Photon Interaction Mechanisms

Question 1

What is the photoelectric effect?

Answer 1

When electrons are released from the target when light of high enough frequency is shone (threshold frequency)

(incident photon is completely absorbed)

Question 2

What causes the photoelectric effect and what has no effect on it?

Answer 2

Increasing intensity of light makes no difference to the threshold frequency (due to wave particle duality)

Increasing the frequency of light increases the current

Question 3

What did the photoelectric effect show?

Answer 3

That light travels in packets and deposits energy in whole amounts

wave-particle duality

Question 4

What is an example of experiment of PE effect?

Answer 4

Light source (UV or x-ray) goes through cathode tube to a target

when photons are above threshold electrons are produced, they complete the circuit (showing that photon leads to e-)

Question 5

What happens to electron when it gains enough energy?

Answer 5

They jump into a higher orbital

Question 6

What is the binding energy of an electron?

Answer 6

The energy required to escape the atom

(below which electrons only move up to higher orbitals)

Question 7

What does the energy of a photon have to be in order for the electron to be ejected?

Answer 7

Incident photon has to have energy equal to or greater than binding energy of inner bound electron

(spare energy is given to photoelectron as kinetic energy)

Question 8

What is the kinetic energy of the photoelectron?

Answer 8

KE = hv - Φ

(Φ = binding energy/work function)

Question 9

What happens to the atom after the electron is ejected?

Answer 9

The atom is now positively charged and in an excited state due to the vacancy left

Question 10

What is the atomic photoelectric attenuation 𝜏?

Answer 10

A measure of the chance of a PE interaction

Question 11

What does 𝜏 depend on?

Answer 11

Atomic number of target (less tightly bound an electron is the lower 𝜏 so e- usually comes from k-shell)

Energy of incident photon (higher hv = lower 𝜏)

Question 12

What is the equation of 𝜏?

Answer 12

𝜏 ∝ Z^4 / E^3

Question 13

At relativistic energy what is the 𝜏 proportional to?

Answer 13

𝜏 ∝ 1/E

(used when rest energy of electron matches binding energies or above

Question 14

What is the K-edge?

Answer 14

Energy above which incident photon has enough energy to free a k-shell electron

Shown as sudden step jumps in graph

Dramatic increase in PE cross section

Question 15

Which direction do photoelectrons come out of the atom?

Answer 15

Never in exact forward direction (θ = 0 degrees) as this would violate conservation of momentum (atom would have to recoil)

Question 16

At what energy does the photoelectron tend to be ejected perpendicular to photon direction?

Answer 16

At low E (< 10keV)

Question 17

At what energy does the photoelectron tend to be ejected in forward direction?

Answer 17

At higher E (>100keV)

(used in radiotherapy to focus electrons forward)

Question 18

What could happen if an electron falls from outer shell to inner shell?

Answer 18

A characteristic photon fluorescence which is more likely when Z > 30 (more electrons so more likely)

hv = BEm - BEk

Question 19

What can the release of a characteristic photon lead to?

Answer 19

A secondary electron (Auger electron) to be released after photon interacts with a loosely bound outer electron

(more likely for Z < 30)

can cause cascade of electrons released

Question 20

What is the energy of the Auger electron?

Answer 20

It has the energy of the characteristic photon (low energy so can’t travel far)

Question 21

How was pair production discovered?

Answer 21

In cloud chamber, charged particles circled in same sized magnetic field

They travelled in opposite directions indicating oppositely charged particles

Question 22

What do matter and anti-matter share?

Answer 22

Equal mass but opposite charge

If the matter and anti-matter counterparts annihilate and release their equivalent-mass energy (E = mc^2)

Question 23

What is the mass of an electron/positron?

Answer 23

0.511 MeV /c^2

Question 24

What is pair production?

Answer 24

Photon produces two particles (a pair)

Question 25

What does the photon energy need to be for pair production to occur?

Answer 25

Greater than 1.022 MeV (photon must have at least energy of pair (0.511 MeV each)

Question 26

What happens to the incident photon before a pair is produced?

Answer 26

It approaches close to nucleus as it interacts with electric field of nucleus and is completely absorbed

Leads to two particles produced

Question 27

What happens after the pair production?

Answer 27

Nucleus recoils with negligible energy (conservation of momentum)

The pair carry spare energy as KE

Question 28

What is the atomic pair production attenuation 𝜅?

Answer 28

A measure of the chance of a PP interaction

Question 29

What does 𝜅 depend on?

Answer 29

Atomic number of target (Z^2): greater the field the photon enters, greater the chance of interaction

Energy of incident photon (E): Above 1.022 MeV, higher chance of interaction

Question 30

What is the equation for 𝜅?

Answer 30

𝜅 ∝ Z^2 E

Question 31

What are the secondary effects of pair production?

Answer 31

Gamma photon pairs produced from positron interactions, each with energy of 0.511 MeV

they travel in opposite directions to conserve momentum

(used for PET scanning)