What are alpha particles

Ernest Rutherford investigated and named alpha rays or particles in 1909. By measuring the charge and mass of the alpha particles he discovered that they are the same as the nuclei of ordinary helium atoms. However a rather neat and simple experiment can be performed to confirm that.

In the diagram below  at first the tube is completely empty - a vacuum. The radioactive source at the base of the tube is emitting alpha radiation. A very high voltage supply is connected to the metal caps at either end of the tube.

To begin with there was no light from the tube but after several days the current passed through the gas emitting light. But after a few days there is a glow. The light is split up using a diffraction grating and a spectrum starts to appear on the screen.  The colours on the screen are what you would expect from helium. We know that alpha particles are positively charged so the conclusion is that an alpha particle is a helium nucleus, that is two protons with two neutrons.

Experiment to demonstrate that an alpha particle is helium nucleus