It is well known that the nucleus consists of nucleons (protons and neutrons) and that has a very high atomic weight. Between the particles in the nucleus has a very strong nuclear interaction, much stronger than the magnetic force or gravity, but it works only over short distances. Therefore, the nucleons in the nucleus and keep it to fall apart.
Some nuclei of atoms - mostly unstable - emit radiation at high frequencies. This radiation is called radioactivity reaches higher frequencies than X-rays. When radiation nucleus disintegrates and turns to the kernel of another element. Unstable elements tend to break down to a more stable elements. Stability of the element is determined based on the ratio of the number of protons and neutrons. Elements with a ratio of 1 are stable (most stable element in the universe is iron), elements with a ratio of up to 1.5 are unstable and elements with a ratio greater than 1.5 are highly unstable. Substances consisting of atoms of these nuclei are called radionuclides - may be either natural or artificial.
AZX → A-4Z-2Y + 42α
From the equation can be deduced that it is a bundle of speeding helium nuclei.
Specific example:23994Pu → 23592U + 42 α
Alpha radiation has the smallest frequency radiation, it's absorbed by sheet of paper.
AZX → AZ-1Y + 01e
Specific example: 3015P → 3014Si + 01e
It consists of a stream of positrons, it's absorbted by the aluminum sheet.
AZX → AZ+1Y + 0-1e
Specific example: 21084Po → 21085At + 0-1e
It consists of a stream of electrons, it's also absorbed by the aluminum sheet.
AX → AY* + 0-1e AY* → AY + γ
Specific example: 6027Co → 6028Ni* + 0-1e 6028Ni* → 6028Ni +γ
This is electromagnetic radiation, it's absorbed by a layer of lead.
Logical it consists of a stream of flying neutrons, it's absorbed by very thick layer of water or concrete.