Particle Physics Flashcards
Who discovered the neutron and how?
James Chadwick discovered the neutron by shooting alpha particles at a sheet of beryllium and a paraffin block. The alpha particles knocked off neutrons off the beryllium, which in turn knocked off protons (H⁺) which were detected by the Geiger counter.
What conclusions were made from the experiment that proved the existence of the neutron?
If the particle can remove H⁺ it must be the same mass
It has neutral charge because it wasn’t detected by a magnet
What is the equation for specific charge?
Specific charge (C kg⁻¹) = charge / mass
What are the forces in a nucleus and where are their differences?
Strong force
Convert metres and femtometres
1 fm = 1*10⁻¹⁵ m
1 m = 1*10¹⁵ fm
What is the equation for the alpha decay of ²²⁶₈₈Ra?
²²⁶₈₈Ra –> ²²²₈₆Rn + ⁴₂He²⁺
What is the equation for the beta decay of ²¹⁰₈₃Bi?
²¹⁰₈₃Bi –> ²¹⁰₈₄Po + ⁰₋₁β + ⁰₀v̅ₑ
What is the difference between beta plus and beta minus decay?
beta minus (β⁻) decay: a neutron is converted to a proton and the process creates an electron and an electron antineutrino
beta plus (β⁺) decay: a proton is converted to a neutron and the process creates a positron and an electron neutrino
What is a proton or neutron inside the nucleus known as?
Nucleon
Discovery of the proton
Rutherford fired alpha particles at nitrogen. Protons were emitted and attracted to the cathode, causing a hydrogen red glow.
¹⁴₇N + ⁴₂α²⁺ –> ¹⁷₈O + ¹₁H¹⁺
¹₁H¹⁺, equal to a proton, was discovered as the constitutent part of an atom.
Discovery of the electron
J J Thompson used thermionic emission to release electrons from a heater. They were attracted to the positive plate in the shape of a cross and made an electron green glow.
It was deduced that the particle must be negatively charged because it was attracted to the positive plate. It was also found that the particle responded to electric and magnetic fields and that it had a low mass.
What was found out from the oil drop experiment?
Physicists found out from the oil drop experiment that the common factor was 1.6*10⁻¹⁹, which is the charge of the electron.
What is Einstein’s equation for energy?
E = mc² - m₀c² where m₀ is the rest mass
Discovery of the positron
Cosmic rays hitting a lead sheet were observed using a cloud chamber - an electron and an unknown particle were emitted.
The unkown particle had the same curvature of the electron, conserving momentum - it must have the same mass.
It gave the exact opposite response to the magnetic field meaning it has an equal but opposite charge to the electron.
This was the first evidence of anti-matter - the positron.
Describe pair production
Occurs when a high energy photon moves near to a nucleus
A particle and its antiparticle are produced
Photon must have equal energy to both of the particles
Describe annihilation
Occurs when a particle and its antiparticle meet
They annihilate each other and produce two gamma rays
Energy produced is equal to the energy of the particles or can be calculated with E = mc²
How do you convert from J to eV?
eV = J / 1.6*10⁻¹⁹
How do you convert from eV to J?
J = eV / 1.6*10⁻¹⁹
How many eV is equal to 1 MeV?
1 MeV = 1*10⁶ eV
Discovery of the muon
Observed to curve in the same direction of an electron but bend less than an electron
Same charge as electron but more mass - 206 times more mass
Muon denoted by μ⁻
Anti-muon denoted by ̅μ̅⁺
Discovery of the kaon
Pions were observed to appear out of nowhere
They must have been produced by the decay of an unknown particle which didn’t leave a trace in the cloud chamber
The unknown particle must therefore be neutral
Further findings:
Roughly half the mass of a proton - named kaons after this
Decayed into muons and pions, or pion pairs
Broke laws of physics
To explain this physicists decided that a new property of matter, strangeness, is needed.
What are fermions?
Matter particles
What are hadrons?
Fermions made of quarks
What are baryons?
Hadrons made of 3 quarks
Protons, neutrons
What are mesons?
Hadrons made of quark-antiquark pairs
Pions, kaons
What are leptons?
Fundamental fermions
Electrons, muons, neutrinos
What are bosons?
Force carrier particles which govern the force interactions between other particles
EM force - photon
Strong interaction - gluon
Weak interaction - W and Z bosons
What are the properties of the up quark?
Charge 2/3
Strangeness 0
Baryon number 1/3
Lepton number 0
What are the properties of the down quark?
Charge -1/3
Strangeness 0
Baryon number 1/3
Lepton number 0
What are the properties of the strange quark?
Charge -1/3
Strangeness -1
Baryon number 1/3
Lepton number 0
Electromagnetic force
Boson - Virtual photon Like charges repel Opposite charges attract Strength inversely proportional to distance Infinite range Affects all charged particles
Strong force
Boson - Gluon Repulsive when at <0.5 fm Attractive between 0.5 to 3-4 fm No magnitude at around 3-4 fm 37 times stronger than electromagnetic Affects only hadrons
Weak force
Boson - W⁺or W⁻ boson
Decay interactions
Very short range
Affects all fermions
Gravity
Boson - Not yet discovered (graviton)
Attractive force
Strength increases with mass
Affects all fermions
Conservation law for energy
Energy is always conserved, and kinetic energy can account for missing mass (E = mc^2)
Conservation law for charge
Charge is always conserved
Conservation law for baryon number
Baryon number is always conserved
Conservation law for lepton number
Lepton number is always conserved
Conservation law for strangeness
Strangeness can only change by +/- 1 in the weak interaction and is always conserved for other interactions
What is the equation linking energy, frequency and Planck’s constant?
E = hf
Feynman diagram rules
Vertical axis is time Horizontal axis is space Straight lines with arrows are fermions Wiggly lines are bosons Antiparticles have arrows going the opposite way
What is electron capture?
One of the protons in a proton rich nucleus interacts with an electron of the inner shell to form a neutron and produce an electron neutrino
Photon
An indivisible unit of light; has energy and no mass, but is a particle and has momentum
Energy stored in a photon = Planck constant x frequency
E = hf
Photoelectric effect
Where electrons are emitted by light shone onto a metal plate; the emitted electrons are called photoelectrons
Each electron absorbs only one photon:
Increasing frequency of the light increase the energy per photon (E = hf), which increases the energy absorbed by each electron, which increases the distance each electron travels
Increasing amplitude or intensity of the light increases the number of photons which increases the number of electrons emitted
For the photoelectric effect to happen the light must exceed the threshold frequency - below this electrons do not have enough energy to leave their atoms
The work function is the minimum energy required to emit photoelectrons
Energy = Planck constant x frequncy = work function + maximum kinetic energy of photoelectrons
E = hf = ϕ + Eₖ ₍ₘₐₓ₎
Stopping potential
The voltage need to push electrons back so that they are not emitted by the photoelectric effect
Maximum kinetic energy of photoelectrons = charge of electron x stopping potential
Eₖ ₍ₘₐₓ₎ = eVₛ
Excitation and ionisation
Electrons only exist in discrete energy levels
Excitation is where the electron gains energy to jump to a higher energy level; eventually it will move back down and emit the energy as a photon
Ionisation is where the electron gains enough energy to leave the atom completely
