7a. What happens overall in Alpha and Beta Radioactive Decay?

• Alpha Decay e.g. the nuclear equation  

  • A helium nucleus, the alpha particle, of 2 protons and 2 neutrons is emitted at high speed/kinetic energy from the nucleus.

  • The residual atom (sometimes referred to as the politically incorrect 'daughter nuclide'!*) has a mass number of 4 less, and an atomic number of 2 less, than the 'parent' or original atom.

  • Most atoms with an atomic number of over 82 (Pb) usually undergo alpha decay.

  • * apart from Marie Curie, in the late 19thC/early 20thC, nuclear physics was dominated by male scientists! 

• ^- Beta^- Decay e.g. the nuclear equation   

  • A neutron in the nucleus changes spontaneously into a proton and a high kinetic energy electron forms the emitted beta particle.

  • Since the proton and neutron have a mass of 1 and the electrons mass is negligible, the mass number stays the same but the atomic (proton) number rises by 1.

  • This tends to happen with isotopes with too many neutrons to be stable (too high an n/p ratio) and lies above the stability curve shown above.

  • By changing a neutron to a proton the n/p ratio is reduced to the nucleus of an isotope lying in the stability band.

• Balancing: The changes can be represented as nuclear equations and they must balance in mass number and nuclear or emitted particle charge (protons in alpha decay, protons and electrons in beta decay).

• In (1) mass = 235 = 231 + 4 and protons = 92 = 90 + 2.

• For (2) mass = 14 = 14 + 0 and for protons/beta charge = 6 = 7 + (-1).  In either case a new element is formed i.e. the 'transmutation' of one element to another has happened. It also means that there can never be a 'pure' Radioisotope.

• Gamma emission: The emission of gamma radiation from a nucleus does not involve any change in the atomic (proton) number or mass number.

  • When a 'new' nucleus is formed it tends to have excess energy making it potentially unstable.

  • To become more 'nuclear stable' the nucleus loses some energy as a burst of gamma radiation but the proton and neutron numbers do not change.

• Positron emission is required by some syllabuses

• ⁺ Positron emission (beta⁺ decay): e.g.

  • A proton changes to neutron  and  a 'positive electron' called a positron is expelled with very high kinetic energy. A positron has the same mass as an electron but carries a positive charge (it is the 'anti-matter' particle of the electron!).

  • Since the proton and neutron have a mass of 1 and the electrons mass is negligible, the mass number stays the same but the atomic (proton) number falls by 1.

  • This tends to happen with isotopes with too few neutrons to be stable (too low an n/p ratio) and lies below the stability curve shown in a previous graph. By changing a proton to a neutron the n/p ratio is increased to an isotope lying in the stability band.

7b. The production of radioisotopes - artificial sources

• To meet the industrial and medical demand for Radioisotopes (as described earlier) many are made by allowing stable isotopes to be hit by neutrons in a small research scale nuclear reactor.

• Note again, the balancing of nuclear equations e.g.

to make carbon-13, used as a chemical tracer carbon in studying the mechanisms or organic chemistry reactions

to make sodium-24, which can be used in tracer studies of animal blood circulation

to make cobalt-60, used as the gamma source for cancer radiotherapy.