6.4 Nuclear Physics (Fundamental Particles, Radioactivity Intro)

Question 1

Compare antiparticles to particles

Answer 1

Equal mass but opposite charge

Question 2

An antiparticle and particle meet at a point in space. What happens?

Answer 2

They annihilate, producing energy in the form of photons.

Question 3

What is pair production?

Answer 3

When a high energy photon creates a matter-anti matter pair (usually electron-positron).

Question 4

What is needed for pair production to work?

Answer 4

The photon must have an energy greater than the combined rest masses of the two particles.

Question 5

What are useful units for expressing the rest mass?

Answer 5

MeV / c2

Question 6

Electrons and positrons have the same rest mass of roughly 0.51MeV/c2. Using pair production, what is the minimum energy of the photon required to create these particles?

Answer 6

1.02MeV

Question 7

Describe Hadrons

Answer 7

Made up of fundamental particles called quarks. They experience the strong and weak force.

Question 8

Hadrons can be split down into two more classes. What are they called?

Answer 8

Mesons and baryons.

Question 9

Describe mesons

Answer 9

Hadron made of a quark-antiquark pair.

Question 10

Describe baryons

Answer 10

Hadron made up of three quarks.

Question 11

Can quarks be isolated?

Answer 11

No. They only exist as mesons or baryons.

Question 12

Describe leptons

Answer 12

Fundamental particles which don’t experience the strong force. They only experience the weak force.

Question 13

Give an example of a lepton

Answer 13

• Electrons
• Positrons
• Neutrinos
• Muons

Question 14

What is radioactive decay?

Answer 14

The spontaneous breakdown of an atomic nucleus, resulting in the release of energy and matter from the nucleus.

Question 15

Radioactive decay is random. What does this mean?

Answer 15

It is impossible to predict which of a number of identical nuclei will decay next.

Question 16

What is the half life?

Answer 16

The time taken on average for half of the active isotopes in a radioactive sample to decay.

Question 17

What is predictable and not predictable in regards to the half life of a substance?

Answer 17

We can be reasonably sure that approximately half the sample will decay.
We cannot be sure which half of the sample decays.

Question 18

Is there a probability that none of an isotope decays in a half life?

Answer 18

Yes. But it is improbably small.

Question 19

Three main forms of radiation

Answer 19

Alpha, Beta (Minus/Plus), Gamma

Question 20

Compare the speed of the three main forms of radiation.

Answer 20

Alpha is slow.
Beta is fast.
Gamma moves at the speed of light.

Question 21

Compare the ionising ability of the three main forms of radiation.

Answer 21

Alpha has high ionising ability.
Beta is medium.
Gamma has no ionising ability

Question 22

Compare the penetrating power of the three main forms of radiation.

Answer 22

Alpha has low penetrating power.
Beta is medium.
Gamma has the highest penetrating power.

Question 23

What material is used to stop each of the three main forms of radiation.

Answer 23

Alpha: Paper
Beta: Aluminium
Gamma: Lead

Question 24

The three main forms of radiation have varying charges. How can we take advantage of this fact to detect them?

Answer 24

Use a magnetic or electric field. They will all respond differently.

Question 25

What three things must be conserved during beta decay?

Answer 25

Charge, Baryon Number, and Lepton number

Question 26

In beta minus decay, what baryon turns into what? What byproducts are produced as a result?

Answer 26

A neutron turns into a proton. This produces an electron and anti electron neutrino.

Question 27

In beta plus decay, what baryon turns into what?

Answer 27

A proton turns into a neutron. This produces a positron and an electron neutrino.

Question 28

In beta minus decay, how does the quark structure change?

Answer 28

A down quark becomes an up quark.

Question 29

In beta plus decay, how does the quark structure change?

Answer 29

An up quark becomes a down quark.

Question 30

Originally we did not have neutrinos in beta decay equations. Why did we add them one day?

Answer 30

We realised that lepton number was not conserved. Neutrinos fix this problem.

Question 31

What can alpha emission be thought of?

Answer 31

Spontaneous fission of unstable nuclei, where the strong force is not great enough to overcome the electrostatic repulsion between protons in the nucleus.

Question 32

Why are helium nuclei in particular emitted in alpha emission?

Answer 32

It has high binding energy per nucleon. This means the total binding energy will increase vastly as the parent nuclei moves closer to the middle of the binding energy per nucleon graph.

Question 33

What is a decay chain?

Answer 33

Many decays happen in succession until a stable atom is reached

Question 34

Why do atoms with a large number of protons have significantly more neutrons, compared to some smaller atoms which have an equal number of protons and neutrons?

Answer 34

As the proton number rises, the EM repulsion between nucleons increases. For these nuclei to not fall apart, there must be more nucleons to produce a strong force attraction. Therefore we have more neutrons.

Question 35

N-Z Graph: What will happen to nuclei with more than 82 protons?

Answer 35

They are more likely to decay via alpha radiation.

Question 36

N-Z Graph: What happens to nuclei on the right of the belt of stability.

Answer 36

Too many protons. They are more likely to decay via beta plus.

Question 37

N-Z Graph: What happens to nuclei on the left of the belt of stability.

Answer 37

Too many neutrons. They are more likely to decay via beta minus.

Question 38

What is gamma decay caused by?

Answer 38

When a nucleus has surplus energy following alpha or beta emission.

Question 39

Results of gamma decay

Answer 39

No change to nucleon composition. Energy is released in the form of a gamma photon.