4.3 Interactions of sub-atomic particles with matter

Question 1

How do electrons, protons, and neutrons interact with matter?

Answer 1

All interact by scatter or absorption

These interactins result in energy deposition

Question 2

Why are interactions important in radiotherapy?

Answer 2

• Treatment and dose deposition
• Can cause physiological damage
• Can be used to detect ionising radiation (relevant to imaging)
• Affects the type of shielding used

Question 3

What type of particles are more likely to interact?

Answer 3

Charged particles due to their electric field

Question 4

What does interaction type and liklihood depend on?

Answer 4

• Energy of the particle
• Distance of the approach of the particle

Question 5

What occurs in ionisation?

Answer 5

The incoming electron transfers enough energy for the atomic electron to be ejected
Incoming electron is scattered

Question 5

What are the two types of collisions?

Answer 5

• Elastic collisions - no loss of kinetic energy (returns to same energy it started with)
• Inelastic collisions - loss of kinetic energy - the wat dose is deposited

Question 5

What occurs in excitation?

Answer 5

1. The incoming electron interacts with an inner shell electron and transfers energy.
2. Incoming electron is scattered
3. If the enegery is greater than the shell energy then the inner shell electron moves out to an outer shell - it is unstable/excited
4. Almost immediately the electron moves back into the inner shell and the difference in energy is released as a photon

Question 5

How are electrons for radiotherapy obtained?

Answer 5

• Excitation
• Ionisation

Question 5

When do soft collisions occur?

Answer 5

• The charged particle passes the atom at a distance greater than the atomic radius
• This is the most probable collision: a coulomb interaction with the whole atom
• Excitation or ionisation occurs
• There is little energy transfer

Question 6

When do hard collisions occur?

Answer 6

• When the particle passes the atom at a comparable distance to the atomic radius
• The incoming electron is likely to interact with the atomic electrons individually
• The orbital electron is ejected with high kinetic energy - a delta wave
• The energy can be omitted as a photon (characteristic x-rays) or an electron (auger electron)

Question 7

When does radiative loss (Bremsstrahlung) occur?

Answer 7

If the intercept distance of the incoming electron is much smaller than the atomic radius

The incoming electron is defelcted and decelerated by the Coulomb field of the nucleus

The incoming electron loses kinetic energy as emitted photons - this conserves energy and momentum

The closer the electron gets to the nucleus (the smaller b is), the greater the energy loss of the electron and therefore the higher the energu of the bremsstrahlung photon lost

Question 8

How does electron energy influence x-ray intensity?

Answer 8

N shaped graph with peak at 1/3 of max energy of electron

Question 9

What factors increase the probability/intensity of Bremsstrahlung radiation?

Answer 9

• Bombarding particle kinetic energy
• Atomic number
• Charge of the bombarding particle

Question 10

What factor decreases the probability/intensity of Bremsstrahlung radiation?

Answer 10

Mass (m) of the bombarding particle

Question 11

Which particle is efficient at Bremsstrahlung radiation?

Answer 11

Electrons - charged with low mass

Question 12

What causes the electron peak?

Answer 12

The electron peak occurs as interaction cross section increases as energy decreases.

As the electron goes deeper it loses energy and therefore loses speed and is therefore more likely to interact.

The low mass of electrons means increase in large angle scattering so there is no well-defined peak

Question 13

What is stopping power?

Answer 13

a material’s ability to stop a particle

Energy loss per unit of length

dE/dZ in MeV/cm (megaelectrons per cm)

Total = sum of collision stopping power + radiative stopping power

Question 14

What is mass stopping power?

Answer 14

The rate of energy loss per square cm

stopping power/density MeVcm² /g

(S/P) collision + (S/P) radiative (radiative losses don’t contribute to local dose deposition because they continue to have interactions elsewhere

Question 15

What is the formla for dose?

Answer 15

DOSE = Partial fluence Φ (S/P) collision

Question 16

How is stopping power related to range?

Answer 16

The greater the stopping power the less the range of the particle

Question 17

What two groups of factors does range depend on?

Answer 17

• Particle factors
• Material factors

Question 18

What are the particle factors that determine range?

Answer 18

Proportional to Z² (squared)

Inversely proportional to velocity squared (v² )

Question 19

What are the material factors that determine range?

Answer 19

• Electron density - number of electrons per unit volume
• Atomic number (z) to atomic mass (a) ratio
• Density of the material

Question 20

W

What is range?

Answer 20

The distance required to travel before the particle stops

The range is the furthest point into the material the particle travels, even if the ‘drunken man’ path total is longer

Question 21

How does high atomic mass of a particle reduce its range?

Answer 21

• Increased deflection
• Reduced forward intensity and particle range

Question 22

How does high energy of a particle increase its range?

Answer 22

• Increased range
• Scatter
• Low angle scatter - don’t bend back as far

Question 23

What are the advantages of proton beam therapy?

Answer 23

Bragg peak:
- protons have finite range
- deposit majority of dose at depth
- maximum dose is delivered to the target, not elsewhere
- almost no exit dose beyond target
- can precisely target and spare OAR

Question 24

Why to Protons have the bragg peak? (Peak of energy deposit at depth)

Answer 24

Protons are so high energy - it takes travelling quite a lot of depth before the particle’s energy has dropped low enough to start to interact

Once the dose is deposited the probabilty of interactions after this reduces rapidly

Question 25

What are the disadvantages of Proton Beam Therapy?

Answer 25

Protons are a mono-energetic beam but tumours are 3D and required dosing all along their depths

This can be by Passive or spot scattering

Question 26

What is passive proton beam scattering?

Answer 26

Single energy beam is scattered to a ‘Spread Out Bragg Peak’ using filters

Question 27

What is Spot scattering of an electron beam?

Answer 27

Manually change the energy of the beam to reach different depths

Question 28

What are the features of neutrons used in RT? (Although actually very rarely used)

Answer 28

• High LET - transfer lots of energy
• Same mass as a proton but no charge so low probability of nuclear interactions

Question 29

What happens in neutron elastic scattering?

Answer 29

The incoming neutron interacts with the nucelus causing it to recoil and scattering the incoming neutron

There is negligible energy transfer

Probability of collision is the same for all nuclei if the nuetron is fast

More energy is transferred to objects of similar mass

Question 30

What happens in neutron inelastic scattering?

Answer 30

The incoming neutron is absorbed into the nucelus causing the nucleus to recoil signficiantly and emitting a ray. A Lower energy neutron is emitted from the nucleus

The transfer of energy in this process is much greater than excitation or ionisation

Question 31

How does energy of a particle affect stopping power and range?

Answer 31

Range is proportional to energy

Stopping power is inversley proportional to energy

Question 32

How does mass of a particle affect stopping power and range?

Answer 32

Less mass = lower stopping = greater range

• For the same kinetic energy an electron moves faster than a proton or alpha particle due to its saller mass
• Higher velocity means it spends less time near orbital electrons and there is therfore less chance of coloumb interactions

Question 33

How does charge of a particle affect stopping power and range?

Answer 33

A greater charge leads to more stopping power and lower range

Inversley proportional to Z²

Question 34

How does density of the material affect stopping power?

Answer 34

Stopping power increases and range decreases with increasing density

Range is inversley proportional to density of the material