Radioactivity

Towards the end of the 19th century various experiments indicated that different types of hitherto unknown radiation were emitted by heavy atoms. In 1896 Becquerel discovered that uranium compounds emitted a form of radiation (beta  or α-radiation) which could be deflected by a magnetic field. This implied that the radiation consisted of a stream of charged particles and these were subsequently identified to be electrons. Subsequently, positron emission was also observed. Then, in 1897-8, Pierre and Marie Curie found another form of much less penetrating charged particle radiation (alpha or α-radiation) which, is now known to consist of ionized helium, i.e. helium nuclei. Finally, in 1900, Villard identified a third form of highly penetrating radiation (gamma or γ-radiation) which was un deflected by a magnetic field and turned out to be akin to x-rays but of much shorter wavelength. Each of these radioactive decay processes happen spontaneously and can be understood to take place when an atomic nucleus is in an excited or unstable state and is able to release energy. Natural occurring radioactivity arises in very heavy nuclei such as uranium and radium which, since they contain many protons all repelling each other, are liable to disintegrate into more stable structures. Artificial radioactive nuclei are produced as the products of nuclear reactions. Quantum mechanics does not allow us to predict exactly when such a radioactive decay will take place, only the probability of it happening in a given time. The standard unit of activity of a radioactive substance is the becquerel which corresponds to one decay per second. All radioactive decays are then characterized by a time known as the mean life, which is the average lifetime of a nucleus before it decays. Mean lives can vary from very high values of the order of 10¹⁰ years for naturally occurring alpha emitters through to 10^-15s for some gamma-emitters. Let us now consider the three forms of radioactivity in turn.

Alpha decay (α-decay). We have seen that the helium nucleus is a very stable structure and that, correspondingly, when it is assembled by bringing two neutrons and two protons together a great deal of energy is released. This means that, since nuclear systems are always seeking to be in the lowest possible energy state, one way of achieving this for an excited or very heavy nucleus may be by ejecting two protons and two neutrons in the form of a helium nucleus-an a-particle, the energy released in forming the helium nucleus being used to enable it to escape from the ‘parent’ nucleus. In this escape the a-particle has to penetrate a potential barrier. To conserve energy, the α-particle and the ‘daughter’ nucleus in such a decay carry away a definite amount of kinetic energy between them equal to the differerence in mass energy (remember E = mc²) between the parent nucleus and the daughter nucleus plus the helium nucleus. This is observed experimentally and in the foregoing terms the process of a-decay is now well understood.

Beta decay (β-decay). This similarly involves the emission of a charged particle an electron (e^-) or a positron (e⁺). This happens when there are respectively too many neutrons or too many protons in a nucleus for it to be stable. Consequently, a neutron changes into a proton, emitting an electron, or a proton into a neutron, emitting a positron. Given that nuclei have definite energies it would be expected that, as with α-decay, the β-particle would also have a definite energy. It was, therefore, a great surprise to find that in any β-decay process the        β-particles were emitted with a wide spread of kinetic energies from zero up to the maximum available. The solution to this problem was suggested by Pauli in 1930 who proposed that in β-decay two particles were emitted so that the definite energy released was shared between them. The electron or positron could then have all possible values of the energy up to the maximum as observed the rest being taken away by the second particle. Since all the electric charge released in β-decay is carried away by the electron or positron it follows that the new particle is electrically neutral, like a neutron. It also has a very small, if not zero, mass since electrons and positrons are observed sometimes to have essentially all the energy released in the decay there is no spare energy for the new particle to have a significant mass. For these reasons the new particle was seen as a ‘little’ neutron and was called a neutrino (denoted by v_e). It also became clear that, if angular momentum is to be conserved in β-decay then, like an electron, it must have spin ½. Similarly, it also has a corresponding antiparticle called an antineutrino (denoted by ῡ_e) . The introduction of the neutrino enabled the β-decay process to be well understood in terms of two basic processes-a neutron converting into a proton with the creation of an electron and an antineutrino (n → p + e^- + ῡ_e) or a proton converting into a neutron with the creation of a positron and a neutrino (n → p + e^- + v_e). Since a neutron is slightly heavier than a proton, the first process can also happen to a free neutron and its mean life is measured to be around 15 minutes, but the second process can only happen when the proton is embedded in a nucleus, which, if it is radioactive, provides the energy to create the positron and the neutrino. However, it was not until 1953 that the neutrino (actually it was the antineutrino) was detected experimentally (by Reines and Cowan) in the vicinity of a nuclear reactor. This was possible because near a reactor there are many free neutrons and, as we have just seen, they decay reasonably quickly into a proton, an electron and an antineutrino. The antineutrino was detected by letting it be captured by a proton, so converting the proton into a neutron and a positron (p + ῡ_e → n + e⁺), the latter being easily observed. This capture process is known as inverse β-decay. Here it should be mentioned that sometimes a nucleus, instead of emitting a positron, captures an orbiting atomic electron, so converting a proton into a neutron and emitting a neutrino (p + e^- → n + v_e)^*.

Gamma decay (γ-decay). This process can easily be understood as the nuclear equivalent of the emission of photons from an atom. Like an atom the nucleus has a series of energy levels and if the nucleus is excited to a higher one, for example in a nuclear reaction or following radioactive decay, it will make a series of jumps down to the lower ones finally ending up in the ‘ground’ state. Each jump involves the release of energy and this is carried away as a photon the γ-radiation. Since nuclear energies are on a much larger scale than atomic energies (millions of electron volts rather than electron volts) it follows that the corresponding frequencies of the emitted photons (energy hv) are some millions of times larger than those of visible radiation and the wavelength of the radiation some millions of times smaller  as observed.

* Here it should be noted that when particles are switched from one side of a reaction arrow (→) to the other. they are replaced by their antiparticles.