X-rays
The name X-rays designates ionizing electromagnetic radiation, invisible to the human eye, capable of passing through opaque bodies and printing photographic films. Its ability to penetrate matter is all the greater the higher the voltage, the lower the density of the matter and the lower the average atomic number of said matter traversed. The current digital systems allow the obtaining and visualization of the radiographic image directly on a computer (computer) without the need to print it. The wavelength is between 10 and 0.01 nanometers, corresponding to frequencies in the range of 30 to 30,000 PHz (50 to 50,000 times the frequency of visible light).
Definition
X-rays are corpuscular 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 another of lower energy and in the disintegration of radioactive isotopes, while X-rays they arise from extranuclear phenomena, at the level of the electronic orbit, fundamentally produced by deceleration of electrons. The energy of X-rays generally falls between ultraviolet radiation and naturally produced gamma rays. X-rays are ionizing radiation because when interacting with matter it produces the ionization of its atoms, that is, it originates charged particles (ions).
Discovery
The history of X-rays begins with the experiments of the British scientist William Crookes, who investigated in the 19th century the effects of certain gases when applying energy discharges to them. These experiments were carried out in an empty tube, and electrodes to generate high voltage currents. He called it a Crookes tube. This tube, being close to photographic plates, generated some blurred images in them. 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 posed by exposure to these radiations. [citation needed ]
German physicist Wilhelm Conrad Röntgen discovered X-rays in 1895 while experimenting with Hittorff-Crookes tubes and the Ruhmkorff coil to investigate the violet fluorescence produced by cathode rays. After covering the tube with black cardboard to eliminate visible light, he observed a faint yellow-green glow coming from a screen coated with platinum-barium cyanide, which disappeared when the tube was turned off. He determined that the rays created highly penetrating, but invisible radiation, which penetrated great thicknesses of paper and even thin metals. He used photographic plates to show that objects were more or less transparent to X-rays depending on their thickness, and made the first human X-ray, using his wife's hand. He called them "mystery rays," or "X-rays." because he didn't know what they were, only that they were generated by cathode rays hitting certain materials. Despite later discoveries about the nature of the phenomenon, it was decided to retain that name. In Central and Eastern Europe, the lightning is called Röntgen rays (German: Röntgenstrahlen i>).
The news of the discovery of X-rays spread very quickly to the world. Röntgen was the object of multiple awards: Emperor Wilhelm II of Germany awarded him the Order of the Crown and was awarded the Rumford Medal of the Royal Society of London in 1896, the Barnard Medal of Columbia University, and the Nobel Prize. Physics in 1901.
X-ray production
The X-rays can be observed when a beam of very energetic electrons (of the order of 1 keV) slow down 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 dependent on the energy of the electrons. This type of radiation is called Bremsstrahlung, or 'braking radiation'. In addition, the atoms of the metallic material also emit monochromatic X-rays, which is known as the characteristic emission line of the material. Another source of X-rays is synchrotron radiation emitted in particle accelerators.
For the production of X-rays in laboratories and hospitals, X-ray tubes are used, which can be of two kinds: tubes with filament or tubes with gas.
The filament tube is a vacuum glass tube in which two electrodes are located at its ends. The cathode is a tungsten filament and the anode is a metal block with a characteristic emission line of the desired energy. The electrons generated at the cathode are focused towards a point on the target (which generally has a 45° inclination) and X-rays are generated as a product of the collision. The total radiation that is achieved is equivalent to 1% of the energy emitted; the rest are electrons and thermal energy, so the anode must be cooled to avoid overheating of the structure. Sometimes the anode is mounted on a rotary motor; by continuously rotating the heating is distributed over the entire surface of the anode and it can be operated at higher power. In this case the device is known as a "rotating anode". Finally, the X-ray tube has a window transparent to X-rays, made of beryllium, aluminum or mica.
The tube with gas is at a pressure of approximately 0.01 mmHg and is controlled by a valve; It has a concave aluminum cathode, which allows the electrons to be focused, and an anode. 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 to later produce X-rays. The refrigeration mechanism and the window are the same as those found in the filament tube.
The X-ray tube generates beams of heterogeneous radiation, especially in the so-called, "anodic effect" or "heel effect" and this refers to the fact that the beams produced at the anode end of the tube are of lower energy, which has an impact and representation in conventional diagnostic images. In this sense, some manufacturers of medical devices included stabilizers in conventional radiology equipment, whose objective is to try to homogenize the emitted ionizing radiation beam.
X-ray detectors
There are various detection systems for X-rays. The first detector used for this purpose was photographic film, prepared with an emulsion appropriate for the wavelength of the X-rays. The sensitivity of the film is determined by the coefficient of mass absorption and is restricted to a range of spectral lines. The disadvantage of these films is a very limited dynamic range and the long time and manipulations required to develop them, which is why they have fallen into disuse.
In the last decades of the XX century, new two-dimensional detectors capable of directly generating a digitized image began to be developed. Among these are the «image plates» (image plates), covered with a phosphorescent material, where the electrons increase their energy by absorbing the diffracted X-rays and are trapped at this level in color centers. The electrons release the energy when the plate is illuminated with laser light, emitting light with an intensity proportional to that of the X-rays incident on the plate. These detectors are an order of magnitude more sensitive than photographic film and have a dynamic range several orders of magnitude greater. Another widely used type of digital two-dimensional detector consists of a phosphorescent plate coupled to a CCD camera. In the 2000s, photodiodes aligned to form a plate began to be used, called PADs (Pixel Array Detectors).
Other detectors commonly used for X-ray detection are ionization devices, which measure the amount of ionization resulting from the interaction of X-rays with gas molecules. 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 can be made of the number of X-ray photons per unit time. Counters that use this principle are the Geiger counter, the proportional counter, and the scintillation detector. These detectors differ from each other in the mode of signal amplification and the sensitivity of the detector.
Spectres
Continuous spectrum
The X-ray tube is made up of two electrodes (cathode and anode), a source of electrons (hot cathode) and a target. The electrons are accelerated by a potential difference between the cathode and the anode. The radiation is produced right in the zone of impact of the electrons and is emitted in all directions.
The energy acquired by the electrons will be determined by the voltage applied between the two electrodes. As the speed of the electron can reach speeds of up (1/3)c{displaystyle (1/3)c} we must consider relativistic effects, so that,
- E=mec21− − v2c2=eV{displaystyle E={frac {m_{e}c^{2}}{sqrt {1-{frac {v^{2}}{c^{2}}}}}}}}}}{ev}
The different electrons do not hit the target in the same way, so it can give up its energy in one or several collisions, producing a continuous spectrum.
The energy of the emitted photon, by conservation of energy and taking Planck's postulates is:
- h.. =K− − K♫{displaystyle hnu =K-K',}
Where K and K' are the energy of the electron before and after the collision respectively.
The x-axis intersection point of the continuous spectrum graph is the minimum wavelength that an electron reaches when accelerated to a certain voltage. This can be explained from the point of view that the electrons collide and give up all their energy. The minimum wavelength is given by:
- λ λ =hc/eV{displaystyle lambda =hc/eV,}
The total energy emitted per second is proportional to the area under the continuous spectrum curve, the atomic number (Z) of the target and the number of electrons per second (i). So the intensity is given by:
- I=AiZVm{displaystyle I=AiZV^{m},}
Where A is the constant of proportionality and m is a constant around 2.
Characteristic spectrum
When the electrons being accelerated in the X-ray tube have a certain critical energy, they can pass close to an inner subshell of the atoms that make up the target. Due to the energy that the electron receives, it can escape from the atom, leaving the atom in a highly excited state. Finally, 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 indisputably depend on the composition of the material on which the X-ray beam hits, to molybdenum, the graph of the continuous spectrum shows two peaks corresponding to the K series of the spectrum of lines, these are superimposed with the continuous spectrum.
The line intensity depends on the difference between the applied voltage (V) and the voltage necessary for excitation (V') to the corresponding line, and is given by:
- I=Bi(V− − V♫)N{displaystyle I=Bi(V-V')^{N},}
Where n and B are constants, and i is the number of electrons per unit time.
For X-ray diffraction, the K series of material is usually used. Because experiments using this technique require monochromatic light, the 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.
Interaction of X-rays with matter
When X-rays interact with matter, they can be partly absorbed and partly transmitted. This characteristic is used in medicine when taking X-rays.
The absorption of X-rays will depend on the distance they cross and their intensity. is given by
- Ix=Iore(− − μ μ /ρ ρ )ρ ρ x{displaystyle I_{x}=I_{o}e^{({-mu /rho }}rho x},}
Where μ μ /ρ ρ {displaystyle mu /rho } is characteristic of the material and independent of the physical state. μ μ {displaystyle mu } is the linear coefficient of absorption and ρ ρ {displaystyle rho } the density of the material.
If a material is composed of different elements, the coefficient of absorption masic μ μ /ρ ρ {displaystyle mu /rho } is additive, in such a way that:
- μ μ ρ ρ =w1(μ μ ρ ρ )1+w2(μ μ ρ ρ )2+...{displaystyle {frac {mu }{rho }}}=w_{1}left({frac {mu }{rho }}{right)_{1}+w_{2}left({frac {mu }{rho }}{right)_{2}+...}
Where w{displaystyle w} means the fraction of the constituent element.
Health risks
The effects of X-radiation on biological organisms depend on the value of the dose. In general, exposure to low doses of X-rays, such as those received during a conventional radiography, are not harmful. Higher doses can cause the characteristic damage of ionizing radiation.
Digital radiographs and especially computed tomography of the thorax or abdomen, together with interventional studies (fluoroscopies, hemodynamics, among others) imply in some cases high doses of radiation, so the principle must be strictly followed for them. known as ALARP ("As Low As Reasonably Practicable", or, in Spanish, "as low as reasonably feasible"): the benefits of the study must be justified by the prescribing physician and the intervening technicians must optimize the dose used.
The biological effects that ionizing radiation can generate are classified as:
- complete body deterministics: typical of very serious accidents, correspond to situations given in nuclear power plants and therefore are far from the use of X-rays in medical practice.
- localized deterministics: may occur in patients who receive high doses of high-energy x-rays in radiation therapy treatments, or in too long interventional studies, frequently treating skin effects (the so-called “radioinduced skin syndrome”).
- other types of organic effects, such as eye overdose cataracts: very unlikely in patients, should involve care and controls in the area's workers. X-ray-induced cataracts, for example, are almost entirely avoided by the use of lumped lenses.
- Radioinduced cancer: may result from receiving small doses for long periods of time, such as radiation technicians and doctors. However, the probability of radioinduced cancer is low and much lower, for example, than that of tobacco-induced cancer.
- effects on pregnant women: they depend heavily on the period of pregnancy being considered. The most risky periods are from the sixth day to the eighth week, when malformations can occur—which, however, have a low probability—and especially from the eighth to the fifteenth week even, when radiation can affect the nervous system and generate mental retardation. In any case, X-ray studies in pregnant women should, whenever possible, be avoided.
In short, each of the effects (ranging from skin burns, hair loss, nausea, cataracts, sterility, birth defects, mental retardation, cancer, to death) is related to the value of the equivalent dose, which is measured in sieverts or rem and must be kept below the so-called threshold dose. The exposure limit is set at 100 mSv every 5 years for radiological workers, not to exceed 50 mSv per year. For the general public, lower exposure limits are set and it is recommended to avoid equivalent doses greater than 5 mSv (0.5 rem)/year in exposures to artificial radiation sources.
Applications
Medical
Since Röntgen discovered that X-rays can capture bone structures, the necessary technology has been developed for its use in medicine. Radiology is the medical specialty that uses radiography as an aid in medical diagnosis, in practice, the most widespread use of X-rays.
X-rays are especially useful in detecting skeletal diseases, although they are also used to diagnose soft tissue diseases, such as pneumonia, lung cancer, pulmonary edema, abscesses.
In other cases, the use of X-rays has more limitations, such as observing the brain or muscles. Alternatives in these cases include computed tomography, nuclear magnetic resonance, 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 through X-ray diffraction experiments because their wavelength is similar to the distance between the atoms of the crystal lattice. 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), inclusions of material, we will have an uneven pattern.
This possibility allows dealing with all kinds of materials, even with compounds, referring to the formulas that deal with the mass absorption coefficient. The only limitation resides in the density of the material to be examined. For materials denser than lead we will not have transmission.