Interactions of Charged Particles Flashcards

(52 cards)

1
Q

What are the two types of charged particle radiation?

A

1) Fast electrons
2) Heavy charged particles

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2
Q

What are fast electrons?

A

Beta particles emitted in nuclear decay

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3
Q

What are heavy charged particles?

A

All energetic ions with a mass greater than 1 amu. This includes alpha particles, protons, and fission objects.

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4
Q

What is the cause of biological effects in medical physics?

A

Charged particle radiation

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5
Q

Charged particles are termed _________ radiation.

A

Ionising

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6
Q

How can heavy charged particles lose energy?

A
  • Collision
  • Elastic scattering
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7
Q

How can electrons lose energy?

A
  • Collisions with bound atomic electrons (inelastic scattering)
  • Bremsstrahlung (radiative interactions)
  • Elastic scattering
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8
Q

Charged particles mainly transfer energy to ________ via the ________ force when they collide.

A

Electrons
Coulomb

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9
Q

Why does it take several collisions for heavy charged particles to stop?

A

Their mass is much greater than that of an electron, meaning that very little energy is transferred per collision.

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10
Q

Why do heavy charged particles travel in an approximately straight path despite colliding with electrons?

A

Because they are relatively massive, meaning that they aren’t deflected much by the collisions.

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11
Q

When do collisions cause ionisation?

A

If an atomic electron is given enough energy to leave the atom.

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12
Q

When do collisions cause excitation?

A

If an atomic electron is given enough energy to move to an excited state, but not enough for ionisation.

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13
Q

What is a delta ray?

A

An ionised electron with enough energy to cause secondary ionisations.

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14
Q

Give the equation for the Q value of energy transfer from a charged particle to an electron

A

Q = Q value
k = constant
z = primary particle charge
e = electron charge
m = electron mass
b = impact parameter
v = primary particle velocity

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15
Q

What are the type types of collision for fast charged particles?

A
  • Hard (impact parameter ~ atomic radius)
  • Soft (impact parameter&raquo_space; atomic radius)
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16
Q

What is a hard collision?

A

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.

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17
Q

What is a soft collision?

A

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.

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18
Q

Why are the contributions from hard and soft collisions to the total energy loss comparable in magnitude?

A

Because less energy is lost in soft collisions but they occur more often and vice versa.

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19
Q

Define linear stopping power

A

The average energy loss, de, per unit distance, dx, along the track of a particle.

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20
Q

Give the equation for stopping power

A

S = linear stopping power
dE = average energy loss
dx = unit distance

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21
Q

Why does the stopping power equation contain a minus sign?

A

Energy decreases with distance so the minus sign ensures that the calculated stopping power is positive.

22
Q

Define collision stopping power

A

The portion of stopping power that occurs due to interactions with atomic electrons (i.e. ionisation and excitation).

23
Q

Define mass collision stopping power

A

The collision stopping power, normalised by the density of the material.

24
Q

What does mass collision stopping power depend on?

A
  • Atomic number, Z
  • Atomic mass, A
  • Ionisation excitation potential, I
  • Particle velocity
  • Particle charge, ze
  • Relativistic rise (particle velocity at relativistic energies)
25
Define restrictive stopping power
A modified version of stopping power relating to the energy transferred to the material surrounding particle tracks from secondary electrons. It is also known as Linear Energy Transfer.
26
How is collision stopping power different for heavy charged particles and electrons?
An additional shell correction term must be considered when defining the collision stopping power of heavy charged particles. This is not required for electrons.
27
Describe the shape of the electron stopping power curve in air, water, muscle, and bone
28
Describe how the collision and radiative terms of mass stopping power change with increasing electron energy for carbon, copper, and lead
29
Define bremsstrahlung
A radiative process that occurs when free electrons interact with the strong electric field of a nucleus rather than atomic electrons. It causes the electron to be deflected and decelerated, emitting a photon.
30
What is the energy range for photons released in bremsstrahlung?
It ranges from near zero to the full energy of the incident electron based on the extent of particle deflection.
31
What does the intensity of bremsstrahlung radiation depend on?
(Z/m)² where m is the mass of the moving particle
32
Give the equation for the percentage radiation losses for electrons due to bremsstrahlung
E = energy loss (MeV) Z = atomic number E_max = maximum energy of electron
33
Give the equation for the effective atomic number of a mixture of elements
Z_eff = effective atomic number f_i = fraction of total weight Z_i = atomic number
34
Bremmstrahlung makes up a _____ proportion of total energy losses.
Small
35
Why does bremsstrahlung have to be considered when building shielding for radiation protection?
Because the products of bremsstrahlung are more penetrating so may require additional shielding.
36
Define radiative stopping power
The average rate of energy loss per unit path length due to bremsstrahlung.
37
Radiative stopping power increases ______ _______ with kinetic energy.
Almost linearly
38
Define radaition yield
The fraction of initial electron energy lost to the bremsstrahlung deceleration process.
39
Give the equation for total linear stopping power
S = stopping power
40
Define energy loss straggling
The fluctuations in stopping power about the mean value, Γ.
41
What is the Bragg curve?
A plot of the specific energy lost along the track of a charged particle.
42
Describe the Bragg curve for alpha particles
43
How do radiotherapy treatments utilise the Bragg curve?
They are designed so that the maximum energy (the Bragg peak) is released within the tumour volume.
44
Describe the shape of the Bragg curve based on changes in stopping power
1) Initially, speed is maximum for the charged particle 2) Speed decreases with penetration because the particle loses energy 3) The slower particle deposits more energy due to increased interaction with surrounding material 4) Near the end of the track, electron pickup occurs and the curve rapidly falls.
45
Define particle range
The distance traversed by a particle before it comes to rest in the stopping material.
46
Give the equation for particle range
R = range a = constant E_0 = initial particle kinetic energy z = charge M = particle mass v_0 = initial velocity
47
What is the mean range for a particle?
The absorber thickness that reduces the incident intensity to half its initial value.
48
What is the extrapolated range for a particle?
The range obtained by extrapolating the linear portion of the end of the transmission curve to zero.
49
How do the mean range and the extrapolated range vary for alpha particles?
They are very similar
50
How do the mean range and the extrapolated range vary for beta particles?
They are very different because the mean range doesn't consider the multiple scattering events that occur before an electron is stopped, making it a poor measure of absorber thickness. Hence, the extrapolated range is used for beta particles.
51
Why do electrons lose energy more slowly than heavy charged particles?
They are quicker, meaning that they are less densely ionising and can travel further before losing energy.
52
Define elastic scattering
The simplest of all nuclear reactions. It typically occurs when charged particles are scattered by a localised force field like an atomic nucleus.