1. Atoms, Nuclei and Radiation
Atoms and nuclei
From scattering to decay equations
In the α-particle scattering experiment, α-particles were fired at thin gold foil in a vacuum. Most passed straight through: the atom is mostly empty space. A few were deflected through large angles, some more than 90°: there is a very small region containing most of the mass and all the positive charge, the nucleus. The nucleus has a diameter of about 10−14 m against about 10−10 m for the atom.
In the simple nuclear model, protons and neutrons (nucleons) make up the nucleus and electrons orbit outside it. The proton number Z is the number of protons; the nucleon number A is the number of protons plus neutrons. A nuclide is written AZX. Isotopes are forms of the same element (same Z) with different numbers of neutrons.
| Radiation | Nature | Charge | Mass |
|---|
| α | helium nucleus, 42He | +2e | 4 u |
| β− | electron | −e | about 1/1840 u |
| β+ | positron | +e | about 1/1840 u |
| γ | electromagnetic wave (photon) | 0 | 0 |
An antiparticle has the same mass as its particle but opposite charge; the positron is the antiparticle of the electron. An electron antineutrino is emitted in β− decay, and an electron neutrino in β+ decay.
In α-decay the energy released is shared between the α-particle and the recoiling nucleus in a fixed way, so α-particles from one nuclide have discrete energies. In β-decay the energy is shared among three particles (β, (anti)neutrino and nucleus) in varying proportions, so β-particles have a continuous range of energies up to a maximum.
The unified atomic mass unit: 1 u = 1.66 × 10−27 kg (one twelfth of the mass of a carbon-12 atom).
Decay equations (A and Z both balance)
α: 23892U → 23490Th + 42α β−: 146C → 147N + 0−1e + ν β+: 2211Na → 2210Ne + 0+1e + ν
Worked Example
State the numbers of protons, neutrons and electrons in a neutral atom of 22688Ra, and write the equation for its α-decay to radon (Rn).
Practice Question
23892U decays by one α-emission followed by two β− emissions. Give the nucleon and proton numbers of the final nuclide, and its relation to the starting nuclide.
Practice Question
Express the mass of an α-particle (4.0015 u) in kilograms.
2. Fundamental Particles