Nuclear Physics Flashcards
emission spectrum
- expose a container of gas at low pressure to a strong electric field
- light emitted from gas
- light analysed by passing it through a prism or diffraction grating
- definition: set of possible wavelengths that can be emitted by a gas
discrete energy
energy can have one of a specific set of values
energy level diagram
each horizontal value represents a possible energy of the atom
how to explain the emission spectrum?
- atom can make a transition from a state of higher energy to a state of lower energy by emitting a photon
- the energy of the emitted photon is the difference in energy between the two levels
ground state
lowest energy state
excited state
- if energy is supplied to the atom, the electron may move to a higher energy level by absorbing the right amount of energy exactly to move up
- electron immediately makes transition down to lower energy (relaxation)
hydrogen energy level diagrams for all possible transitions from n=3
whether the electron will make a direct or indirect transition is just probability
how is the absorption spectrum produced?
- beam of white light through gas
- majority of atoms in ground state
- electrons may absorb photons in beam and become excited
- only happens if photon that is absorbed has the exact right energy that corresponds to difference in levels
- light that is transmitted through gas will be missing photons - corresponds to dark lines
why are the photons in an absorption spectrum missing?
photons emitted in all directions, not necessarily along the direction of the observer
nucleon
proton or neutron
nuclide
nucleus with specific number of protons and neutrons
isotopes
nuclei that have the same number of protons but a different number of neutrons
unstable nucleus
nucleus that randomly and spontaneously emits particles that carry energy away from nucleus
radioactivity
emission of particles and energy from a nucleus
alpha decay
alpha particle is emitted from the nucleus and the decaying nucleus turns into a different nucleus
beta minus decay
neutron in the decaying nucleus turns into a proton, emitting an electron and an anti-neutrino
alpha particle
helium nucleus
beta minus particle
electron
beta plus decay
nucleus emits positron and neutrino
gamma decay
nucleus emits a gamma ray
gamma particle
photon of high-frequency electromagnetic radiation
penetrative power of alpha, beta minus, gamma particles
- alpha is least penetrating
- beta minus has less charge and travels faster so interacts less with environment so more penetrative
- gamma has no ionising power bc no charge so most penetrative
ionising power of alpha, beta minus and gamma
- alpha has a lot of momentum and double charge so a lot of interaction
- beta minus has less momentum and less charge so less interaction
- gamma is not very ionising - depends on intensity
decay series
set of decays that takes place until a given nucleus ends up as a stable nucleus
random
cannot predict which unstable nucleus in a sample will decay or when there will be a decay
spontaneous
cannot affect the rate of decay of a given sample in any way
law of radioactive decay
rate of decay is proportional to the number of nuclei that have not yet decayed
what is a consequence of the law of radiactive decay?
number of radioactive nuclei decreases exponentially
half-life
time after which the number of radioactive nuclei is reduced by a factor of 2
activity
number of decays per second
becquerel
- unit of activity
- 1Bq = one decay per second
does activity obey the exponential decay law?
yes
background radiation
- activity does not approach zero, it approaches the activity due to all other sources of radiation
- cosmic rays from Sun, radioactive material in rocks and ground, radiation from nuclear weapons testing
electromagnetic interaction
- acts on any particle that has electric charge
- force given by Coulomb’s law
- infinite range
weak nuclear interaction
- acts on protons, neutrons, electrons and neutrinos in order to bring about beta decay
- very short range
strong nuclear interaction
- mainly attractive force acts on protons and neutrons to keep them bound to each other inside the nucleus
- short range
gravitational interaction
- force of attraction between masses
- small mass on atomic particles makes this force irrelevant for atomic and nuclear physics
- infinite range
electroweak interaction
electromagnetic interaction and weak interaction are two sides of the same force
how does strong force explain why stable large nuclei have more neutrons than protons
- as more protons are added to a nucleus the tendency for the nucleus to break apart increases because all protons repel each other through electromagnetic force
- strong force has short range so any one proton only attracts its immediate neighbours
- to keep nucleus together we need more neutrons that will contribute to nuclear binding through strong force but will not add to repulsive force
atomic mass unit
1/12 of the mass of an atom of carbon-12
mass defect
difference between the mass of the protons plus the mass of the neutrons and the mass of the nucleus
binding energy
- energy required to completely separate the nucleons of that nucleus
- 1u=931.5MeVc-2
binding energy curve and its features
- binding energy per nucleon for hydrogen is zero because there is only one particle in the nucleus
- curve rises sharply for low values of A
- curve has a maximum for A=62 (nickel) which makes this nucleus particularly stable
- peaks at position of nuclei He, C and O makes these nuclei unusually stable compared to immediate neighbours
- curve drops gently from peak onwards
why is the binding energy per nucleon roughly constant above a certain value of A?
- short range of force implies that any given nucleon can interact with its immediate neighbours
- for large nuclei any one nucleon is surrounded by the same number immediate neighbours so energy needed to remove that nucleon from the nucleus is the same
nuclear fission
process in which a heavy nucleus splits up into lighter nuclei
nuclear fusion
joining of two light nuclei into a heavier one with the associated production of energy
thomson model
atom is a sphere of positive charge with the electrons moving inside the sphere
Rutherford experiment
- alpha particles were directed at a thin gold foil in an evacuated changer
- number of particles deflected by different angles were recorded
- great majority of alpha particles went straight through foil with little deviation
- occasionally alpha particles were detected at very large scattering angles
elementary particles
particles which are not made out of any smaller component particles
what are the three classes of elementary particles
quarks, leptons, exchange particles
what are the six flavors of quarks?
- up, charm, top
- down, strange, bottom
anti-particles
- have the same mass as quarks but all other properties are opposite
- denoted with a bar on top of symbol
hadron
particle made out of quarks
baryon
combination of three quarks
quark + anti-quark
meson
three anti-particles combine
anti-baryon
baryon number
- quarks assigned +1/3
- antiquarks assigned -1/3
what is conserved in all reactions?
baryon number and electric charge
lepton types
- electron and its neutrino
- muon and its neutrino
- tau and its neutrino
Feynman diagrams
a) photon absorption by an electron
b) photon absorption by a positron
c) electron emitting a proton
d) photon materialises into an electron and a positron - pair production
e) electron-positron annihilation
quark confinement
not possible to observe isolated quarks
what causes quark confinement?
- force between quark and anti-quark is constant no matter the separation
- total energy needed to separate the quark from the anti-quark gets larger as separation increases
- free the quark completely would require an infinite amount of energy and so is impossible
- all that would happen would be the production of a meson-anti-meson pair
higgs particle
responsible, through its interactions, for the mass of the particles of the standard model, in particular the masses of the W and the Z
antiparticle
same mass as its particle but all the quantum numbers are opposite
lepton family number
conserved in all reactions
color interaction
interaction between objects with color
gluon
- force-carrying particle
- eight types each with zero mass
- carries a combination of color and anti-color
- emission and absorption by different quarks causes the color force
