Many experiments in passing radioactive materials through a magnetic field led to the conclusion that - although alpha and beta rays are deflected to side or other - gamma rays travel straight through and are not deflected. For this reason the early experimenters assumed correctly that gamma rays carried no charge. Gamma rays seemed to have properties like those of x rays, but they were more energetic. In fact, gamma rays have a very high energy content and strong penetrating ability. Today we know that because of this penetrating ability, a high concentration of gamma rays can harm human genes. The nucleus of an atom gives off gamma rays only under certain conditions, usually when the nucleus loses energy. And the atom, of course, has to be of the radioactive variety.
When Becquerel allowed beta rays to pass through a magnetic field, he found that they behaved like cathode rays - that is, like rays that come from a negative electrode. Therefore, Becquerel concluded that these rays were composed of speeding electrons. However, beta particles, like gamma and alpha particles, come from the nucleus of the atom, and electrons don't exist in the nucleus.
How, then, are beta particles formed? Scientists believe that they are formed when neutrons, during a nuclear reaction, are transformed into protons:
neutron proton + beta particle
A beta particle, like an electron, has a zero mass number and a charge of -1. Beta particles have only slight penetrating power compared to gamma rays. They can be stopped by a thin sheet of almost any metal.
When scientists of the early 1900's (Bequerel, James Chadwick, and Werner Heisenberg, etc.) investigated alpha particles in a magnetic field, they found that these particles were deflected in the opposite direction from beta rays. They therefore assumed that the alpha particles must be positively charged. Further experiments showed that these particles had a mass of 4 and a charge of +2. Heisenberg suggested that we picture an alpha particle as a helium-4 atom with its two electrons removed; in other words, we can imagine it as composed of two protons and two neutrons. Alpha particles have virtually no penetrating ability. They can be stopped by a then sheet of paper.