Please enable JavaScript.
Coggle requires JavaScript to display documents.
Nuclear and Particle Physics (Particle Detectors (Bubble Chamber (Photo…
Nuclear and Particle Physics
Atomic Structure
History of Atomic Structure:
Concept of atoms around since the Ancient Greeks
1804: John Dalton proposed that matter is made up pf tiny spheres that cannot be broken up (atoms) and each element is made up of different spheres
JJ Thompson: electrons can be removed from atoms - disproves Dalton's theory that atoms cannot be broken up. This suggested atoms were spheres of positive charge with tiny negative electrons stuck in them - plum pudding model/Thompson model
1909: Rutherford (scattering experiment) - studied scattering of alpha particles by thin metal foils
Rutherford's Scattering Experiment
Stream of alpha particles from radioactive source fired at very thin gold foil
Alpha particles strike a fluorescent screen to produce a tiny visible flash of light
Flashes recorded and the number of alpha particles scattered at different angles counted
Thompson Model would suggest that all the flashes should have been seen within a small angle of the beam
Results:
Most alpha particles passed straight through the foil
Some deflected through large angles
Few deflected through angles greater than 90°
Conclusions:
Atom mainly empty space
Nucleus is charged
Most of mass concentrated in the nucleus
Diameter of nucleus tiny compared to diameter of atom
Particles in Magnetic Fields
A current carrying wire can experience a force in a magnetic field due to current being a flow of negative electrons
Force on a moving charge in a magnetic field is always perpendicular to the direction of travel - centripetal
Particle Accelerators
Cyclotron uses circular deflection
Charged particles fired into one of the electrodes where the magnetic field causes them to follow a [semi]circular path and leave the electrode
An applied alternating potential difference across the gap accelerates the particle across the gap until it reaches the next electrode - acceleration causes greater speed ∴ greater momentum ∴ circular path in next electrode has greater radius
Potential difference changes direction just as the electron is about to leave the electrode so it is accelerated again into the first electrode
Repeats until radius so large that particle leaves cyclotron
Made up of two hollow semi-circular electrode with uniform magnetic field applied perpendicular to plane of electrodes, and an alternating potential difference applied between the electrodes
Applications in medicine such as producing radioactive tracers or high energy radiation beams for radiotherapy
KE=QV
Linac
Long, straight tube containing series of tube-shaped electrodes of alternating charge
Electrodes connected to alternating potential difference supply so charge of each electrode continuously changes ∴ electric field between each pair of electrodes continuously switches direction
Speed of the particle increases each time it passes an electrode - the increase in speed is compensated for by increasing the length of the electrodes so that the particle spends the same amount of time in each electrode
The high energy particles that leave the linac collide with a fixed target at the end of the tube
Electron Gun
Thermionic emission - free electrons in the metal gain enough thermal energy to break free from the surface of the metal
Heating coil heats metal cathode
Electrons emitted are accelerated towards cylindrical anode by electric field set up by high potential difference
some electrons pass through a small hole in the anode creating a narrow electron beam
Electrons in beam move at constant velocity as no potential difference ∴ no force beyond anode
Electron beam may be passed through an applied magnetic field to direct electrons towards something e.g. in electron microscope
Particle Detectors
Cloud Chamber
Uses super cooled vapour (something that is still a gas below its usual condensation temperature)
Ions left by particles cause vapour to condense leaving vapour trails
Heavy short tracks = lots of ionisation e.g. alpha particles
Bubble Chamber
Uses superheated liquid hydrogen kept as a liquid above its boiling point by putting it under pressure
If pressure suddenly reduced, bubbles of gas form where there is a trail of ions
Photo must be taken quickly before bubbles grow too big
Straight lines - incoming beam
Several particles will carry on straight through and can be ignored
Spiral from straight track - knock-on electron
Electron kicked out of hydrogen atom - tells you which way particles are travelling and which way negative particles curve
Point with several curved tracks shows reaction - positive and negative particles can be identified from the way they curve
Kept in strong magnetic field so charged particles follow a curved path
Charged particles are deflected by a magnetic field - experience a force so follow a curved path - positive and negative particles curve opposite ways
Spark Chamber
Faster than Bubble Chambers
Set of thin metal plates close together in an inert gas
When high potential difference is applied to plates, sparks can cascade along a trail of ionised particles left by a charged particle
Photo is taken of the spark trail
Can be reused in a fraction of a second as sparking clears ions when the potential difference is removed
Drift Chamber
Consists of separate wires instead of plates (spark chamber)
Wires arranged so that the computer can detect arrival of ions created when they drift to a detection wire
Momentum of particles proportional to the radius of the track
If field directed into paper, negative particles curve clockwise, positive anticlockwise according to Fleming's Left Hand Rule
Neutral particles do not leave tracks as do not produce much ionisation - travel in straight lines, paths often inferred from presence of gaps between visible tracks - possible to determine momentum by applying principle of conservation of momentum
Neutral particles can only be seen when they decay or interact
V Shape starting in middle of nowhere: two oppositely charged particles from the decay of a neutral particle - distance from interaction point to the V depends on half life of neutral particle
Paticles travelling close to the speed of light - relativistic time dilation
time appears to run more slowly for moving particle so seems to survive for longer than normal
Particle Classification
Quarks - Fundamental particles, building blocks of hadrons
Antiquarks have opposite properties of quarks
Bottom and Charm quarks discovered in 1970s and most already discovered quarks and leptons came in pairs ∴ top quark predicted by symmetry of model
Evidence for quarks came from hitting protons with high energy electrons
High energy electrons have short de Broglie wavelength meaning they can e used to probe tiny distances inside a proton - way in which electrons scattered showed three concentrations of charge (quarks) inside proton
Quark confinement - impossible to get quarks by themselves
Even if blasted with enough energy, quarks wouldn't separate as energy gets changed into more quarks and antiquarks - pair production to make mesons
Hadron
Baryon
Made of three quarks, e.g. proton uud, neutron udd
Meson
Quark and antiquark
Pion - combination of up, anti-up, down, anti-down
Kaon - strange/antistrange with up/down/anti-up/anti-down
Lepton
tau
tau neutrino
electron
electron neutrino
muon
muon neutrino
Particle Interactions
When energy converted into mass, same amount of matter and antimatter created - pair production - two photons fired together at high speeds
Only happens if photon has enough energy to produce that much mass - often happens near nucleus to help conserve momentum
Usually electron-positron pairs as have relatively low mass
Photon must have at least combined energy of the two particles die to their masses (assuming particles have negligible KE) - minimum energy = E(λ)
annihilation - particle meets antiparticle - combined mass converted into energy in form of pair of photons
