1.4 - PARTICLES AND ANTIPARTICLES

Question 1

What happens when antimatter and matter particles meet?

Answer 1

they destroy each other and radiation is released

Question 2

How do we make use of the effect when antimatter and matter particles meet?

Answer 2

in a positron emitting tomography (PET) hospital scanner

Question 3

What does the P in PET stand for?

Answer 3

positron, which is the antiparticle of the electron

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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

Question 6

What happens when the positron and electron meet?

Answer 6

they annihilate each other

Question 7

What is produced due to the annihilation?

Answer 7

two gamma photons

Question 8

What senses the two gamma photons produced?

Answer 8

detectors linked to computers

Question 9

What is gradually produced from the detector signals?

Answer 9

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

Question 10

When does positron emission take place?

Answer 10

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

Question 11

What charge does a positron carry?

Answer 11

positive charge

Question 12

Why does the positron carry a positive charge?

Answer 12

as it’s the antiparticle of the electron

Question 13

What is the symbol for the positron?

Answer 13

β⁺

Question 14

What else is emitted during the positron emission?

Answer 14

a neutrino

Question 15

What is the charge of the neutrino?

Answer 15

uncharged

Question 16

What is the symbol of the neutrino?

Answer 16

v

Question 17

Are positron-emitting isotopes naturally occurring?

Answer 17

no

Question 18

How are positron-emitting isotopes manufactured?

Answer 18

by placing a stable isotope, in liquid or solid form, in the path of a beam of protons

Question 19

What happens to some of the nuclei in substance?

Answer 19

it absorbs extra protons and become unstable positron-emitters

Question 20

When was antimatter predicted and by who?

Answer 20

in 1928 by the English physicist Paul Dirac

Question 21

In comparison to the positron when was the first antiparticle discovered?

Answer 21

before the first antiparticle the positron was discovered

Question 22

More than 20yrs earlier to the prediction of antimatter, what did Einstein show?

Answer 22

mass of a particle increases the faster it travels

Question 23

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

Answer 23

E = mc^2

Question 24

What did Einstein most importantly say?

Answer 24

the mass of a particle when it is stationary, its rest mass, corresponds to rest energy locked up as mass

Question 25

What did Einstein show regarding rest energy?

Answer 25

rest energy must be included in the conservation of energy

Question 26

What did Dirac predict?

Answer 26

existence of antimatter particles that would unlock rest energy, whenever a particle and a corresponding antiparticle meet and annihilate each other

Question 27

Dirac's theory of antiparticles predicted that for every type of particle there is a corresponding antiparticle that: (3)

Answer 27

annihilates that particle and itself if they meet, converting their total mass into photons has exactly the same rest mass as the particle has exactly opposite charge to the particle if the particle has a charge

Question 28

What process, opposite to the annihilation process did Dirac predict?

Answer 28

pair production

Question 29

What did Dirac predict in the pair production process?

Answer 29

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

Question 30

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

Answer 30

in millions of electron volts

Question 31

What is the unit for electron volts?

Answer 31

MeV

Question 32

How many joules does 1 MeV equal to?

Answer 32

1 MeV = 1.60 * 10^-13 J

Question 33

What is one electron volt defined as?

Answer 33

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

Question 34

When does annihilation occur?

Answer 34

when a particle and a corresponding antiparticle meet and their mass is converted into radiation energy

Question 35

What is produced in the annihilation process?

Answer 35

2 photons (as a single photon cannot ensure a total momentum of zero after the collision)

Question 36

What is the minimum energy of each photon equated to?

Answer 36

the energy of the two photons, to the rest energy of the particle and of the antiparticle

Question 37

What happens in pair production?

Answer 37

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

Question 38

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

Answer 38

using a cloud chamber

Question 39

What is a cloud chamber?

Answer 39

small transparent container containing air saturated with vapour and made very cold

Question 40

Where else does the same conditions as in the cloud chamber exist?

Answer 40

high in the atmosphere

Question 41

What do ionising particles leave?

Answer 41

a visible trail of liquid droplets when they pass through the air

Question 42

What did the American physicist Carl Anderson do in 1932?

Answer 42

used a cloud chamber and a camera to photograph trails produced by cosmic rays

Question 43

What did Carl Anderson decide to see?

Answer 43

if the particles could pass through a lead plate in the chamber

Question 44

What did he apply to the chamber and why?

Answer 44

with a magnetic field applied, he knew the trail of a charged particle would bend in the field

Question 45

What would happen to a positive particle?

Answer 45

deflected by the magnetic field in the opposite direction to a negative particle travelling in the same direction

Question 46

How did the speed affect the particle?

Answer 46

the slower it went, the more it would bend

Question 47

What did he think would happen if a particle went through the plate?

Answer 47

he thought it would be slowed down so its trail would bend more afterwards

Question 48

What did he actually discover?

Answer 48

a beta particle that slowed down but bent in the opposite direction to all the other beta trails he had photographed

Question 49

What momentous discovery did he make?

Answer 49

a positron, the first antiparticle to be detected