Notes

alpha particle Particle radiating from the nucleus of some atoms; symbolized by He or α. Identical to a helium nucleus.
beta particle Negative particle identical to an electron but radiating from a decaying nucleus; it is symbolized by β.
gamma ray High intensity bundle of energy being emitted from some decaying nuclei; it is symbolized by γ.

The nuclei of some atoms are naturally unstable. They fly apart of their own accord, and we cannot predict which one will fly apart next. At the same time, certain characteristics of a nucleus can be studied and predicted. Our study in this section will be to see how decaying nuclei can be detected and how we can measure that decay.

As is true of many of the great discoveries in science, the discovery of radioactivity was an accidental, unplanned discovery. The setting was Forborne, in Paris, France, 1896. The scene was a lab in which a man by the name of Professor Henri Becquerel was studying phosphorescent substances, those substances which, after being exposed to sunlight, will glow in the dark. The weather was cloudy, and Professor Becquerel wrapped some uranium ore in papers and put it in his desk drawer because without the sunshine, he had to delay his experimentation. He was not aware that he had placed the ore sample on a sealed envelope containing a photographic plate.

Several days later, when Becquerel examined the photographic plate, he found that on the place where the ore had lain, the plate had been developed as if it had been exposed to light. After further experimentation, he concluded that the uranium gave off rays continuously, and that these rays could penetrate the paper and expose the film without its being exposed to sunlight. He called this process by which a substance gives off penetrating rays radioactivity.