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The first studies on radiation: Geissler, Crookes and Rontgen.

The second half of the nineteenth century was particularly fruitful for the development of Physical Science, which began to shed light on the intimate constitution of matter with constant and meticulous efforts of a large group of scientists from various European nations.

The principle of self-reliance was released, as the scientific research, then, as now, needed quiet and devotion. Funds were limited, and the findings of those years were not usually an immediate technological and industrial application, as it happens very often nowadays.

Almost no scientific research was financed by the industries that existed, and a lot of the resulting frontiers were never to appear to have practical applications until many years after their achievement.

Figures of the scientists of the time are entirely anachronistic when compared to those of the present, but in those years were so common as to constitute a norm: the research laboratories were not, in fact, the exclusive preserve of universities but often were adjacent to the local housing of those who used them and among the employees of the scientists there were their own relatives.

If the laboratory assistants were not the just family members, then they were artisans of the highest level, real artists in the manufacture of many different instruments and tools: without the ability of a Heinrich Geissler, for example, it would not have been possible to improve pumps in order to obtain a vacuum inside of glass tubes or make them into the most varied forms, closed at both ends.

So it was not possible to study the phenomenon of electric discharge in rarefied gases contained within the tubes.

The Geissler’s tube, in its simplest form, is a glass cylinder with a positive electrode (anode) at one end and a negative electrode (cathode) to the other: inside the tube the air is removed and a partial vacuum is created. The addition of small amounts of chemical elements different from those that make the air we breathe, allows, once applied to electric voltage, to obtain a discharge of light whose characteristics depend on factors such as the residual pressure inside the tube, the type of chemical element added, the voltage applied to the electrodes.

Geissler’s tube was turned on for the first time in 1857, and since then has never stopped working and being produced, almost immediately finding application outside of research labs: the device known to the general public as the “neon tube” and still used today to brighten the rooms of a home or decorate shops and restaurants with eye-catching signs is nothing if not the Geissler’s tube. Neon is only one of the chemical elements that can be placed inside the tube: in its place you can use the sodium or mercury or other chemicals, to change the color of the light emitted.

Even telecommunications has benefited from a similar device: in 1906 Lee De Forest invented his own radio valve drawing inspiration from the Geissler’s discharge tube.

It can be said that few inventions in human history have had a happier fate than that of the Geissler’s tube and yet, what inspired the scientific research in those years was still a genuine and unselfish love of knowledge, understood as a force for the liberation and elevation of the human being.

It was in chasing this “need to know” that William Crookes improved the Geissler’s tube by adding a second negative electrode between the cathode and the anode: the aim was to study the fluorescence of the phosphor coated on the end of the discharge tube placed behind the anode, a phenomenon already known to Geissler but hardly to be studied with the original version of the device.

With this modification, seemingly trivial, from 1861, Crookes discovered a set of facts that were unbelievable.

The discharge that was created inside the tube, visible as a thin glowing line, hit the main cathode causing the emission of something directed toward the positive pole and partially obscured by the secondary cathode (often in the form of small Maltese cross). This “something” induced fluorescence in the phosphor layer on the end of the glass behind the anode: the visible image was exactly the perspective of the Maltese cross in the secondary cathode!

Crookes gave the name “cathode rays” to these mysterious emissions: it was soon clear that these rays are not electromagnetic radiation, like radio waves for example, but particles that are propagated in a straight line, with mass and negative electric charge. These particles were extracted from the metal surface through a mechanism that was explained in detail only years later.

The visible discharge was not simply constituted by a flow of heated gas which emitted light as any more or less hot object, but it was a completely new state of matter. In 1879, William Crookes discovered that matter can exist not only in solid, liquid or gaseous form, but also under that of a particular gas at high temperature and electrically charged: the plasma gas.

If cathode rays were actually particles with mass and characterized by negative electrical charge, plasma was endowed with mass and positive electric charge, thereby suggesting that there could be a phenomena of recombination and dissociation between plasma and corpuscles. The cathode rays were nothing more than electrons, as the plasma gas flow of positive ions that were created spontaneously in the early, very fast, stages of the discharge.

Studies of those years soon led to the abandonment of the idea of an ​​atom as the ultimate constituent of matter, not further divided, and allowed for understanding how it actually consists of different kinds of particles.

The technological application of Crookes’ tubes closest to the general public is undoubtedly the … television! Whether it’s bulky and obsolete CRT screens or more subtle and modern plasma screens, it is thanks to the early studies by Crookes on the tubes of the same name and the phenomena that they make possible that we can enjoy TV at home.

William Crookes is known in chemistry for the discovery of the chemical element thallium in 1861, while in 1903, he managed to separate uranium from one of its decay products, then known as uranium-X, or, with a modern term, protactinium. He never won the Nobel prize, but a lot of British awards.

The most important application that arose from the studies of Crookes, however, was a medical one and the result of a discovery made by accident by Conrad Rontgen in 1895: X-rays.

Even Rontgen, in fact, was engaged in the study of the phenomena in discharge tubes, only in a lab attached to a large apartment where he lived with his family. He performed experiments often at night, so he can better analyze the discharges in the pipes that were available. One night, the 8th of November 1895, he noticed a greenish light coming from a piece of cardboard that was, this one by chance too, on the trajectory of “cathode rays” emitted from the tube he was using at that moment, not far from it.

The cardboard was covered by a chemical compound reactive to light, but the phenomenon just observed was not due to light in the discharge tube, and in the laboratory, there wasn’t any other illumination.

The luminescence disappeared when the tube was switched off and returned when it was rekindled.

Rontgen placed his hand between the tube and the cardboard and noticed on it, with astonishment, the shadow of his bones: it was as if something from the tube could penetrate through the skin and the bones of the hand, but only partially, just enough so that a shadow is formed on the cardboard.

Or on a photographic plate, as all scientists learned to do soon, and how the same Rontgen did, impressing one with the image of the hand of his wife, who often lent herself as a laboratory assistant.

The name X-ray was chosen by Rontgen to indicate the unknown nature of this penetrating radiation which allowed seeing inside objects without having to break or open them, and thus, had very different applications \to explore.

The exact mechanism of X-ray emission was included only a few years after their discovery, thanks to the first studies of quantum mechanics.

The electrons emitted by the cathode of a discharge tube hits the glass behind the anode with sufficient energy to excite the electrons of the constituent atoms. The X-ray emission is due to the return of atomic electrons to a non-excited state. The effect is more marked if the end struck by the cathodic beam has been previously coated with a reactive element, for example, phosphorus.

This is the reason why, at a certain moment, all industries began to produce television sets with CRT tubes made ​​of lead glass: lead, added to the mass of molten glass, allows X-rays to be absorbed that are produced by electrons hitting the screen, avoiding harm to a person who looks too closely at the television (or computer monitor).

A difference in the cathode rays which, remember, are a flow of electrons, that is to say, corpuscles having mass and electric charge, X-rays are a real electromagnetic radiation characterized by a wavelength shorter than that of ultraviolet rays, also invisible to the naked eye, but with higher energy. How many scientists learned to their cost from the earliest times, X-rays can produce severe burns and diseases on living tissues exposed for short periods due to their action.

The Crookes’ tubes, modified and improved to emit X-ray beams in a controllable way, found immediate applications in hospitals throughout Europe and the world, thus establishing the birth of a new branch of medicine: radiology.

In 1901, Wilhelm Conrad Rontgen won the Nobel Prize for his discovery of X-rays, but the entire premium amount, about 50,000 crowns at the time, was donated to the University of Wurzburg.

Afterward, Rontgen even lost interest in research on radiation X: faithful to an ideal of science in the service of all humanity, he refused to patent anything that was relevant to his findings. This allowed an extensive and fruitful study of both the X-rays and the devices that could produce them artificially, but it also had, as a sad consequence, that the father of such a great discovery died in poverty in 1923.

The Federal Republic of Germany recognized the greatness of Rontgen with a commemorative stamp in 1951, putting an end once and for all to the controversy about the authorship of the discovery of X-ray, relieved by the German physicist, Phillip von Lenard, still in the early twentieth century.


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