Directly Ionising Radiation

Question 1

Why can electrons scatter off other electrons?

Answer 1

They are approximately (only approx due to relativistic effects) the same sizes

Question 2

What are terms a and b

Answer 2

a is Atomic radius (10^-10m)
b is impact parameter, measure of distance of incoming electron from atomic nucleus

Question 3

What kind of electron interaction processes are there?

Answer 3

Collisional (excitation, ionisation)
Radiative (brem, cerenkov)

Question 4

When do you have excitation?

Answer 4

b»a

Question 5

When do you have ionisation?

Answer 5

a~b

Question 6

What happens during excitation?

Answer 6

Closest approach of electron is large
Incoming electron loses small amount of energy to atom
Electron scattered by small angle
Electron in inner shell moves to outer shell
Only needs a few eV
Excited atom dissipates energy by emitting light photon if gas, or as heat in solid

Question 7

What happens during ionisation?

Answer 7

Distance of closest approach same order as atomic dimension
Incoming electron loses a larger amount of energy to atomic electron
Electron in inner shell gets enough energy to overcome binding energy and is removed
Incoming electron loses energy and is deflected

Question 8

What amount of energy transfer is more frequent in ionisation?

Answer 8

Small energy transfer

Question 9

What is the name given to the ejected electron in ionisation and how much energy does it have?

Answer 9

Delta ray
delta E > 100eV
Has sufficient energy to produce ionisation and excitation at a distance from path of original incoming electron

Question 10

Quantum mechanics dE/dl predictions

Answer 10

e^2
1/v^2
rho
Z/A

Question 11

What is dE/dl

Answer 11

Linear stopping power
Rate of kinetic energy loss per unit path length of the particle
S
(S/rho is mass stopping power)

Question 12

How many events will an electron undergo?

Answer 12

10^4 - 10^6 interactions before all energy is gone, depositing few eV of energy each time

Question 13

When would Bremsstrahlung occur?

Answer 13

b«a
radiative

Question 14

What does electron interact with in Bremsstrahlung?

Answer 14

Coulomb field of nucleus

Question 15

What happens during Bremsstrahlung?

Answer 15

Electron interacts with coulomb field, decelerates and changes direction
Photon emitted with energy between 0 and E

Question 16

What is cerenkov radiation?

Answer 16

When charged paticles passing through dielectric at velocity greater than phase velocity of light of that medium emits photons. Light emitted in optical region of spectrum,, only small amount, usually ignored

Question 17

What is approximate ratio of radiative to collisional?

Answer 17

EZ/800
E in MeV

Question 18

Total stopping power

Answer 18

Stot = Scoll + Srad

Question 19

Effect of Z and velocity on Scoll

Answer 19

Rate of energy loss greatest when particle moving slowly
Rate of energy loss greater in low Z materials

Question 20

Why is energy loss greater in low Z materials in Scoll?

Answer 20

They have higher electron density than high Z materials
Electrons are more loosely bound compared to higher Z materials

High Z materials have more tightly bound electrons not available to this type of interaction

Question 21

Bragg peak rate of energy loss

Answer 21

Inversely proportional to square of velocity of charged particle
Greatest when particle is moving slowly

Question 22

Why is Bragg peak not observed in electron PDD?

Answer 22

Energy and range straddling

Question 23

Factors affecting radiative stopping power Srad

Answer 23

Z^2
1/m^2
E

Question 24

path length vs range for electrons

Answer 24

Path length is total distance travelled before coming to rest
Range is sum of individual path lengths in the original direction of travel

Question 25

How does mass angular scattering power vary with Z and E for electron beam?

Answer 25

Z^2/E^2

Question 26

How does variation of mass angular scattering power affect shape of isodose?

Answer 26

Causes bulging of isodoses which can be clinically significant - dose under skin spreads out

Question 27

What is origin of electron build up?

Answer 27

Electron beam at surface predominantly forward scattered Increasing lateral scatter with depth effectively increases electron pathlength through deeper layers

Question 28

What is the practical range?

Answer 28

The depth of the intersection of the linear portion of the curve with the bremsstrahlung tail

Question 29

What does general expression for absorbed dose in medium exclude?

Answer 29

Electron interactions which deposit dose at distance from a point - radiative energy loss and energy transferred to delta rays

Question 30

What is absorbed dose per unit mass of gas?

Answer 30

J.W/e where J is total charge of one sign resulting from ionisation (not exposure, which is photons) W is mean energy to create ion pair in gas e is electronic charge

Question 31

What are conditions of Bragg Gray Cavity Theory?

Answer 31

Infinite uniform medium Small gas filled cavity Cavity sufficiently small not to perturb electron fluence or electron energy fluence Electrons only lose a small fraction of energy crossing cavity Absorbed dose in cavity is solely due to charged particles crossing cavity

Question 32

Assumptions for Bragg Gray in electrons

Answer 32

No secondary electrons (delta rays) produced in cavity No electrons stopped in cavity Mass stopping power ratios energy independent Discount energy lost by electrons for Brem

Question 33

Ratio of dose in gas and medium in Bragg Gray

Answer 33

Ratio of dose deposited in gas cavity and in medium in absence of gas cavity is same ratio as mass stopping power of gas and medium respectively Dmed = Smed_gas.Dgas

Question 34

Equation for Dmed, Bragg Gray

Answer 34

Dmed = Smed_gas . JW/e

Question 35

Special consideration Bragg Gray

Answer 35

Energy spectrum of electrons as they slow down (average stopping power ratios over energy spectrum) Density or polarisation effect (effect of polarisation of medium near to charged particle tracks which affect stopping powers)

Question 36

What is mass stopping power?

Answer 36

S/rho = 1/ rho . dE/dl

Question 37

What is linear energy transfer

Answer 37

L_delta = (dE/dl)_delta Restricted to interactions where losses less than delta, as delta rays deposit energy at a distance (often delta 10keV)

Question 38

What is continuous slowing down approximation

Answer 38

Can consider electrons as continuously slowing down as they undergo many interactions before losing all of their energy, losing energy each time

Question 39

Why, for high energy electrons, is the magnitude of de/dl greater for liquids than for gases

Answer 39

Relativistic electrons cause polarisation of atoms in medium, screening E field which reduces strength of interactions and reduces dE/dl Magnitude of effect increases with increasing density dE/dl is therefore higher in vapour than condensed phases

Question 40

What is J?

Answer 40

Total charge of one sign per unit mass from ionisation dQ/dm Not exposure which is specific to photons in air

Question 41

What is W?

Answer 41

Mean energy required to create one ion pair

Question 42

What is equation for Dgas?

Answer 42

J.W/e