1. Topic 7.2 The ABC’s of Radioactivity
Types of Nuclear Decay
Topic 7.2
The ABC’s of Radioactivity

2. Nuclear Decay Review Describe the nuclear decay process.
Name the 2 key items involved
What is the relationship between the rate of decay and the number nuclei in the sample?
Describe the term half-life
Why is the nuclear activity is more meaningful than the number of particles in the nuclei?
nuclear decay is a random process and that the probability of decay is related to the amount of energy that will be released when it happens.
Understand that the number of decays per second in a sample of material is directly proportional to the number of nuclei.
Understand how half life gives a measure of the rate of decay of a sample.
Understand why the activity is also exponential and that the activity is more useful to us than the number of particles.

4. During the decay process…
the nucleus is ‘broken up’ into smaller pieces
How does this happen?
Where does the initial energy come from?
How would the masses compare before and after a decay?
Are there certain types of particles that come out of a decay or is it random?
classical ideas of potential energy do not explain where the energy is transferred to when a nucleus is split up.
Accept that increased mass would give an answer if mass and energy were equivalent E=mc2.
Introduce the electron Volt as a unit of energy.
Have a feeling for the scale of difference between energy associated with chemical and nuclear reactions.
Understand that binding energy is not in the nucleus but has been released.
Introduce the Unified mass unit.
Understand how to calculate binding energy from tables of mass and plot the BE/nucl curve.
Deduce from knowledge of mechanics that physical systems will tend to a state of lowest energy.

5. Binding energy
This is the work required to completely separate the nucleons of the nucleus. Note: Binding Energy is NOT the energy contained by the nucleus that holds the nucleons together.

6. Binding energy
This is the work required to completely separate the nucleons of the nucleus. What happens if the nucleus is unstable?

7. Binding energy per nucleon
This is the work required to completely separate the nucleons of the nucleus divided by the number of nucleons. It is a measure of how stable the nucleus is.
This is an important graph for this topic

8. The Binding Energy curve

9. Types of Nuclear Decay
They are defined by the 3 main types of particles that can be ejected from an unstable nuclei.
Aims
Understand that if a nucleus can change into one with higher BE then it will as this will release energy.
Distinguish between α, β and γ radiation
Identify and α particle as a Helium nucleus.
Write the nuclear equation for α decay and calculate the amount of energy released.
Apply the conservation of momentum and energy to predict that α radiation is mono-energetic.
Understand why α particles are so ionising and see how this property enables them to be easily detected by ionisation and cloud chambers but means they have short range.

10. Types of Nuclear Decay
What is the main reason that make unstable isotopes ‘decay’?
It is important to remember that all atoms (and its nucleus) and all physical systems will tend to a state of lowest energy.

11. Alpha particles α

12. He Alpha particles 2+ 4 2 2 protons and 2 neutrons joined together
The same as the nucleus of a helium atom
Stopped by paper or a few cm of air
Highly ionising
Deflected by electric and strong magnetic fields He 2 4 2+

13. Alpha Decay

14. Th U He + Alpha Decay 231 235 2+ 4 2 90 92 Atomic mass goes down by 4
Atomic number goes down by 2

15. Ionization by alpha particles

16. Ionization by alpha particles

17. Beta particles β

18. e Beta particles -1 Fast moving electrons Effectively massless
Stopped by about 3 mm of aluminium
Weakly ionising
Deflected by electric and magnetic fields e -1

19. Beta Decay

20. Beta Decay
In the nucleus a neutron changes into an electron (the beta particle which is ejected) and a proton (which stays in the nucleus)
During beta decay the mass number stays the same but the proton number goes up by 1.
Remember the electron comes from the nucleus! e Th 90
231 Pa 91
231 + -1

21. Beta Decay – Mono energetic?
Unlike alpha particles, beta particles can be emitted with different energies
How do we conserve the energy with each reaction?
antimatter
e + עe
antineutrino Th 90
231 Pa 91
231 + -1

22. e + עe Th Pa + Antiparticles
Produced naturally  symmetry in the universe!
Particle + Antiparticle = ENERGY (photons)
How did matter dominate in our universe?
antimatter
positron
e + עe
neutrino Th 90
231 Pa 91
231 + +1

23. Gamma rays

24. Gamma rays High frequency electromagnetic radiation
Stopped by several cm of lead
Very weakly ionising
NOT affected by electric or magnetic fields

25. Gamma rays
Associated with daughter nucleus of alpha decay α Th 90
231 U 92
235 +

27. Types of Nuclear Decay

28. Particles in a Magnetic Field
If the magnetic field is coming out of the page, name the three types of radiation that’s present in this diagram

29. Particles in a Magnetic Field
Use the right hand rule for alpha particle! α

30. Particles in a Magnetic Field
Use the left hand rule for beta particle! β α

31. Particles in a Magnetic Field
Gamma rays have no charge! β ϒ α

32. Cloud Chamber Utilizes ionization properties of alpha & beta particles
Vapour condenses around small particles of ‘dust’  cloud
Similar to vapour trails left by airplanes
The α‑radiation produces dense straight tracks showing intense ionisation.
Notice that all the tracks are similar in length.
The high‑energy β‑ray tracks are thinner and less intense.
The tracks vary in length and most of the tracks are much longer than the α ‑particle tracks.
The γ‑rays do not produce continuous tracks.

33. PhET Simulations
Alpha Decay
Beta Decay