Particle Physics Flashcards
Nucleon
proton or neutron found in nucleus
Suggest what you should remember when calculating mass of an atom
Mass of electron is negligible
Isotopes
Atoms with the same number of protons and a different number of neutrons
Specific Charge
Charge per unit mass for a particle
Describe how the strong nuclear force between two nucleons varies with their separation and its role in the nucleus of an atom
- strong nuclear force has a short range
- repulsive below 0.5fm
- attractive up to 3fm
- overcomes electrostatic repulsion between proton and neutron
- holds protons and neutrons together in the nucleus
Explain why the specific charge of an electron is approximately 2000 times that of a hydrogen nucleus
- hydrogen nucleus consists of a proton
- proton and electron have the same magnitude of charge
- proton has a relative mass 1 and electron is 1/2000
- specific charge of proton is approx 2000 times electron due to higher charge density
Suggest how charge and specific charge are different for isotopes
- charge remains the same as neutrons have no charge
- specific charge is greater for the isotope with the smallest mass number
Nuclide Notation
- A = nucleon number
- Z = proton number
Describe alpha decay
- unstable nucleus emits an alpha particle (helium nucleus)
- decays into a new daughter nucleus
- mass number reduced by four and atomic number reduced by 2.
Suggests what happens to alpha particle from alpha decay
Shoots off and eventually acquires electrons to become a helium atom
Describe beta - decay
- neutron changes into proton in nucleus
- unstable nucleus emits a beta particle (fast moving electron)
- antineutrino released
- new daughter nucleus formed
- same mass number and atomic number plus one
Describe beta + decay
- proton changes into neutron in nucleus
- unstable nucleus emits a beta particle (fast moving positron)
- neutrino released
- new daughter nucleus formed
- same mass number and atomic number minus one
Describe gamma decay
- excited nucleus releases gamma radiation (surplus of energy)
- after alpha or beta decay
- same nuclide just less energetic
How did beta decay lead to discovery of neutrino
- realised energy was not conserved in beta decay
- existence of neutrino hypothesised to account for conservation of energy
Photon
Packet or quantum of electromagnetic waves
Antimatter
Sub-atomic particle having the same mass but opposite charge of a given particle
Annihilation
- sub-atomic particle and its antiparticle collide
- convert their total mass into photons
- two photons are released to conserve momentum
Rest Mass/Energy
- energy an object has when it is stationary
- E=mc^2
- hypothesised mass of an object increases with its velocity so rest mass is its minimum energy
Electron volt
- energy transferred when an electron is moved through a potential difference of one volt
- eV = magnitude of charge of electron
1. 6x10-19 J
Pair production
- photon with sufficient energy interacts with orbital electron (to conserve momentum)
- converts its energy into a particle-antiparticle pair which separates
Name gauge boson and particles affected by electromagnetic force
Virtual photon
Charged particles only
Name gauge boson and particles affected by weak interaction
W+ and W-
All affected
Name gauge boson and particles affected by strong force
Pion (Gluon)
Hadrons only
Name gauge boson and particles affected by gravity
Graviton
Any particles with mass
Define gauge boson and briefly describe their role
- force carriers that are exchanged when forces act
- transfer energy / momentum
Model for repulsion
Ball passed between particles moving them further apart
Model for attraction
Boomerang passed between particles moving them closer together
Key points to remember when drawing Feynman diagrams
- incoming particles are drawn upwards in time
- baryons and leptons must stay on their own side
- charges must balance
- gauge bosons represented by a wiggly line and drawn at an angle upwards
Compare exchange particles of electromagnetic and weak force
- photon vs W boson
- massless vs non zero rest mass
- infinite range vs very short range
- does not carry charge vs charged
Weak Force
- responsible for changing the nature of particles
- for hadrons it means changing their flavour of quark e.g. in beta decay.
Describe Feynman diagram for B- decay
- weak force causes neutron to change into proton
- W - boson is produced and decays into electron and antineutrino
- conserve charge and energy
Describe Feynman diagram for B+ decay
- weak force causes proton to change into neutron
- W + boson is produced and decays into positron and neutrino
- conserve charge and energy.
Neutron-neutrino interaction*
- neutron can interact with a neutrino
- through emission of a W- boson changing the neutron into a proton
- neutrino turns into a B- particle
Proton-antineutrino interaction*
- proton can interact with an antineutrino
- through the emission of a W+ boson changing the proton into a neutron
- antineutrino turns into a B + particle
Electron Capture
- proton in proton rich nucleus interacts with inner shell electron through weak interaction
- proton emits a W+ boson which changes electron into neutrino
- proton turns into a neutron
Electron-Proton collision
- proton collides with a high speed electron
- weak interaction
- electron emits W - boson and turns into a neutrino
- proton changes into neutron.
Hadron
Particles affected by strong force
Lepton
Fundamental particles not affected by strong force, e.g. electrons, muons and neutrinos
Baryons
Hadrons made up to three quarks, e.g. protons and neutrons
Mesons
Hadrons made up of a quark and an antiquark, e.g. pions and kaons
Properties that must be conserved in interactions
Energy Charge Baryon no. Lepton no. Strangeness (only in strong interaction - may change by 0, +1 or -1 in weak interaction)
Muon
Heavy electrons (decay into electrons)
Quarks
Fundamental particles that make up hadrons
Suggest what all baryons decay into
Proton
Suggest what kaons decay into
Pion
Strangeness
quantum property only conserved in strong interaction
Strange particle interactions
- produced through strong interaction
- decay through weak interaction
Explain what happens to electrons in an atom to produce a line spectrum
- electrons are excited to higher energy levels
- electrons fall (de-excite) to lower energy levels
- release photons with discrete frequencies/energies
- produces discrete wavelengths of light
Explain how a fluorescent tube works
- ionisation/excitation of mercury through collisions with electrons
- mercury atoms de-excite and emit UV photons
- UV radiation absorbed by powder coating
- powder atoms excited so electrons move to higher energy levels
- de-excite and release photons with discrete frequencies
- emitted frequencies are within he range of visible light
Excitation
- process by which atom absorbs EXACTLY the right amount of energy
- electron moves to higher energy level from ground state
Suggest why emitted photons have specific wavelengths
Electrons fall from one fixed energy level to another so emitted photons have a discrete amount of energy
Ionisation
Electron is removed from the outer shell of an atom
An atom in the ground state is given energy by electron impact. Suggest how this energy might be used.
- energy could be used to liberate electron in ground state (ionise atom)
- extra energy would be converted into kinetic energy of electron
Explain the difference between an electron and photon of the same energy colliding with an electron in an atom at ground state
- for electron impact
- electron would be excited to a higher energy level
- even if the energy does not correspond with a particular energy level
- extra energy is converted into kinetic energy
- photons are only absorbed if they have EXACT energies allowing electron to be excited to a higher energy level
Properties of electrons and electromagnetic waves that suggest they could be particles or waves
Waves
- electrons diffract
Particles
- photoelectric effect for waves
- electrons can be accelerated by electric field so carry momentum/KE
- electrons can be deflected by magnetic fields so carry charge
Suggest a suitable material to observe diffraction pattern of electrons and explain your choice
- crystals
- spacing between atoms is small enough to behave like an aperture
- gaps similar in size to the de Broglie wavelength of an electron
Photoelectric effect
- emission of electrons from a metal surface
- when it is illuminated by light
- of frequency greater than threshold frequency
Suggest issues with wave theory shown by photoelectric effect
- lack of time delay with electron emission (should take time for electrons to gain energy)
- threshold frequency (electrons should gain some energy from wave regardless of frequency)
Suggest what impact intensity of radiation has on electrons in the photoelectric effect and explain why it does not affect their kinetic energy
- intensity affects rate of electron emission as more photons can liberate more photoelectrons
- kinetic energy is unaffected since energy gained by a photoelectron is due to one photon only
Work Function
- minimum amount of energy
- required to release an electron from a metal surface
State which factor affects kinetic energy of photoelectrons
Frequency
Stopping Potential
- minimum potential applied to metal plate
- to attract all photoelectrons emitted from surface back to surface of metal
De Broglie Hypothesis
All matter has wavelike nature characterised by its de Broglie wavelength
Suggest what can alter de Broglie wavelength of a particle
Its velocity since de Broglie wavelength is calculated based on momentum of the particle
Explain the reason we often do not experience the wavelike nature of particles
Wavelike behaviour is only evident when the aperture the particle passes through is almost equal to the size of its de Broglie wavelength which is rarely the case
Explain why alpha particle is no longer affected by the strong nuclear force once outside nucleus
- short range
- no effect for distances larger than 3fm
Antiparticle for photon
Photon since it does not carry charge
Properties of strange particles that make them different from those that are not strange
- strange quark
- longer half life than expected
- decays by weak interaction
Explain why kinetic energy of photoelectrons emitted has a range of values up to certain maximum
- energy of all photons is the same since E=hf
- more energy required to remove deeper electrons
- Ek cannot exceed hf - Φ
Explain what is meant by validated evidence
- predictions have been tested through experiments
- peer reviewed
- results are repeatable/reproducible
Ground state
Electrons in lowest energy state
Suggest why photoelectron emission does not take place below threshold frequency
E=hf so photon does not have enough energy to release electrons
Suggest why a high voltage is needed in a fluorescent tube
Needed to start the flow of current as it gives electrons enough kinetic energy to ionise mercury atoms
Threshold Frequency
- minimum frequency of photons (light) to overcome work function
- below which no photoelectrons are emitted
Suggest why low pressure is needed in a fluorescent tube
Too many atoms means electrons cannot reach the anode
Explain why electrons used to diffract around atomic nuclei would require larger kinetic energy if they are to show a diffraction pattern compared to when diffracted through a thin specimen of graphite
- atomic nucleus is smaller than atom
- wavelength of electron must be smaller for noticeable diffraction
- smaller wavelength requires larger momentum according to de Broglie’s equation
- larger momentum requires larger kinetic energy
Explain how the diffraction pattern observed changes when electrons travel through crystal with smaller velocity
- electrons have smaller momentum
- associated de Broglie wavelength larger
- more diffraction observed with larger wavelength
- spacing of fringes would increase and pattern covers larger area
Evidence for weak interaction
- strangeness not conserved
- decay in quark flavour
Suggest difference between hadrons and leptons
- hadrons are not fundamental but leptons are
- hadrons are affected by the strong force but leptons are not
Suggest what happens to a positron created from pair production
- collides with another electron and annihilates
- releases two photons
