Interaction Of Electrons With Matter

Question 1

Stopping power

Mass stopping power

Answer 1

1. Amount of energy lost by particle per unit length of travel
2. Stopping power divided by density

Question 2

How do photons interact with matter

Answer 2

-photoelectric effect
-Compton effect
-pair production
Result: high energy e set in motion (special e )

Question 3

Interaction of special e with other e

Energy transfers lead to:

Answer 3

Ionization: energy transfer sufficient to overcome BE

Excitation - energy transfer not sufficient

Question 4

Interaction of special e with nuclei

Answer 4

-special e passing close to nucleus will experience force (attraction)
-special e is light and pulled off original course
-curved path (deceleration)
-referred to as Bremsstrahlung (breaking radiation)
-nucleus heavy and does not move
Important with:
-high Z
-light incident particles
- as energy of special e increases

Question 5

Interaction of special e with living cells

Answer 5

• dominant interaction is with orbiting e of atoms making up cells
• energy transfer leads to ionizations and excitations
• ionizations lead to production of hydroxyl radicals
• hydroxyl radicals interact with DNA to change behaviour of cells

Question 6

Interaction of special r with tungsten (X-ray tube target)

Answer 6

-special e may undergo a number of different types of collisions with high Z material
Type a, b, c, d

Question 7

Restricted stopping power and linear energy transfer (LET)

Answer 7

• different terms for same phenomenon
  -closely related to stopping power
• ## focus on way energy is deposited along e path

Question 8

Delta ray

Answer 8

• orbital e has enough energy to create its own path (ionize and excite-acts like special e )
• energy of delta ray carried away from special e path
• energy of delta ray excluded in calculation of restricted stopping power

Question 9

Bragg peak

Answer 9

Large LET at the end of the path is called Bragg peak

Peak observed for all charged particles

Question 10

Electron equilibrium

Answer 10

• energy transfer from photon to medium is two stage process
  1. Interaction of photon with atom (fast moving e)

2. Transfer energy from electron to medium ( excite/ionize )

Question 11

KERMA

Answer 11

Kinetic energy released in medium

How much are these photons leaving behind in the medium