8. Nuclear and Particle Physics Flashcards
What did Thomson’s plum pudding model show?
That the atom was made up of a sphere of positive charge.
- With small areas of negative charge evenly distributed throughout.
What was done in the Rutherford scattering experiment?
- an alpha source supplied alpha particles.
- There were fired at gold foil in an evacuated chamber that was covered in a fluorescent coating.
- A microscope was used to observe the path of alpha particles.
What was observed from the Rutherford experiment?
- Most alpha particles passed straight through the foil with no deflection.
- A small number of particles were deflected by a large amount
- very few particles were deflected by more than 90ᵒ
What was concluded from the Rutherford experiment
The atom has a small, dense, and charged nucleus at its centre.
What is thermionic emission?
Where a metal is heated until free electrons on its surface gains enough energy and are emitted.
How can electrons be accelerated?
- Electric fields: to increase their velocity.
- Magnetic fields: to increase their radial acceleration.
Why does an electron accelerate radially in a magnetic field?
The force experienced by an electron moving in a magnetic field is perpendicular to its motion.
How do electron guns work?
- Use a potential difference to accelerate electrons.
- A cathode is heated to thermionically emit the electron.
- I then accelerate towards the anode.
- Once it passes the anode it moves at a constant velocity.
Describe what happens in a linear accelerator.
- Several electrodes (drift tubes), progressively increase in length along with the accelerator.
- Adjacent electrodes are connected to the opposite polarity of an alternating voltage.
- So an alternating electric field is formed in the gaps between electrodes.
- At the source charges particles are released so that their polarity of the voltage is opposite to the polarity of the charged particle.
- This accelerated the particle towards the first electrode.
Why does the length of each drift tube in a LINAC increase?
- So particles spend the same amount of time in each tube.
- so as the particle passes through the electrode, the polarity of voltage can reverse, meaning the particle is accelerated towards the next electrode
What components make up a cyclotron?
- Two semi-circular electrodes called ‘dees’, with a uniform magnetic field acting perpendicular to the plane of the electrodes.
- A high-frequency alternating voltage applied between the electrodes.
Describe what happens in a cyclotron?
- A charged particle moves from the centre of one of the electrodes
- It is deflected in a circular path by the magnetic field.
- Once the particle reaches the edge of the electrode they begin to move across the gap between the electrodes.
- They are accelerated by the electric field.
- This increases the radius of their circular path as it moves through the other electrode.
- When the particle reaches the gap again, the alternating electric field changes direction, allowing the particle to be accelerated again.
- This repeats several times until the required speed is reached by the particle and they exit the cyclotron.
How are electric and magnetic fields used in particle detectors, such as mass spectrometers?
- a sample is vaporised (turned into gas).
- An electron gun is used to create a beam of electrons that is directed towards the vapour.
- The electrons collide with the atom in the vapour causing them to become ionised. Now they are charged.
- The ions are accelerated using an electric field.
- They then pass into the velocity selector.
- This is where an elastic and magnetic field act perpendicularly to each other.
- The fields exert forces on the ions in opposite directions and only the ions in which the forces are balanced travel in a straight line.
- As they pass through the separation chamber, the uniform magnetic field exerts a force perpendicularly to their direction of motion.
- This causes them to follow a circular path and hit a screen, where the radius of path can be measured.
How do you find the radius of the path of a charged particle in a magnetic field?
F = BQv F = mv²/r
BQv = mv²/r BQ = mv/r r = mv/BQ
p=mv
r = p/BQ
During particle interactions what must always be conserved?
- Charge
- Mass-energy
- Momentum
- Baryon number
- Lepton number
How can charged particle tracks be observed?
Cloud or bubble chambers
How do bubble chambers work?
- A tank filled with superheated liquid hydrogen.
- This forms bubbles around any ionised particles.
- Therefore by observing the path created by the visible bubbles, you can see the path taken by moving charged particles.
- These will take circular paths as it is placed in a magnetic field.
How can u analyse a bubble chamber?
Find the radius of curvature of tracks:
- this allows you to find out the characteristics of the particle using the equation
r = mv/BQ.
Find the direction of curvature:
- this allows you to find out the polarity of charge using Fleming’s left hand rule.
Analyse interactions:
- if the track stops suddenly = the particle has collided.
- if the track abruptly changes direction = particles have collided.
- If track appears from nothing = charged particles have been created from an uncharged particle.
What happens in a creation event?
- a photon is converted into an equal amount of matter and antimatter.
- This only occurs if the photon has an energy greater than the total energy of both particles.
- Any excess energy is transferred into kinetic energy of the particels.
What happens in an annihilation event?
- where a particle and its corresponding antiparticle collide.
- its masses are converted into energy.
- This, along with the kinetic energy of the particles, is released in the form of 2 photons in the opposite directions in order to conserve momentum.
What is an electronvolt?
The kinetic energy of an electron accelerated across a potential difference of 1 V.
What is time dilation?
- occurs as a consequence of special relativity (relativistic effects).
- causes time to run at different speeds depending on the motion of an observer.
- as a result the lifetime of a particle moving at relativistic speeds recorded by a stationary observer would be longer than the actual time.
What experimental evidence is there of time dilation?
Muon decay.
Why might a particle travelling at close to the speed of light collide with more particles?
- as particles are moving close to the speed of light…
- their lifetimes are longer due to time dilation (relativistic effects).
- So particle travel for longer than expected…
- and so they interact with more particles.
