Chadwick's collision experiment
To prove the existence of neutrons using Nitrogen nuclei in secondary collisions.
Until 1932, the atom was known to consist of a positively charged nucleus surrounded by enough negatively charged electrons to make the atom electrically neutral. Most of the atom was empty space, with its mass concentrated in a tiny nucleus. It was known that hydrogen, the simplest atom, contains only one proton and that the helium atom contains two protons. Therefore, the ratio of the mass of a helium atom to that of a hydrogen atom should be 2 : 1. (as electrons are much lighter than protons, their contribution to atomic mass can be ignored). In reality, however, the ratio is found out to be 4 : 1. Rutherford and others postulated that there must be another type of subatomic particle in the atomic nucleus; the proof was provided by another English physicist, James Chadwick, in 1932. When Chadwick bombarded a thin sheet of beryllium with α particles, a very high–energy radiation similar to γ rays was emitted by the metal. Later experiments showed that the rays actually consisted of a third type of subatomic particles, which Chadwick named neutrons, because they proved to be electrically neutral particles having a mass slightly greater than that of protons. The mystery of the mass ratio could now be explained. In the helium nucleus there are two protons and two neutrons, but in the hydrogen nucleus there is only one proton and no neutrons, therefore, the ratio is 4 : 1.
Radio active source emitting Alpha, Beta and Gama rays.
Radioactivity:
After the discovery of cathode rays, a German Scientist Wilhelm Rontgen noticed that cathode rays caused glass
and the metals to emit very unusual rays. This, highly energetic radiation penetrated matter, darkened covered photographic plate
and caused a variety of substances to fluoresce. Because these rays could not be deflected by a magnet, they could not contain
charged particles as cathode rays. As their nature was unknown at that time Rontgen called them as X–rays.
Henri Becquerel, a French Scientist then made an extensive research on uranium mineral called pitchblende. He discovered that it spontaneously emits high–energy radiation. This spontaneous emission of radiation is called radioactivity. At Becquerel's suggestion Marie Curie and her husband, Pierre, began their famous experiments to isolate the radioactive components of the mineral. They have found that three types of rays are produced by the decay or breakdown, of radioactive substances such as uranium. Two of the three are deflected by oppositely charges metal plates. Alpha (α) rays consist of positively charged particles called as a particles, therefore are deflected by the positively charged plate. Beta (β) rays or β particles, are electrons and are deflected by the negatively charged plate. The third type of radioactive radiation consists of high energy rays called gamma (γ)rays. Like X rays, γ rays have no charge and are not affected by an external field.