Particles Flashcards
For a reaction why is the mass of the products larger than the mass of the reactants
The K.E. of the reactants is converted into mass
What are Cosmic Rays
Charged particles which have the highest energies ever to be observed (10^8 times CERN)
What was the problem associated with the origin of cosmic rays
B-fields in extragalactic space, the milky way and surrounding the earth deflect and distort the parths of the charged particles
How was the origin of cosmic rays figured out
High-energy neutrinos produced by the same cosmic accelerators preserve their directional information. These were traced back to a Blazar.
What is a blazar and how does it produce cosmic rays
A feeding super massive black hole at the centre of galaxies which launch powerful jets which accelerate protons (ie produce cosmic rays). When it is directed in the direction of eath the feeding black hole is called a Blazar. The Collisions between accelerated protons produce pions which leave neutrinos and gamma rays as decays products which move in the direction of the jet.
What are the exchange particles of the strong force and when are the mediators used
Mesons - interact between nuclei
Gluons - interact between quarks
Why can bound n-p states exist but not p-p or n-n states not
Due to pauli’s exclusions principle for both identical fermions to exist in the same quantum state. Therefore the total spin will be equal to zero. S = 0 potential well, however, is not deep enough to form a bound state whereas an S = 1 state (like deuteron) is.
Definition: Binding energy
The energy required to split nucleus into its elementary constituents. (or energy released when free protons and neutrons form a nucleus)
What are the 3 types of beta decay
Beta minus
Beta plus
K capture
In the Semi- empirical mass formula what do the following terms represent aA -bA^2/3 -s(N-Z)^2 /A -dZ^2 /A^1/3 Pairing term/A^1/2
aA - Bulk Term
a: Basic Binding energy per nucleon
analogy with bulk cohesive energy of liquid
-bA^2/3 - Surface term
Nucleons near the surface are less well bound (fewer
neighbours)
-ve as there is less binding energies for these nucleons
Contribution = proportional to SA = prop. to R^2 = prop
to A^2/3
-s(N-Z)^2 /A - Symmetry Term
Favours N = Z
Q-M explanation
-dZ^2 /A^1/3 coulomb term
Nucleus has a charge Ze in small vol. this is
energetically unfavourable. therefore this term is
proportional to the energy of the uniformly charged
sphere
Pairing Term
Empirical favours even-even nuclei over odd-odd and
odd-even
What does the liquid drop model not cover
Light Nuclei
Magic Numbers
Spin
Excited States
How is Nuclear spin-orbit coupling different to the one in atomic physics
i) not electromagnetic in origin
ii) Produces large splits which increase with l
iii) l +1/2 level is lower for nuclear coupling
What do magic numbers correspond to
Filling up to levels with a larger average gap to the next level there for it is more stable
Why does nuclear spin-orbit coupling explain the presence of magic numbers
S-O coupling splits the energy levels into discrete bands. Bands with a large gap in between are considered shells and therefore extra stable.
Rules when filling up shells of nuclear models
(a) Filled LEvels have a total angular momentum of 0
(b) Successive protons and neutrons pair off to give
angular momentum = 0
All even-even nuclei: J= 0
Even - odd: J=j of unpaired nucleon
odd - odd: no general rule
What are the different classification of particles
fermions and bosons ( half and integer spin respectively) Bosons: mesons and gauge bosons Fermions: Baryons, leptons,
Baryons and Mesons give hadrons
Explain why the cross section against energy spectrum (in neutron reactions) for light nuclei feature spikes at very large energies, compared to the heavy nuclei spectra which feature many peaks closely spaced but at much lower energies.
Heavy nuclei have more bound energy levels and so transitions between energy levels result in smaller changes in energies hence the spikes at smaller energies and more frequent spikes in the spectra.
Light nuclei feature less energy levels and so the jumps between give much higher energies.
How can the Omega - baryon (SSS ) exist? Why doesnt it violate paulis exclusion principle?
new quantum number ‘colour’ required to explain the existence. Each S quark in the omega - baryon has a different colour, so no violation of pauli as not all quantum numbers identical.
How do you work out baryon number and lepton number?
Baryon number= 1/3 ( no of quarks - no of anti quarks)
Electron Lepton number = Ne - Ne+ + N(electron neutrino) - N(electron anti neutrino).
etc
List the 4 fundamental forces and their associated properties
Gravity - mediator graviton - range inf
Couloumb force / electro magnetic force - mediator photons - range inf
Weak force - mediator w+ w- z0 gauge bosons - range 10^-18m
Strong force - mediators: gluons in quark and mesons in nuclei - range 10^-15m strong force additional property of asymptotic freedom (like a spring, increases in strength as separation increases).
Which of the fundamental forces can leptons feel
All except the strong force
What is the difference between a decay and a reaction
decay- going from 1 entity to multiple products
reaction - going from two entities to reaction products
List the conservation laws
Conservation of energy -angular momentum - quark flavour (except in weak interactions)- charge - baryon number - lepton number
Derive an expression for the lifetime of a virtual particle of mass m0, comment on the case of virtual photons.
HUP delta E delta T = h bar /2
delta E = m0c^2
solve for delta T