C24 - Particle Physics

Question 1

How was Rutherford’s alpha-scattering experiment carried out?

Answer 1

A narrow beam of electrons (of the same kinetic energy) from a a radioactive source were targeted at a thin piece of gold foil (only a few atomic layers thick).

The alpha particles were scattered by the foil and detected on a zinc sulfide screen mounted in front of a microscope.
Each alpha particle hitting the fluorescent screen produced a tiny speck of light.

The microscope was moved around in order to count the number of alpha particles scattered through different values of angle per minute (from 0° to 180°).

Question 2

What were the observations made from Rutherford’s alpha-scattering experiment?

Answer 2

• Most alpha particles passed straight through the thin gold foil with very little scattering. Approx 1/2000 alpha particles were scattered.
• Very few particles were deflected through angles greater than 90° (1/10,000).

Question 3

What were the conclusions made from Rutherford’s alpha-scattering experiment?

Answer 3

(Since most of the particles passed straight through the gold) - most of the atom is empty space with most of the mass concentrated in a small region [the nucleus].

(Few particles were deflected above 90° therefore) - the nucleus has a positive charge as it repelled the few positive alpha particles that came near it.

Question 4

What is the simple nuclear model of the atom?

Answer 4

It has a nucleus with protons and neutrons.

Question 5

What does the letter A represent? (Nucleus)

Answer 5

The nucleon number / total number of protons and neutrons / atomic mass

Question 6

What does the letter Z represent? (Nucleus)

Answer 6

Atomic number / number of protons

Question 7

What equation shows the radius of a nucleus?

Answer 7

R = r0A¹/³

R is radius
r0 has an approximate value of 1.2 fm (1.2 * 10 ⁻¹⁵ m)
A is the nucleon number / atomic mass

Question 8

What is 1 fm?

Answer 8

1 * 10 ⁻¹⁵ m

Question 9

What force overcomes the force of repulsion between protons in nuclei and stops them flying apart?

Answer 9

The strong nuclear force / strong force

Question 10

What is the strong nuclear force?

Answer 10

A strong form which acts between all nucleons.
It’s very short range, effective over just a few femtometres (1 * 10⁻¹⁵ m).

It keeps the protons and neutrons together.

Question 11

How does the strong nuclear force vary between 2 nucleons with distance/separation r?

Answer 11

(Like an l shape from positive to negative then towards zero N)

• At very small separation, the force is large and repulsive but quickly decreases with increasing separation (up to approx 0.5 fm).The force then increases/becomes more negative and attractive (min at approx 1 fm). It then decreases/becomes less negative (still attractive) and tends towards zero newtons.

Question 12

At what separation (between nucleons) is the strong nuclear force repulsive?

Answer 12

Below approx 0.5 fm (0.5 * 10 ⁻¹⁵ m).

It is shown as a positive force in the graph.

Question 13

At what separation (between nucleons) is the strong nuclear force attractive?

Answer 13

Approx between 0.5 - 3 fm (10 ⁻¹⁵ m).

Force is 0 above 3 fm.

Question 14

What’s an antiparticle?

Answer 14

The antimatter counterpart of a particle, with the opposite charge to the particle (if the particle has charge) and exactly the same rest mass as the particle.

If a particle and its corresponding antiparticle meet, they completely destroy each other by annihilation and the masses of both are converted to a high energy pair of photons.

Question 15

What’s the antiparticle of an electron?

Answer 15

The positron (with a charge of +e).

(Most antiparticles are symbolised by a bar over the letter for the particle).

Question 16

What are the 4 fundamental forces / interactions (in order of relative strength)?

Answer 16

Strong nuclear (experienced by nucleons)

Electromagnetic (experienced by static and moving charged particles)

Weak nuclear (responsible for beta decay)

Gravitational (experienced by all particles with mass)

Question 17

What is a fundamental particle?

Answer 17

A particle with no internal structure which can not be divided into smaller bits.

Question 18

What are the 2 families of classification for subatomic particles?

Answer 18

Hadrons

Leptons

Question 19

What are hadrons?

Answer 19

Particles and antiparticles that are affected by the strong nuclear force.
They decay by the weak nuclear force.
Any particle containing quarks is called a hadron.

Hadrons, if charged, also experience electromagnetic force.

Examples include protons, neutrons and mesons.

Question 20

What are leptons?

Answer 20

Particles and antiparticles that are not affected by the strong nuclear force.

Leptons, if charged, also experience electromagnetic force.

Examples include electrons, neutrinos and muons.

Question 21

What’s a quark?

Answer 21

An elementary particle that can exist in six forms (plus their antiparticles) and joins with other quarks to for hadrons.

Question 22

What is the standard model of elementary particles?

Answer 22

It requires 6 quarks and their 6 anti-quarks.

All quarks gave a charge, a fraction of the elementary charge e.

Question 23

What are the 6 quarks and how are they represented?

Answer 23

Up (u)

Down (d)

Charm (c)

Strange (s)

Top (t)

Bottom (b)

(Their anti-quarks and named by adding anti- before their name and are represented by their letter with a dash above it).

Question 24

What are baryons?

Answer 24

Any hadron made with a combination of 3 quarks.

(E.g. protons uud and neutrons udd).

Question 25

What are mesons?

Answer 25

Hadrons made with a combination of a quark and antiquark.

Question 26

What happens in beta- decay?

Answer 26

A neutron in an unstable nucleus decays into a proton, electron and electron antineutrino. n → p + e− + /νₑ

Question 27

What’s a neutrino?

Answer 27

A lepton (fundamental particle) that carries no charge and may have a tiny mass (less than a millionth of the mass of an electron).

Question 28

What happens in beta+ decay?

Answer 28

A proton decays into a neutron, positron and an electron neutrino. p → n + e+ + νₑ

Question 29

What happens to the quarks in beta- decay?

Answer 29

One of the down quarks becomes an up quark and, in the process, an electron and electron anti neutrino are emitted. d → u + e- + /νₑ

Question 30

What happens to the quarks in beta+ decay?

Answer 30

One of the up quarks becomes a down quark and, in the process, a positron and electron neutrino are emitted. u → d + e+ + νₑ

Question 31

What are the 2 groups of hadrons?

Answer 31

Baryons (made of a combination of 3 quarks) Mesons (made of a quark and antiquark)