Crookes tube

X-rays were first observed in discharge tubes known as "Crookes tubes," named after one of their inventors, British physicist William Crookes.[8] When the uses of X-rays in medicine and science were discovered, specialized Crookes tubes began to be manufactured for the production of

In Crookes tubes, the electrons necessary to generate The cathode, consisting of a concave-shaped aluminum plate, created a beam of electrons. In early models, the anode was used to accelerate the electrons, which collided with the glass at the end of the tube. This caused the tube to break after a certain time; This problem was solved by adopting the anode as the target of the electron beam, although other models used a third electrode, the anti-cathode, for the production of X-rays. Normally the anti-cathode or, in its absence, the anode, was made of platinum, positioned obliquely so that the Thanks to the curvature of the cathode, the electrons were focused on an area of ​​the anode of around 1 mm in diameter, to produce sharper images.[9]​.

The tubes worked by applying a direct current voltage of up to about 100 kilovolts between the anodes and cathode. The voltage was generated by an inductor coil, or, in the case of large tubes, an electrical generator. The voltage generated and accelerated some ions in the gas present in the tube, which in turn ionized other atoms of the gas in a chain reaction. The cathode emitted electrons when positively charged ions collided with the metal. These electrons, together with those emitted by the gas, caused X-rays in the anode or anticathode through Bremsstrahlung or fluorescence.

Crookes tubes were unstable. As time went by, the residual gas inside was absorbed by the walls of the tube; As a consequence, the pressure was reduced, the voltage increased and higher energy X-rays were produced, until the tube stopped working. To prevent this process, a small conduit with mica or other material was attached to the tube that gave off gas when heated, thus restoring the initial pressure. The glass walls of the tube would blacken after continued use of the tube.[9]​.

Coolidge tube

In 1913, William Coolidge made several improvements to the Crookes tube. The Coolidge tube, also known as a "hot cathode tube," has been in use ever since with some modifications to the basic design. It operates in a high vacuum, about 10 Pa "Pascal (unit)"), or 10 Torr, and the electrons are generated by thermionic emission in a tungsten filament—the cathode—heated by an electric current. The electron beam emitted by the cathode is accelerated by applying a potential difference between the cathode and the anode; When they collide with the anode, the electrons produce X-rays by the same processes as in the Crookes tube.[10]​.

rotating anode

The rotating anode tube is a Coolidge tube in which the anode is rotated by electromagnetic induction generated by stators located around the tube. When rotating, the heat generated by the impact of the electron beam is distributed over a larger surface area, which allows the intensity of the electron beam to be increased in applications that require a high dose of X-rays.[11]​.

Microfocus tubes

Certain techniques, such as microtomography, require very high resolution images that can be achieved using a small-section X-ray beam. Microfocus tubes produce beams with a typical diameter of less than 50 µm in diameter. Solid anode microfocus tubes are similar to a conventional Coolidge tube, but with the electron beam incident on a very small area of ​​the anode, typically between 5 and 20 µm; The power density of the electron beam is limited to a maximum value of 0.4-0.8 W/µm so as not to melt the anode,[12]​ so these sources are not very powerful, for example, 4-8 W for an electron beam of 10 µm diameter.

Liquid metal anode tubes, on the other hand, can operate with a power of 3-6 W/µm. In these instruments, the solid anode is replaced by a jet of liquid metal, generally gallium in continuous circulation.[13]​[14]​ The total power is an order of magnitude greater than in fixed anode sources, which allows the focus to be reduced to 5 µm in diameter, accompanied by an improvement in image resolution and a shorter exposure time.

Carbon nanotube cathode

The cathode used in conventional tubes can be replaced by a series of carbon nanotubes that emit electrons when a voltage is applied to them, instead of by heating, like the tungsten filament, so they can operate at room temperature. This design was conceived by a group of scientists from the University of North Carolina and patented in 2000. In addition to improving energy consumption, this design has advantages in applications that require images of moving objects: the electron beams from different nanotubes emit X-rays in different directions, so it is not necessary to move the device, as is the case with tubes with a single filament, which would result in more images. sharp.[10]​[15]​.

Medicine

The first uses of X-ray tubes in medicine and dentistry date back to the end of the century. The first gas tubes were already used to perform fluoroscopy and x-rays, exploiting the contrast in absorption of In addition to their role as diagnostic instruments for bone and dental injuries, afflictions of the digestive system and in angiography, they are part of the equipment used in some surgical procedures, especially to visualize the correct implantation of devices. Another important application, especially in the past, was in the field of radiotherapy, especially in the treatment of cancer and tumors, made possible by the ability of X-rays to cause cell death. While Crookes tubes could be used to treat superficial tumors, it was not until the development of vacuum tubes that radiation of sufficient energy could be obtained to reach internal tumors. X-ray tubes for this application require a very high voltage and have gradually been replaced by other X-ray sources, such as linear accelerators.[17]​.

Business and safety inspections

X-ray tubes are part of the security devices in airports and public buildings and for goods inspection. In baggage checks, the X-ray generator emits broad-spectrum radiation and two detector plates separated by a metal sheet, which can only pass through the highest energy In the century, lower-energy X-ray scanners began to appear, which can pass through clothing but are reflected by dense objects. The X-ray beam moves horizontally and vertically and the reflected rays at each position make up a two-dimensional image of the outside of the body.[19]​.

X-ray tubes are part of product inspection and quality control equipment in many industries using various techniques, such as fluoroscopy or computed tomography.[20]​ Microfocus tubes are particularly useful for visualizing electronic components on integrated circuits.[21]​.

Materials analysis

X-rays are widely used to examine the structure, properties and composition of all types of organic and inorganic materials. X-ray tubes are used in diffractometers, instruments used to study crystalline material using X-ray diffraction, with the aim of identifying minerals and inorganic compounds and determining the structure of matter at atomic resolution. These experiments are crucial for research and development in disciplines as diverse as geology, biology, physics of matter and environmental science.[22]​[23]​ They are also used as a source of

In analytical experiments it is common to discard the X-rays generated by braking radiation and use only the monochromatic beam corresponding to the characteristic emission of the anode. This can be achieved through the use of monochromators and filters that are poorly absorbent at the wavelength of interest, but more opaque to X-rays of shorter wavelengths, normally a metal with an atomic number Z lower than the metal used in the anode.[25]​.

When x-rays were discovered, they were not suspected of being dangerous to health, and for a time, high-voltage ocular.[16]​[26]​ It was finally established that a dose of 3 mSv") can cause redness and irritation of the skin. Since some tubes can result in exposures between 10 and 10,000 mSv/h, it is necessary to adopt measures to minimize the dose received during the use and handling of X-ray tubes.[27]​.

In all modern sources, the tube is surrounded by a protective lead shell, which absorbs all X-rays except those directed toward the exit window.[28] Devices are also used to regulate the maximum voltage in the tube, and filters and collimators to confine the Even when the dose received in a single exposure is not high enough to cause short-term effects, the accumulation of repeated exposures increases the risk of developing cancer, so safety protocols are typically implemented—for example, a requirement to occlude the the area to be treated; In diagnostic medicine, patients are placed at a certain distance, to reduce the dose per unit area, and exposure times are used as short as possible.[27]​.

Crookes tube

X-rays were first observed in discharge tubes known as "Crookes tubes," named after one of their inventors, British physicist William Crookes.[8] When the uses of X-rays in medicine and science were discovered, specialized Crookes tubes began to be manufactured for the production of

In Crookes tubes, the electrons necessary to generate The cathode, consisting of a concave-shaped aluminum plate, created a beam of electrons. In early models, the anode was used to accelerate the electrons, which collided with the glass at the end of the tube. This caused the tube to break after a certain time; This problem was solved by adopting the anode as the target of the electron beam, although other models used a third electrode, the anti-cathode, for the production of X-rays. Normally the anti-cathode or, in its absence, the anode, was made of platinum, positioned obliquely so that the Thanks to the curvature of the cathode, the electrons were focused on an area of ​​the anode of around 1 mm in diameter, to produce sharper images.[9]​.

The tubes worked by applying a direct current voltage of up to about 100 kilovolts between the anodes and cathode. The voltage was generated by an inductor coil, or, in the case of large tubes, an electrical generator. The voltage generated and accelerated some ions in the gas present in the tube, which in turn ionized other atoms of the gas in a chain reaction. The cathode emitted electrons when positively charged ions collided with the metal. These electrons, together with those emitted by the gas, caused X-rays in the anode or anticathode through Bremsstrahlung or fluorescence.

Crookes tubes were unstable. As time went by, the residual gas inside was absorbed by the walls of the tube; As a consequence, the pressure was reduced, the voltage increased and higher energy X-rays were produced, until the tube stopped working. To prevent this process, a small conduit with mica or other material was attached to the tube that gave off gas when heated, thus restoring the initial pressure. The glass walls of the tube would blacken after continued use of the tube.[9]​.

Coolidge tube

In 1913, William Coolidge made several improvements to the Crookes tube. The Coolidge tube, also known as a "hot cathode tube," has been in use ever since with some modifications to the basic design. It operates in a high vacuum, about 10 Pa "Pascal (unit)"), or 10 Torr, and the electrons are generated by thermionic emission in a tungsten filament—the cathode—heated by an electric current. The electron beam emitted by the cathode is accelerated by applying a potential difference between the cathode and the anode; When they collide with the anode, the electrons produce X-rays by the same processes as in the Crookes tube.[10]​.

rotating anode

The rotating anode tube is a Coolidge tube in which the anode is rotated by electromagnetic induction generated by stators located around the tube. When rotating, the heat generated by the impact of the electron beam is distributed over a larger surface area, which allows the intensity of the electron beam to be increased in applications that require a high dose of X-rays.[11]​.

Microfocus tubes

Certain techniques, such as microtomography, require very high resolution images that can be achieved using a small-section X-ray beam. Microfocus tubes produce beams with a typical diameter of less than 50 µm in diameter. Solid anode microfocus tubes are similar to a conventional Coolidge tube, but with the electron beam incident on a very small area of ​​the anode, typically between 5 and 20 µm; The power density of the electron beam is limited to a maximum value of 0.4-0.8 W/µm so as not to melt the anode,[12]​ so these sources are not very powerful, for example, 4-8 W for an electron beam of 10 µm diameter.

Liquid metal anode tubes, on the other hand, can operate with a power of 3-6 W/µm. In these instruments, the solid anode is replaced by a jet of liquid metal, generally gallium in continuous circulation.[13]​[14]​ The total power is an order of magnitude greater than in fixed anode sources, which allows the focus to be reduced to 5 µm in diameter, accompanied by an improvement in image resolution and a shorter exposure time.

Carbon nanotube cathode

The cathode used in conventional tubes can be replaced by a series of carbon nanotubes that emit electrons when a voltage is applied to them, instead of by heating, like the tungsten filament, so they can operate at room temperature. This design was conceived by a group of scientists from the University of North Carolina and patented in 2000. In addition to improving energy consumption, this design has advantages in applications that require images of moving objects: the electron beams from different nanotubes emit X-rays in different directions, so it is not necessary to move the device, as is the case with tubes with a single filament, which would result in more images. sharp.[10]​[15]​.

Medicine

The first uses of X-ray tubes in medicine and dentistry date back to the end of the century. The first gas tubes were already used to perform fluoroscopy and x-rays, exploiting the contrast in absorption of In addition to their role as diagnostic instruments for bone and dental injuries, afflictions of the digestive system and in angiography, they are part of the equipment used in some surgical procedures, especially to visualize the correct implantation of devices. Another important application, especially in the past, was in the field of radiotherapy, especially in the treatment of cancer and tumors, made possible by the ability of X-rays to cause cell death. While Crookes tubes could be used to treat superficial tumors, it was not until the development of vacuum tubes that radiation of sufficient energy could be obtained to reach internal tumors. X-ray tubes for this application require a very high voltage and have gradually been replaced by other X-ray sources, such as linear accelerators.[17]​.

Business and safety inspections

X-ray tubes are part of the security devices in airports and public buildings and for goods inspection. In baggage checks, the X-ray generator emits broad-spectrum radiation and two detector plates separated by a metal sheet, which can only pass through the highest energy In the century, lower-energy X-ray scanners began to appear, which can pass through clothing but are reflected by dense objects. The X-ray beam moves horizontally and vertically and the reflected rays at each position make up a two-dimensional image of the outside of the body.[19]​.

X-ray tubes are part of product inspection and quality control equipment in many industries using various techniques, such as fluoroscopy or computed tomography.[20]​ Microfocus tubes are particularly useful for visualizing electronic components on integrated circuits.[21]​.

Materials analysis

X-rays are widely used to examine the structure, properties and composition of all types of organic and inorganic materials. X-ray tubes are used in diffractometers, instruments used to study crystalline material using X-ray diffraction, with the aim of identifying minerals and inorganic compounds and determining the structure of matter at atomic resolution. These experiments are crucial for research and development in disciplines as diverse as geology, biology, physics of matter and environmental science.[22]​[23]​ They are also used as a source of

In analytical experiments it is common to discard the X-rays generated by braking radiation and use only the monochromatic beam corresponding to the characteristic emission of the anode. This can be achieved through the use of monochromators and filters that are poorly absorbent at the wavelength of interest, but more opaque to X-rays of shorter wavelengths, normally a metal with an atomic number Z lower than the metal used in the anode.[25]​.

When x-rays were discovered, they were not suspected of being dangerous to health, and for a time, high-voltage ocular.[16]​[26]​ It was finally established that a dose of 3 mSv") can cause redness and irritation of the skin. Since some tubes can result in exposures between 10 and 10,000 mSv/h, it is necessary to adopt measures to minimize the dose received during the use and handling of X-ray tubes.[27]​.

In all modern sources, the tube is surrounded by a protective lead shell, which absorbs all X-rays except those directed toward the exit window.[28] Devices are also used to regulate the maximum voltage in the tube, and filters and collimators to confine the Even when the dose received in a single exposure is not high enough to cause short-term effects, the accumulation of repeated exposures increases the risk of developing cancer, so safety protocols are typically implemented—for example, a requirement to occlude the the area to be treated; In diagnostic medicine, patients are placed at a certain distance, to reduce the dose per unit area, and exposure times are used as short as possible.[27]​.