Particle physics DONE Flashcards
Explain why the tubes gradually increase in length in the first part of the linac accelerator.
The speed of the electrons increases along the accelerator
Thus the tubes have to increase in length so that the electrons spend the same time in each one
This is needed because the alternating voltage has a fixed time period so tubes switch polarity at fixed time intervals
What is connected across the terminals of a linac?
A high frequency
Alternating voltage
Why do electrons travel with constant velocity in the tubes of a linac?
Because there is no electric field in the tubes
So the electrons experience no force in the tubes and thus don’t accelerate
Why are the tubes the same length at the end of a linac?
At the end of the accelerator the electrons approach the speed of light and their mass increases
The tubes are the same length because the speed of the electrons have become a maximum/ constant
State the relationship between time period of particle’s circular motion and radius of circle. Include conditions on this relationship.
T is independent of r
Assuming m is constant/ relativistic effects don’t apply
State the purpose of the electric field in the cyclotron.
The electric field between the dees accelerates the particles
State the purpose of the magnetic field in the cyclotron
The magnetic field keeps the particles moving in a circular path
Why is the p.d. across the dees in the cyclotron alternating?
Protons need to be accelerated between the dees
Every half rotation, polarity of dees needs to change to attract the proton from one side to another
With reference to the magnetic field and the alternating potential difference, explain how the cyclotron produces a beam of high speed particles.
Electric field accelerates particles between dees
Polarity of dees switches every half cycle
Magnetic field provides a force perpendicular to the path of the particles
This is the centripetal force that causes the particles to move in a circular motion whilst in the dees
How does the kinetic energy of the protons increase as they follow a circular path in the cyclotron?
Electric field between the dees causes protons to accelerate
Dee facing proton is always negative/ p.d. reversed each time proton reaches gap
KE increases each time proton crosses the gap
What are 2 differences between how the linac and cyclotron work?
Cyclotron uses magnetic field, linac doesn’t
They manage increasing speed differently: linac increases length of drift tubes, cyclotron increases radius of particles’ path
When energy is converted into mass you have to make equal amounts of matter and antimatter. What is this process called?
Pair production
If the photon has energy above the minimum required for pair production, states what happens to the excess energy.
Converted into kinetic energy of particle and antiparticle
State the condition for pair production to occur for a particular type of particle and antiparticle..
Photon must have enough energy to produce the total mass of the particle and antiparticle
E= 2mc^2
Where does pair production tend to happen to help conserve momentum?
Near a nucleus
Describe what is likely to happen to an antiparticle shortly after its creation.
Likely to meet their particle and they will annihilate
Releasing two photons
When all the mass of the particle and antiparticle gets converted into energy, a pair of photons are formed. State what this process is called.
Annihilation
Describe the conclusions Rutherford reached about the structure of gold atoms as a result of the alpha particle scattering experiments.
Atom mainly empty space
Nucleus is charged
Mass concentrated in centre of atom
Describe what happens when a charged particle passes through a substance.
It causes ionisation
Electrons are knocked out of atoms
The particle leaves a trail of ions as it goes
Cloud chambers and bubble chambers show up charged particles. Describe what can be inferred about short and heavy tracks and a particle which may cause this track.
Short and heavy means the particle causes lots of ionisation
Alpha particle
Cloud chambers and bubble chambers show up charged particles. Describe what can be inferred about faint and long tracks and a particle that may cause this track.
Faint and long shows the particle causes less ionisation
Beta particle
State why a photon leaves no track in a bubble chamber.
Because a photon causes no ionisation
Positive and negative particles curve opposite ways. State how we distinguish which is which.
Fleming’s left hand rule
Explain why particles follow a curved track instead of a circular track.
Interactions with the detector decreases the kinetic energy and hence momentum of the particles
In a bubble chamber what does a straight line show?
Incoming beam of charged particles that pass through chamber without interacting
A little spiral comes off one of the straight tracks in the bubble chamber. State what this shows and what can be deduced from this.
Little spiral shows a knock-on electron
Tells you which way the particles are going
And which way negative particles curve
Neutral particles don’t make tracks. State the 2 events when you can see them.
When they decay
When they interact
What can be deduced from a V shape starting in the middle of nowhere in a bubble chamber/ cloud chamber?
A neutral particle has decayed
Into 2 oppositely charged particles
State why the tracks are sometimes almost straight in the V shape instead of curved.
Because the particles have such a large amount of momentum
Radius of curvature is so large
Explain why some neutral particles seem to survive much longer than normal/ what we would expect. Name this effect.
Because the particles are travelling close to the speed of light
They experience relativistic time dilation
State the structure of a baryon.
3 quarks
or 3 antiquarks
State the structure of a meson.
A quark and an antiquark
All baryons, except protons, are unstable. State what happens to all baryons except protons.
They decay to a proton
State the main 2 groups of particles: the group of fundamental particles and the group with particles that are made up of quarks.
Hadrons
Leptons
Name the 6 types of quarks.
Top Bottom Up Down Charm Strange
State what is meant by quark confinement.
Its not possible to isolate a quark.
Conservation laws tell us if a reaction/ interaction is possible. State the 5 conservation laws.
Conservation of momentum Conservation of mass- energy Conservation of charge Conservation of baryon number Conservation of lepton number
Explain what a charge of +2/3 means.
2/3 the charge of a proton
State a similarity between a particle and its antiparticle.
Same mass/ magnitude of charge
State one difference between a particle and its antiparticle.
Opposite charge
State what is meant by relativistic.
Travelling close to the speed of light
State why it is necessary to use very high energies in experiments where particles collide and produce other particles.
High energy needed to create antimatter/ overcome large repulsive forces/ break particles into their constituents
State the strangeness value of the strange quark.
-1
State what is meant by a muon or tau particle, what happens to them eventually and why and what comes with them when they are produced.
Heavy electrons
Eventually decay to ordinary electrons because they are unstable
Come with individual neutrinos when they are produced
