6.4 Nuclear Physics (Activity, Fusion and Fission)

Question 1

Define the activity of a source

Answer 1

The rate at which nuclei decay, or number of decays per second.

Question 2

What is activity measured in?

Answer 2

Becquerel (s^-1)

Question 3

What two things does activity depend on?

Answer 3

Half life and number of active nuclei in the sample

Question 4

What is the equation for activity?

Answer 4

A = 𝜆N

A: Activity (Bq)
𝜆: Decay constant (s-1)
N: Number of nuclei

Question 5

What is the decay constant (𝜆)?

Answer 5

The probability that an individual nucleus will decay per unit time.

Question 6

Why is recorded activity in an experiment always higher than the actual activity of a sample? What can be done about this?

Answer 6

The experiment is also measuring background radiation.
First measure the background without the sample nearby. Then the background must be deducted from the activity measured in the presence of the sample.

Question 7

What are the two radioactive decay equations?

Answer 7

N = N₀e^-𝜆t [Number of Nuclei]
A = A₀e^-𝜆t [Activity]

Question 8

What is the half life?

Answer 8

The average time it takes for half of the sample to decay.

Question 9

What equation gives us the half life?

Answer 9

𝜆t = ln(2)

t: Half life

Question 10

I have a graph of activity against time, using a logarithmic scale. How can I determine the half life from this?

Answer 10

ln(A) = ln(A₀) + ln(e^-𝜆t)
ln(A) = ln(A₀) - 𝜆t
y = c + mx
The gradient is -𝜆.

Question 11

Naturally occuring carbon in the atmosphere contains three main isotopes. What are they, and which one is radioactive?

Answer 11

99% : C-12
1% : C-13
Trace: C-14 [Radioactive]

Question 12

Living organisms absorb C14 in their lifetimes. How do we take advantage of this in carbon dating? [3]

Answer 12

• Since living organisms absorb C14 while they are alive, the ratio of C-14 to C-12 matches the atmospheric ratio.
• When an organism dies, the number of C-14 atoms is capped. C-14 is a radioactive isotope that decays via beta emission with a half life of 5700 years.
• By measuring the ratio of C14 to C12 in the dead tissue and comparing it to the atmosphere, an estimation for the time since the organism’s death can be calculated.

Question 13

Fusion reactions require extreme heat and pressure. Why is extreme heat required? Why is extreme pressure required?

Answer 13

Heat –> Greater chance of fusion when a collision occurs.
Pressure –> Greater chance of collisions

Question 14

Stage 1 of the proton proton chain

Answer 14

Lone protons (hydrogen nuclei) fuse to form an unstable 2/2 He. This particle then undergoes beta decay forming deuterium nuclei (an isotope of hydrogen, 2/1H).

H1 + H1 –> H2 + Beta-Plus + Ve

Question 15

What is the proton-proton chain?

Answer 15

How stellar fusion occurs in main sequence stars such as the sun.

Question 16

Stage 2 of the proton proton chain

Answer 16

Deuterium nucleus + Lone proton –> Tritium + Gamma

H2 + H1 –> H3 + Gamma

Question 17

Stage 3 of proton proton chain

Answer 17

Tritium + Tritium –> Helium4 + two Hydrogen1

H3 + H3 –> He4 + 2H1

Question 18

What is nuclear fission?

Answer 18

When a heavy nucleus splits apart into two lighter nuclei.

Question 19

How do we induce nuclear fission using U235?

Answer 19

A low speed thermal neutron is fired at the U235 nuclei. It absorbs it and becomes the unstable U236 isotope. It then either decays via fission, producing two smaller daughter nuclei and more fast neutrons, or decays via gamma emission.

Question 20

[Fission] How does the mass of the two daughter nuclei compare to the mass of the original heavy nucleus?

Answer 20

It is always less. The excess mass is transformed intro energy. The total energy released is the combination of the kinetic energy of the neutrons and daughter nuclei, and the energy of the photons.

Question 21

Describe a fission reactor

Answer 21

• Fissile materials sit inside a reactor core surrounded by a thermal coolent.
• This coolant absorbs the thermal energy from fission, and transforms it into kinetic energy in turbines.
• The turbines turn the generator, transferring this energy into electrical energy (alternating current).

Question 22

Fuel rods

Answer 22

They contain uranium fuel.

Question 23

Control Rods

Answer 23

Absorbs some of the thermal neutrons to control the rate of nuclear reaction.

Question 24

Moderator

Answer 24

Slows down fast moving neutrons to create thermal neutrons

Question 25

What is a good moderator which is often used?

Answer 25

Water

Question 26

Why is a moderator important?

Answer 26

Fission is more likely to occur with slower neutrons. This means neutrons released by the fission process are too high energy to continue the reaction. A moderator slows them down, increasing the chance of a chain of U235 fission reactions.

Question 27

Benefits of using control rods

Answer 27

Can be used to keep the power output constant
Can be used to prevent the reactor core from overheating

Question 28

How is toxic waste disposed of, and why?

Answer 28

It is buried deep underground.

Question 29

Why dispose of toxic waste?

Answer 29

It is radioactive and has a very long half life.