1.4 - PARTICLES AND ANTIPARTICLES

Question 1

What happens when antimatter and matter particles meet?

Answer 1

Annihilation.
All mass from the particle and antiparticle gets converted back to energy in the form of two gamma ray photons

Question 2

What must the particles be for annihilation?

Answer 2

Corresponding particle and antiparticle

Question 3

How long do antiparticles last before they are annihilated?

Answer 3

Only exist for a fraction of a second before this happens, so you don’t usually get them in ordinary matter.

Question 4

How do we make use of the effect of Annihilation?

Answer 4

in a positron emitting tomography (PET) hospital scanner

Question 5

What happens when a PET scanner is used for a brain scan?

Answer 5

a positron-emitting isotope is administered to the patient and some of it reaches the brain via the blood system

Question 6

How long does each positron travel before it meets an electron?

Answer 6

each positron emitted travels no further than a few millimeters before it meets an electron

Question 7

What happens when the positron and electron meet?

Answer 7

they annihilate each other

Question 8

What is produced due to the annihilation?

Answer 8

two gamma photons

Question 9

What senses the two gamma photons produced?

Answer 9

detectors linked to computers

Question 10

What is gradually produced from the detector signals?

Answer 10

an image is built up from the detector signals of where the positron-emitting nuclei are inside the brain

Question 11

How do you distinguish gamma rays due to annihilation compared to other gamma rays?

Answer 11

Always produced in pairs moving in the opposite directions

Question 12

When does positron emission take place?

Answer 12

when a proton changes into a neutron in an unstable nucleus with too many protons

Question 13

What charge does a positron carry?

Answer 13

positive charge

Question 14

Why does the positron carry a positive charge?

Answer 14

as it’s the antiparticle of the electron

Question 15

What is the symbol for the positron?

Answer 15

β⁺

Question 16

What else is emitted during the positron emission?

Answer 16

a neutrino

Question 17

What is a neutrino’s antiparticle?

Answer 17

antineutrino

Question 18

What is the charge of the neutrino?

Answer 18

uncharged

Question 19

What is the symbol of the neutrino?

Answer 19

v

Question 20

Neutrino’s are so small, what can you assume about them?

Answer 20

zero mass

zero rest energy

Question 21

Are positron-emitting isotopes naturally occurring?

Answer 21

no

Question 22

Which one of Einstein’s equations relate the energy supplied to the particle to its increase in mass?

Answer 22

E = mc^2

Question 23

What did Einstein most importantly say?

Answer 23

the rest mass (when stationary) of a particle corresponds to rest energy locked up as mass.

Energy can turn into mass, Mass can turn into energy.

Question 24

For every type of particle with a corresponding antiparticle they (3 rules about what antiparticles and particles have/don’t have in common, 1 rule about what happens when they meet)

Answer 24

annihilate if they meet, converting their total mass into photons

have exactly the same rest mass

have exactly the same rest energy

have exactly opposite charges to the particle (if they have a charge)

Question 25

What process is opposite to the annihilation process

Answer 25

pair production

Question 26

Where does pair production happen?

Answer 26

When a high enough energy photon interacts with an orbital electron / nucleus / atom.

Question 27

What is pair production?

Answer 27

**Energy is converted to mass**… you get **equal** amounts of **matter** and **antimatter**

Question 28

When does pair production only happen?

Answer 28

If there is enough energy to produce the masses of the particles.

Question 29

Why can't pair production occur if the photon's **frequency** is too low

Answer 29

Energy of photon depends on frequency. If energy/frequency is below a certain value… …there is not enough energy to provide mass/rest energy of particles.

Question 30

What must pair production produce in order to keep certain quantities conserved?

Answer 30

particle and it's corresponding antiparticle

Question 31

Why must a photon produce a particle and antiparticle pair?

Answer 31

To conserve momentum the photon needs to interact with interacting particles

Question 32

What often happens when photons are passing near a nucleus?

Answer 32

a photon with sufficient energy passing near a nucleus or an electron can suddenly change into a particle-antiparticle pair, which would then separate from each other. Usually electron - positron pair

Question 33

How does the positron and the electron produced in a detector move away from each other after pair production?

Answer 33

curve away from each other due to the applied magnetic field they're in and they have opposite charges

Question 34

Where does excess energy from pair production go?

Answer 34

To the kinetic energy of the particle + antiparticle?

Question 35

How is the energy of a particle or antiparticle often expressed?

Answer 35

in millions of electron-volts : Mega Electron-volts

Question 36

What is the unit for electron volts?

Answer 36

MeV

Question 37

How many joules does 1 MeV equal to?

Answer 37

1 MeV = 1.60 \* 10^-13 J

Question 38

What is one electron volt defined as?

Answer 38

the energy transferred when an electron is moved through a potential difference of 1 volt

Question 39

Why are two photons produced in annihilation

Answer 39

A single photon cannot ensure a total momentum of zero after the collision. Two gamma ray photons going in opposite directions conserve momentum

Question 40

What is the minimum energy of each photon from annihilation equated to?

Answer 40

The energy of the two photons equate to the rest energy of the particle and of the antiparticle

Question 41

How do you calculate the minimum energy of a photon produced in annihilation?

Answer 41

Interaction = between a particle and an antiparticle. Both have a rest energy of **E_0.** The two photons need to have a total energy of at least 2**E₀** _… … for energy to be conserved. **Emin** = minimum energy of photons produced. **E₀** = rest energy of particle type annihilated. **2Emin = 2E₀** **Emin = E₀**

Question 42

What is **E₀** measured in?

Answer 42

MeV

Question 43

What happens in pair production?

Answer 43

a photon creates a particle and a corresponding antiparticle, and vanishes in the process

Question 44

What is the minimum energy needed for pair production?

Answer 44

Total rest energy of the particles that are produced.

Question 45

What is the rest energy of the particle the same as in pair production?

Answer 45

The rest energy of the particle is just the amount of energy that would be produced if all of it's mass was transformed into energy.

Question 46

What is the minimum energy needed for pair production (explain the equation)

Answer 46

Produces an antiparticle and particle… Both have rest energy, **E₀** … Minimum energy needed is at least **2E₀** … …for there to be enough energy to produce the particles. **Emin = 2E₀**

Question 47

Rest energy of electron = 0.510999 MeV. what is the min energy needed to produce an electron positron pair?

Answer 47

2 x 0.5109999 = 1.021998 MeV

Question 48

Why are electron - positron pairs more common?

Answer 48

relatively low mass = low rest energy = less energy needed for pair production.

Question 49

How can the path of alpha and beta particles be seen?

Answer 49

using a cloud chamber

Question 50

How did the speed affect the particle?

Answer 50

the slower it went, the more it would bend

Question 51

At what speed do all electromagnetic waves travel in a vacuum?

Answer 51

at the speed of light

Question 52

What is the speed of light?

Answer 52

3.00 \* 10^8ms^-1

Question 53

Equation for wavelength

Answer 53

λ = v/f wavelength = velocity/frequency

Question 54

In what measurement is light wavelengths often expressed in?

Answer 54

nanometres (nm)

Question 55

How is an electromagnetic wave emitted?

Answer 55

as short bursts of waves

Question 56

How does each burst of EM wave leave from the source?

Answer 56

each burst leaves the source in a different direction

Question 57

What is each burst of EM wave?

Answer 57

a packet of electromagnetic waves

Question 58

What is each burst of EM wave referred to as?

Answer 58

photon

Question 59

What is the photoelectric effect?

Answer 59

emission of electrons from a metal surface when light is directed at the surface

Question 60

What did Einstein assume the energy E of a photon depend on?

Answer 60

its frequency f in accordance with the equation: photon energy E = hf

Question 61

What is the h in the equation?

Answer 61

a constant referred to as the Planck constant

Question 62

How else can you write E = hf?

Answer 62

E = hc/λ

Question 63

What is the value of h?

Answer 63

6.63 \*10^-34 Js

Question 64

What does a laser beam consist of?

Answer 64

photons of the same frequency