08 - Nuclear And Particle Physics

Question 1

What is nucleon number?

Answer 1

The sum of the number of protons and neutrons in the nucleus.

Question 2

What is atomic number?

Answer 2

The total number of protons in the nucleus.

Question 3

Outline Rutherford’s alpha scattering experiment.

Answer 3

High speed alpha particles were fired at a very thin sheet of gold foil. The deflections of the particles were measured and conclusions were drawn.

Question 4

What was observed in the Alpha scattering experiment?

Answer 4

• Most of the alpha particles passed straight through the gold atoms
• Some of them were deflected
• A few of them were deflect backwards

Question 5

What is thermionic emission?

Answer 5

Thermionic emission is the release of electrons due to heating.

Question 6

Explain why electrons are released from a heated filament.

Answer 6

As the filament heats up, free electrons inside the metal gain kinetic energy. When the surface electrons gain sufficient energy, they are released from the surface.

Question 7

What will happen to a beam of electrons if it is passed through a potential difference?

Answer 7

The beam of electrons will be accelerated since work is done by the potential difference.

Question 8

How do you calculate the energy transferred to an electron, when it is accelerated across a potential difference?

Answer 8

Energy = Charge x Potential Difference
Energy = eV

Question 9

What happens when a beam of electrons is directed into a magnetic field?

Answer 9

The electron beam will be deflected, since magnetic fields apply forces on moving charges.

Question 10

What is the magnitude of the force experienced by a moving electron in a magnetic field?

Answer 10

Force = Magnetic Flux Density × Charge
× Velocity
F = Bev

Question 11

If the electrons are moving perpendicular to the field lines, which direction will the magnetic force act?

Answer 11

The force will act perpendicular to both the electron and field directions.

Question 12

Describe the shape of the path of a beam of electrons passing through a magnetic field.

Answer 12

The beam will produce a circular path since the magnetic force always acts perpendicular to the electrons’ motion. This means it acts as a centripetal force and produces a circular path.

Question 13

What is a cyclotron?

Answer 13

A cyclotron is a particle accelerator that uses magnetic fields to accelerate particles in circular paths. This allows higher speeds to be reached, without the limitation of the accelerator’s length.

Question 14

Describe the basic composition of a cyclotron.

Answer 14

Cyclotrons consist of two D-shaped paths which are separated by a small gap. An alternating potential difference is applied across the gap.

Question 15

How does a cyclotron work?

Answer 15

An electron beam is passed into the cyclotron, where it is deflected into a circular path by a perpendicular magnetic field. When the beam reaches the gap, it is accelerated by a potential difference. This increases the speed of the beam, causing the radius of the path to increase. This process repeats every half circle.

Question 16

State the equation used to calculate the circular radius of an electron beam deflected in a magnetic field.

Answer 16

Radius =mV / BQ

Question 17

What two equations must you combine to derive the radius equation?

Answer 17

1. Centripetal Force = mv^2/r
2. Magnetic Force = BQv
   Centripetal Force = Magnetic Force

Question 18

State the mass-energy equation.

Answer 18

E = mc^2

Question 19

What is 1 MeV in joules?

Answer 19

(1.6×10^-19) × 10^6 = 1.6 x10^-13 J

Question 20

In the quark-lepton model, what are the four main categories of particles?

Answer 20

1. Baryons
2. Mesons
3. Leptons
4. Photons

Question 21

Describe the quark composition of a baryon.

Answer 21

Baryons are made up of three quarks.

Question 22

Describe the quark composition of a meson.

Answer 22

Mesons are made up of a quark and antiquark pair.

Question 23

Which category of particles are classed as fundamental particles?

Answer 23

Leptons

Question 24

Give two examples of leptons.

Answer 24

1. Electrons
2. Neutrinos

Question 25

What category of particles do pions belong in?

Answer 25

Mesons.

Question 26

Give two examples of baryons.

Answer 26

1. Protons 2. Neutrons

Question 27

What did the symmetry of the quark-lepton model predict the existence of?

Answer 27

The top quark.

Question 28

What is an antiparticle?

Answer 28

An antiparticle is one that has the same mass but opposite charge and conservation numbers to its corresponding particle.

Question 29

What is the antiparticle of a proton?

Answer 29

An antiproton.

Question 30

What is the antiparticle of an electron?

Answer 30

A positron.

Question 31

Name four things that are always conserved in a particle interaction.

Answer 31

1. Mass/Energy 2. Baryon Number 3. Lepton Number 4. Charge

Question 32

Describe the conservation of lepton number.

Answer 32

The lepton number for each specific type of lepton must be the same before and after an interaction.

Question 33

what was concluded from Rutherford’s gold foil experiment

Answer 33

atom is mostly empty space mass is concentrated in a very small dense nucleus (concluded as most went through) could tell nucleus was positive as some reflected

Question 34

why does a cathode ray tube have to be in a vaccum

Answer 34

as otherwise te electrons would lose eneergy and ionise atoms in the air and the electron beam would not be produced

Question 35

what is relativistic mass

Answer 35

as velocity increases mass increases at a certain point

Question 36

explain how a cyclotron produces a high energy proton beam

Answer 36

- alternating p.d/electric field - the electric field between the 2 dees causes the acceleration of the protons across the gap (between the 2 dees). alays accelerating across the gap as charge of 2 dees keeps flipping - magnetic field is perpendicular to pare of dees - proton path is curved by the magnetic field - as velocity of proton increases, radius of path in dees increases - therefore the time for which a proton is in a dee remains constant

Question 37

how does a bubble chamber detector work?

Answer 37

As particles enter the chamber, a piston suddenly decreases its pressure, and the liquid enters into a superheated(maintained just below boiling). as the particle passes through the liquid it ionises particles in the liquid knocking of electrons leaving an ionised track, around which the liquid vaporizes, forming microscopic bubbles as these th electrons dissipate energy into the surrounding liquid causing localised boiling. Bubble density around a track is proportional to a particle's energy loss. The path of the ions is curved as the chamber has a magnetic field, so the radius of curvature can be measured

Question 38

how does a geiger muller tube work

Answer 38

wall of tube is a cathode as is a conducting material anode carrying wire radial electric field between the wall and the wire gas is ionised by radiation causing an electron to be repelled by the cathode leaving charged particles which gain energy so they ionise other gas atoms causing an ionising avalanche

Question 39

describe the set up of a Linear particle accelerator (LINAC)

Answer 39

set of metal drift tubes in a line electrons accelerated by electrc field between drift tubes acceleration between gaps adjacent tubes connected to opposite terminals of a power supply and opposite charge charge of particle is opposite to the first drift tube charge p.d / electric field is alternating so that the next tube is of opposite charge to particle so its accelerated time spent in each tube must be the same so as the electron accelerates, tubes must be longer

Question 40

explain how a cloud chamber works

Answer 40

It consists of a sealed container filled with a supersaturated vapour, typically a mixture of alcohol and water. When a charged particle passes through the vapour, it ionizes the molecules along its path, causing them to release electrons.. the ionised gas particles align surrounding alcoholk particles, condensing them into doplets the greater the charge, the thicker the trail.

Question 41

what is a quark

Answer 41

an elementary particle and a fundamental particle consistent of matter. quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons

Question 42

Derive r = p / BQ

Answer 42

mv^2 / r = BQv mv = BQr p = mv r = p / BQ

Question 43

why are high energies required to investigate the structure of nucleons

Answer 43

High energies are required to investigate the structure of nucleons due to their small size. This is because the higher the energy of matter (such as electrons), the smaller their De Broglie wavelength, and so they are more suitable for investigating smaller objects (as the resolution increases).

Question 44

What is the relationship between how curved the path of the particle and its velocity

Answer 44

The greater the velocity, the lower the radius of curvature due to F= mv^2 / r

Question 45

Relationship between radius of curvature and momentum

Answer 45

r = p / BQ Therefore the straighter the path if the particle, the greater the velocity and therefore the greater the momentum. Higher radius means less curved

Question 46

In a bubble chamber, what does it mean if the particle leaves no track

Answer 46

If the particle leaves no track, this means it has no charge

Question 47

How is the top quark predicted

Answer 47

The Standard Model organizes quarks into three generations, each containing a pair of quarks. The first generation includes the up and down quarks, the second generation includes the charm and strange quarks, and the third generation includes the bottom (or beauty) quark and its predicted partner, the top quark

Question 48

What does it mean if particle tracks apear from “nowhere” in a particle detector

Answer 48

This indicates particle-antiparticle creation These paths are in opposite directions because the particle-antiparticle pair is oppositely charged Therefore, the magnetic force on them is oppositely directed However they have the same radius because they each have the same mass (and hence, momentum)

Question 49

What does it mean if two particles spiral off in opposite directions in a particle detector

Answer 49

This indicates each particle is oppositely charged, because the (centripetal) magnetic force acts on them in the opposite directions

Question 50

Why do particles spin inwards in a particle detector

Answer 50

you expect the radius to be decreasing, because charged particles will tend to continue ionising other particles around them - hence losing kinetic energy. As their kinetic energy decreases, so does their momentum - and hence, track radius will also decrease.

Question 51

Explain why electrons must be accelerated to very high energies if they are to be used to probe the internal structure of a proton.

Answer 51

The proton diameter ∼ 10^-15 m so the de Broglie wavelength of the electrons must be at most this size in order to resolve the internal structure of the proton Since the de Broglie wavelength is inversely proportional to the momentum of the electrons, then they must be accelerated to very high velocity (and hence, energy) in order to obtain very short wavelengths

Question 52

what happens to mass as velocity increases closeto the speed of light

Answer 52

mass would increase as velocity increases

Question 53

what is pair production and what is required for it to happen

Answer 53

This is the fomation of an electron and a positron from a pulse of em energy travelling in the vicinity of an atomic nucleus. For this reaction to take place the energy of the em wave has to be greater than the rest-mass energy of the pair produced

Question 54

what is electron - positron annihilation and what does it produce

Answer 54

it occurs when an electron and positron collide. the result is the conversion of the electron and positron and the creation of gamma ray photons so that it satisfies the conservation laws.

Question 55

Define a photon

Answer 55

A discrete packet of energy

Question 56

Explain how a mass spectrometer works

Answer 56

1. A sample is vaporised, meaning it is converted into a vapour (gas) 2. Next, an electron gun is used to create a beam of electrons which are directed at the vapour. The fast-moving electrons collide with the ions in the vapour causing them to become ionised (lose electrons), causing them to become charged. 3. Then, the ions are accelerated using an electric field as they are now charged. 4. Once they have been accelerated, they pass into the velocity selector, where an electric and magnetic field are acting perpendicular to each other. The fields exert forces on the ions in opposite directions and only the ions for which the forces are balanced travel in a straight line and then pass through into the separation chamber. This results in only particles travelling at a particular speed progressing into the next part of the mass spectrometer. 5. In the separation chamber, there is a uniform magnetic field which exerts a force on the ions perpendicular to their direction of travel, causing them to follow a circular path and hit a screen, where the radius of their circular path can be measured.

Question 57

Explain the process of pair production and the circumstances required

Answer 57

Pair production is where a photon is converted into an equal amount of matter and antimatter. This can only occur when the photon has an energy greater than the total rest energy of both particles, any excess energy is converted into the kinetic energy of the particles.

Question 58

Describe annihilation

Answer 58

Annihilation is where a particle and its corresponding antiparticle collide, as a result their masses are converted into energy. This energy, along with the kinetic energy of the two particles is released in the form of 2 photons moving in opposite directions in order to conserve momentum.

Question 59

What is time dilation

Answer 59

Time dilation occurs as a consequence of special relativity. Time dilation causes time to run at different speeds depending on the motion of an observer. As a consequence of time dilation, the lifetime of a particle moving at relativistic speeds recorded by a stationary observer would be longer than the actual time

Question 60

Explain how muon decay provides evidence for time dilation

Answer 60

muons enter the atmosphere at very high speeds and so experience significant time dilation, which affects how quickly they decay. Muons are formed in the upper atmosphere and have a lifetime of around 2 us, which suggests that as they travel to the surface of the Earth, most would decay before reaching sea level, however experimental evidence showed the opposite to be true. Most muons (around 80%) were still present upon reaching sea level, even though more than 2 microseconds had passed to an external observer. This can only be explained by time dilation as the muons are travelling at close to the speed of light.

Question 61

Explain how time dilation is used in accelerator collision experiments

Answer 61

Time dilation also occurs in the context of accelerator collision experiments as particles are moving are speeds comparable to the speed of light. This means that their lifetimes (as observed by a stationary observer) are longer so particles travel for longer than expected (when taking their usual lifetimes into account) and so can interact with more particles.