The Discovery of Gamma Rays
Department of Physics, Building 307
Radiation physics and chemistry flourished in the years around 1900. Wilhelm Conrad Röntgen's sensational discovery of X rays in 1895 was soon followed by Henri Becquerel's discovery of radioactivity and by J. J. Thomson's proof of the independent existence of negative electrons of small mass. Marie and Pierre Curie discovered the radioactive elements polonium and radium. A new kind of extremely penetrating rays, later called gamma rays, was observed by Paul Villard, but his discovery is almost never discussed in any detail. He stands, one might say, in the shadow of the giants Becquerel and the Curies.
Paul Villard was born in 1860 in a village near Lyon, France. In 1881 he entered the École Normale Supérieure in Paris. After his agrégation, which gave him the license to teach at any secondary school financed by the government, Villard taught at various lycées in the province, and finally at the Lycée of Montpellier. Here he became Maitre de Conférences at the University. He liked scientific research but soon felt that he had to work in Paris, which was the centre of physical science in France. Having a modest fortune that was sufficient for his needs, he asked for leave from his teaching position. He went to Paris where he enjoyed the hospitality of chemistry professor Henri Debray and his successors at the École Normale. Villard now devoted himself exclusively to science, spending the rest of his professional life in the chemistry department of the École Normale in rue díUlm.
Villard preferred independent research, and most of his papers are single-authored. He also had little concern for fame. Nevertheless, the Académie des Sciences awarded him its Wilde prize in 1904 and its La Caze prize in 1907. In 1908 he succeeded physicist Eleuthère Mascart as a member of the Academy. During the last years of his life, Villard was forced to spend extended periods of time outside of Paris because of his deteriorating health. He died in Bayonne on January 13, 1934.
Villard's earliest studies were in the field of physical chemistry, where he investigated the combination of water with various gases under pressure, forming hydrates of them. He published his first papers in the Comptes rendus des Séancesde líAcadémie des Sciences in 1888 together with Robert Hippolyte de Forcrand. They repeated, with improved accuracy, some earlier studies on gaseous hydrates, but Villard soon reported on a further series of completely new hydrates, which had been considerably more difficult to produce. This work formed the basis for his doctoral thesis. In 1897 Villard gained access to a Crookes tube and started publishing a long series of papers on cathode rays and X rays. Villard's publication rate peaked in the years 1898-1900. During this period he published about ten major papers each year, including his two papers on radium radiation in 1900.
Radioactivity was a hot topic in Villard's day, and its investigation was pursued vigorously by a number of prominent scientists, such as Ernest Rutherford, Becquerel and the Curies. Transmission experiments indicated that the radiation emitted by radioactive bodies was heterogeneous. Rutherford named the then distinguishable types of radiation alpha and beta rays. At this time beta rays were more often studied than alpha rays because of their higher penetrating power and marked photographic action. Becquerel as well as Friedrich Giesel, Stefan Meyer and Egon Ritter von Schweidler demonstrated that radium beta rays are affected by a magnetic field. This brought out their strong resemblance to cathode rays.
Using his sensitive electrometer based on a piezoelectric crystal, Pierre Curie demonstrated that radium radiation consists of two distinct types: rays that are deviable in a magnetic field (beta rays), and rays that are non-deviable in a magnetic field (alpha rays). Transmission experiments by Marie Curie verified that the non-deviable rays are much less penetrating than the deviable rays. At this time, alpha rays were believed to be non-deviable by a magnetic field. Therefore, the two kinds of radiation, alpha and beta, were initially distinguished as non-deviable and deviable in a magnetic field.
The predictable identification of the deviable rays (beta rays) with cathode rays (electrons) required two more verifications. It was necessary to demonstrate that beta rays carry a charge of negative electricity, and that they are deflected by an electrostatic field. The Curies showed that the deviable radium rays impart a negative charge to an insulated conductor. Becquerel and Ernst Dorn independently verified the electrostatic deflection. Thus, beta rays were definitely identified with cathode rays, i.e., it was proved that they are streams of rapidly moving, negatively charged electrons. For the velocity of the beta particles Becquerel got an astounding figure, between one-half and two-thirds of the velocity of light.
The Curies also observed some chemical effects produced by radium rays. They found that the rays emitted by highly radioactive salts of barium are capable of converting oxygen into ozone. They also observed a coloring action of the rays on glass and on barium platinocyanide commonly used for fluorescent screens. These results evoked Villardís interest because he had made similar observations with X rays. Villardís interest in radioactivity was now aroused, and he wanted to compare the reflection and refraction properties of cathode rays and beta rays. As it turned out, he would make an unpredictable discovery.
Paul Villard presented his paper, "Sur la réflexion et la réfraction des rayons cathodiques et des rayons déviables du radium," at the Monday session of the Paris Académie des Sciences on April 9, 1900. It follows from the title that Villard originally set out to study the deviable rays (beta rays) but his work led to his discovery of a new kind of penetrating rays. The description of his experiment is hard to follow without a diagram, but none is supplied. Villard addressed himself to a few scientists who were familiar with his experimental methods and he considered a verbal description perfectly adequate.
Villard emphasized that the deviable rays (beta rays) behave in all respects like cathode rays and that he wanted to measure the refraction of these rays. During the course of his work, Villard noticed that in almost every experiment the photographic plate revealed traces of a non-refracted beam, which obviously had been propagating in a straight line. This beam was superimposed on the refracted beam, making it difficult to interpret the photographs. Next, Villard tried to deflect the non-refracted rays in a magnetic field, but they were unaffected. Moreover, these rays were penetrating enough to affect the photographic plate protected by several layers of black paper as well as an aluminium foil. The rays were even able to traverse a 0.2-mm thick lead foil when placed in the beam.
The Curies kindly placed a much stronger radium sample at Villardís disposal, and three weeks later he presented new and more detailed results on the radium rays to the Académie des Sciences. His comparative study of the penetrating power of beta rays and his new type of rays, "Sur le rayonnement du radium," was read by Academy member Jules Violle at the Monday meeting on April 30, 1900. Villardís experimental arrangement was about the same as in his first radium experiment. The radiation from the radium sample was collimated by a long groove in a lead block and sent consecutively through two photographic plates stacked on top of each other. The deviable rays were bent in a magnetic field before hitting the photographic plates.
Villard reported that the first photographic plate showed traces of two distinct beams. One had been deflected by the magnetic field and broadened. The other had propagated along an absolutely straight line and produced a sharp impression. On the second plate there was only one trace, that from the non-deflected beam. It produced an impression that was as sharp and intense as on the first plate. It was even more visible because of the lower background radiation on the second plate. The plates were made of glass, and because of the grazing incidence the non-deflected beam had traversed 1 cm of glass before reaching the second plate. It followed that the non-deflected rays were able to penetrate at least 1cm of glass without any noticeable attenuation. Even a lead foil, 0.3mm thick, was found to attenuate the rays only slightly. Villard appears already to have associated the penetrating radiation with X rays. He concluded that the "X rays" emitted by radium had a considerably larger penetrating power than the deviable rays (beta rays).
Less than three weeks later, Villard expressed himself more boldly. At the Friday meeting of the Société francaise de physique on May 18, 1900, he demonstrated that radium emits rays that are non-deviable and extremely penetrating. These new rays, said Villard, were different from the radium rays observed so far. He went on to suggest that the extremely penetrating rays, discovered by him, were a kind of X rays. Furthermore, as he pointed out, the readily absorbed radium rays (alpha rays) were analogous to the non-deviable cathode rays (positive ions or Kanalstrahlen) previously observed by J. J. Thomson, Wilhelm Wien and others. The deviable rays (beta rays) had already been shown by Becquerel to be identical to a stream of electrons. Villard concluded that "on retrouverait ainsi les trois rayonnements des tubes de Crookes," i. e., the three kinds of radiation (ions, electrons and X rays) known from experiments with cathode-ray tubes were all present in radium rays. Thus, from the beginning, Villard gave a correct interpretation of the three components of radium rays. Unfortunately, his discovery was largely overlooked by his contemporaries.
Villard performed his radium experiments under the watchful eyes of Becquerel. In fact, both men reported on transmission experiments with radium radiation at the April 9, 1900, session of the Académie des Sciences. Becquerel found it necessary to repeat Villardís experiment, and he delivered his comments three weeks later at the April 30 meeting. He disputed the apparent refraction of beta rays. Regarding the very penetrating rays, he simply denied their presence, arguing that the existence of these rays could not possibly have escaped attention in the experiments carried out by him or the Curies. Gradually, however, Becquerel had to accept the experimental facts. The Curies seemed to treat Villardís findings with more interest. As mentioned above, they placed a stronger radium source at his disposal, thus enabling him to produce more detailed and reliable observations. They also supported his interpretation of the penetrating rays as a kind of X rays.
Contrary to common belief, Villard did not introduce the designation "gamma rays." It is characteristic of the weak contemporary interest in these penetrating rays that they went unnamed for nearly three years. The name gamma rays was probably invented by Rutherford, but I have been unable to determine where they are explicitly named. Rutherford is still using the descriptive form "rays nondeviable in character, but of very great penetrating power" in the January 1903 issue of the Philosophical Magazine, but in the subsequent February issue, he introduces the trio alpha, beta and gamma. Marie Curie notes in her doctoral thesis that one can distinguish between three types of radiation, which are denoted by the letters alpha, beta and gamma, following the notation of Rutherford.
At first sight, it seems surprising that no one apparently took much care in 1900 and the following years of Villardís new kind of very penetrating rays. Rutherford measured the absorption of gamma rays in various materials, but for the time being did not pay much attention to these new rays. Turning to the alpha rays, Rutherford discovered their electric and magnetic deviability and proved that they consist of positively charged particles. From measurements of their charge-to-mass ratio, the alpha particles were provisionally identified as positive ions of hydrogen or helium. They finally were established as helium ions. Rutherford recognized that the alpha particles, since their mass is much larger than the mass of the beta particles, carry virtually all of the energy released in radioactive processes. Therefore he considered alpha rays more important than beta and gamma rays.
Consequently, he concentrated his research on the investigation of alpha particles. His studies culminated in the transformation theory of Rutherford and Frederick Soddy of 1903, and in the alpha -scattering experiments by Hans Geiger and Ernest Marsden, which led Rutherford to postulate the existence of the atomic nucleus in 1911.
Although studies of alpha rays yielded remarkable results, beta rays continued to attract considerable interest. Becquerelís numerical result for the charge-to-mass ratio of the beta particle was adequate for its interpretation as an electron and, if not at first very accurate, was soon improved upon by other experimenters. Walter Kaufmann was even able to show that its mass increases with increasing velocity of the particle.
Thus prominent and influential physicists and chemists were busy investigating alpha and beta rays (particles), yet they paid little attention to Villard's discovery of gamma rays. The available experimental possibilities for investigating gamma rays were limited, and their nature was difficult to determine. Marie Curie included a gamma-ray radiograph in her doctoral thesis, thereby demonstrating a potential application of Villardís discovery. She also noted, however, the weak contrast between bone and soft tissue in gamma radiographs, and the long exposure times required. It was much easier and faster to produce X-ray radiographs, and gamma rays remained a scientific curiosity for many years.
Apart from the limited experimental techniques available, a major reason for the small interest in gamma rays was that they apparently did not fit into contemporary views in radiation physics and chemistry. After J. J. Thomson's proof of the independent existence of the electron of small mass in 1897, and in particular after his measurements of its charge and mass in 1899, contemporary scientists focused much of their interest on the material nature of atomic radiations. The view that atomic radiations are material and particulate proved to be successful in interpretating the nature of cathode rays, and it continued to deliver remarkable results when applied to alpha and beta rays. That picture was disturbed with gamma rays, which did not seem to fit into this established view of radiation and matter.
The electromagnetic wave nature of X rays was firmly established in 1912, when Max von Laue conceived the idea of employing a crystal as a space diffraction grating for X rays. The successful realization of this idea by Laue and his assistants Walther Friedrich and Paul Knipping opened up a wide field of research. In the hands of William and Lawrence Bragg, father and son, X-ray diffraction soon became a powerful tool for crystal-structure determination, but also for X-ray spectroscopy. It would not be long before Rutherford applied crystal-diffraction techniques to confirm the wave nature of gamma rays. He and E. N. da C. Andrade first determined the wavelength of relatively soft gamma rays. Later, they employed an ingenious transmission method to measure the small angles of reflection (about 1.5º) of harder gamma rays. Thus, it was finally established that gamma rays as well as X rays are electromagnetic radiations of short wavelength. Meanwhile, high-voltage X-ray generators had made it possible to produce X rays with wavelengths in a range overlapping those of gamma rays, the only distinction between the two types of radiation being their origin. A few years later, Arthur Holly Comptonís studies of the scattering of X rays led to the concept of X rays acting as particles. Thus, it was shown that X rays and gamma rays can indeed be viewed as streams of particles or quanta moving with the velocity of light. These particles, however, are not massive electrons but light (sic) quanta of zero rest mass (photons).
After publishing his two papers of 1900 on his discovery of gamma rays, Villard made no further studies of them. Was he disappointed by the little interest that his discovery aroused in the contemporary scientific community? In particular, was he disappointed by the reluctant acceptance of his results by Becquerel? Whatever the case may be, Villard decided to withdraw from the highly competitive arena of research in radioactivity and to devote his efforts to the more familiar cathode rays and X rays. Villard also developed an interest in the aurora borealis whose streamers are caused by the interaction between charged particles and molecules in the upper atmosphere and have some bearing on the electric discharges in a rarefied gas in a cathode-ray tube.
By necessity, but also following his own inclination, Villard constructed all of his experimental equipment himself. He devised several instruments useful for practising radiologists. The idea of using the ionization of air as a measure of the output of an X-ray tube was suggested by Villard in 1908. The principle laid down by Villard became internationally recognized twenty years later, when the roentgen (r) unit of radiation exposure was recommended by the Second International Congress of Radiology in Stockholm in 1928.
Today, the name of Paul Villard has disappeared almost without a trace. Nevertheless, as I have shown here, Villard made important contributions to radiation science and technology. In particular, many of his practical inventions had a lasting impact. Villard also made a crucial contribution to radiation science when he discovered gamma rays. However, the nature of these rays unfolded over several years through the work of several people. This clarifying process had to await the development of new concepts, such as the quantum theory of radiation and the existence of high-frequency electromagnetic waves.
Benoit Lelong, "Paul Villard, J.J. Thomson et la composition des rayons catodique", Revue díHistoire des Sciences 50 (1997) 89-130.
Leif Gerward, "Paul Villard and his Discovery
of Gamma Rays", Physics in Perspective 1 (1999)367-383.
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