a hard collision?
b~R - the particle collides at a distance of the order of the atomic radius. More energy is transferred and electrons are ejected (potentially creating delta-rays). These are rarer than soft collisions but more energy is transferred.
What is the impact parameter, b?
Distance of closest approach.
What properties of an ion beam will affect how it losses energy to its surroundings?
Charge
Mass/size
What is a delta ray?
Energy transfer to atomic electron is sufficient to ionise and cause futher ionisations in other atoms
What is the relationship of stopping power to energy and Z?
prop to Z^2 and prop to 1/E
What is elastic scattering?
charged particle scattered by localised force field when particle passes close to atomic nucleus (b <atomic radius) - electrons change direction but no loss of energy
