Among the scientists who explored radioactivity immediately after Becquerel’s discovery was Ernst Rutherford, then at McGill University in Montréal, Canada. He and Frederick Soddy found that uranium and thorium emitted two different kinds of rays. One could be stopped by paper, the other required metal about a centimeter thick. Rutherford named them alpha and beta rays after the first two letters in the Greek alphabet. In 1907 he named the even more penetrating rays produced by radium gamma rays.

In 1900 Pierre (1859-1906) and Marie Curie (1867-1934), using an electroscope, found that beta rays are negative particles. Becquerel used the same kind of apparatus J.J. Thomson (1856-1940) had used (see the chapter “What Is the World Made Of?”) to measure the ratio of mass to charge of an electron to determine that betas typically travel at about half the speed of light and are identical to electrons. In late 1907 Rutherford demonstrated that alpha rays were helium atoms with the two electrons removed. (He had not yet discovered the nucleus. Today we say that an alpha particle is the helium-4 nucleus.) Gamma rays were later found to be very high energy photons.

Alpha, beta, and gammas all are hazardous to our health. Alphas are blocked by skin, but if a radioactive material, such as the gas radium, is inhaled, the alpha particles can cause damage to the lungs. Betas can penetrate skin and tissue and, if they strike a cell, can cause mutations to the DNA. Gammas can cause mutations and kill cells; they are used in cancer radiation therapy.