A unique lab is hidden right beneath Prague’s Vítkov Hill

28. 04. 2025

An inconspicuous door can be found just a few dozen meters from the mouth of the pedestrian tunnel connecting Prague’s Karlín and Žižkov districts. Beyond it? An outpost of the Nuclear Physics Institute of the Czech Academy of Sciences (CAS) – the Microtron MT25 laboratory, home to a circular electron accelerator. So what is it used for, and what makes it one of a kind?

Written and prepared by: Markéta Wernerová, External Relations Division, CAO of the CAS
Translated by: Tereza Novická, External Relations Division, CAO of the CAS
Photo: Jana Plavec, External Relations Division, CAO of the CAS

The text and photos are released for use under the Creative Commons license.

The word “NEBOJ” (DON’T WORRY) is emblazoned in big bold letters right above the Karlín entrance to the Žižkov tunnel. But it’s not worry that we’re feeling – it’s curiosity. What’s hidden inside the tunnel? We can find a small clue that there’s something “sciencey” going on in here just outside the tunnel entrance: a trashcan labeled “NUCLEAR PHYSICS INSTITUTE.”

That’s because the Microtron MT25 lab, our destination today, belongs to the Nuclear Physics Institute of the Czech Academy of Sciences (CAS). Just a few dozen meters past the mouth of the tunnel, we come across an inconspicuous door. After knocking, we come face to face with the lab director, David Chvátil.

This is no ordinary lab, as suggested by the warning sign on the door: “Controlled Area” along with the classic radiation symbol.

So where exactly are we? “This is part of a civil defense shelter built in 1942, but never actually used. The pedestrian tunnel itself wasn’t finished until the 1950s. Our colleagues acquired the space from the Prague City Council, and in the late 1970s, under the leadership of Professor Čestmír Šimáně, they began building the lab,” David Chvátil explains.

The lab’s circular electron accelerator, the Microtron MT25, requires a hefty power supply and – when running at full capacity – requires 2.5 meters of concrete shielding. The underground bunker beneath Vítkov Hill in the Karlín district of Prague fits the bill on both counts.

The lab team walks us through how electrons move in circular paths inside the accelerator.

One of the lab’s main methods is photon activation analysis. This method allows researchers to precisely determine which elements – and in what quantities – are present in an irradiated sample. It can detect levels down to just a few parts per million (ppm).

Safety is a top priority in the lab. A forgotten or improperly stored tool could cause serious damage.

David Chvátil explains that they also work with other institutes of the Czech Academy of Sciences, with universities, museums, and even companies – focused on medicine and food safety to archaeology and rock analysis.

The lab has been in operation since 1981. It underwent partial modernization between 2003 and 2005.

The electron accelerator was built DIY style by a team of researchers led by Čestmír Šimáně, a pioneer of nuclear research in former Czechoslovakia. The device is so unique that any upgrades and repairs have to be done entirely in-house by the lab’s own staff.

A subtle reminder of just how precise and delicate this instrument is – handle with care… (The instrument knob label reads “GENTLY”).

The Czech Academy of Sciences (the CAS)

The mission of the CAS

The primary mission of the CAS is to conduct research in a broad spectrum of natural, technical and social sciences as well as humanities. This research aims to advance progress of scientific knowledge at the international level, considering, however, the specific needs of the Czech society and the national culture.

President of the CAS

Prof. Radomír Pánek started his first term of office in March 2025. He is a prominent Czech scientist specializing in plasma physics and nuclear fusion.

A unique lab is hidden right beneath Prague’s Vítkov Hill

This is no ordinary lab, as suggested by the warning sign on the door: “Controlled Area” along with the classic radiation symbol.

The lab’s circular electron accelerator, the Microtron MT25, requires a hefty power supply and – when running at full capacity – requires 2.5 meters of concrete shielding. The underground bunker beneath Vítkov Hill in the Karlín district of Prague fits the bill on both counts.

The lab team walks us through how electrons move in circular paths inside the accelerator.

One of the lab’s main methods is photon activation analysis. This method allows researchers to precisely determine which elements – and in what quantities – are present in an irradiated sample. It can detect levels down to just a few parts per million (ppm).

Safety is a top priority in the lab. A forgotten or improperly stored tool could cause serious damage.

David Chvátil explains that they also work with other institutes of the Czech Academy of Sciences, with universities, museums, and even companies – focused on medicine and food safety to archaeology and rock analysis.

The lab has been in operation since 1981. It underwent partial modernization between 2003 and 2005.

The electron accelerator was built DIY style by a team of researchers led by Čestmír Šimáně, a pioneer of nuclear research in former Czechoslovakia. The device is so unique that any upgrades and repairs have to be done entirely in-house by the lab’s own staff.

A subtle reminder of just how precise and delicate this instrument is – handle with care… (The instrument knob label reads “GENTLY”).

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The Czech Academy of Sciences (the CAS)