Guide

Associate Professor, Yoshinari Hayato
Yumiko Takenaga
（Kamioka Observatory, ICRR,
The University of Tokyo）

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## What if we put Hyper-Kamiokande in a dome stadium?

Hyper-Kamiokande, a huge experimental detector, is scheduled to be completed in 2027 underground at the Kamioka Mine.
How big is Hyper-Kamiokande compared to the first generation Kamiokande and the currently running Super-Kamiokande?
Let’s compare their scale as clearly as possible.

Tokyo Dome, you might see it often at baseball games.

### Hyper-Kamiokande won’t fit in the Tokyo Dome?

#### Super-Kamiokande that completely fits the infield

The size of the Hyper-Kamiokande is 68 m in diameter by 71 m high. Why do we need such a huge device? I will explain in detail later. …… Can you imagine how big it really is?
To make it easier to understand, let’s imagine that it is placed on the ground of the Tokyo Dome. Let’s start with the Super-Kamiokande, which is currently in operation.

The height from the ground surface of the Tokyo Dome is about 62 meters. The Super-Kamiokande tank is 39 m in diameter by 42 m in height, so it can somehow fit there. The height corresponds to about the 14th floor in an apartment building.
The diameter is about the same as the distance from the home base to the second base (39 m), so you might imagine that the infield would be about the same size as the length of a 25-m swimming pool.

Incidentally, the size of the first Kamiokande is 19 m in diameter and 16 m in height. The diameter is a little shorter than the distance from the mound to the home base, which is about 18 m. Compared to the length of a 25-m swimming pool, it is about 0.6 times as long.

It’s gotten bigger and bigger and more powerful since the first Kamiokande!

#### Hyper-Kamiokande is so big it goes through the ceiling.

Now let’s place the Hyper-Kamiokande tank. The height of the tank is 71 m, so it will go through the ceiling. Even a relatively large dome such as Sapporo Dome is 68 meters above the ground, so the tank will not fit in the dome. For an apartment building, it would be about the height of the 23rd floor.
The diameter of the dome is 68 m, which is about 1.7 times the diameter of the Super-Kamiokande, so it would reach from the home base to the front of the center. The width of a soccer court is exactly 68m……, which may be easier for soccer lovers to understand.

### Why do we need such a big tank?

#### Hyper-Kamiokande to be installed in a large underground cavity

Hyper-Kamiokande is constructed underground in Kamioka-cho, Hida City, Gifu Prefecture. We can use the mountain as a filter underground, while various particles are flying around on the earth’s surface. Unlike other particles, neutrinos can easily pass through one or two earth, so it is easier to observe them underground. The underground cavern that will be excavated in 2022 to install the giant tank will be 69 m in diameter and 88 m high. It is one of the world’s largest artificial underground cavities.

The planned site for the Hyper-Kamiokande installation. A huge cavity will be excavated from here.

#### Experiment with particles in large amounts of water

The huge tank of the Hyper-Kamiokande will be filled with ultrapure water, as in previous generations of the Kamiokande series. This is to make use of the particles in the water in various ways. For example, neutrinos, which can pass through anything, can collide with particles in matter in rare cases, so water (particles in water), which is relatively inexpensive to prepare, is used. The amount of water is 260,000 tons in Hyper-Kamiokande, compared to 50,000 tons in Super-Kamiokande.

Another purpose is to observe the “proton” in the nucleus of an atom in a water molecule. Although protons have been thought to be unbreakable, there is a theory that they may actually have a finite life span. However, its life span is tremendously long, 10 billion times 10 billion times 10 billion years, so we are trying to collect as many protons as possible and observe their decay.

#### Photosensor that would capture even the light from a flashlight on the moon’s surface

Incidentally, neutrinos and protons are “seen” by the photomultiplier tubes installed inside the tank. The photomultiplier tubes in Super-Kamiokande were so good performance that they could “detect the light from a flashlight on the moon,” but the photomultiplier tubes in Hyper-Kamiokande have been further improved to twice the sensitivity! The number of photomultiplier tubes installed in the Hyper-Kamiokande will be 40,000, approximately four times the 11,129 installed in the Super-Kamiokande.
In addition to the increased size of the tank, these various innovations will make it possible for Hyper-Kamiokande to collect data equivalent to 100 years of Super-Kamiokande data in 10 years. We may soon be able to understand the mysteries of the universe and elementary particles. Please look forward to it.

Photomultiplier tubes to be installed inside the tank.

#### Is Hyper-Kamiokande a giant camera?

Photomultiplier tubes are
like the sensors (pixels of) an ultra-high performance camera.
Thanks to this,
the Hyper-Kamiokande is a giant camera that can capture even a single photon.

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