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

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

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

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

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

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

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

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

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

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

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

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

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

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

Valley of Stability Saturday, 16 February 2019 Leeds City College