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 a 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 second type of decay 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 emitted from the nucleus. That is why the gamma radiation often occurs with other types of radiation.
REMEMBER:
In the alpha decay the nucleus transforms into a new one having two protons and two neutrons fewer than the initial one.
In the beta decay the nucleus transforms into a new one having one proton more and one neutron fewer than the initial one.
In the beta decay there are an electron and a neutrino emitted from the nucleus.
The emission of a gamma quantum is connected with the excited state of the nucleus.
