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In 1896, the French physicist Antoine Henri Becquerel accidentally found that a uranium-rich mineral called pitchblende emitted invisible, penetrating rays that could darken a photographic plate enclosed in an opaque envelope.While working on phosphorescent materials, he happened to place the pitchblende on black paper that he had used to cover a piece of ﬁlm.The types we will discuss here are: alpha, beta, and gamma (listed in increasing ability to penetrate matter).Alpha decay is seen only in heavier elements greater than atomic number 52, tellurium.
Such isotopes are radioactive, and are referred to as “radioisotopes.” There are many types of emmitted particles and radiation that radioisotopes produce when they decay.
This means that rays carry energy, but pitchblende emits them continuously without any energy input.
This is an apparent violation of the law of conservation of energy.
When he looked more carefully, he noticed that the ﬁlm had lots of patches on it, and that this did not happen when other elements were placed on the paper.
He eventually concluded that some rays must be coming out of the uranium crystals to produce this eﬀect.