Interactions of Charged Particles Flashcards
(52 cards)
What are the two types of charged particle radiation?
1) Fast electrons
2) Heavy charged particles
What are fast electrons?
Beta particles emitted in nuclear decay
What are heavy charged particles?
All energetic ions with a mass greater than 1 amu. This includes alpha particles, protons, and fission objects.
What is the cause of biological effects in medical physics?
Charged particle radiation
Charged particles are termed _________ radiation.
Ionising
How can heavy charged particles lose energy?
- Collision
- Elastic scattering
How can electrons lose energy?
- Collisions with bound atomic electrons (inelastic scattering)
- Bremsstrahlung (radiative interactions)
- Elastic scattering
Charged particles mainly transfer energy to ________ via the ________ force when they collide.
Electrons
Coulomb
Why does it take several collisions for heavy charged particles to stop?
Their mass is much greater than that of an electron, meaning that very little energy is transferred per collision.
Why do heavy charged particles travel in an approximately straight path despite colliding with electrons?
Because they are relatively massive, meaning that they aren’t deflected much by the collisions.
When do collisions cause ionisation?
If an atomic electron is given enough energy to leave the atom.
When do collisions cause excitation?
If an atomic electron is given enough energy to move to an excited state, but not enough for ionisation.
What is a delta ray?
An ionised electron with enough energy to cause secondary ionisations.
Give the equation for the Q value of energy transfer from a charged particle to an electron
Q = Q value
k = constant
z = primary particle charge
e = electron charge
m = electron mass
b = impact parameter
v = primary particle velocity
What are the type types of collision for fast charged particles?
- Hard (impact parameter ~ atomic radius)
- Soft (impact parameter»_space; atomic radius)
What is a hard collision?
A collision where there is a small impact parameter, causing electrons to be ejected as delta rays. They are more rare than soft collision because the nucleus is small so is hit less often.
What is a soft collision?
A collision where there is a large impact parameter, causing excitation or ionisation of valence shell electrons. This is becasue the Coulomb field affects the atom as a whole.
Why are the contributions from hard and soft collisions to the total energy loss comparable in magnitude?
Because less energy is lost in soft collisions but they occur more often and vice versa.
Define linear stopping power
The average energy loss, de, per unit distance, dx, along the track of a particle.
Give the equation for stopping power
S = linear stopping power
dE = average energy loss
dx = unit distance
Why does the stopping power equation contain a minus sign?
Energy decreases with distance so the minus sign ensures that the calculated stopping power is positive.
Define collision stopping power
The portion of stopping power that occurs due to interactions with atomic electrons (i.e. ionisation and excitation).
Define mass collision stopping power
The collision stopping power, normalised by the density of the material.
What does mass collision stopping power depend on?
- Atomic number, Z
- Atomic mass, A
- Ionisation excitation potential, I
- Particle velocity
- Particle charge, ze
- Relativistic rise (particle velocity at relativistic energies)