X-ray - SpeedyLook encyclopedia

History

At the end of the 19th century, Wilhelm Röntgen, like many physicists of the time, impassions for the Cathode rays which were discovered by Hittorf in 1869; these new rays had been studied by Crookes (see the article Tube of Crookes ). At that time, all the physicists can reproduce the experiment of Crookes but nobody had idea of application of these radiations.

In 1895, Wilhelm Röntgen reproduces the experiment with many recoveries by modifying its experimental parameters (standard different targets, tensions, etc). The November 8th 1895, it manages to make luminescent a platinocyanide screen of Baryum. It is an intuition that one can qualify the “brilliant one” who will carry out Röntgen in the direction of his discovery: he decides to make the experiment in the darkness while plunging his tube of Crookes in an opaque box. The result is identical to the normal situation. Röntgen places then various objects of various densities between the Anode and the screen fluorescent, and from of deduced that the radiation crosses the matter all the more easily as this one is not very dense and not very thick. More disconcerting still, when it places metal objects between the tube and a photographic plate, it manages to visualize the shade of the object on the negative one.

Röntgen manages to deduce from it that the rays are produced in the direction of the electrons of the tube and that this radiation is invisible and very penetrating.

As it does not find denomination adequate for its rays, Röntgen baptizes them “X-rays”. Let us note in the passing that this radiation is still often called Röntgen Strahlen (litt. rays of Röntgen) in Germany.

The first stereotype is that of the Main of Anna Bertha Röntgen (December 22nd 1895, poses of 20 min.); it is about the first Radiographie, the Radiologie was born.

One month later, Bergonié reproduces with Bordeaux the experiment of Röntgen, before this last publishes officially.

The December 28th 1895, Röntgen publishes his discovery in an article entitled “ Über eine neue Art von Strahlen ” (“In connection with a new kind of rays”) in the bulletin of the physicochemical Company of Würzburg.

It is this discovery which will be worth to him the first Nobel Prize of physics in 1901.

It draws four conclusions in its article:

“x-rays are absorbed by the matter; their absorption is according to the atomic mass of the atoms absorbents;
x-rays are diffused by the matter; it is the radiation of fluorescence;
x-rays impress the photographic plate;
x-rays discharge the electrically charged bodies.”

The search for Röntgen is quickly developed in dentistry since two weeks later, Dr. Otto Walkhoof carries out with Braunschweig the first dental radiography.

It takes 25 minutes of exposure. It uses a photographic plate out of glass, covered with black paper and a dam (operative field) in rubber. Six months after, appear the first book devoted to what will become the radiology whose applications multiply - within the framework of medical physics, for the diagnosis of the diseases then their treatment (Radiothérapie which gives an extraordinary expansion to what was up to that point the electrotherapy).

Röntgen left his name to the measuring unit used in radiology to evaluate an exposure to the radiations. The symbol of the roentgens is R.

The discovery of Röntgen made it tower of the ground quickly. In 1897, Antoine Béclère, pediatrist and considered clinician, created, with its expenses, the first hospital Laboratory of radiology.

Everyone wanted to make photograph its skeleton. But for a long time, the amounts were too strong. For example, Henri Simon, photo hobbyist, left his life to the service of radiology. Charged with taking radiographies, the symptoms due to ionizing radiations appeared after only two years of practice. One cut down initially the hand to him (which was constantly in contact with the fluorescent screen) but then, a Cancer generalized was declared.

At the beginning of radiology, x-rays were used with fine multiples: in the fun fairs where one exploited the phenomenon of Fluorescence, in the stores where one studied the adaptation of a Chaussure to the foot of the customers thanks to the radiation and of course, one used them for medical radiography. Still there, one made some errors, for example by radiographing the expectant mothers.

With the years, one decreased the duration of the examinations and the quantities managed. Hundred years after their discovery, one still makes use of x-rays in modern radiography. One also uses them in the scanners, to carry out cuts of the human body. Several other techniques came to supplement the apparatuses of the doctors: the Ultrasound S, the imagery by nuclear Magnetic resonance, the Scintiscanning or the Tomography by emission of Positron S.

But one does not make use of x-rays only in medicine; the security services use them to examine the contents of the bags or the air and maritime Conteneur S on screen. The police officers exploit them in order to analyze textile fibers and paintings being on the place of a disaster. In Mineralogy, one can identify various crystals using the Diffraction of x-rays.

Production of x-rays

X-rays are an electromagnetic radiation like the radio waves, the visible Lumière, or the infra-red one. However, they can be produced in two very specific ways:

by changes of orbit of electron S coming from the electron shells; x-rays are produced by electronic transitions utilizing the internal layers, close to the core; the excitation giving the transition can be caused by x-rays or by a bombardment of electrons, it is in particular the principle of the Spectrométrie of x-ray fluorescence and of the Microsonde of Castaing;
by acceleration of electrons (Acceleration in the broad sense: braking, change of trajectory); two systems are used:

the braking of the electrons on a target in a tube with x-rays: the electrons are extracted from a Cathode of Tungstène heated, accelerated by an electric tension in a vacuum tube, this beam is used to bombard a metal target (called Anode or anticathode); the deceleration of the electrons by the atoms of the target causes a continuous Rayonnement of braking (or Bremsstrahlung , German term adoptee internationally);
voir the article Tube with x-rays ;
the curve of the trajectory in particle accelerators, it is the radiation known as “synchrotron”.

Note that in the case of a tube with x-rays, one has at the same time a continuous radiation ( Bremsstrahlung ) and a phenomenon of fluorescence of the target.

The photograph used in the insert to above illustrate at the same time physical sciences and quantum is a diffractometer with x-rays.

If electromagnetic ionizing radiation is derived from a natural source, the Photon S are not regarded as being x-rays, but rather as being Gamma rays.

Properties of x-rays

Historically, x-rays were known to make shine certain crystals (Fluorescence), ionize gases and impress the photographic plates.

The principal properties of x-rays are the following ones:

they penetrate the “soft matter easily” (solid matter not very dense and made up of light elements like the Carbone, the Oxygène and the Azote); they are easily absorbed by the “hard matter” (dense solid matter made up of heavy elements);

it is what allows the Medical imagery (Radiographie, scanner): they cross the flesh and are stopped by the bones;

they are easily absorbed by the Air, by the atmosphere;

in fact, the telescopes with x-rays (which detect x-rays emitted by stars) must be placed in satellites, and medical radiographies, the source of x-rays must be close to the patient;

the order of magnitude their wavelength being that of the interatomic distances in the crystals (metals, rocks…), they can diffract on these crystals;

this makes it possible to make chemical analysis, and more precisely of the analysis of phase by Diffraction of x-rays (or Radiocristallographie);

because of important energy of the photons, they cause ionizations of the atoms, they are radiations known as “ionizing”;

this gives rise to the phenomenon of Fluorescence X, which allows a chemical analysis, but that modifies also the alive cells (cf will infra ).

Effects on health

X-rays are ionizing radiations. An exposure prolonged to x-rays can cause Brûlure S (radiomes) but also of the Cancer S. These effects were really taken into account rather late. Thus in a work of 1954, one did not read any recommendation of safety, but on the other hand:

“ It was shown that X-rays produce year effect, though has small one, directly upon the retina, giving small channel to has faint illumination off the whole field off view. ”

“It was shown that x-rays cause an effect, certainly not very important, directly on the Rétine, causing a light illumination in all the field of view. ”

what seems to indicate that the authors or their collaborators were subjected for this purpose occasionally.

The personnel working with x-rays must follow a specific training, to be protected and followed médicalement (these measurements can be not very constraining if the apparatus is well “tight” with x-rays)

Detection

X-rays are invisible with the eye, but they impress the photographic films. If one places a virgin film protected from the light (in a Darkroom or wrapped in an opaque paper), the figure revealed on film gives the intensity of x-rays having struck the film to this place. It is what made it possible Röntgen to discover these rays. This process is used in medical radiography like in some Diffractomètre S (stereotyped of Laue, rooms of Debye-Scherrer). It is also used in the system of follow-up of the manipulators: those must permanently carry a badge, called “film Dosimètre”, locking up a virgin film; this badge regularly is changed and developed by the departments of health to control that the manipulator did not receive from excessive amount of x-rays.

Like all the radiation ionizing, x-rays are detected by the meters Geiger-Müller (or meter G-M). If one decreases the biasing of the meter, one obtains a meter known as “proportional” (still called “” or “meter gas meter with gas flow”); whereas meter G-M works with saturation, in the meter proportional, the generated electric impulses are proportional to the energy of photons X.

X-rays cause also luminous Fluorescence on certain materials, as iodide NaI the sodium. This principle is used with the “scintillation counters” (or “Scintillateur S”): one places a photodetector after a crystal of NaI; the intensities of the electric impulses collected by the photomultiplier are they also proportional to energies of the photons.

Just as they can ionize a gas in a meter G-M or proportional, x-rays can also ionize the atoms of a crystal Semi-conducteur and thus generate pairs electron-positron pair of loads. If one subjects a semiconductor to an high voltage of prepolarisation, the arrival of a photon X will release an electric charge proportional to the energy of the Photon. This principle is used in the detectors known as “solid”, in particular for the dispersive Analyze in energy ( EDX or EDS ). To have a correct resolution, limited by the energy of threshold necessary to the creation of loads, the solid detectors must be cooled, either with a platinum Peltier, or with the Azote liquid. The semiconductors used are in general Silicium doped with the Lithium If (Li), or of the Germanium doped with lithium Ge (Li).

Let us note in the passing that the low temperature does not have a direct effect on the value of energy of threshold, but on the background noise. It is possible on the other hand to use superconductive maintained at very low temperature in order to make use of really small energy of threshold. For example the energy of threshold necessary to the creation of “free” loads in silicon is about 3 eV, whereas in the superconductive tantalum, say below 1 Kelvin, it is of 1 meV, that is to say 1.000 times weaker. The reduction in the threshold value causes to increase the number of loads created at the time of the deposition of energy, which makes it possible to reach a better resolution. The latter is indeed limited by the statistical fluctuations of the number of load created. The amplitude of these fluctuations can be estimated with the Loi of Poisson. Recent experiments of detection of a photon X using a maintained calorimeter at very low temperature (0,1 K) make it possible to obtain an excellent resolution in energy. In this case, the energy of the absorptive photon makes it possible to heat an absorber, the difference in temperature is measured using a sensitive ultra thermometer.

In order to compare the approaches: if allows a measuring accuracy of about 150 eV for a Photon of 6.000 eV. A sensor with Your makes it possible to approach 20 eV, and a Calorimètre maintained to 0,1 K recently showed a resolution from approximately 5 eV, that is to say a resolution of about 0,1%. It is useful to mention that the cryogenic methods of detection do not make it possible yet to manufacture sensors having a great number of elements of images (Pixel), whereas the sensors based on the Semi-conducteur S offer “cameras” to x-rays with several thousands of elements. Moreover, the counting rates obtained by the cryogenic sensors are limited, 1.000 to 10.000 cps per pixel.

X-rays in Crystallography

The analysis of the crystals by Diffraction of x-rays is also called Radiocristallographie . This makes it possible either to characterize crystals and to know their structure (one works then in general with monocrystals), or to recognize already characterized crystals (one works in general with polycrystalline powders).

To work with a monocrystal, one uses the apparatus opposite:

x-rays leave by the vertical tube in top;
the Cristal in the center of the photograph is too small to be considering; it is fixed at the end of a fine needle of glass handled by the goniometric head on the line (which resembles the Mandrin of a Perceuse) and makes it possible according to three successive axes (vertical, with 45° and horizontal) to turn the Cristal in all the orientations all while maintaining it in the beam of x-rays;
a Caméra video (in black in top on the left) makes it possible to control that the crystal is well centered;
a well in bottom in the medium is held by a blade: the well is used to stop the direct x-rays which did not interact with the crystal;
a cooling system (on the left, tube with letters in red) makes it possible to cool the crystal;
is not visible on the photograph the detector of x-rays which has been for a few years a camera CCC making it possible to replace at the same time the plates photographs and the meters;
is not visible also the source of x-rays and its Monochromateur focalisor which is composed of a Multicouche mirror with x-rays;
is not visible the data processing of acquisition of the experimental data.

Used in Geology and Metallurgy, it is also a tool of Biophysique, very much used in Biologie to determine the structure of the molecules of alive, in particular in Cristallogénèse (it is art to manufacture monocrystals with a pure Molécule); within this framework, a monocrystal of the molecule is put in a beam of x-rays Monochromatique S and the Diffraction observed for different position from the Cristal in the beam of x-rays (handled by a Goniomètre) makes it possible to determine not only the structure of the crystal, but more especially the structure of the molecule. It is in particular by X-ray crystallography that James Watson, Francis Crick and their collaborators could determine the helicoid structure of DNA in 1953.

Regulation

In the European Union, the use of x-rays is subjected to the Norme S Euratom 96/29 and 97/43. Directive 97/43/Euratom of June 30th, 1997 should have been transposed in French Internal rights at the latest on May 30th, 2000.

In France, it is necessary to refer:

with the Public health code (CSP), L.1333-1 articles in L.1333-20 (legislative left news), R.1333-17 articles in R.1333-93 (lawful left news);
with the Labor regulation (CT), R.231-73 articles in R.231-116 (left lawful - decrees in Council of State);
to standards NFC 74-100 (design), NFC 15-160, NFC 15-164 (installation); these standards are obsolete and are in the course of actualization in 2006.

The organism in charge of control is the DGSNR, the Head office of nuclear safety and protection against radiation, created n°2002-255 by the decree of February 22nd, 2002, modifying the decree n°93-1272 of December 1st, 1993, and which replaces the DSIN (Direction of the safety of nuclear installations). The DGSNR is a branch of the nuclear Autorité of safety (ASN).

code of santée public

Another meaning

“X-ray” is also the name of the letter X in the Alphabet radio operator international.

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