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.
