Particle Physics

Question 1

What is the nucleon/mass number?

Answer 1

Nucleon/Mass number = number of protons + number of neutrons

Question 2

What is the proton/atomic number?

Answer 2

Proton/Atomic number = number of protons (=number of electrons)

Question 3

What is an isotope?

Answer 3

The isotopes of an elementary all have the same proton/atomic number but different number of neutrons. So the same number of nuclear protons and surrounding electrons, but different numbers of neutrons in the nucleus.

Question 4

What is Rutherford’s alpha particle scattering experiment and what are the conclusions from it?

Answer 4

Alpha particles from a natural radioactive source were fired at a vert thin sheet of gold. Most passed through undeflected from their path; others were deflected through small angles. A tiny fraction - fewer than 0.01% were deflected by more than 90˚.

Conclusions from the experiment:

1. Atoms contain a nucleus which is very small (very small percentage of alpha particles are scattered back)
2. Therefore most of atoms is empty space
3. Nucleus has the same charge (+) as the alpha particle
4. The nucleus must have a large mass in comparison to the alpha particle (otherwise it would get knocked out of the way)

Question 5

Describe Millikan’s oil drop experiment

Answer 5

Millikan’s oil drop experiment (to find the charge on an electron)

An atomiser is used to spray oil drops into the chamber. The drops become charged by friction with the nozzle of the atomiser (or from ionising radiation). Some of the oil drops fall through a hole in the top plate and enter the region between the parallel plates. A p.d. is then applied to the plates and changed until one of the drops becomes stationary, then the forces are balanced:
Weight = electrostatic force + upthrust (weight of displaced air)
Then the supply is turned off and the drop is allowed to fall freely:
Weight = upthrust + viscous drag
By rearranging these equation a value for the charge of one drop can be found. Then from different values of charge Millikan found a common factor, e, charge on an electron (so charge was quantised).

Question 6

Describe the process of thermionic emission

Answer 6

Thermionic emission - when a piece of metal is heated up to a high temperature, negatively charged electrons ‘bubble’ out of its surface. Of course, they will be attracted back to the surface by the positively charged proton they leave behind. But if a positively charged plate is placed near the piece of metal in a vacuum, the electrons accelerate towards it and can be made into a narrow beam.

Question 7

Describe the electron gun

Answer 7

Thermionic emission from the heater, made of tungsten filament, which is connected to an a.c. supply. This produces a cloud of electrons/a space charge. An accelerating voltage is connected to a hollow cylinder, which becomes positively charged. The electric field strength inside the cylinder is 0.

1/2 mv^2 = eV

Relativity may need to be considered as v is close to c,
m=mo/√(1-v^2/c^2)

Question 8

What happens to an electron in an electric field?

Answer 8

An electron will experience a force Fe towards the positive plate. This force is constant.
The force is perpendicular to the velocity of the electron in the x-direction, and so there is no change in the speed in the x-direction.

From NII: F=ma => EQ=ma => V/d = ma => a =eV/md

Then use SUVAT

Question 9

Describe a Cathode Ray Oscilloscope

Answer 9

The electrons feel a constant force towards the positive plates, this does not change its horizontal speed.
When the electron leaves the region between the plates there is no force acting on it (ignoring the weight) and so it continues to move in a straight lien at constant speed.
The plate sensitivity is the p.d. applied to the CRO to move the spot on the screen.

Question 10

What happens to an electron in a magnetic field?

Answer 10

The electrons are released during thermionic emission and then enter the magnetic field. The magnetic force (Bev) is always perpendicular to the velocity and therefore does not change the speed. Therefore the electrons travel in a circular motion.

F = ma
Bev = mv^2/r

1/2 mv^2 = eV

Use Pythagoras to find radius.

Question 11

Explain what happens to an electron when a magnetic and an electric fields are crossed?

Answer 11

There is a uniform magnetic field from Helmholtz coils and a uniform electric field from parallel plates, magnetic into the page, electric down the page.

1. The electric force eE (from QE) acts upwards
2. The magnetic force Bev (BQv) acts downwards

So, for no deflection FE = FB
eE=Bev => v=E/B

Question 12

Describe a linac

Answer 12

Linac (linear acceleration) - electrons are accelerated in a series of tubes of increasing length.
Alternate tubes are connected to a radio frequency (rf) supply. The frequency has to be high in order to change the polarity as quickly as the electrons are moving at high speeds.
Inside the tubes the electrons move at constant speed as E=0 inside a hollow charged cylinder conductor. The tubes are in a vacuum.
The electrons accelerate across the gaps.
The length of the tubes must increase so that the electrons arrive at the next gap in the time taken for the signal to reverse.
Time spent in a tube is the time taken for the voltage to reverse (1/2 a period)

Question 13

Describe a cyclotron

Answer 13

Cyclotron - consists of two evacuated metal dees. Larger accelerators can produce higher energies but are limited by the size and cost of magnets. The magnets produce a field perpendicular to the plane containing the dees. When the protons move inside the dees, there is no electric force (E=0 inside a hollow charged conductor).
The magnetic force on the protons is constant (BQv) inside the dee so they move in a circular path.
The protons get accelerated across the gap, so the readies of the circle gets larger each time (r proportional to v).
To obtain large energies we would need large radii, which means a larger cyclotron, therefore generally cyclotrons only produce low energy particles (e.g. 1 -> 10 MeV)

Time spent in one of the dees: time = distance/speed
Time = πr/(BrQ/m)

Question 14

What is a spark chamber?

Answer 14

Spark chamber - a series of aluminium plates, each 2.5 cm thick, arranged vertically. They are separated by 1 cm gaps, filled with gas, there is a high voltage between neighbouring plates. As a cosmic ray or another high energy particle passes through, it ionises the gas and causes sparks to jump between the plates, a track is seen. High energies are require to pass through many layers of metal.

Question 15

What is a bubble chamber?

Answer 15

Bubble chamber - a chamber containing liquid hydrogen. When a particle passes through, it leaves invisible ions in the liquid (which is slightly below the boiling point). The piston is then withdrawn, rapidly reducing the pressure in the liquid, which is now above its boiling point. Bubbles of gas form on the ions. The chamber is photographed from many angles. The piston is pushed back upwards, restoring the pressure so that the liquid is above its boiling point once more. The bubbles dissolve back into the liquid.

Question 16

What is a drift chamber?

Answer 16

Drift chamber - similar to a Geiger counter. The chamber contains an array of fine wires, with high voltages between them. As particles pass through, they cause ionisation and a pulse of current flows into the nearest wire. Electronic circuits record the arrival of these pulses, and from their positions and the times at which they arrive, the track of the original particle can be reconstructed by a computer, hence they are more useful than other detectors.

Question 17

What is ionisation?

Answer 17

Ionisation is the process by which an atom or a molecule acquires a negative or a positive charge by gaining or losing electrons to form ions (an atom or molecule with a net electric charge due to the loss or gain of one or more electrons)

Question 18

What’s an expression for the radius of the path of a particle in a magnetic field?

Answer 18

From F = ma

BQv = mv^2/r => r = mv/BQ => r =p/BQ

Question 19

What is conserved in interactions between particles?

Answer 19

Interactions between particles are examples of elastic collisions hence kinetic energy and momentum are always conserved.
Conservation of change also applies.

Question 20

What different types of particle tracks are there? (7)

Answer 20

1. Two curves in opposite directions - oppositely charged ionising particles
2. Curve that gets thicker at the end - particles stopping, denser track as more ionisations are caused when they’re moving slower
3. Straight line with a spiral - dealt rays are the short tracks of electrons, branching off from the main track, a result of the ionisation process
4. Small spiral - an electron that appears as a small isolated spiral is known as a Compton electron. an electron has been knocked out of an atom by a game ray passing through the chamber
5. Big spirals - electron spirals appear because moving electrons eradicate energy as they cause ionisations. As they travel more slowly, their tracks become more curved
6. V shape - when an uncharged particle decays into charged particles, these are likely to show up as a V shape (same as 1 but faster)
7. Kink - kinks show where a particle has emitted one or more neutral particles (which leave no tracks)

Question 21

Why are high energies needed to break particles into their constituents?

Answer 21

The strong nuclear force overcomes electrostatic forces and holds the protons and neutrons together in a nucleus. Therefore high energies are needed to overcome it.

Also high energies are needed to create new particles as some of energy goes to the increase in mass.

Question 22

What is ∆E=∆mc^2?

Answer 22

The theory of special relativity, proposed by Einstein, predicted that the faster things go, the heavier they become.

∆E=c^2∆m - in the sub-atomic world the conservation of mass-energy applies. When particles have high energies (high speeds, close to the speed of light) they have a greater mass (∆m+mo) that is greater than their rest mass mo.

Question 23

What are the non-SI units for mass and energy?

Answer 23

Mass: eV/c^2; unified atomic mass unit, u = 1/12 th of the mass of one atom of the isotope of carbon 12C6
Energy: Electron volts - eV (from E=QV)

Question 24

What are relativistic effects?

Answer 24

1. At speeds near the speed of light, the theory of special relativity applies, so the mass of the objects increases.
2. At speeds near to the speed of light time dilation occurs, so the particles ‘last’ longer than their half-life (ie time appears to be moving slower from the point of view of the object moving close to the speed of light)

Question 25

How were quarks discovered?

Answer 25

Deep inelastic scattering of high energy electrons
High energy electrons from a linac are directed towards protons. Some of the high energy electrons are able to “penetrate” deep into the proton, and are scattered. The KE of the electron after the intersection is less than that before. The scattering pattern shows that there are 3 scattering centres in the protons. Those are called quarks.

Question 26

What are the 12 quarks and what are their properties?

Answer 26

Up, down, charm, strange, top, bottom.
All of the above have “antiquarks”.

Only up and down are stable.

u, c, t have charge of +2/3; d, s, b have -1/3; for antiquarks signs reverse

Quarks are held together by the strong nuclear force.

Question 27

What is a baryon?

Answer 27

A baryon is a particle that contains 3 quarks. Either 3 quarks or 3 antiquarks.

Question 28

What is a meson?

Answer 28

A meson is a particle that contains 2 quarks. Quark and an antiquark

Question 29

What is the quark structure of a proton and a neutron?

Answer 29

Proton: uud
Neutron: udd

Question 30

What conservation laws do quarks obey?

Answer 30

1. Conservation of mass-energy
2. Conservation of momentum
3. Conservation of baryon number (B)
   Antibaryons: B=-1; Mesons: B=0; Baryons: B=1
4. Conservation of charm (C), strangeness (S), topness (Tau) and bottomness (ß)
   Charm quark C=1
   Strange quark S=-1
   Top quark quark Tau=1
   Bottom quark ß=-1

Question 31

What are leptons?

Answer 31

Leptons are the ‘light’ particles, as opposed to hadrons, which means (‘bulky’ or ‘heavy’)
Electron e-
Muon µ-
Tauon Tau-
All of the above have neutrinos
Also have antiparticles, which have antineutrinos (positron)
Electrons, positrons and all the neutrinos and antineutrinos are stable

Question 32

What conservation law do lepton interactions obey?

Answer 32

The interactions involving leptons conserve lepton number (L)
for leptons L=1, for antiparticles L=-1

Question 33

What force is involved in changing quarks from one type into another?

Answer 33

When quarks change from one type to another, the weak nuclear force is involved (range ~ 10^-18) and some of the of the conservation laws may not apply (eg strangeness, bottomness)

Question 34

What’s de Broglie’s equation?

Answer 34

Wave-particle duality: particles can sometimes behave like waves and waves can sometimes behave like particles

De Broglie’s equation links waves and particles: λ=h/p

Also: E=hƒ; E - energy of a photon, ƒ - frequency of the radiation