Physics__Nuclear and Particle Physics Flashcards
What is the nucleon number?
Mass number - the number of protons and neutrons in nucleus
Whats the proton number?
Atomic number - number of protons in nucleus
What are isotopes?
Atoms of an element with the same number of protons but a different number of neutrons.
What evidence is there for the nuclear model of the atom?
- Alpha Particle Scattering
- Alpha particles were fired at thin gold foil with a detector on the other side to count how many particles deflected at different angles
What were the results of alpha particle scattering?
- The majority of alpha particles went through foil without detection - this suggested mainly empty space
- Some alpha particles were deflected at angles <10* - this suggested a positively charged nucleus was concentrated at the centre (2*+ve charges repel)
- Only very few alpha particles deflected straight back at angles > 90* - this suggested the nucleus is very small and is where the mass and charge of the atom is concentrated (massive)
- it was concluded: Atoms are small, dense, positively charged nuclei
How has our understanding of atomic structure changed over time?
- Rutherford scattering demonstrated the existence of a nucleus Before this scientists believed in Thomson’s plum pudding model
- where the atom was made up of a sphere of positive charge, with small areas of negative charge evenly distributed throughout
What’s thermionic emission?
Where a metal is heated until the free electrons on its surface gain enough energy and are emitted.
How can electrons be accelerated?
- Electric fields can be used in order to increase their velocity
- Magnetic fields can be used to accelerate radially (as the force experienced by an electron moving in a magnetic field is always perpendicular to its motion).
How do electron guns work?
- Potential difference accelerates electrons which are released from the cathode by thermionic emission.
- The electrons accelerate towards anode which has a small gap
- The electrons pass through and form a narrow electron beam, travelling at a constant velocity past the anode
What are the two types of particle accelerators?
Linear accelerator (LINAC) - uses an alternating electric field
Cyclotron - uses a magnetic field and an alternating electric field
Why are electron guns useful?
They produce electron beams of relatively low energy to be used in particle accelerators.
How does a linear accelerator work?
- Several cylindrical electrodes (drift tubes) gradually increase in length along the LINAC
- Particles accelerate due to electric field
- Acceleration takes place in gaps between tubes
- Adjacent electrodes are connected to the opposite polarity of an alternating voltage
- Alternating electric field so that as electron emerges from one tube, the next is positive
- The polarity of the voltage of the first electrode is opposite to the polarity of the charged particles
- Time spent in each tube must be constant, as electrons travel faster, tubes must be longer
How does a cyclotron work?
- There is an alternating E-Field
- E- field accelerates particles in gap between dees
- Magnetic field perpendicular to plane of dees
- Particle path is curved by magnetic field (circular motion)
- As velocity of particles increases, radius of path in dees increases (r=p/BQ)
- Time the particle is in a dee is constant
How is circular motion possible in the cyclotron?
- The force exerted by the magnetic field is always perpendicular to the direction of travel,
- NOTE: the particle’s speed will not increase due to the magnetic field, which is why there is an alternating electric field between the electrodes
How do particle detectors work?
- A sample is vaporised
- An electron gun fires electron beam at the vapour and electrons collide with the particles in the vapour, which become ionised.
- Ions are accelerated using an electric field Ions enter velocity selector, where an electric and magnetic field are acting perpendicular to each other.
- Fields exert forces on the ions in opposite directions, so only ions for which the forces are balanced travel in a straight line and pass into the separation chamber.
- So only particles travelling at a particular speed progress into the separation chamber.
- In the separation chamber, there is a uniform magnetic field which exerts a force on the ions perpendicular to their direction of travel
- This causes them to follow a circular path and hit a screen, where the radius can be measured.
- Radius is used to determine mass to charge ratios - thus sample is identified.
Derive the equation for the radius of a charged particle in a magnetic field
Where p is momentum, Q is charge, and B is magnetic flux density
What must always be conerved during particle interactions?
- Charge
- Energy
- Momentum
- This can be used to interpret particle tracks in bubble chambers
Why are high energies required to investigate the structure of nucleons?
- When investigating an object, you must use waves with a similar wavelength to that of the object you are investigating
- The de Broglie wavelength shows a particle’s momentum and wavelength are inversely proportional
- The smaller the de Broglie wavelength needed, the higher the energy (momentum) required
- Nucleons are tiny (10-15m), so small wavelengths are used to investigate, thus v. high energies required
What did Einstein prove in the theory of special relativity?
mass and energy are interchangeable
E = mc^2
What is pair production?
- A photon is converted into an equal amount of matter and antimatter.
- (Only occurs when photon has energy>total rest energy of both particles, excess converted to KE)
What’s annihilation?
- A particle and its antiparticle collide,
- So: Their masses are converted into energy.
- This energy, along with the KE of the 2 particles, is released as 2 photons
- Moving in opposite directions to conserve momentum
What is the relativistic increase in particle lifetime?
- When particles travel at relativistic speeds, the length of their lifetimes APPEAR to change.
- This is due to time dilation…which causes time to run at different speeds depending on the motion of an observer.
- So, the lifetime of a particle moving at relativistic speeds recorded by a stationary observer is longer than the actual time.
What evidence is there for time dilation?
- Muon decay (experimental evidence)
- Muons enter the atmosphere at relativistic speeds, so experience significant time dilation.
- They have a lifetime of ~2microseconds which suggests they would decay before reaching Earth from the atmosphere
- However most muons are still present upon reaching sea level.
- This can only be explained by time dilation In accelerator collision experiments
- Time dilation grants particles a longer lifetime, allowing them to travel longer than expected, and so able to interact with more particles
What can particles be classified as in the standard model?
- Baryons (e.g. neutrons and protons)
- Mesons (e.g. pions)
- Leptons (e.g. electrons and neutrinos)
- Photons
- Note: both Baryons and Mesons are hadrons - they contain quarks and experience the strong nuclear force.
What are baryons?
Particles made from 3 quarks
What are mesons?
Particles made from a quark and an antiquark
What are leptons?
Fundamental particles
What information did the standard model help provide?
The symmetry of the model predicted the top quark
What are the charges of the different quarks (in terms of 1.6x10-19)?
- Up 2/3
- Down -1/3
- Charm 2/3
- Strange -1/3
- Top 2/3
- Bottom -1/3
What are the charges of the leptons (in terms of 1.6x10-19)
- Electron -1
- Muon -1
- Tau -1
- Electron neutrino 0
- Muon neutrino 0
- Tau neutrino 0
What are the charges of the different force carriers?
Gauge Bosons
* Gluon 0
* Photon 0
* Z boson 0
* W boson +/- 1
Scalar bosons
* higgs 0
What are the baryon and lepton numbers of the quarks?
Baryon number = 1/3
Lepton number = 0
How can we determine if a particle interaction is possible?
Use the laws of conservation of:
* Charge
* Baryon number
* Lepton number
Proton and neutron quark composition?
- Proton: Up Up Down
- Neutron: Down Down Up
