Physics

X-ray Discovery (continued)


The fortuitous November 8, 1895

In the last decade of the last century, research on cathode rays was the most effervescent theme in all of Europe, so it seems natural that Roentgen, then director of the Institute of Physics at the University of Würzburg, wanted to repeat some of the experiments published. . According to Fuchs and Romer, Roentgen's experiments began in 1894, but almost all historical literature realizes that these works began in 1895. Later we will discuss this little mystery. We will present here what is known of the facts that occurred as of that Friday, November 8, 1895.

There is some controversy in the literature on the evolution of the facts, but one thing seems certain: Roentgen has not worked with X-rays for more than 3 years. Moreover, in less than 8 weeks he discovered virtually all of their fundamental properties, wrote three papers on the subject, and by 1897 was back to his favorite subjects, abandoning a subject of such fertility, which earned him the Nobel Prize. Physics, not only to him (1901), but also to Lenard (1905), Thomson (1906), Laue (1914), WH Bragg and WL Bragg (1915), Barkla (1917) and Siegbahn (1924).

In a letter sent in February 1896 to his great friend Ludwig Zehnder, Roentgen says that during the experiments he did not tell anyone about his work except his wife. Thus, the opening paragraph of this article, taken from an account of Manes, may be false; It was used here as a force for dramatic expression. What is known is that on December 28, 1895 Roentgen forwarded to the president of the Würzburg Society of Physics and Medicine (SFMW) a manuscript entitled "About a New Type of Lightning" ("On a new kind of rays "or in German "Ueber eine neue art von strahlen "), which he regards as a "preliminary communication". Given the depth and conciseness with which the results are presented, it is not surprising that this was the most important of Roentgen's three works. On March 9, 1896, he sent his second communication to the same society under the same title as the first. In his article, Watson transcribes these two communications; the original versions in German and the translations in English. According to Jauncey, the third article is dated March 10, 1897. In the issue of January 23, 1896, Nature publishes an English version of the first communication and is immediately reproduced in Science, Scientific American Supplement, Journal of the Franklin Institute and in the popular magazine Review of Reviews (similar to Reader's Digest). The german magazine Annalen der Physik, in its edition of January 1, 1898, reproduces the three articles. Copies of the first work, with a one-hand x-ray, were sent from late December to early January to the leading scientists in Europe, who learned of the breakthrough, as SFMW's annals were widely circulated. limited, practically local.

Roentgen has received numerous conference invitations, but seems to have declined from all but one, presented at SFMW on January 23, 1896, in which he was extremely successful despite his acknowledged shyness. At this conference he took several radiographs, including one made famous by the great anatomist, professor at the University of Würzburg, A. von Kölliker. With each x-ray he got, the audience responded with enthusiasm and loud applause.

The first two communications

Roentgen's first two communications, which he considered to be one, are fine examples of objectivity and conciseness, without neglecting the depth that the subject requires. It impresses the amount of data obtained in such a short time, but frustrates the expectation of the reader interested in the heuristic of the investigation and the assembly of the equipment; There is no detailed information to that effect. He reports that he used a large Ruhmkorff coil, but does not specify what type of vacuum tube he used; We will discuss this issue later.

The results are presented in 21 topics, many of which contain a single paragraph, along which Roentgen discusses virtually all fundamental X-ray properties. In the order in which they appear in the communications, these properties are as follows. Firstly, the rays can be detected by flickering on a phosphorescent screen or by printing on a photographic plate. Unlike cathode rays, X-rays can be observed even when the screen is placed at a distance of approximately two meters from the vacuum tube (cathode rays do not exceed eight centimeters in air). Roentgen tests the transparency of a huge amount of materials, finding that two properties are important: material density and thickness; the denser and thicker the less transparent. After testing transparency, Roentgen investigates refractive and reflection effects. He does not observe either, although he was doubtful about the reflection. It tries to deflect X-rays with the aid of a magnetic field, but fails to do so, and here it establishes one of the fundamental experimental differences between x-rays and cathode rays, as they are easily deflected by a magnetic field.

In topic 12 he discusses one of the most fundamental issues for X-ray identification. He concludes that these rays are produced by the cathode rays on the glass wall of the discharge tube! He then reports that he has observed X-rays produced by the shock of cathode rays on an aluminum plate, and promises to test other materials. A year later, on December 17, 1896, English physicist Sir George Stokes demonstrated that X-rays are produced by the deceleration of charged particles, a phenomenon that occurs when, for example, high energy electrons penetrate a heavy material! Or, in the language of the time, when cathode rays penetrate heavy material!

In topic 17, which concludes the first communication, he discusses the nature of x-rays. Obviously it discards identity with cathode rays. It suggests that it could be something like ultraviolet light, due to the fluorescent effects and printing of photographic plates, but in comparing other properties, it concludes that x-rays cannot be of the same nature as usual ultraviolet light. Concludes the article by suggesting that x-rays could be longitudinal vibrations in the ether. As is well known, this hypothesis was used by the Germans (Goldstein, Hertz, Lenard, and others) to explain cathode rays.

At the beginning of the second communication, topic 18, Roentgen examines the issue of the effect of x-rays on electrified bodies, referring to the results published by Lenard. It immediately suggests that the effects attributed by Lenard to cathode rays were in fact due to the X-rays produced on the aluminum window of his vacuum tube. (Lenard had the X-rays there in front of him, and didn't know it!)

In the final topics, 19, 20 and 21, it discusses practical questions: induction coil operation, vacuum maintenance and difference between aluminum and platinum, regarding the intensity of the produced beam.

What else but chance?

To understand the discovery of X-rays as the result of a planned scientific work, much more than a chance event, would require knowledge of the heuristics that guided the research planning. Unfortunately, Roentgen gives no insight into this heuristic. As we have seen above, their accounts objectively describe the results obtained, without much elucidation or theoretical conjecture. The historian is left with the alternative of speculating from known facts in an attempt to construct a plausible rational scheme for the great discovery. Two doubts have never been answered in the literature:

Had Roentgen used various types of vacuum tubes? If Fuchs and Romer's information is correct, why did Roentgen replace Lenard's tube with a conventional tube (Hittorf or Crookes)?

Why wrap the tube with black cardboard?

In an interview with journalist Dam in January 1896, Roentgen reports that he was using a Crookes tube at the time of discovery (November 8, 1895). In a letter sent to Zehnder (February 1896), he says he used a 50/20 cm Ruhmkorff coil, with Deprez switch, and approximately 20 amps of primary current. The system is evacuated with a Raps pump over several days. Best results are obtained when the discharge electrodes are approximately 3 cm apart. Again, it does not specify the type of pipe used; It only says that the phenomenon can be observed in any type of vacuum tube, including incandescent lamps.

That Roentgen discovered X-rays by chance seems to be no doubt. How else could something so unexpected be discovered? Now, what is not sure is what accident caused the discovery, and when it occurred. It is hard to imagine that in the first experimental arrangement Roentgen wrapped the tube with the cardboard. What did he expect to see through the black cardboard, if not X-rays? How is it possible, in less than two months, for anyone to approach that huge amount of fundamental aspects of an unknown phenomenon, no matter how brilliant? On the other hand, if the "true" moment of discovery is not November 8th, why is Roentgen making us believe this is the correct date?

Whether it was an accident or not, the fact is that the impact of the discovery was such that, quite rightly, the first Nobel Prize in Physics (1901) was awarded to Roentgen.

The immediate repercussion

In terms of immediate repercussion, the discovery of x-rays seems to be a unique case in the history of science. The observation of the solar eclipse of 1919, which proved part of Einstein's theory of general relativity, is a rival of respect when considering repercussions in the press, but it does not even compete lightly when considering repercussions in the scientific world. (The recent discovery of superconducting ceramics has also had a strong impact on the press and the scientific community, but we have no quantitative knowledge of that impact.) The remarkable applications in medicine were immediately noticed by Roentgen himself, who made a radiograph of his hand. Researchers around the world have come to repeat Roentgen's experiment, not only in trying to discover new applications, but also in order to understand the phenomenon, a task that has challenged human intelligence for nearly three decades.

The first major question concerned the nature of radiation. In fact, Jauncey's survey of the news showed the confusion between X-rays and cathode rays. Not only did the newspapers use these two terms interchangeably, but also some physicists. Importantly, the discovery that cathode rays were electrons was made by Thomson two years after Roentgen's discovery. Even scientists who did not confuse cathode rays with X-rays did not know what this thing Roentgen had discovered. There were two schools of thought. One, to which the English Thomson and Stokes belonged, believed that X-rays were transverse vibrations in the ether, just as ordinary light. The other school, to which the German Lenard belonged, held that X-rays were longitudinal vibrations in the ether. After extensive experiments, the controversy was decided favorably to the English school.

When, in 1905, Einstein proposed the idea of ​​the energy photon, a concept that admitted a corpuscular character to light, it was possible to calculate the wavelength associated with x-rays, but experimental evidence of the corpuscular character came only from Bragg's work. , after 1908. Around 1912, more confusion surfaced. That year, Laue and her students W. Friedrich and P. Knipping discovered X-ray diffraction in zinc sulfide (ZnS) crystals, a definitive experiment for establishing the undulatory character of X-rays. The confusion caused by this duality only was resolved with the work of from Brogliefrom 1923. Therefore, the current view of X-rays is that they belong to the electromagnetic spectrum, and as such present the particle-wave duality: depending on the circumstances, they show corpuscular or wave properties. The electromagnetic spectrum includes visible light, radio waves, ultraviolet, infrared and gamma radiation. Fundamentally, what differentiates one radiation from another is wavelength. To give you an idea, the wavelength of visible light is a thousand times longer than that of x-rays.

In addition to this enormous interest aroused in the scientific community, it is interesting to evaluate the interest aroused in the lay community, which greatly contributed to the creation of a folklore around the phenomenon. By way of illustration, let's look at some of the most colorful news published by the US newspaper. St. Louis Post-Dispatch. On February 11, 1896, a note came out of an invention by a professor from Perugia (Italy) that allowed the human eye to see x-rays. On February 13, the newspaper reported that Roentgen had illuminated his brain. and seen your pulse. The next day, a story related the opinion held by some scientists that Roentgen's discovery could establish new theories about the creation of the world.

Other extravagant news is reported in Jaucey's article. In an unidentified newspaper, a story warned of the vulnerability to which everyone was subjected after the discovery of X-rays. Anyone armed with a vacuum tube, the newspaper said, could have a complete view of the interior of a residence. Other news suggested fantastic applications for x-rays, such as resuscitating electrocuted people. One famous electrical engineer, arguing that X-rays or cathode rays were sound waves, claimed to have heard the rays emitted. Another electrical engineer made attempts to photograph the human brain, but was unsuccessful.

The sensationalist character that the subject was arousing motivated the New York Times to be warned on March 15, 1896: "Whenever something extraordinary is discovered, a multitude of writers take hold of the subject and, not knowing the scientific principles involved but driven by sensational tendencies, make conjectures that not only surpass the understanding of the phenomenon, but also in many others. cases transcend the limits of possibilities. This has been the fate of Roentgen's X-rays. ".

This enormous curiosity led many people to take serious health risks as they attempted new X-ray applications. On March 29, 1896, the newspaper St. Louis Globe-Democrat made the first public warning about the danger of x-rays to the eyes. By the way, there is a seemingly folk story that a New York shoe store had great marketing appeal that custom shoes were tested with the aid of X-rays!

How x-rays are produced

In his publications Roentgen does not specify the type of equipment used, but it is not difficult to imagine the possible components of his experimental arrangement: a direct current battery, an induction coil, a vacuum tube and a vacuum pump. Incremented by fantastic technological developments, and given different names, these components continue to be used in modern scientific research. In Roentgen's time, they were known by the names of their discoverers. Thus, the main batteries were those of Volta (invented in 1800) and those of Bunsen (1843). Among the induction coils, Ruhmkorff's (1851) were the most famous.

Regarding the use of vacuum, the first known experiment was carried out by the Italian Gasparo Berti, circa 1640. From these experiments, passing the Torriceli barometer (1644) and the first vacuum pump built by Guericke (1650), we come to the various pumps available at the end of the last century, among which stand out: the Hauksbee (1709) double-piston pump, the Geissler (1855), Toepler (1862) and Sprengel (1873), and the Fluess Oil Pump (1892). In a letter sent to Zehnder, Roentgen reports that he used a Raps bomb, the description of which is not found in the relevant literature.

The design of vacuum tubes for electric discharge observation began with the work of William Morgan, around 1785, and experimental consistency with the results obtained by Faraday, around 1833. However, it was only after the development of the vacuum pumps. vacuum, occurring after 1850, that research on electric discharges in rarefied gases had considerable momentum. As a result, the most well-known vacuum tubes bear the names of the researchers of this era. Noteworthy are the pipes of: Geissler, Pluecker, Hittorf, Crookes and Lenard.

As a historical recovery, we will present brief descriptions of the possible equipment used by Roentgen.

The Ruhmkorff coil, operating on the current transformer principle, is capable of producing high voltages. It contains two coils wrapped in an iron core, isolated from each other. The inner (primary) bobbin is made with a relatively short wire (30 to 50 meters), while the outer (secondary) bobbin is made with a very long wire (hundreds of kilometers). For operation of the equipment, a direct current battery (eg Lap battery) is used to supply a certain voltage to the primary coil. When the current is suddenly interrupted, a higher voltage is induced in the secondary coil. The voltage transformation factor is proportional to the ratio of the wire lengths. The coils used at the end of the last century produced tensions of thousands of volts. Power interruption can be performed, for example, with the aid of a switch used for Morse code telegraphic transmissions. The powers of these coils, measured by the length of the spark they produced, served to classify the laboratories of the time. To get an idea of ​​the order of magnitude, the Royal Institution of London preserves a large 280-mile-long Ruhmkorff coil in the secondary coil, capable of producing sparks 42 inches long.

It seems certain that Roentgen's first vacuum tube was a Lenard tube, but apparently he bought other conventional cathode ray tubes. The essential difference between one and the other type of tube is that Lenard's has an aluminum window designed to allow the study of cathode rays on its exterior. Made of glass, these tubes had only two electrodes inside. With the increasing use of x-rays, other tubes began to be built. Until 1913, the most commonly used was the focusing tube, but soon afterwards the Coolidge tube was widely accepted, a model still used today.

From what is known, we can imagine the following procedure adopted by Roentgen: the Ruhmkorff coil terminals were connected to the vacuum tube electrodes; by manipulating a high voltage telegraph switch was produced between the terminals; the shock of the cathode ray beam (electrons) with the anode (positive electrode) produced x-rays. In essence, the procedure currently used is the same. It is usually distinguished two types of x-rays produced in this process. One is the continuous spectrum, bremsstrahlung in German, and results from the deceleration of the electron during anode penetration. The other type is the characteristic x-ray of the anode material. Thus each X-ray spectrum is the superposition of a continuous spectrum and a series of characteristic anode spectral lines.

X-rays and the Periodic Table

By 1913 Moseley measured the frequencies of characteristic X-ray spectral lines of about 40 elements. From the square root frequency graph versus the atomic number Z of the element, he obtained a relation that became known as Moseley's law (see details in the text about elementary x-ray concepts). The immediate repercussion of this result was the alteration of the periodic table. Moseley's work played a major role in the consolidation and international acceptance of Bohr's model. In fact, it was the first of the experimental work to confirm Bohr's predictions. In a letter written to Bohr on November 16, 1913, Moseley notes that his formula could be written in a form identical to that obtained with Bohr's model.

Prior to Moseley's work, the atomic number was associated with the position of the atom in Mendelev's periodic table, which distributed the elements according to their weight. Moseley showed, for example, that argon should have Z = 18 instead of Z = 19 (according to Mendelev's table). On the other hand, potassium should have Z = 19 instead of Z = 18. He also showed that cobalt must precede nickel, although Co's atomic weight is higher than Ni's. According to Mendelev, the atomic number was approximately equal to half the atomic weight. Moseley defined the atomic weight as equal to the number of electrons in the electrically neutral atom.

Comparing the expressions obtained by Moseley with the Bohr-derived Balmer-Rydberg formula, we see that they differ by the presence of a subtractive constant to the value of Z. Moseley explained it as being due to the shielding effect of the nuclear charge. by the most intense orbital electrons.

Moseley's law presented results quite different from those of the current scientific paradigm. From this Moseley deduced that between hydrogen and uranium there should be exactly 92 kinds of atoms whose chemical properties were governed by Z, not by atomic weight. This meant that the periodic table should follow the ascending order of the atomic number and not the atomic weight. Obeying this sequence, some places in that table became vacant, corresponding to Z = 43, 61, 75, 85, and 87. By this time, there was a great controversy among chemists about the exact number of rare earths; It was discussed whether these ranged from Z = 58 to Z = 71 or to Z = 72.

The great rare earth scholar was Georges Urbain, and he was even the discoverer of one of them, the Lutetium (Z = 71), in 1907. In 1911, Urbain thought he isolated another rare earth, with Z = 72, which he called Celium. However, the chemical analysis methods hitherto used were complicated and uncertain. Upon hearing of Moseley's method in 1914, Urbain moved from France to England, taking samples of rare earths, including one of the probable Celsius. Within a few hours Moseley examined them and classified them without, however, confirming the cleft. His sample, Moseley noted, was nothing more than a mix of known rare earths. Urbain was so impressed by Moseley's work that he decided to publicize it in the chemist community. Despite this stance, Urbain continued to believe that the element Z = 72 was a rare earth, and pursued his quest. This belief was strongly renewed when in May 1922, Alexandre Dauvillier announced that he had isolated certium by analyzing the L-type X-ray spectrum of samples containing the ytterbium rare earths (Z = 70) and lutetium. This news was so fantastic that it even impressed Rutherford, since since 1914 he had been following with great interest the controversy over whether or not to be a rare earth, element 72. Believing that this controversy had ended, Rutherford wrote a letter to Nature (17/06/1922) in which it said that one of the vacant posts in the Moseley periodic table had just been filled.

Danish physicists, based on Bohr's model, claimed that element 72 must be a metal similar to zirconium. Bohr himself had made this statement on his sixth lecture WolfskehlMinistered at Göttingen on June 21, 1922. While reading Rutherford's letter from Nature On the 17th, Bohr thought his statement was wrong, so much so that he expressed this opinion in a letter sent to James Franck on July 15 of the same year. However, hearing that Dirk Coster, an expert in X-ray spectroscopy, did not agree with Dauvillier's interpretation, Bohr decided to invite him to work in Copenhagen so that together with von Hevesy the three could settle such a controversial issue. . Coster arrived in Copenhagen in September, immediately starting the search for element 72 in zirconium ores. On December 11, just minutes before uttering his Nobel lectureBohr received a call from Coster reporting positive results. At the end of his "Nobel class," Bohr announced the important discovery. In volume 111 of Nature (20/01/1923), in a letter signed by Coster and von Hevesy, the scientific world learns of the discovery of hafnium, the atomic number 72. The name was given in honor of Copenhagen, which in Latin means hafniae. According to Mehra and Rechenberg, this discovery was Niels Bohr's greatest triumph.

With regard to the elements provided by Moseley, it should be noted that element 75, the rhenium, was discovered in 1925 by the Noddack couple. Element 87, discovered in 1939 by Marguerite Perey, is called francium and belongs to a natural radioactive family. The other elements (43, 61 and 85) were obtained artificially. Since their average lives were very short, these elements could not be naturally produced, or at least observed.

Source: Institute of Physics page - UFRGS