X-rays 

X-rays . It is electromagnetic radiation , invisible, capable of passing through opaque bodies and of impressing photographic films . The Wavelength is between 10 to 0.1 Nanometers , corresponding to frequencies in the range of 30 to 3,000 PHz (50 to 5,000 times the frequency of visible light ).

Summary

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  • 1 Definition
  • 2 Discovery
  • 3 Features
  • 4 X-ray production
  • 5 Spectra
    • 1 Continuous spectrum
    • 2 Characteristic spectrum
  • 6 Interaction of X-rays with matter
  • 7 Health risks
  • 8 Applications
    • 1 Medical
    • 2 Others
  • 9 Sources

Definition

X-rays are electromagnetic radiation of the same nature as radio waves, microwave waves , infrared rays , visible light , ultraviolet rays, and gamma rays . The fundamental difference with gamma rays is their origin: gamma rays are radiations of nuclear origin that are produced by the de-excitation of a nucleon from an excited level to a lower energy level and in the disintegration of radioactive isotopes, while X-raysthey arise from extra-nuclear phenomena, at the level of the electronic orbit, mainly produced by deceleration of electrons. X-ray energy is generally between ultraviolet radiation and naturally produced gamma rays. X-rays are ionizing radiation because when interacting with matter, it produces ionization of its atoms, that is, it causes charged particles ( ions ).

Discovery

The history of X-rays begins with the experiments of the British scientist William Crookes , who investigated the effects of certain gases in the 19th century by applying energy discharges to them. These experiments were carried out in an empty tube, and electrodes to generate high voltage currents. He called it Crookes Tube . Well, this tube, being near photographic plates, generated in them some blurred images. Despite the discovery, Crookes did not continue to investigate this effect.

This is how Nikola Tesla , in 1887, began to study this effect created by means of Crookes tubes. One of the consequences of his research was to warn the scientific community of the danger to biological organisms that exposure to these radiation poses.

But until 8 November as as 1895 no X – rays were discovered; Physicist Wilhelm Conrad Röntgen performed experiments on the Hittorff-Crookes tubes (or simply Crookes Tube ) and the Ruhmkorff Coil . It analyzed the cathode rays to avoid the violet Fluorescence that the cathode rays produced in the walls of a glass of the tube. To do this, create an atmosphere of darkness, and cover the tube with a black cardboard sleeve.

When he connected his equipment for the last time, at nightfall, he was surprised to see a faint greenish-yellow glow in the distance: on a nearby bench was a small cardboard with a solution of platinum-barium cyanide crystals, in which he observed a darkening when turning off the tube. When you light the tube again, the glare was produced again. He further removed the crystal solution and found that the fluorescence was still occurring, thus he repeated the experiment and determined that the rays created a very penetrating but invisible radiation . He observed that lightning strikes through large layers of paper and even less dense metals than Lead .

In the following seven weeks, he studied with great rigor the characteristic properties of these new and unknown rays. Thought about Photographingthis phenomenon and that’s when he made a new discovery: the photographic plates he had in his box were veiled. He intuited the action of these rays on the photographic emulsion and set about checking it. He placed a wooden box with weights on a photographic plate and the result was surprising. The ray went through the wood and impressed the image of the weights in the photograph. He did various experiments with objects such as a compass and a shotgun barrel. To check the distance and range of the rays, he went to the next room, closed the door, and placed a photographic plate. He obtained the image of the molding, the door hinge, and even the traces of the paint that covered it.

A hundred years later, none of his research has been considered casual. On December 22, a memorable day, he decided to practice the first test with humans. Since he could not simultaneously handle his reel, the glass photographic plate, and expose his own hand to lightning, he asked his wife to place the Hand on the plate for fifteen minutes . By revealing the glass plate, a historical image appeared in science. The bones of Berta’s hand, with the ring floating over them: the first radiographic image of the human body . Thus is born one of the most powerful and exciting branches of Medicine : Radiology .

The discoverer of these types of lightning also had the idea of ​​the name. He called them “unknown rays”, or what is the same: “X-rays” because he did not know what they were, nor how they were caused. Unknown rays, a name that gives them a historical meaning. Hence, many years later, despite the discoveries about the nature of the phenomenon, it was decided that they should retain that name.

The news of the discovery of “X-rays” spread very quickly in the world. Roentgen was object of multiple recognitions, the Emperor Wilhelm II of Germany awarded him the Order of the Crown, he was honored with the Rumford medal of the Royal Society of London in 1896, with the Barnard medal of Columbia University and with the Nobel Prize of Physics in 1901.

The discovery of “X” rays was the product of research, experimentation, and not by accident as some authors claim; WC Roentgen, a man of science, a keen observer, investigated the smallest details, examined the consequences of a perhaps accidental act, and therefore he succeeded where others failed. This genius did not want to patent his discovery when Thomas Alva Edison proposed it to him, stating that he bequeathed it for the benefit of humanity.

features

  • X-rays also occur when the electronbeam strikes a heavy metal, for example the anode of the cathode ray tube.
  • X-rays do not have a charge, since they do not deviate under the action of electric and magnetic fields.
  • These rays are capable of penetrating solid bodies. Bodies made of light elements are more transparent to X-rays than those made of heavy elements.

 

X-ray production

X-rays are products of the rapid deceleration of very energetic electrons (of the order of 1000eV) when colliding with a metallic target. According to classical mechanics, an accelerated charge emits electromagnetic radiation, thus, the shock produces a continuous spectrum of X-rays (from a certain minimum wavelength). However experimentally, in addition to this continuous spectrum, there are characteristic lines for each material. These spectra – continuous and characteristic – will be studied in more detail below.

X-ray production occurs in an X-ray tube that can vary depending on the electron source and can be of two kinds: tubes with filament or tubes with gas.

The filament tube is a glass vacuum tube in which two electrodes are located at its ends. The cathode is a hot tungsten filament and the anode is a block of copper in which the target is immersed. The anode is continuously cooled by circulating water, since the energy of the electrons when hit with the target, is transformed into thermal energy in a large percentage. The electrons generated at the cathode are focused towards a target point (which generally has a 45 ° inclination) and as a result of the collision the X-rays are generated. Finally, the X-ray tube has a window which is transparent to this type of radiation made from beryllium, aluminum or mica.

The gas tube is at a pressure of approximately 0.01 MmHg and is controlled by a valve; It has a concave aluminum cathode, which allows focusing the electrons and an anode. The ionized nitrogen and oxygen particles present in the tube are attracted to the cathode and anode. Positive ions are attracted to the cathode and inject electrons into it. Subsequently, the electrons are accelerated towards the anode (which contains the target) at high energies and then produce X-rays. The cooling mechanism and the window are the same as those found in the filament tube.

The most common detection systems are photographic films and ionization devices.

The emulsion of photographic films varies depending on the wavelength to which you want to expose. The film sensitivity is determined by the mass absorption coefficient and is restricted to a range of spectral lines. The disadvantage of these films is, due to their hailstorm nature, the impossibility of a detailed analysis since it does not allow a large resolution.

Ionization devices measure the amount of ionization of a gas produced by interaction with X-rays. In an ionization chamber, negative ions are attracted to the anode and positive ions to the cathode, generating current in an external circuit. The relationship between the amount of current produced and the intensity of the radiation are proportional, so an estimate of the number of X-ray photons per unit time can be made. The counters that use this principle are the Geiger Counter , the Proportional Counter and the Flash Counter. The difference between them is the amplification of the signal and the sensitivity of the detector. PICO PAL JEREMY

Spectral

Continuous spectrum

The X-ray tube is made up of two electrodes (cathode and anode), an electron source (hot cathode) and a target. The electrons are accelerated by a potential difference between the cathode and the anode. Radiation is produced just in the impact zone of the electrons and is emitted in all directions.

The energy acquired by the electrons is going to be determined by the voltage applied between the two electrodes. As the speed of the electron can reach speeds of up to <math> (1/3) c </math> we must consider relativistic effects, in such a way that

<math> E = \ frac {m_ {e} c ^ 2} {\ sqrt {1- \ frac {v ^ 2} {c ^ 2}}} = eV </math>

The different electrons do not collide with the target in the same way, so it can yield its energy in one or several collisions, producing a continuous spectrum.

The energy of the emitted photon, by conserving energy and taking Planck’s postulates is

<math> h \ nu = K-K ‘</math>

where K and K ‘is the energy of the electron before and after the collision respectively.

The cut-off point with the x-axis of the continuous spectrum graph is the minimum length that a photon reaches when accelerated to a certain voltage. This can be explained from the point of view that the electrons collide and deliver all their energy. The minimum wavelength is given by <math> \ lambda = hc / eV </math>, the total energy emitted per second, is proportional to the area under the curve of the continuous spectrum, of the atomic number (Z) of the target and the number of electrons per second (i). So the intensity is given by

<math> I = AiZV ^ {m} </math>

where A is the constant of proportionality and m a constant around 2.

Characteristic spectrum

When the electrons that are accelerated in the X-ray tube possess some critical energy, they can pass close to an internal sublayer of the atoms that make up the target. Due to the energy received by the electron, it can escape from the atom, leaving the atom in a supremely excited state. Eventually, the atom will return to its equilibrium state emitting a set of high-frequency photons, which correspond to the spectrum of X-ray lines. This will undoubtedly depend on the composition of the material in which the X-ray beam falls, to Molybdenum, the continuous spectrum graph shows two peaks corresponding to the K series of the line spectrum, these are superimposed with the continuous spectrum.

The intensity of any line depends on the difference of the applied voltage (V) and the necessary voltage for the excitation (V ‘) to the corresponding line, and is given by

<math> I = B i (V-V ‘) ^ {N} </math>

where n and B are constants, and i is the number of electrons per unit of time.

For X-ray Diffraction , the K series of the material is the one usually used. Because experiments using this technique require monochromatic light, electrons that are accelerated in the X-ray tube must have energies above 30 keV. This allows the width of the K line used to be very narrow (on the order of 0.001 Å). The relationship between the length of any particular line and the atomic number of the atom is given by Moseley’s Law

X-ray interaction with matter

When X-rays interact with matter, they can be partly absorbed and partly transmitted. This feature is used in medicine when performing radiographs.

The absorption of X-rays will depend on the distance they travel through and their intensity. It is given by

<math> I_ {x} = I_ {o} e ^ {({- \ mu / \ rho}) \ rho x} </math>

<math> \ mu / \ rho </math>, it is characteristic of the material and independent of the physical state. µ the linear absorption coefficient and rho the density of the material.

If a material is composed of different elements, the mass absorption coefficient <math> \ mu / \ rho </math> is additive, in such a way that

<math> \ frac {\ mu} {\ rho} = w_ {1} \ left (\ frac {\ mu} {\ rho} \ right) _ {1} + w_ {2} \ left (\ frac {\ mu} {\ rho} \ right) _ {2} + … </math>

where w means the fraction of the constituent element.

Health risks

How radiation affects health depends on the size of the radiation dose. Exposure to the low doses of X-rays to which humans are exposed daily is not harmful. On the other hand, it is known that exposure to massive amounts can cause serious damage. Therefore, it is advisable not to expose yourself to more ionizing radiation than necessary.

Exposure to high amounts of X-rays can produce effects such as skin burns , hair loss, birth defects, cancer , mental retardation, and death . The dose determines if an effect is manifested and with what severity. The manifestation of effects such as skin burns, hair loss , Sterility , Nausea and Cataractsrequires that you be exposed to a minimal dose (the threshold dose). If the dose is increased above the threshold dose, the effect is more serious. An increase in psychological pressure has been observed in groups of people exposed to low doses of radiation. Impaired mental faculties (central nervous system syndrome) have also been documented in people exposed to thousands of rads of ionizing radiation.

Applications

Medical

Since Röntgen discovered that X-rays allow to capture bone structures , the necessary technology has been developed for its use in Medicine . The Radiology is the medical specialty that uses x – ray as an aid to diagnosis in practice, the most widespread use of X – rays

X – rays are especially useful in detecting diseases of the skeleton , but are also used to diagnose diseases of tissues soft, such as pneumonia , lung cancer , pulmonary edema , abscesses .

In other cases, the use of X-rays has more limitations, such as in the observation of the Brain or Muscles . Alternatives in these cases include computed tomography , magnetic resonance imaging, or ultrasound .

X-rays are also used in real-time procedures , such as Angiography , or in contrast studies .

Others

X-rays can be used to explore the structure of crystalline matter by means of X-ray Diffraction experiments because their wavelength is similar to the distance between the atoms of the crystal lattice. The X – ray diffraction is one of the most useful tools in the field of crystallography .

It can also be used to determine defects in technical components, such as pipes, turbines , motors, walls, beams, and in general almost any structural element. Taking advantage of the absorption / transmission characteristic of X-Rays, if we apply an X-ray source to one of these elements, and this is completely perfect, the absorption / transmission pattern will be the same throughout the entire component, but If we have defects, such as pores, loss of thickness, cracks (they are not usually easily detectable), material inclusions we will have an uneven pattern.

This possibility allows dealing with all kinds of materials, including compounds, referring to the formulas that deal with the mass absorption coefficient. The only limitation is the density of the material to be examined. For materials denser than Lead we will not have transmission.

 

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