Particles

Question 1

What is are isotopes?

Answer 1

Atoms with the same number of protons but different numbers of neutrons (and so varying mass numbers).

Question 2

Why is the nuclear strong force neccessary?

Answer 2

The electrostatic repulsion between nucleons is much stronger than the gravitational attraction between them, so another attractive force is needed to hold the nucleus together.

Question 3

Describe how the strong nuclear forcee varies with nucleon separation.

Answer 3

0-0.5 fm: repulsive force otherwise it would crush the nucleus to a point.

0.5-3.0 fm: short-range attractive force (maximum lies between these two values).

>3.0 fm: falls rapidly to zero

Question 4

What are the conditions neccessary for alpha emission?

Answer 4

• large nuclei
• strong force unable to keep nucleus stable

Question 5

What are the conditions neccessary for beta emission?

Answer 5

• neutron-rich nuclei
• One of the neutrons is changed into a proton, with a beta particle and an antineutrino being emitted.

Question 6

What observations led to the hypothesis of a the existence of the neutrino?

Answer 6

• After beta decay, the total energy of the particles was lower than before the beta particle was emitted, seemingly contradicting the law of conservation of energy.
• It was suggested that another particle was being emitted too, which carried the missing energy. This particle had to be neutral to conserve charge and have zero or almost zero mass, as it hadn’t been detected yet.

Question 7

What are the similarities and differences between a particle and its antiparticle?

Answer 7

• same mass and rest energy
• opposite charge

Question 8

What is the antiparticle of the electron called?

Answer 8

positron

Question 9

Describe pair-production.

Answer 9

• particles collide, releasing energy in the form of photons
• this energy is converted to matter and antimatter (a single photon needs to have enough energy to undergo pair production)
• the minimum energy for a photon to undergo pair production is the total rest energy of the particles produced (only gamma ray photons will have enough energy)
• electron-positron pairs are the most commonly produced as they have the lowest mass and so lowest rest energy

Question 10

What is the formula for the energy of a photon?

Answer 10

E = hf = hc/_lambda

Question 11

What happens when a particle meets its antiparticle?

Answer 11

• process is called annihilation
• all the mass of the particle and antiparticle is converted into energy
• energy is emitted in the form of two gamma ray photons to conserve momentum
• The minimum total energy of the two gamma ray photons is equal to the sum of the rest energies of the particle and antiparticle (which have equal rest energies):

2E_min = 2E₀ so E_min = hf_min = E₀

Question 12

Explain what a virtual particle is.

Answer 12

• exchange particles which exist for very short periods of time and are the mechanism by which ‘real’ particles exert forces at a distance on each other.
• exchange particles are called gauge bosons
• e.g. the repulsion between two protons is caused by the exchange of virtual photons, which are the gauge bosons of the electromagnetic force

Question 13

List the 4 fundemental forces in nature, their exchange particles and what types of particles they affect

Answer 13

• electromagnetic, virtual photon, charged particles
• weak, W⁺ & W^-, all types
• strong, pions (π⁺, π^-, π⁰), hadrons only
• gravity, *not tested*

Question 14

Explain the relationship between the mass of the gauge boson and the range of the force.

Answer 14

The heavier the mass of the gauge boson, the more energy it takes to create it and so it exists for a shorter time and can’t travel far.

E.g. the W boson is 100x heavier than a proton, so the weak force has a very short range, but photons have no mass so the electromagnetic force has infinite range.

Question 15

What are the rules for drawing particle interaction diagrams?

Answer 15

• Incoming particles start at the bottom and move upward
• baryons and leptons can’t cross from one side to the other (L to R or vv)
• The charges on both sides (top to bottom) have to balance. W bosons carry charge from one side of the diagram to the other (they aren’t counted when balancing the charges).
• A W^- particle going left has the same effect as a W⁺ particle going right
• Exchange particles generally transfer from L to R unless otherwise indicated by an arrow.

Question 16

Draw the diagrams for electromagnetic repulsion

Answer 16

Question 17

Draw the diagrams for electron capture and electron-proton collisions.

Answer 17

Question 18

Draw the diagrams for beta-plus and beta-minus decay.

Answer 18

Question 19

What are the main features of a hadron?

Answer 19

• subject to strong interation
• two classes: baryons and mesons
• made up of quarks therefore not fundemental particles

Question 20

What are the main features of baryons?

Answer 20

• protons and neutrons
• all baryons except protons are unstable i.e. they all decay to a proton along with other particles.
• The baryon number, B, is the number of baryons: protons and neutrons have B = +1 and antibaryons have B = -1. Any particle which isn’t a baryon has B = 0.
• The total baryon number in any particles interaction never changes.

Question 21

What are the main features of mesons?

Answer 21

• all mesons are unstable
• pions and kaons are the two types of mesons
• pions (π-mesons) are the lightest meson and come in 3 flavours: π^-, π⁺, π⁰ depending on their electric charge
• kaons (K-mesons) are heavier and more unstable than pions. There are K⁺ and K⁰ kaons, and they have a very short lifetime, decaying into pions.
• both found in cosmic rays
• mesons interact with baryons via the strong force
• pion interactions swap protons with neutrons and neutrons with protons, changing the charge on the pion but not the overall baryon number.

Question 22

What are the main features of leptons?

Answer 22

• Fundemental particles which don’t feel the strong force.
• Interact via the weak force.
• Types of lepton:
  
  • electrons, e^- - stable
  • muons, μ^- - unstable, decay into e^-
  • electron-neutrino and muon-neutrino - zero or almost zero mass & zero electric charge

Question 23

How is lepton number counted?

Answer 23

• two types which have to be counted spearately:
  
  • L_e & L_μ
  • electron and electron-neutrino both have L_e = +1
  • muon and muon-neutrino both have L_μ = +1
  • antiparticles of each have equal and opposite lepton numbers.

Question 24

What are strange particles?

Answer 24

• Created through the strong interaction but decay via the weak interaction
• e.g. kaons
• Strangeness is given by the number of strange quarks which make up the hadron (the s quark has S = -1 , and is conserved in the strong interaction, but not in the weak interaction (can change by +1, 0 or -1). This means that strange particles are always created in pairs to conserve strangness e.g. K⁺ and K^-.

Question 25

What are quarks?

Answer 25

• Fundemental particles which make up hadrons (antiparticles of hadrons are made from antiquarks).

Question 26

What combination of quarks forms a proton?

Answer 26

uud

Question 27

What combination of quarks forms a neutron?

Answer 27

udd

Question 28

What combinations of quarks make up mesons?

Answer 28

• Always a quark and an antiquark
• Kaons:
  
  • K⁺ : us̄
  • K⁰: ds̄
  • K^-: sū
• Pions:
  
  • π⁺: ud̄
  • π^-: dū
  • π⁰: uū or dd̄

Question 29

What happens during beta-plus and beta-minus emission?

Answer 29

• beta-minus: neutron to proton i.e. d quark to u quark via weak interaction
• beta-plus: proton to neutron i.e. u quark to d quark via weak interaction

Question 30

What quantities are conserved during particle interactions, and what are the rules governing the conservation?

Answer 30

• Charge is always conserved
• Baryon number is always conserved
• The only way to change the type of quark is through the weak interaction, therefore there must be the same number of (anti)strange quarks at all times during a strong interaction, i.e. strangeness is always conserved in strong interations. In the weak interaction, the strangeness can change by +1, 0 or -1
• Both the electron and muon lepton numbers are always conserved.