Particle Physics - accelerators, detectors and particle tracks

Question 1

What are the Relativistic Effects on a particle?

Answer 1

Particles sometimes have a longer lifetime

Question 2

What is a ‘Linac’?

Answer 2

Linear Accelerator

Question 3

What is a ‘Cyclotron’?

Answer 3

Circular Accelerator

Question 4

Explain how a linac accelerates a PROTON

Answer 4

1. The proton is released from its source.
2. The drift tubes are connected alternately to each end of the parallel circuit which has an alternating current power supply, which charges them.
3. A potential difference is produced between the tubes and the source, in the vacuum surrounding the tubes and source.
4. As the electrostatic forces of attraction (due to the next tube being oppositely charged) and the electrostatic forces of repulsion (due to the tube it is in being equally charged) the particles accelerate through the tubes.
5. The tubes get longer the further you go from the source of the particle and the frequency of the alternating current remains constant, SO it accelerates. (the particle spends the exact same time between the tubes each time)

• Go to slide 119 of Miss’ presentation on Particle Physics for useful diagram

Question 5

Explain how a cyclotron accelerates an ELECTRON

Answer 5

1. The electron is released from its source in the centre.
2. Two dees (dee - a semi-circular hollow structure which can be charged) are connected at either end of a circuit, which has an alternating power supply. There is a gap between the dees which is a vacuum.
3. As the electron is released from the source, into the vacuum gap, the initial potential difference between the two dees (as they will be oppositely charged) causes the electron to be attracted into the positively charged dee.
4. As there is a static magnetic field also passing through the dees, this causes the electron to initially start travelling in a spiral pattern.
5. As the magnetic flux density (strength of magnet) does not change, the particle’s radius does NOT change when it is in either of the dees.
6. The radius of the particle increases due to the potential difference between the dees and the radius of the path of the electron ONLY increases when it travels through the gap between the dees. This is because between the gap, the electrostatic forces from the potential difference increases the velocity of the electron, so as the ‘v’ increases in F = mv^2 / r and if ‘m’ and ‘F’ stay constant, then ‘r’ must increase. SO, in the gap, the radius slightly increases, with each rotation.
7. Finally, it is sent out a window at the edge of a dee, directing the electrons to form a beam.

(sorry it’s a bloody long answer)
* Go to slide 110 of Miss’ presentation on Particle Physics for useful diagram

Question 6

What is the purpose of the magnetic field through the cyclotron?

Answer 6

Initiate the centripetal force (turning course) of the particle

Question 7

What is the purpose of the vacuum gap between the dees in a cyclotron?

Answer 7

• Potential Difference is induced across the gap
• Thus accelerating the particle, within the gap
• Then velocity of the particle must increase as the radius increases BUT the frequency at which the alternating power supply does not change
• So particle speeds up (accelerated)

Question 8

What is the purpose of the drift tubes getting longer, the further you go from the particle source?

Answer 8

• Frequency of the alternating power supply remains at a constant value
• So by making tubes longer, particle must accelerate
• BECAUSE of the electrostatic forces of attraction/repulsion that will be induced in the gap/tubes

Question 9

Which rule do you use to know what effects a specific magnetic field will have on a particle (in terms of force)?

Answer 9

Fleming’s Left Hand rule

F - thumb - force
B - index - magnetic field
I - middle - conventional current

Question 10

If a proton is in a cyclotron with a magnetic field passing into the page (say we are looking at it from above), in which direction will the proton spiral?

Answer 10

To the left, an anti-clockwise spiral