29 June 2009
The invention: the first electronic device able to detect and measure radioactivity in atomic particles. The people behind the invention: Hans Geiger (1882-1945), a German physicist Ernest Rutherford (1871-1937), a British physicist Sir John Sealy Edward Townsend (1868-1957), an Irish physicist Sir William Crookes (1832-1919), an English physicist Wilhelm Conrad Röntgen (1845-1923), a German physicist Antoine-Henri Becquerel (1852-1908), a French physicist Discovering Natural Radiation When radioactivity was discovered and first studied, the work was done with rather simple devices. In the 1870’s, Sir William Crookes learned how to create a very good vacuum in a glass tube. He placed electrodes in each end of the tube and studied the passage of electricity through the tube. This simple device became known as the “Crookes tube.” In 1895, Wilhelm Conrad Röntgen was experimenting with a Crookes tube. It was known that when electricity went through a Crookes tube, one end of the glass tube might glow. Certain mineral salts placed near the tube would also glow. In order to observe carefully the glowing salts, Röntgen had darkened the room and covered most of the Crookes tube with dark paper. Suddenly, a flash of light caught his eye. It came from a mineral sample placed some distance from the tube and shielded by the dark paper; yet when the tube was switched off, the mineral sample went dark. Experimenting further, Röntgen became convinced that some ray from the Crookes tube had penetrated the mineral and caused it to glow. Since light rays were blocked by the black paper, he called the mystery ray an “X ray,” with “X” standing for unknown. Antoine-Henri Becquerel heard of the discovery of X rays and, in February, 1886, set out to discover if glowing minerals themselves emitted X rays. Some minerals, called “phosphorescent,” begin to glow when activated by sunlight. Becquerel’s experiment involved wrapping photographic film in black paper and setting various phosphorescent minerals on top and leaving them in the sun. He soon learned that phosphorescent minerals containing uranium would expose the film. Aseries of cloudy days, however, brought a great surprise. Anxious to continue his experiments, Becquerel decided to develop film that had not been exposed to sunlight. He was astonished to discover that the film was deeply exposed. Some emanations must be coming from the uranium, he realized, and they had nothing to do with sunlight. Thus, natural radioactivity was discovered by accident with a simple piece of photographic film. Rutherford and Geiger Ernest Rutherford joined the world of international physics at about the same time that radioactivity was discovered. Studying the “Becquerel rays” emitted by uranium, Rutherford eventually distinguished three different types of radiation, which he named “alpha,” “beta,” and “gamma” after the first three letters of the Greek alphabet. He showed that alpha particles, the least penetrating of the three, are the nuclei of helium atoms (a group of two neutrons and a proton tightly bound together). It was later shown that beta particles are electrons. Gamma rays, which are far more penetrating than either alpha or beta particles, were shown to be similar to X rays, but with higher energies. Rutherford became director of the associated research laboratory at Manchester University in 1907. Hans Geiger became an assistant. At this time, Rutherford was trying to prove that alpha particles carry a double positive charge. The best way to do this was to measure the electric charge that a stream of alpha particles would bring to a target. By dividing that charge by the total number of alpha particles that fell on the target, one could calculate the charge of a single alpha particle. The problem lay in counting the particles and in proving that every particle had been counted. Basing their design upon work done by Sir John Sealy Edward Townsend, a former colleague of Rutherford, Geiger and Rutherford constructed an electronic counter. It consisted of a long brass tube sealed at both ends from which most of the air had been pumped. A thin wire, insulated from the brass, was suspended down the middle of the tube. This wire was connected to batteries producing about thirteen hundred volts and to an electrometer, a device that could measure the voltage of the wire. This voltage could be increased until a spark jumped between the wire and the tube. If the voltage was turned down a little, the tube was ready to operate. An alpha particle entering the tube would ionize (knock some electrons away from) at least a few atoms. These electrons would be accelerated by the high voltage and, in turn, would ionize more atoms, freeing more electrons. This process would continue until an avalanche of electrons struck the central wire and the electrometer registered the voltage change. Since the tube was nearly ready to arc because of the high voltage, every alpha particle, even if it had very little energy, would initiate a discharge. The most complex of the early radiation detection devices—the forerunner of the Geiger counter—had just been developed. The two physicists reported their findings in February, 1908. Impact Their first measurements showed that one gram of radium emitted 34 thousand million alpha particles per second. Soon, the number was refined to 32.8 thousand million per second. Next, Geiger and Rutherford measured the amount of charge emitted by radium each second. Dividing this number by the previous number gave them the charge on a single alpha particle. Just as Rutherford had anticipated, the charge was double that of a hydrogen ion (a proton). This proved to be the most accurate determination of the fundamental charge until the American physicist Robert Andrews Millikan conducted his classic oil-drop experiment in 1911. Another fundamental result came froma careful measurement of the volume of helium emitted by radium each second. Using that value, other properties of gases, and the number of helium nuclei emitted each second, they were able to calculate Avogadro’s number more directly and accurately than had previously been possible. (Avogadro’s number enables one to calculate the number of atoms in a given amount of material.)The true Geiger counter evolved when Geiger replaced the central wire of the tube with a needle whose point lay just inside a thin entrance window. This counter was much more sensitive to alpha and beta particles and also to gamma rays. By 1928, with the assistance of Walther Müller, Geiger made his counter much more efficient, responsive, durable, and portable. There are probably few radiation facilities in the world that do not have at least one Geiger counter or one of its compact modern relatives.