In 1896, Antoine Henri Becquerel investigated X-rays, which had just been discovered by Wilhelm Conrad Röntgen, in more detail. It was already known that X-rays could blacken photographic plates as if light were falling on them. During his work, Becquerel discovered almost by chance that uranium salts could also blacken photographic plates in the dark. The significant difference was that X-rays were produced artificially, as they are today, but the radiation from the radioactive substances was purely natural.

Together with Marie and Pierre Curie, Becquerel was soon able to identify other radioactive substances and discover some of the properties of radiation: It can penetrate opaque materials and is independent of temperature changes and chemical treatments of the sample. It can also discharge both positively and negatively charged objects and make uncharged substances, such as air, conductive for a short time (ionising property).

The new type of radiation also aroused the interest of Ernest Rutherford. He initially investigated the penetrating power of the radiation and was able to establish that it consisted of two components. He labelled them α and β radiation. He wrote:

These experiments show that uranium radiation is complex and that at least two different kinds of radiation occur - one which is absorbed very rapidly, called for convenience α-radiation, and the other, which has a more penetrating character, is called β-radiation (translation from Rutherford (1899)).

It was not long before a third component of radiation was identified. This was named γ-radiation in reference to the already known components. It can penetrate even deeper into matter than α and β radiation.

These properties of the types of radiation are summarised in the following table

In addition to the penetrating power of the individual types of radiation, further distinctions have been found. If the radiation is sent through an electric field, the α radiation is attracted to the negative pole and the β radiation to the positive pole. Similarly, the two types of radiation are also deflected in different directions in a magnetic field. In contrast, γ-radiation is not influenced in either field.

The behaviour of the radiation in the electric and magnetic field therefore shows that α-radiation must be positively charged and β-radiation must be negatively charged and both consist of particles. The strength of the deflection in the electric and magnetic field can also be used to determine the ratio of charge to mass of the particles. This ratio is called the specific charge and is like a fingerprint for each particle.

For the β-particles, the same specific charge was found as for electrons, which J. J. Thomson had already determined in 1897. They behave in the same way in electric and magnetic fields. This was a strong indication that β-radiation consists of electrons. They have a single negative charge and a mass of 0.00055 u.

The situation was similar for the α particles: the specific charge and other experimental findings were clear indications that it consists of doubly positively charged helium atoms. The mass of these particles is significantly greater than that of the electrons, it is 4 u. Rutherford and his colleague T. Royds succeeded in proving this assumption in 1909.

γ-radiation, on the other hand, does not consist of particles, it carries no charge and is not deflected in an electric or magnetic field. It behaves in a similar way to the aforementioned X-rays. This radiation is pure electromagnetic radiation.