X-RAYS AND RADIOACTIVITY BASIC INFORMATION AND TUTORIALS

Cathode-ray research had many important spin-offs. In particular, two natural phenomena of immense theoretical and practical significance were discovered in the course of other investigations.

In 1895, Wilhelm Roentgen (1845 1923) noticed that when cathode-ray tubes were operating, certain materials outside the tubes glowed or fluoresced.

He showed that this fluorescence was caused by radiation emitted by the cathode-ray tubes. Because of the unknown nature of this radiation, Roentgen coined the term X-ray. We now recognize the X-ray as a form of high-energy electromagnetic radiation.

Antoine Henri Becquerel (1852 1908) associated X-rays with fluorescence and wondered if naturally fluorescent materials produce X-rays. To test this idea, he wrapped a photographic plate with black paper, placed a coin on the paper, covered the coin with a uranium-containing fluorescent material, and exposed the entire assembly to sunlight.

When he developed the film, a clear image of the coin could be seen. The fluorescent material had emitted radiation (presumably X-rays) that penetrated the paper and exposed the film.

On one occasion, because the sky was overcast, Becquerel placed the experimental assembly inside a desk drawer for a few days while waiting for the weather to clear. On resuming the experiment, Becquerel decided to replace the original photographic film, expecting that it may have become slightly exposed.

He developed the original film and found that instead of the expected feeble image, there was a very sharp one. The film had become strongly exposed because the uranium-containing material had emitted radiation continuously, even when it was not fluorescing. Becquerel had discovered radioactivity.

Ernest Rutherford (1871 1937) identified two types of radiation from radioactive materials, alpha and beta Alpha particles carry two fundamental units of positive charge and have essentially the same mass as helium atoms.

In fact, alpha particles are identical to ions. Beta particles are negatively charged particles produced by changes occurring within the nuclei of radioactive atoms and have the same properties as electrons.

A third form of radiation, which is not affected by electric or magnetic fields, was discovered in 1900 by Paul Villard. This radiation, called gamma rays.