Hans Geiger and Ernest Rutherford developed the first electronic radiation counter able to detect atomic particles.
When radioactivity was discovered and first studied, researchers used 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 it. This simple device became known as the Crookes tube.
Antoine-Henri Becquerel heard of the discovery of X rays, and in February, 1886, he set out to discover whether nature might also reverse the process—that is, whether glowing minerals might emit X rays. Some minerals begin to glow when activated by sunlight. If they are then placed in the dark, the glow fades; the swiftness of the fading depends on the mineral involved. Such minerals are called phosphorescent. Becquerel’s testing procedure was to wrap photographic film in enough black paper that setting it in the sun all day would not expose it. He then placed various phosphorescent minerals on top of the film package and left them in the sun.
Becquerel soon learned that those phosphorescent minerals containing uranium would expose the film. To make certain that the film was not being exposed by chemicals from the uranium minerals, he covered the film with glass and also placed various pieces of metal on the film. He discovered that metal seemed to protect the film from exposure. The greatest surprise, however, came during a series of cloudy days. Eager to continue his experiments, he decided to develop film from a test that had not been exposed to sunlight. He was astonished to discover that the film was deeply exposed. Some emanation had to be coming from the uranium, and it had nothing to do with sunlight. Natural radioactivity was thus discovered with a simple piece of photographic film.
Hans Geiger.
Ernest Rutherford joined the world of international physics at about the same time radioactivity was discovered. He came from New Zealand for advanced study in physics at Cambridge in 1895. His previous work had been with radio waves, but now he plunged into a study of radioactivity. A remarkably talented man, he became a dominant figure in the field in only a few years. Studying the “Becquerel rays” emitted by uranium, Rutherford eventually distinguished three different types of radiation, which he named alpha, beta, and gamma (from the first three letters of the Greek alphabet). He showed that alpha particles
Rutherford became director of the associated research laboratory at Manchester University in 1907, and Hans Geiger became an assistant there. Up to that time, Rutherford had been unable to prove his conjecture that the alpha particle carries a double positive charge, but he would soon prove it with Geiger’s assistance. The obvious way to proceed would be to allow a stream of alpha particles to fall on a target and to measure the electric charge the particles brought to the target. By dividing that charge by the total number of alpha particles, one would have the charge of an alpha particle. The problem lay in counting the particles and in proving that every particle had been counted.
Basing their design on 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. When the voltage was turned down a little from this “sparkover” value, 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, which freed new electrons. These would ionize still more atoms and so on, until a veritable avalanche of electrons would strike the central wire and the electrometer would register the voltage change. Two key points became evident: First, because 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. Second, because of the electron avalanche, each electric discharge would be several thousand times larger than the alpha particle alone could have caused and would be easily measured with the electrometer.
Geiger and Rutherford completed their apparatus by connecting their alpha-detector tube to a “firing tube” that was 4.5 meters (approximately 14.8 feet) long. The air had been evacuated from the firing tube, and a small film of radium was fixed at the far end. Only a few alpha particles per minute were headed in the right direction to travel the length of the firing tube, pass through a small hole in a stopper, and pass through a thin mica window into the detector tube. There they traveled parallel with the central wire while they lost energy to ionization, which in its turn caused an electron avalanche. It was necessary to restrict the number of alpha particles to a few per minute because the electrometer was somewhat slow to respond and to recover.
On February 11, 1908, Geiger and Rutherford reported on their electrical method of counting alpha particles to the Manchester Literary and Philosophical Society. The most complex of the early radiation detection devices—the forerunner of the Geiger counter—had just been developed.
Geiger and Rutherford’s first measurements showed that one gram of radium emitted 34 thousand million alpha particles per second. Soon, they refined the number to 32.8 thousand million per second. Next, they 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 Robert Andrews Millikan’s classic oil-drop experiment in 1909.
Another fundamental result came from a 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, Geiger and Rutherford were able to calculate the Avogadro number (which enables the calculation of the number of atoms in a given amount of material) more directly and accurately than had previously been possible.
One of the most important applications of the “proto” Geiger counter was in proving that a different way to count alpha particles worked. In 1903, Crookes invented a device he called a spinthariscope,
Using both photographic film and an advanced model of the spinthariscope, Geiger and Rutherford noted that a beam of alpha particles tended to spread out after penetrating a thin mica window or a metal foil. Rutherford came to see this as a key to the structure of the atom. His approach could be compared to that of a traveler in unfamiliar territory. Suppose the traveler suddenly comes to a fog bank so thick that he cannot see anything that lies ahead, whether trees, a cliff, or a haystack. The traveler could throw pebbles into the fog to learn what lies ahead. That is, by throwing pebbles in various directions and listening to hear whether they strike something, the traveler could gain some notion of what is there. In just this fashion, Rutherford threw alpha particles at atoms. Using a zinc sulfide screen, he could observe where the alpha particles came out after interacting with the atoms of a target foil. The “proto” Geiger counter was used to prove that when appropriate precautions were taken, observers could accurately count the alpha particles that struck the zinc sulfide screen. In 1911, Rutherford announced that the atom must consist of a tiny but relatively massive nucleus surrounded by a vast space through which the electrons move.
The true Geiger counter evolved when Geiger replaced the central wire of the early counter’s 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,
As for the other early instruments, X-ray-sensitive film is still used today. Both the television tube and the X-ray tube are direct descendants of the Crookes tube. The zinc sulfide screen is closely related to the phosphor-coated television screen, which glows under a beam of beta particles.
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Andrade, Edward Neville da Costa. Rutherford and the Nature of the Atom. Garden City, N.Y.: Doubleday, 1964. This delightful book is easy to read but does not stint on science. Andrade, a physicist, joined Rutherford’s group from 1913 to 1914, about the time Geiger changed the “proto” Geiger counter into the Geiger counter. Includes some interesting photographs. Highly recommended. -
Berks, I. B., ed. Rutherford at Manchester. London: Heywood, 1962. This volume resulted from a commemorative conference held at Manchester University in 1961 to mark the fiftieth anniversary of Rutherford’s discovery of the nucleus. Presents several speeches by colleagues of Rutherford who reminisce about him and his work as well as several key scientific papers by Rutherford and Geiger. Includes historical photographs, a complete bibliography of Rutherford’s publications, and a bibliography of papers published by other members of his group. -
Chown, Marcus. The Magic Furnace: The Search for the Origins of Atoms. New York: Oxford University Press, 2001. A history of humankind’s attempts to understand the atom. Discusses all of the major theories and experiments that have furthered knowledge in this area. Includes glossary, select bibliography, and index. -
Keller, Alex. The Infancy of Atomic Physics: Hercules in His Cradle. Oxford, England: Clarendon Press, 1983. Describes the works of Rutherford, Geiger, and many others. The major portion of the book deals with the time period from about 1880 to 1920. Places the scientists’ works within the individuals’ cultural and historical backgrounds. Includes a useful bibliography. -
Mann, Wilfrid B., R. L. Ayres, and S. B. Garfinkel. Radioactivity and Its Measurement. 2d ed. Elmsford, N.Y.: Pergamon Press, 1980. Presents exceptionally complete descriptions of some of the early experiments in the discovery of radioactivity, including the work of Geiger and Rutherford. Readers with some background in physics, chemistry, and algebra will be most likely to appreciate this work. -
Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986. Presents brief descriptions of the key experiments by Geiger and Rutherford, placing them in historical context and including interesting personal details. Written for a wide audience. -
Rowland, John. Ernest Rutherford. New York: Philosophical Library, 1957. An easily read biography of Rutherford. Describes his work with Geiger and the advent of the Geiger tube.
Elster and Geitel Study Radioactivity
Becquerel Wins the Nobel Prize for Discovering Natural Radioactivity
Boltwood Uses Radioactivity to Determine Ages of Rocks
Rutherford Discovers the Proton
First Artificial Radioactive Element Is Developed