Physics

X-ray Discovery


Introduction

In the late afternoon of November 8, 1895, when everyone had finished their workday, the German physicist Wilhelm Conrad Roentgen (1845-1923) was still in his small laboratory under the watchful eyes of his servant. As Roentgen, in that dark room, busied himself with the observation of the conduction of electricity through a Crookes pipe, the servant, in a high state of excitement, caught his eye: "Teacher, look at the screen!".

In the vicinity of the vacuum tube was a barium platinum-cyanide-coated screen, on which projected an unexpected brightness resulting from the fluorescence of the material. Roentgen rotated the screen so that the face without fluorescent material faced Crookes tube; yet he observed the fluorescence. It was then that he decided to put his hand in front of the tube, seeing his bones projected on the screen. Roentgen observed for the first time what came to be called X-rays.

The above paragraph may be a dramatization of what actually happened that day, but the fact that history records is that this fantastic discovery had a resounding repercussion, not only in the scientific community, but also in the mass media. For example, by 1896, less than a year after the discovery, approximately 49 books and pamphlets and 1,000 articles had already been published on the subject. A survey by Jauncey in the US newspaper St. Louis Post-Dispatch, shows that between January 7 and March 16, 1896, fourteen notes were published on the discovery and other related studies.

However, the best-known references to this finding tend to downplay its author's merit, emphasizing the fortuitous aspect of observation. This distorted view of Roentgen's work is only eliminated when one becomes aware of his accounts. 50 years old at the time of the X-ray discovery, and less than 50 published papers, Roentgen's favorite subjects were the physical properties of crystals and applied physics (in 1878 he presented a telephone alarm and in 1879 an aneroid barometer). ). On X-rays, he published only three works, and by the end of his life was no more than 60. For a Nobel Prize holder in Physics, this is a relatively inexpressive amount. This "small" production may be a consequence of its strict criteria for evaluating the results obtained. As far as anyone knows, he was so careful that he never had to review the published results. Reading your first two articles on x-rays, you can see the accuracy of your work.

In addition to the undeniable importance in medicine, technology and current scientific research, X-ray discovery has a history full of curious and interesting facts that demonstrate Roentgen's enormous insight. For example, Crookes even complained about the Ilford photographic input factory for sending him "veiled" papers. These light-protected papers were usually placed near their cathode ray tubes, and the X-rays produced there (not yet discovered) veiled them. Other physicists have observed this "phenomenon" of veiled papers, but have never related it to being near cathode ray tubes! More curious and intriguing is the fact that Lenard "stumbled" on x-rays before Roentgen, but didn't notice. So it seems that it was not just chance that favored Roentgen; The discovery of the X-rays was "falling mature," but it needed someone subtle enough to identify its iconoclastic aspect. To understand why, it is necessary to follow the history of cathode rays.

Cathode rays and Lenard rays versus X ray

In 1838 Faraday conducted a series of experiments with electric discharges in rarefied gases, definitively linking his name to the discovery of cathode rays. However, due to the technical difficulties with producing good quality vacuum, these works only got new impetus twenty years later. This new phase, begun around 1858 by German physicist Julius Plücker (1801-1868), produced results that challenged human intelligence for nearly forty years until a good understanding of the phenomenon was gained. The denomination cathode rays (Kathodenstrahlen) was introduced by the German physicist Eugen Goldstein (1850-1931) in 1876, when he presented the interpretation that these rays were waves in the ether. A contrary interpretation, defended by the English, also caught the attention of the scientific world of the time. For Crookes, cathode rays were charged molecules, which constituted the fourth state of matter (This name is used today when we refer to plasma, which is exactly what you get when you produce an electric discharge in a thin gas!). In 1897 Thomson ended the controversy by demonstrating that cathode rays were electrons. Over the course of these 40 years, a number of observations, comments, and hypotheses suggest that several researchers have been "prowling the door of X-ray discovery." Anderson lists some of these indications; In his first two works, Roentgen refers to the possibilities that Lenard had to make the discovery.

In an article published in 1880, Goldstein mentions that a fluorescent screen could be excited even when protected from cathode rays. Published in German and English, this work must have come to the knowledge of almost all the researchers involved in these studies, however, for the next fifteen years no one questioned the fact that the screen fluoresced even without being hit by cathode rays! Also Thomson came close; A year before the discovery of X-rays, he reported that he had observed phosphorescence in glass pieces several inches away from the vacuum tube.

Of all the researchers, Lenard seems to have been the closest to Roentgen's discovery. Continuing the work of his teacher, Heinrich Hertz, Lenard conducted experiments to verify whether cathode rays produced inside a Crookes tube could be observed outside. To this end, he constructed a Crookes tube with a small aluminum window (approximately 0.0025 mm thick) on the opposite side of the cathode, and began to observe cathode rays outside the tube through its interaction with phosphorescent materials. Later these rays became known as lenard rays. In 1894 Lenard publishes, in the German magazine Annalen der Physik, his first observations, among which stand out:

  1. Lenard rays sensitized a photographic plate.
  2. An electrically charged aluminum disc would discharge when placed in the path of these rays, even when this disc was placed more than 8 cm (the maximum range of cathode rays in the air). When the hand was placed in front of the beam, the electric shock effect disappeared. Commenting on these results, Lenard wrote: "It cannot be said whether we are observing an action of the cathode rays on the aluminum window surface, or on the air, or finally on the charged disc! However, the last action is very unlikely at great distances from the window ".
  3. The rays were continuously deflected by a magnetic field; that is, some rays were deflected more than others, and there were some that did not deflect.

From all that is known today, it follows that Lenard rays consisted of cathode rays (electrons) and x-rays, but he believed they were only cathode rays! It was enough that he had used a rather thick aluminum window so that the electrons could not pass through it to have an X-ray beam! According to Anderson, Lenard was deeply disappointed to have missed this discovery, and never used Roentgen's name when referring to X-rays.