The Atmosphere of the Former RIKEN Institute

Located across the river from the remaining ruins of the Army

Gunpowder Plant, this laboratory was once a hub for advanced research.

Scientists here focused on observing cosmic rays—high-energy particles that shower down from space. It is said that the institute started with a small team of only about ten people.

What made this place remarkable was its culture; unlike the rigid hierarchy common in Japanese society at the time, this lab offered an environment where men and women of all ages could freely exchange opinions. Photographs displayed in the central conference room offer a glimpse into this open and progressive atmosphere.

One of the main research themes at the institute was the study of cosmic rays, and a massive device used for this purpose still remains in one of the rooms.

This structure, which looks like a giant box, is part of a plastic scintillator.

This device is constructed using a transparent plastic called polyvinyl toluene mixed with trace amounts of two types of fluorescent materials.

When electrically charged particles pass through the plastic, the fluorescent material emits light, allowing the detection of elementary particles. Since the light generated by the passing particles is incredibly faint and invisible to the naked eye, the plastic scintillator was designed to convert that faint light into electrical signals for measurement.

Although I am not an expert and cannot identify exactly which parts of the remaining structure correspond to the technical components of the plastic scintillator, the square structure itself possesses a strange allure.

Standing before it, one feels a peculiar presence, as if wandering inside the world of “Minecraft” or becoming a character in a sci-fi novel or video game.

It creates a mysterious and immersive space. In addition to this device, other apparatuses such as Geiger-Müller counters were reportedly stationed in separate rooms, though they no longer remain today.

To observe cosmic rays falling from space, it is best to have as few obstacles as possible between the instruments and the sky.

Surprisingly, the reason this building was chosen for the laboratory lies in its unique architecture.

During the army era, this building was used as an explosives physics laboratory. Because of this dangerous function, the ceiling was intentionally constructed to be thin.

The design ensured that in the unfortunate event of an explosion, the blast would be directed upward toward the sky rather than outward, minimizing damage to the surrounding area.

Ironically, this safety feature for explosives made the building perfectly suited for cosmic ray observation.

The institute was also home to what we would call a supercomputer in modern terms: an electronic calculator.

While there were no detailed stories available about exactly what it looked like, the facility included a waiting room and stationed engineers dedicated to maintaining the machine.

Considering the era, one can imagine a computer that resembled a wall of modular synthesizers with exposed cables.

It is fascinating to think that such cutting-edge technology of the time was operating right here in this quiet corner of Itabashi.

The Legacy of Hideki Yukawa

One of the most prominent figures associated with this institute was Dr. Hideki Yukawa, the first Japanese national to win a Nobel Prize.

He received the award in 1949 after overseas researchers proved his “Meson Theory.

” Mesons act like a glue that holds protons and neutrons together within an atomic nucleus.

Although protons naturally repel each other because they both carry a positive charge, the meson binds them together.

Yukawa’s theory predicted the existence of these particles before they were actually found.
His prestige as a Nobel laureate is believed to have been instrumental in securing the massive funding required to establish the electronic computer room and equip the new RIKEN facility.

While Dr. Yukawa is famous for his Meson Theory, his work at the RIKEN Itabashi Branch focused on a different phenomenon known as “Air Showers.”

He used the facility’s electronic computers to theoretically analyze how high-energy cosmic rays collide with the Earth’s atmosphere and scatter, much like a meteor breaking apart upon entry.