At the end of the 19th century Thomson and Rutheford studied the phenomenon of the ionisation of gases illuminated with the rays discovered by Becquerel. During the experiment Rutheford discovered that there were two kinds of such radiation: The first one called the alpha rays was easily absorbed even by paper; the second one called the beta rays could permeate through even thick metal brasses like for example 0.25 centimetres of aluminium. Soon the third kind of radiation was discovered. It had high penetrating power - it could go through even a couple of centimetres thick layers of lead. This third kind of radiation was called the gamma rays.
A few next years scientists spent explaining the nature and properties of those three kinds of radiation.
The alpha rays (having low penetrating power) consist of positively charged particles - in a magnetic field, and in an electric field alpha rays are deflected in the same direction as other positively charged particles. It turned out that the ratio q/m (charge to mass) of the alpha particles is two times smaller than q/m of hydrogen nuclei. Scientists concluded that alpha particles are helium nuclei whose mass equals 4*(hydrogen mass), and whose positive charge equals 2*e. As you know the helium nucleus consists of two protons and two neutrons.
The beta particles (having much higher penetrating power) can be deflected in a magnetic field, and in an electric field, and the deflection direction shows that they are charged negatively. In the further study it turned out that they are simply electrons.
Various radioactive elements emit different amounts of those three kinds of radiation.
Why do nuclei emit the alpha, beta, and gamma radiation? As we have said before, when the number of nucleons in the nucleus increases, it reaches some point at which Coulomb's repelling force (between the protons) predominates the attracting nuclear force. Such nucleus is unstable and sooner or later it decays.
An unstable nucleus emits an alpha or beta particle and so it transforms into a nucleus of some other element. A and Z are the symbols for the mass number and the atomic number (here of the nucleus before it decays) respectively. The nucleus of helium (which is the alpha particle) consists of two protons and two neutrons. So, if decaying atom emits the alpha particle, the mass number of the daughter atom equals A-4, and the atomic number equals Z-2. That can be schematically written like this:
The transformation proceeds, of course, in accordance with the energy conservation law and the charge conservation law. The charge of the initial nucleus is equal to the charge of the daughter nucleus plus the charge of the emitted particle. And the mass of the initial nucleus is equal to the mass of the daughter nucleus plus the mass of the emitted particle plus - according to Einstein's equation E=mc2 - the energy emitted in the decay and divided by c2. The alpha particles emitted in the decay can then collide with the atoms of the medium and ionise them. The effect was used by many experiments. Finally the alpha particle gets neutralised and turns into a helium atom.
The second type of decays is the beta decay in which the decaying nucleus emits an electron. And that can be schematically written like this:
But where did the electron get from, as there are not any in the atomic nucleus? In the twenties of the 20th century scientists tried to find the answer to this question. But as the result of their researches they got only successive contradictions and unknowns. Just in 1931 Wolfgang Pauli found the explanation of the process. His idea was that during the beta decay one of the neutrons turns into a proton, an electron, and a neutrino (the last one we call today the antineutrino). The neutrino, just like the photon, doesn't have any mass. (Well, the latest experiments show that perhaps it has some tiny, little mass, however.) It also doesn't have any charge. And so the electron and the neutrino are ejected from the nucleus, while the proton stays inside. The final proof of this theory was the discovery of the neutrino in 1954.
As you already know, the gamma radiation is the highly energetic photons. Very often they occur with other kinds of radiation. The construction of the atomic nucleus is a bit similar to the construction of the atom as a whole; the nucleons are situated on some orbits. The nucleon can also get to a higher orbit and then get down again emitting the gamma photon. The nucleon can get to a higher orbit if there is some particle (beta or alpha) emitted from the nucleus. That is why the gamma radiation often occurs with the other types of radiation.
