1919 Discovery of Protons
The [nucleus] is the place where radioactivity takes place. In order to understand radioactivity, man should first understand what is in the nucleus.
In 1919, Rutherford bombarded gases with alpha particles. This is the diagram of the apparatus used in Rutherford's investigations.
The thin silver foil at one end could stop the alpha particles from the radioactive source. Any other particles that penetrated through the foil would fall on the zinc sulphide screen and produced [scintillation]. Various gases such as oxygen, nitrogen and carbon dioxide have been used to fill up the apparatus but scintillation was only detected when nitrogen was used. The particles which have passed through the silver foil were shown to have a similar range, and same charge as protons.
Later investigations of the reaction in a [cloud chamber] gave rise to photographs showing clearly that the alpha particle actually entered the nitrogen nucleus which immediately ejected a proton.
This was the first occasion on which one element was changed into another one. This process was called nuclear transmutation.
Rutherford's students nicknamed him "the crocodile" because " the crocodile cannot turn its head... it must always go forward with all devouring jaws."
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