1. Nuclear Physics 2 Radiation Properties Saturday, 16 February 2019
Leeds City College

2. Starter Radiation Particle Range in air Stopped by Alpha Beta Gamma
Helium nucleus
Few mm
Paper, skin
High speed electron
Less than 1 m
3 mm Aluminium
EM radiation
Infinite
2 cm lead
Strictly speaking the lead attenuates the gamma radiation so that it is little above the background radiation.
Saturday, 16 February 2019
Leeds City College

3. Equations for Decay X Y + He Q Th Ra + He Q n p + e Q ne X Y + e Q ne
Alpha X Y + He Q
A Z
A – 4
Z – 2 4 2
e.g. Th Ra + He Q
228 90
224 88 4 2
Beta n p + e Q
1 0 1 -1 ne X Y + e Q
A X A
Z + 1 -1 ne
e.g. Al Si + e Q
29 13 29 14 -1 ne
Saturday, 16 February 2019
Leeds City College

4. Production of Alpha Particles
They come from heavy elements of mass greater than 106 atomic mass units.
In classical physics, the strong force balances the electro-magnetic force, so the alpha does not have the energy to get out.
In quantum physics, there is a small chance that the alpha can get out by a process of quantum tunnelling.
At this level we assume that all the energy is kinetic. Alpha particle energy is between 3 and 7 MeV
Escape route by quantum tunnelling
Energy well
Maximum energy available
Saturday, 16 February 2019
Leeds City College

5. Range of Alpha Particle
Ref: Helmut Paul
Saturday, 16 February 2019
Leeds City College

6. Path of Alpha in a Magnetic Field
Use Fleming’s Left Hand Rule to determine the path.
Saturday, 16 February 2019
Leeds City College

7. Beta Particles In beta minus decay:
Inductiveload
Wikimedia commons
Joe Holdsworth
Wikimedia commons
Beta minus decay involves the change of a neutron to a proton via the W- boson. It happens in smaller nuclei that have an excess of neutrons.
Saturday, 16 February 2019
Leeds City College

8. Energy in Beta
All energy is kinetic, as the beta is a high speed electron (no sub-structure).
The energy is precise within an isotope.
There is a variable sharing between the electron, electron antineutrino, and the nucleus itself.
Momentum is conserved.
We are only interested in the maximum energy level.
Number of b particles
Max kinetic energy = Q
Energy
Saturday, 16 February 2019
Leeds City College

9. Valley of Stability The black line shows the stable isotopes.
Unstable nuclides are radioactive.
If there is an excess of neutrons, beta minus decay happens.
If there are too few neutrons, beta plus decay happens.
The higher up the sides of the valley, the less stable the nuclide.
Saturday, 16 February 2019
Leeds City College

10. Alpha and Beta in a Magnetic Field
The magnetic field is going into the board. Which path does each particle follow?
Alpha
Source
Gamma
Beta
Saturday, 16 February 2019
Leeds City College

11. Force on the Charge F = B I l I = Q t l = v t We know:
Velocity = distance  time
Current = charge  time
F = BIl
The t terms cancel out to give: F = B I l I = Q t l = v t
Saturday, 16 February 2019
Leeds City College

12. Motion in a magnetic field
B field going into the screen
The force at 90 o acts as a centripetal force, keeping the electron in a circular path.
Force always acts at 90 o
Saturday, 16 February 2019
Leeds City College

13. Bear Trap
Remember that the direction of the electrons’ movement is in the opposite direction to the conventional current. So if the electrons are going from left to right, the conventional current is going from right to left.
When using Fleming’s Left Hand Rule, the current is conventional.
Saturday, 16 February 2019
Leeds City College

14. Equation For centripetal force:
The v on the left cancels to get rid of the v2 term on the right:
For a charge in a magnetic field:
Rearranging:
We equate these to give:
Saturday, 16 February 2019
Leeds City College

15. Valley of Stability
Saturday, 16 February 2019
Leeds City College