08 - Nuclear And Particle Physics Flashcards
What is nucleon number?
The sum of the number of protons and neutrons in the nucleus.
What is atomic number?
The total number of protons in the nucleus.
Outline Rutherford’s alpha scattering experiment.
High speed alpha particles were fired at a very thin sheet of gold foil. The deflections of the particles were measured and conclusions were drawn.
What was observed in the Alpha scattering experiment?
- Most of the alpha particles passed straight through the gold atoms
- Some of them were deflected
- A few of them were deflect backwards
What is thermionic emission?
Thermionic emission is the release of electrons due to heating.
Explain why electrons are released from a heated filament.
As the filament heats up, free electrons inside the metal gain kinetic energy. When the surface electrons gain sufficient energy, they are released from the surface.
What will happen to a beam of electrons if it is passed through a potential difference?
The beam of electrons will be accelerated since work is done by the potential difference.
How do you calculate the energy transferred to an electron, when it is accelerated across a potential difference?
Energy = Charge x Potential Difference
Energy = eV
What happens when a beam of electrons is directed into a magnetic field?
The electron beam will be deflected, since magnetic fields apply forces on moving charges.
What is the magnitude of the force experienced by a moving electron in a magnetic field?
Force = Magnetic Flux Density × Charge
× Velocity
F = Bev
If the electrons are moving perpendicular to the field lines, which direction will the magnetic force act?
The force will act perpendicular to both the electron and field directions.
Describe the shape of the path of a beam of electrons passing through a magnetic field.
The beam will produce a circular path since the magnetic force always acts perpendicular to the electrons’ motion. This means it acts as a centripetal force and produces a circular path.
What is a cyclotron?
A cyclotron is a particle accelerator that uses magnetic fields to accelerate particles in circular paths. This allows higher speeds to be reached, without the limitation of the accelerator’s length.
Describe the basic composition of a cyclotron.
Cyclotrons consist of two D-shaped paths which are separated by a small gap. An alternating potential difference is applied across the gap.
How does a cyclotron work?
An electron beam is passed into the cyclotron, where it is deflected into a circular path by a perpendicular magnetic field. When the beam reaches the gap, it is accelerated by a potential difference. This increases the speed of the beam, causing the radius of the path to increase. This process repeats every half circle.
State the equation used to calculate the circular radius of an electron beam deflected in a magnetic field.
Radius =mV / BQ
What two equations must you combine to derive the radius equation?
- Centripetal Force = mv^2/r
- Magnetic Force = BQv
Centripetal Force = Magnetic Force
State the mass-energy equation.
E = mc^2
What is 1 MeV in joules?
(1.6×10^-19) × 10^6 = 1.6 x10^-13 J
In the quark-lepton model, what are the four main categories of particles?
- Baryons
- Mesons
- Leptons
- Photons
Describe the quark composition of a baryon.
Baryons are made up of three quarks.
Describe the quark composition of a meson.
Mesons are made up of a quark and antiquark pair.
Which category of particles are classed as fundamental particles?
Leptons
Give two examples of leptons.
- Electrons
- Neutrinos
What category of particles do pions belong in?
Mesons.
Give two examples of baryons.
- Protons
- Neutrons
What did the symmetry of the quark-lepton model predict the existence of?
The top quark.
What is an antiparticle?
An antiparticle is one that has the same mass but opposite charge and conservation numbers to its corresponding particle.
What is the antiparticle of a proton?
An antiproton.
What is the antiparticle of an electron?
A positron.
Name four things that are always conserved in a particle interaction.
- Mass/Energy
- Baryon Number
- Lepton Number
- Charge
Describe the conservation of lepton number.
The lepton number for each specific type of lepton must be the same before and after an interaction.
what was concluded from Rutherford’s gold foil experiment
atom is mostly empty space
mass is concentrated in a very small dense nucleus (concluded as most went through)
could tell nucleus was positive as some reflected
why does a cathode ray tube have to be in a vaccum
as otherwise te electrons would lose eneergy and ionise atoms in the air and the electron beam would not be produced
what is relativistic mass
as velocity increases mass increases at a certain point
explain how a cyclotron produces a high energy proton beam
- alternating p.d/electric field
- the electric field between the 2 dees causes the acceleration of the protons across the gap (between the 2 dees). alays accelerating across the gap as charge of 2 dees keeps flipping
- magnetic field is perpendicular to pare of dees
- proton path is curved by the magnetic field
- as velocity of proton increases, radius of path in dees increases
- therefore the time for which a proton is in a dee remains constant
how does a bubble chamber detector work?
As particles enter the chamber, a piston suddenly decreases its pressure, and the liquid enters into a superheated(maintained just below boiling). as the particle passes through the liquid it ionises particles in the liquid knocking of electrons leaving an ionised track, around which the liquid vaporizes, forming microscopic bubbles as these th electrons dissipate energy into the surrounding liquid causing localised boiling. Bubble density around a track is proportional to a particle’s energy loss.
how does a geiger muller tube work
wall of tube is a cathode as is a conducting material
anode carrying wire
radial electric field between the wall and the wire
gas is ionised by radiation causing an electron to be repelled by the cathode
leaving charged particles which gain energy so they ionise other gas atoms causing an ionising avalanche
describe the set up of a Linear particle accelerator (LINAC)
set of metal drift tubes in a line
electrons accelerated by electrc field between drift tubes
acceleration between gaps
adjacent tubes connected to opposite terminals of a power supply and opposite charge
charge of particle is opposite to the first drift tube charge
p.d / electric field is alternating so that the next tube is of opposite charge to particle so its accelerated
time spent in each tube must be the same so as the electron accelerates, tubes must be longer
explain how a cloud chamber works
It consists of a sealed container filled with a supersaturated vapour, typically a mixture of alcohol and water. When a charged particle passes through the vapour, it ionizes the molecules along its path, causing them to release electrons.. the ionised gas particles align surrounding alcoholk particles, condensing them into doplets
the greater the charge, the thicker the trail.
what is a quark
an elementary particle and a fundamental particle consistent of matter. quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons
Derive r = p / BQ
mv^2 / r = BQv
mv = BQr
p = mv
r = p / BQ
why are high energies required to investigate the structure of nucleons
High energies are required to investigate the structure of nucleons due to their small size. This is because the higher the energy of matter (such as electrons), the smaller their De Broglie wavelength, and so they are more suitable for investigating smaller objects (as the resolution increases).
What is the relationship between how curved the path of the particle and its velocity
The greater the velocity, the lower the radius of curvature due to F= mv^2 / r
Relationship between radius of curvature and momentum
r = p / BQ
Therefore the straighter the path if the particle, the greater the velocity and therefore the greater the momentum. Higher radius means less curved
In a bubble chamber, what does it mean if the particle leaves no track
If the particle leaves no track, this means it has no charge
How is the top quark predicted
The
Standard Model organizes quarks into three generations, each containing a pair of quarks. The first generation includes the up and down quarks, the second generation includes the charm and strange quarks, and the third generation includes the bottom (or beauty) quark and its predicted partner, the top quark
What does it mean if particle tracks apear from “nowhere” in a particle detector
This indicates particle-antiparticle creation
These paths are in opposite directions because the particle-antiparticle pair is oppositely charged
Therefore, the magnetic force on them is oppositely directed
However they have the same radius because they each have the same mass (and hence, momentum)
What does it mean if two particles spiral off in opposite directions in a particle detector
This indicates each particle is oppositely charged, because the (centripetal) magnetic force acts on them in the opposite directions
Why do particles spin inwards in a particle detector
you expect the radius to be decreasing, because charged particles will tend to continue ionising other particles around them - hence losing kinetic energy.
As their kinetic energy decreases, so does their momentum - and hence, track radius will also decrease.
Explain why electrons must be accelerated to very high energies if they are to be used to probe the internal structure of a proton.
The proton diameter ∼ 10^-15 m so the de Broglie wavelength of the electrons must be at most this size in order to resolve the internal structure of the proton
Since the de Broglie wavelength is inversely proportional to the momentum of the electrons, then they must be accelerated to very high velocity (and hence, energy) in order to obtain very short wavelengths
what happens to mass as velocity increases closeto the speed of light
mass would increase as velocity increases
what is pair production and what is required for it to happen
This is the fomation of an electron and a positron from a pulse of em energy travelling in the vicinity of an atomic nucleus. For this reaction to take place the energy of the em wave has to be greater than the rest-mass energy of the pair produced
what is electron - positron annihilation and what does it produce
it occurs when an electron and positron collide. the result is the conversion of the electron and positron and the creation of gamma ray photons so that it satisfies the conservation laws.
