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Microtron under the Vítkov Hill

The Vítkov Hill in the center of Prague hides a pedestrian tunnel under it, in addition to several old tunnels for trains. But only few people know what lies behind completely inconspicuous spray-painted metal doors.

Just outside the door, you can see a 22kV transformer station, which supplies the necessary power for all the equipment inside. It was originally intended for Karlín district.

To the left, you can walk through narrow concrete corridors. These lead to a small room with several doors.

Behind the blue door on the left is a warehouse and a huge concrete lens that was once movable, allowing the tunnel to be closed to increase its resistance to atomic explosion.

The door on the right feel considerably more interesting thanks to the pictograms:

The first narrow room behind the door hides a mechanical workshop.

The next room is an electronic laboratory:

The penultimate room forms a kind of crossroad, in which there are two other rooms besides the doors through which we entered. One huge, armored, one normal. Behind those two tons of lead-filled, hydraulic-operated doors, is a microtron.

Behind the normal ones on the left is the spectrometer where the samples are analyzed. On the right, there a control panel, which has hardly changed since 1981:

(The plaquette with the schema diagram roughly in the middle is worthy of note.)

The gray box in the picture on the right is an electromagnetic wave modulator, like the kind used in RADARs. It is a one-off production from St. Petersburg, Russia. The resulting wave is routed through the wave-guide to the last and most important room, hidden behind an airlock consisting of a pair of huge lead doors. Those provide shielding equivalent to three meters of concrete:

This is where the Microtron MT25 is placed.

How the Microtron works

A microtron is a particle accelerator accelerating a group of electrons into variable speed (=energy). Microwaves are fed into the red cylindrical chamber, where they interact with the hot voltage-induced crystal of the cavity resonator (?hexaboride?). This gives the electrons in the valence spheres of the crystal initial energy and pushes them out. The charged particles then make a circular movement in the magnetic field.

(Cavity resonator component. There's a crystal in the crevice in the middle. Sorry about the blurry photo.)

Electrons are accelerated in groups called "bunches" along circular paths, in which they stick because the entire chamber is one large magnet, affecting their trajectories.

The entire device is timed and calculated so that the length of the electron circulation along the circle is a multiple of the wavelength of incoming electromagnetic radiation. As a result, electrons are repeatedly given about 1 MeV of energy in each orbit, causing them to move to a higher orbit with a larger radius.


This particular microtron allows to select several internal diameters of runways and thus also output energies scaled by 1MeV steps from 6 MeV to 25 MeV. The bunch of electrons is then at some point led out and directed through other magnets to a specific target.

The outlet channel is basically a sliding pipe that can be placed on a specific track. It must be set before switching on the device, which results in selecting the energy level of the bunch.

(CHVÁTIL, David, Miroslav VOGNAR a Pavel KRIST. Mikrotron MT 25 - zdroj tvrdého záření gama [online]. , 1 [cit. 2020-04-01]. Available from:

Interestingly, thanks to the internal design, it is not possible to regulate the electron current under 13 MeV because the pipe cannot be shifted to the appropriate track. Change to lower energy states is done by replacing a cavity resonator by one, which produces an energy of only 0.5 MeV. This effectively divides the obtained energy levels of the particles by two.

Electrons are then brought into electron guides, which are basically pipes with electromagnets attached:

Dipoles, which act as output selectors, allow you to select a specific electron tube by using the same rule as in the microtron - the charged particle curves its trajectory in an external magnetic field. The more energetic the particle, the stronger magnets are needed. In this way, a straight-flying particle can be led into one of several tubes.

You can see the electromagnets on the pipes, which our guide refers to as "quadruple duplets". These allow the bunch to be focused or defocused, from a few millimeters to about twelve centimeters. The principle is a bit similar to the focusing technique used in old CRT TVs.

I tried to capture the principle in key details in the animation here:

The output of the electron guide looks like this (middle of the image):

Basically, a pipe terminated with some material. On the photo above, it is a film with a luminophore layer, which causes it to shine where electrons pass through it. This light is monitored by a heavily shielded camera placed above the microtron, which is pointing at this film through the mirror.

Additional filters can be placed behind the foil. A material with a large proton number causes electrons to break in the Coulomb field of filter particles. Some materials (tungsten, for example) are calculated to stop electrons completely by knocking an electron out of the nucleus, creating a hole that fills up with another electron while emitting gamma photon emissions. Other materials, such as lead or depleted uranium, cause gamma photons to convert into neutrons (the electron has such energy that a neutron comes out of the nucleus).

With this simple principle, it is possible to get a stream of electrons, gamma and neutron radiation from one device.

Electrons are mono-energetic because all others are eliminated when they fall out of synchronization of the accelerated "bunch" in circular orbits. Emitted photons have a continuous energy spectrum.

Here, the electron ray is directed to a temporarily built wall of lead bricks, which is used to shield unwanted radiation, except for a small narrow space in the tube which you can see in the middle of the wall.

A rotating face with an engine on which the irradiated sample rotates is also often used. If we look at the end of the second electron guide, it looks like it leads into the void in front of a green box:

It is worth noting the aluminum-looking tube to the left of the orange pillar. It is a pipe for pneumatic tube capsules. These are irradiated through a narrow crack in the middle and then sucked out with a dark gray hose, which is at the moment disconnected in the photo above. The capsule pipeline goes to the room next door, where is a spectrograph that measures the spectrum of the irradiated sample, thus gaining, for example, information about its composition.

The pneumatic capsule system makes possible detection and analysis of samples with a very short half-life (seconds or less).

To lift the lid, there is a small crane with a travel track above the microtron. It is used for maintenance, replacement of the cavity resonator and so on. The ingot forming the body of the microtron is used as a pole attachment of the magnet, which creates a homogeneous magnetic field. However, there are also correction coils that modify imperfect circular pathways affected by material inhomogeneities.

What is the microtron used for

The electron is about 1000 times lighter than the proton, i.e. the acceleration of electrons is 1000 times easier than acceleration of protons. CERN's most famous particle accelerator has 7 TeV for protons. This gives energy differences about a billion times greater (6 orders of magnitude per tera +1,000 times more per proton) compared to the microtron. As our guide explains, you can't use a comparison to a snail and a Formula One because there's only a few orders of magnitude difference in speed.

In theory, it might seem that the microtron does not have much use when there are instruments capable of achieving significantly greater energy all over the world. However, the microtron has a great advantage in the huge configurability of the bunch, energies and kinds of emitted particles. But also in the fact that it achieves small energies that can be used, for example, for chemical changes.

From the particle physic follows, that if the particle's energy is less than 10 MeV, the charged particles are unable to cross the Coulomb barrier, and thus there will be no nuclear reaction. In other words, all changes that take place are only on a chemical basis, supported by the energy of the microtron. As a result, there is no induced activity in the irradiated object when the irradiation is done (the sample is not radioactive).

A lot of the uses are for biologists and food producers. For example, the Food Research Institute said it wanted to improve the material of the sausage casings, so they will crack less. According to our guide, this was actually achieved by low-energy irradiation, where after several attempts they were able to find a configuration causing networking at the material level. In this way, it is possible to get different physical and chemical behaviors from the same particles of material by reordering its structure.

To lighten the mood, we were told about an experiment with the aging of cognac by irradiation, which was eventually a success. The physical principle is based on releasing ester and ketone molecules from the end of chains of complex molecules that would otherwise have to be released by cosmic radiation, Brownian motion, heat, and generally the oscillation of molecules. Esters and ketones are said to make the best tastes and are bound at the ends of molecules by weak forces, from where they fall off on their own over time. The equivalent of 20 years of maturation is said to take about ninety seconds on microtron. It should be added that in this way it is not possible to create, for example, the taste of an oak barrel, only to release already present taste molecules.

Often, the microtron is used for medicine. Among the things that made it to the media was the development of diamond nano-technology, which was also developed thanks to irradiation on the microtron. This technology allows for example to render cancer cell by cell, or even the processes inside cells.

The irradiation of a special biological gelatine of animal origin was also interesting because they managed to create a solid and biocompatible material usable as bone substitutes. These replacements, for example, of titanium clavicle joints, do not cause the body's immune response.

Further, our guide lists the use for electronics testing and describes the verification of various particle detectors, as well as the radiation resistance of electronics used in the military or space industries. The lead-brick fence was used to verify the function of the sensitive detector a fortnight before our visit, and served to reduce radiation to a smaller level.

As an example, he also mentions the ability to determine whether irradiated materials have changed, so, for example, different parts of Temelín Nuclear Power Station were examined here.

Irradiation of various crystals is also common, for example for the Crytur company, which is said to be the only company outside Russia and China capable of producing crystals of the required quality, so the Germans approached it about the construction of a scintillation detector for a newly built particle accelerator in Darmstadt.

Furthermore, they help to prototype special electrode materials for new types of batteries because they can create different chemical or physical structures by irradiation that could not be produced by other processes. For example, irradiation can transfer molecules into a specific structure, cause them to be semi-permeable or to create small holes in them.

This is probably the most interesting part of the work for David Chvátil because it is necessary to carry out interdisciplinary cooperation with chemists and material physicists. They often do not understand nuclear physics, so it is necessary to prepare tailor-made solutions for them and develop various creative procedures.

However, the most common use is the so-called photon activation analysis. It works by an electron being knocked out of the nucleus of the target material by radiation. This makes the nucleus unstable, and β-decay turns it into an element that is to the left of it in the periodic table of elements. In doing so, characteristic photon radiation is emitted. This is specific to each element by its energy and spectrum. By measuring these photons and their spectrum, it is possible to find out exactly what elements, or even their isotopes, are present in the material.

This method has a very high sensitivity for some elements - they can be detected with a resolution of up to 1 ppm, i.e. detect one particle in a million of others. During the communist regime in the Czech Republic, the microtron was used in this way to search for gold, by measurement of percentage of gold in soil samples.

Different elements need to be analyzed for different lengths of time, depending on the material, it can be up to hours. Photon analysis is interesting because the workplaces where it would be performed are few, unlike neutron analysis, which is widely practiced in the world.

More recently, the microtron made headlines, for example, when it was involved in analyzing Tycho de Brahe's beard after historians speculated about his poisoning. And also when it was used to analyze the bracelet that archaeologists found on a prehistoric man.

The role of microtron is mainly in research and prototyping, not in production. Once a process is fine-tuned and explored, a radioactive source such as Cobalt 60 is typically used. These are also used for example for sterilizing all spices and dressing materials in the European Union, but also irradiating furniture against worms and so on.

Interesting history

The Czech Republic is the only post-Soviet country to build its own microtron, thanks to Professor Čestmír Šimáně. Here is a translation from Czech wikipedia:

After studying at the grammar school in Opava-Kateřinky and at the 3rd Grammar School in Brno, he graduated from the Technical University of Dr. Edvard Beneš in Brno. After the war he worked as an assistant at the Faculty of Science of Masaryk University. In 1947 he succeeded in applying for a scholarship to study in France. There he studied for two years at the Collège de France under the leadership of Professor Frédéric Joliot-Curie. He worked, among other things, in a scientific group dealing with cyclotrons and also on the accelerator in Ivry. In Paris, he had the opportunity to attend lectures by the best scientists of the time, such as Louis de Broglie.

After returning from France, he became the first employee of the Institute for Atomic Physics. He built a laboratory of nuclear physics in Hostivař, Prague. In 1954 he was director of the Institute of Physics of the Czechoslovak Academy of Sciences and then the first director of the Institute of Nuclear Physics in Řež, which he has a great deal of credit for building. In 1964 he began to lecture at the Faculty of Nuclear Sciences and Physics engineering of the Czech Technical University in Prague, and from 1967 to 1972 he was dean there. He worked at this faculty for a total of forty years.

From 1961 to 1964 he was director of the Technical Supply and Nuclear Materials Division of the International Atomic Energy Commission in Vienna. At that time, he also worked as an interpreter in the negotiations of Russian and American scientists. In 1956 he became a long-time member of the Scientific Board of the Joint Institute for Nuclear Research (SÚJV) in Dubna near Moscow, where he served as deputy director from 1973 to 1977.

Professor Šimáně was one of the absolutely key people who was behind the birth of all nuclear research in the Czech Republic. According to the story of our guide, it was he who realized during his time in Dubna, that the microtron was the ideal type of accelerator for the then Czecho-Slovak Republic. Thanks to him, a design was created in our country and subsequently the construction of the microtron was realized.

This required the blast furnace to stop in Vítkovice and pour iron ingot specially for the microtron, which was then machined in Pilsen Skoda to the desired shape. All other equipment except a few pieces was then made directly in the premises under Vítkov.

The Czech microtron, unlike the Russian ones, is in the shape of a roughly three-ton pot with a one-ton lid on it. The Russians then use a design where the lid is removable from above and below. To this day, they say, there are disputes about which design is better. Ours doesn't bend as much from below when an operating vacuum of 10^-4 pascal is produced in the microtron.

Microtrons in the world

According to the story of our guide, historically there was a Russian and Swedish branch of development. The Swedes used an electron gun to inject electrons, which is why their microtrons were very expensive. The electron gun itself, for energy levels in mega-electron-volts (MeV), could cost more than the rest of the microtron. In contrast, the Russians developed a method with a cavity resonator, where the crystal costs a few dollars.

There are generally few microtrons left in the world. In the west, there are practically no microtrons of the classical type. Most microtrons of the Swedish type have been converted into a so-called racetrack, which can reach higher energy levels. Racetrack is a combination with a linear accelerator, where the microtron seems to be cut in two, and linear accelerators are inserted between them. It is generally estimated that there are only a few dozen, maybe only about a dozen still working microtrons in the world.

(Obrázek pochází z wikipedie:

Today, linear accelerators are mostly used instead of microtrons. For comparison, it is estimated, that there are about fifteen thousand of them in the world. Just in Prague, there is at least ten. The vast majority is used in medicine, specifically in oncology centers. Linear accelerators are manufactured by about two or three companies in the world, doctors have experience with them, and they are basically as user-friendly as possible. You can just press a few buttons and it works as expected.

Basically, all microtrons of the original type are of Russian origin because the Soviet Union equipped the RVHP (Comecon) countries with them. Microtrons remained, for example, in East Germany, Poland, Hungary, Romania, Bulgaria and Ukraine. The Vietnamese and Mongols also got their own microtrons. There is also a microtron in a hospital in Moscow, at the Institute of Nuclear Joint Institute for Nuclear Research in Dubna, one is said to be somewhere in Siberia and one in Canada.

Thanks to the small number of pieces, there are no companies that provide commercial support for microtrons, so if someone operates it, they have to do everything by themselves. You could say that the people around the microtron have to understand everything to the last screw and wire, and they have to manufacture most of the components by themselves. This requires people with a mix of knowledge so broad that they can handle everything from particle to material physics, engineering and electronics. Mechanical and electronic workshops in the first two rooms, where students from several faculties participate in maintenance and development, are also used for this purpose.

Interestingly, in virtually all countries of the former RVHP (Comecon), except for Ukraine, microtrons are no longer used due to complexity. For example, Poles, Hungarians, Romanians, and Bulgarians have gave up microtrons, and Vietnamese and Mongols use them only in a simplified mode.

In addition to Russia, the Czech Republic is one of the few countries that has sufficient experts for its own research and further improvement of microtron technology and the resulting advantages (photon activation analysis) over high-energy accelerators.

It seems to me to be a remarkable trace of Professor Šimáně, who, like Kolben, has raised our small republic to equal to world powers, regarding nuclear technology.

Questions we had

Of course, we had a lot of questions, here are some of the remaining ones that I have not been able to dissolve into the rest of the article;

How much does an hour of irradiation costs approximately

The microtron is said to have four different accounting modes:

Computer control

The microtron is not easy to control with a computer because the control theory is not fully described by a few equations. Work is currently underway to transfer the control to Tecomat PLCs.

According to David Chvátil, he is currently the only one in Central Europe who can manually start, control with use of analog-electronic instruments, and calibrate the microtron by hand. It is expected that even after the computer control is put in place, the operation will still take place in semi-automatic mode, like the take-off of the aircraft. The operator will first start and calibrate the machine manually, and only after reaching the operating parameters will pass the control to the computer, which will maintain the operation.


Huge thanks belong to our guide, Ing. David Chvátil from the Institute of Nuclear Physics of the Czech Academy of Sciences. His guide and explanation skills are absolutely amazing. He answered all our questions to the last detail in a sophisticated technical way, but at the same time in such a way that everyone understood.

I have to say, I wasn't the only one who was excited about the excursion, and I wouldn't be afraid to call it epic. Personally, it gave me so many stimuli and things to think about that even months later, I still think about the physics around the microtron. The enthusiasm even went so far that I wondered if I might want to profile myself in this direction in the future, and maybe go to remotely study some kind of associated field.

What impressed me the most is the complete configurability of the whole machine. Absolutely everything can be set up and modified, from the biggest nonsense to the very primary functions. The people around the microtron know perfectly what they're doing at a level that I can only dream of. They are able to use Microtron in thousands of different ways, and play on it like you could play a musical instrument.

When I was on the excursion, I realized quite clearly that this is how science is supposed to be done and how a new generation of students should learn. Don't give them expensive toys bought somewhere abroad, but let them build a complex device from scratch. The network effect creates a huge benefit for the whole republic, which causes these people, and the people around them, to become experts, who then spread their expertise among others.

I got to the microtron through a colleague, whom I recommended an excursion to the LVR-15 nuclear reactor in Řež, where I went with a few brmlab guys. You can read about it here: LVR-15 research reactor near Prague.


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