Particle Physics Flashcards
What observation from the Rutherford scattering experiment provided evidence for the atom being mainly empty space?
Most alpha particles passed straight through
What observation from the Rutherford scattering experiment provided evidence for the centre of the atom (nucleus) being charged?
Some alpha particles were deflected through very large angles
What observation from the Rutherford scattering experiment provided evidence for the size of the nucleus being much smaller than the whole atom?
Very few particles were deflected through angles greater than 90 degrees
What observation from the Rutherford scattering experiment provided evidence for most of the mass being concentrated in the nucleus?
A large mass would be required to deflect alpha particles at an angle greater than 90 degrees
What were the main conclusions from the Rutherford scattering experiment?
- Most of the atom is empty space
- The nucleus at the centre of the atom is charged
- The nucleus is much smaller than the size of the whole atom
- Most of the mass of the atom is concentrated in the nucelus
Why are high energies required in particle accelerators?
To overcome repulsive forces between particles
Why do anti-particles produced in particle accelerators only survive for a couple of seconds?
They annihilate
How do cyclotrons accelerate charged particles?
- There is an alternating pd/electric field
- This accelerates the charged particles between the dees
- The presence of a magnetic field causes the particles to move in a circular path
- As the velocity of the particles increases, the radius of the circular path increases
- The time for which the particle is in the dee remains constant
What fields are used in a cyclotron and what are they used for?
- Magnetic field: force at right angles to create circular motion
- Electric field: accelerates particles to maintain circular motion
What are the key features of the linac?
- Consists of a long straight tube containing a series of electrodes
- The charge on each electrode alternates between positive and negative
- Electrodes are connected to an alternating pd supply so the charge of each electrode is continuously changing between positive and negative
How does a linac accelerate charged particles?
- The alternating pd supply and alternating electrodes means that the electric field between each pair of electrodes is constantly changing over time
- The alternating pd means the particle is always attracted to the next electrode along and repelled from the previous one
Why does the length of each electrode in the linac increase?
To compensate for the increasing speed so that the particle stays in each one for the same amount of time
How does thermionic emission work?
- When the surface of a metal is heated, its free electrons gain thermal energy
- When they have enough energy, they break free from the surface
- Once emitted, they can be accelerated using an electric field
What accounts for the circular path of electrons in the electron gun?
- The magnetic force on electrons acts perpendicular to the plane containing B and v
- Therefore force acts towards the centre of the circle
- This provides a centripetal force on the electron that maintains circular motion
How many quarks make up a baryon?
3 quarks or 3 anti-quarks
What is the baryon number for a baryon?
1
What is the baryon number for an anti-baryon?
-1
What is the baryon number for a quark?
1/3
What is the baryon number for an anti-quark?
-1/3
What is the baryon number for a meson?
0
How many quarks make up a meson?
2, a quark and anti-quark pair
What is the charge of an electron and muon?
-1
What is the charge of an electron neutrino and a muon neutrino?
0
What quantities always have to be conserved in an equation for decay etc?
- Baryon number
- Mass-energy
- Momentum
- Charge
- Strangeness
What is a bubble chamber and how does it show the presence of particles?
- Hydrogen kept in a liquid form above the normal boiling point at high pressure
- Pressure suddenly reduced when a particle passes through and bubbles of gas form, showing a trail of ions
- Particles with opposite charges curve in opposite directions
What is a relativistic particle?
One travelling close to the speed of light, so a change in mass occurs
What type of particle is a proton?
Baryon
What type of particle is a neutron?
Baryon
What type of particle is a pion?
Meson
What type of particle is a kaon?
Meson
What is the strangeness of a proton?
0
What is the strangeness of a neutron?
0
What is the strangeness of a pion?
0
What is the strangeness of a kaon?
±1
What do kaons decay into?
Pions
What do pions decay into?
Muons
What does a muon decay into, and how is this done?
Into electrons and anti-neutrinos via weak interaction
What does an antimuon decay into?
Positrons
What is quark confinement?
Quarks are never found in isolation, only in pairs or triplets
What happens to quarks in beta decay?
A down quark turns into an up quark
What happens to quarks in beta plus decay?
An up quark turns into a down quark
