The magnetic deflection of alpha-particles is difficult to
produce; a field of 1 tesla gives a path of radius 39 cm for an alpha particle of energy 1.8
MeV, while a similar field acting on beta-particles of the same energy would give a radius of
0.75 cm. This suggests either that alpha particles are heavier than electrons or that they
move more slowly. It was found that both are true! They were also found to be very heavily
ionising particles (this accounts for their short range in air) and to have a positive charge of
Rutherford and Royds made a direct identification of the alpha particle in 1909.
The apparatus they used is shown in Figure 1 Radon gas, an alpha-emitter, was enclosed in
a glass tube A that had walls only 0.01 mm thick. The tube B was originally
After a week the
mercury level in B was raised and a discharge struck, forming a spectrum. On examination it
was found to be the spectrum of helium. When the experiment was repeated with A initially
filled with helium no spectrum was seen when a discharge was struck in
Rutherford reasoned that the alpha particles produced by the radon collected electrons from the glass as they passed through the walls of tube A, thus becoming helium atoms.
The alpha particles emitted by a source have a well-defined energy. Sometimes a source will emit alpha particles of two different energies, but these two values are distinct and separate.
The distinct nature of these energies is very good evidence for the fact that the alpha particle is emitted on its own, not in conjunction with another particle. Only two particles are involved, the alpha particle and the nucleus that has emitted it. The tracks of alpha particles observed in cloud chambers and bubble chambers provide very good evidence for this.
A graph of the results from a source emitting alpha particles with two distinct energies is shown in Figure 2.