Topic 8 - Particle/Nuclear Flashcards
Nucleon number
number of neutrons and protons in the atom
Proton Number
number of protons in the nucleus
Rutherford’s scattering experiment
A stream of alpha particles from a radioactive source were fired at a very thin gold foil sheet. The number of alpha particles at different angles was recorded
Rutherford’s scattering experiment conclusions
- most (fast charged) alpha particles went straight through therefore an atom in mostly empty space
- some alpha particles deflected at an angle greater than 90 therefore part of the atom must be more massive than the alpha particle this is the nucleus
- alpha particles were repelled therefore the nucleus must be positively charged
- since atoms are neutral overall so electrons must be in the outside of the atom
Nuclear model
- concentrated mass in centre
- strong positive charge in centre
- negative charge spread across remaining atom
Thermionic emission
the process by which free electrons are emitted from the surface of a metal when external heat energy is applied
How do electron guns work
- thermionic emmisionrelease electrons
- electrons accelerated by electric field
- passed through a small hole so the electrons are in a beam
energy gained by electron (eV) =
accelerating voltage
how does a cyclotron work
two semi circular electrodes with alternating charge have a gap between them. The electric field between the electrodes accelerates the charged particle across. A perpendicular magnetic field is applied to keep the particle moving in a circular path.
Why does the radius of a charged particle in a cyclotron increase
because velocity is proportional to radius so as it is accelerated the radius increases.
How does a LINAC work
A high frequency AC current is applied to the electrodes so that their charge changes from + to - . The charged particle is always repelled from the previous electrode and attracted to the next one hence causing it to be accelerated through the electric field to the next electrode.
Why does the length of electrodes on an LINAC increase
the length of the electrodes increase so that the particle has the same acceleration even when it is moving faster.
Tubes switch polarity.
Hadrons
particles that feel the strong interaction
Baryons
hadrons made of 3 quarks
Mesons
hadrons made of two quarks: a quark and an anti-quark
proton
Baryon: uud
Neutron
Baryon: udd
K+
Meson: u ŝ
K
Meson: d ŝ
K -
Meson: s û
π+
Meson: u antidown
π
Meson: u û
OR
d antidown
π-
Meson: d û
Anti Mesons
K and π are their own anti particles
Whereas K+ is the antiparticle of K-
Leptons
fundamental particles that don’t feel the strong interaction. They interact with other particles via the weak interaction, gravity and the electromagnetic force.
Electrons
stable leptons
Muons
heavy unstable leptons (eventually decays to an electron)
Tau
heaviest least stable lepton
Neutrino
Electrons, Muons and Taus have their own neutrino which has zero mass and zero charge
Neutron decay
unstable so decays to a proton via beta decay
Antiparticles
each particle has a corresponding antiparticle with identical mass and opposite charge, baryon and lepton numbers
E=mc^2
energy can turn into mass and mass can turn into energy.
when energy is converted to mass you make equal amounts of matter and antimatter
pair production
if a particle is produced an anti-particle must also be produced
Relativity
the mass of an object increases as velocity increases due to relativistic effect
converting from kg to MeV/c^2
- convert mass to energy (e=mc^2)
- convert to MeV
converting from MeV/c^2 to kg
- Convert to J/C^2
- divide by C^2
eV -> joules
- e
joules -> eV
/ e
production of an anti particle pair
only happens if gamma proton has enough energy to create mass. happens near a nucleus to conserve momentum.
Annilihation
occurs when a particle meets an anti-particle. All mass is converted into energy
Conservation Laws in Particle Reactions
- charge
- Baryon Number
- Lepton Number
- Mass/energy
- momentum
detecting charged particles
charged particles cause ionisation therefore leave a trail of ions
Cloud chambers
supercooled vapour condenses when a particle passes through
Bubble chambers
Hydrogen kept as a liquid (above normal boiling point). If you quickly reduce the pressure bubbles of gas form where there are trails of ions
Charged particles in magnetic field
circular paths
Charged particles in electric fields
parabolic paths
spiral paths
the particle is interacting and losing energy
Neutral particle tracks
there aren’t any
Why are collisions high energy
Energy required to overcome electrostatic repulsion. Since particles move fast the energy/momentum must be high, shorter de broglie wavelength.
Why are linac tubes at the end the same length?
the speed of the particle has become a maximum
ionisation
Electrons have been removed/added from a molecule
charged particle in magnetic field
circular
charged particle in electric field
parabolic (tries to get in line with electric field)
Why don’t photons leave a trail
they’re neutral
When two particles are formed from a photon why do their tracks curve away from each other?
they are in a magnetic field implying one is positively charged and one is negatively charged, charge is conserved.
what does the curvature of the spirals for a charged particle in a magnetic field tell you?
the momentum of the particle (more momentum means bigger radius)
difference between electric field and magnetic field on a charged particle
electric might do work but magnetic field never does work
When alpha particles are fired at a gold foil what happens to most of them/ what does this mean?
- they pass straight through
- most of the atom is empty space
Why are some alpha particles scattered through 180 when fired at gold foil?
- most of the mass is in the centre which is charged positively hence deflecting the positive charge of the alpha particle
what is the force that causes deflection of charged particles?
electrostatic repulsion
What happens to the path of deflection if the charge is twice as much?
- deflection starts earlier
- the final deflection is greater
Use of electric fields in particle detectors
- used to accelerate/deflect particles
- direction of deflection indicates charge (work is done to make particle move in same direction as the field)
derive a = EQ/m
F = EQ F=ma a = EQ/m
use of magnetic fields in particle detectors
- produces circular motion
- direction of curvature indicates force (flemings LHR)
- momentum found from radius of curvature
Kinetic energy transferred when a charge accelerates across a potential difference
E = QV
why are only a low proportion of decays detected?
- emmisions in all directions
- some emitted particles may be absorbed by the material in the sample
- some emitted particles may be absorbed by the window
- some pass through the detector
creation
creates a particle and an antiparticle. E = mc^2
annilhation
a particle and its anti-particle are destroyed simultaneously in a conversion to energy (2 photons)
electronvolt
energy required to accelerate an electron through a pd of 1v
Antimatter
same mass, opposite charge (+other properties)
electric fields…
accelerate particles (speed up + change direction)
magnetic fields…
accelerate particles (change the direction into a circular path)
what can particle tracks be used to work out?
- charge
- mass
- energy
LINAC
a series of electrode tubes of increasing length with an AC pd applied across them
what does firing electrons at a hydrogen target tell us
- proton is not uniform, it has some empty space since some electrons passed through
- made up of smaller particles called quarks
fundamental particle
has no internal structure / not made up of other particles
atomic process that produces emission spectra
electron drops down energy levels and de-excites releasing energy
why were scientists able to predict the 6th quark?
- standard model symmetry
- quarks came in pairs
- 6 known leptons by only 5 known quarks
why is GeV/c a unit of momentum
GeV/c^2 is a unit of mass
p = mv
Why did it take a long time to find experimental evidence for the top quark
large mass, needs a lot of energy
quark order
Up, Down, Charm, Strange, Top, Bottom
Last quark to be discovered
Top
lightest quark
up
heaviest quark
top
lambda particle
baryon: u d s
