Muons, also known as mu mesons, are elementary particles that enter the earth's atmosphere as cosmic rays, and are everywhere around us, just like electrons.
"If the 20^th century was the electron age, the 21^st century could be the muon age," says Dr. Kanetada Nagamine, Chief Scientist in the Muon Science Laboratory.
In fact, Dr. Nagamine and his staff have been engaged for 25 years in a wide range of research into the structure of substances, the development of nuclear fusion technology, observation of electron transport of proteins and DNA, and an exploration of the inside of volcanoes and the earth's crust using muons. Let's step into the front line of muon science, which embraces every field of science including substances, energy, life, and the earth.

The Muon and the Nobel Prize in Chemistry

In 1983, Dr. Nagamine and his staff started their own study, using muons to investigate how the electrons work in an organic molecule. They focused on the conducting polymer "polyacetylene" that had been just developed. The pioneering chemist Hideki Shirakawa, the chemist Alan G. MacDiarmid, and the physicist Alan J. Heeger were jointly awarded the 2000 Nobel Prize in chemistry for their work on polyacetylene.
"This substance has two isomers: trans and cis. Dr. Heeger formulated a theory that electrons of the trans form exhibit a motion based on solitons, showing stable, nonlinear undulations. It therefore flashed through my mind that I really had to prove his theory through muon implantation."
There are two types of muons: one that has a charge of +1 and the other that has a charge of -1. The +1 type is used to investigate the motion of electrons. The mass of a muon is 1/9 the mass of a proton. A muon can therefore be thought of as a light proton. Once this muon is implanted into a soft substance such as a high polymer, it gradually loses its energy. The muon captures one electron in the substance and forms muonium like a hydrogen atom. The muonium loses more energy and finally makes a chemical bond with one atom in the substance (Figure 1).
"At that time, the electron captured by the muon is liberated, having a unique motion to the organic molecule. The state of the moving electron can be measured via the muon."
Both the muon and the electron have a spin of 1/2 and are so-called micromagnets. When the electron is localized or moves around with the property of a molecule, it has an impact upon the muon, thus causing a magnetic reaction.
By applying an external magnetic field, the muon spin begins to precess. When the precession speed is synchronized with the electron speed, the "spin fluctuation," which is a magnetic reaction, maximizes.
Plotting the external magnetic field on the horizontal axis and the fluctuation on the vertical axis gives the relationship between the "spin fluctuation" and the electron speed. It is also possible to clarify the form of motion of electrons from the change in fluctuation and its relation to the size of the external magnetic field. If electrons have one-dimensional motion, for example, the fluctuation is proportional to the inverse of the strength of the external magnetic field.
"Our muon-based measurement verified that cis-form polyacetylene had localized electrons and trans-form polyacetylene had quick one-dimensional motion along the chain in a soliton motion form. We published our paper on this subject with Professor Shirakawa in 1984."
Dr. Nagamine calls this electron transport measurement method, in which the muon captures an electron that is then labeled and serves as a sensor for detecting its behavior, "muon electron labeling".
"We pioneered this method and applied it to polymer electron transport. After that, the British group who are our joint researchers in the muon experiments took over our project and examined a variety of polymer electron transport forms, because we were busy with our muon nuclear fusion project and did not have the time to get involved in it."
However, in 1997, about 15 years after the work on polyacetylene,Nagamine's group restarted their research on muon electron labeling,targeting electron transport phenomena in biomolecules.

Investigating the Motion of Biomolecular Electrons

The first target was a huge protein molecule called cytochrome c with a molecular weight of 100,000. This protein is an electron transport system that exists in the mitochondria of an organelle that takes in oxygen. The heme protein forms the center, and it is surrounded by a hydro-carbon ring, and surrounded by more than 100 amino acids outside the carbon ring.
"Cytochrome c is one of the few proteins that can be crystallized in bulk. It was an appropriate target for our muon electron labeling experiments."
Being implanted into cytochrome c, a muon stops at a place with a negatively charged several atoms away from the central heme iron, and liberates electrons.
"Our experiment demonstrated that the liberated electrons are moving around the carbon ring; some of the electrons were directed towards the heme iron and some moved to the next chain. (Figure 1)."
As they observed, the electrons were transmitted along the chain at very high speeds, traveling between atoms in the order of picoseconds (10^-12), and jumped between chains at lower speeds.
Moreover, they found an interesting fact when comparing cytochrome c with the protein molecule myoglobin, which has an oxygen transport function. Myoglobin has the same heme iron at its center as cytochrome c, which is also surrounded by hydro-carbon ring. However, myoglobin does not play an important role in electron transport in biological environments. Once a muon is implanted into myoglobin, its electrons trigger electron transport in the myoblobin. Consequently, the electrons travel along the chain at very high speeds like cytochrome c, but there is a difference in the way that the electrons jump between chains (Figure 2).
With cytochrome c, its electrons hibernate in one chain until a certain temperature and are suddenly activated above that temperature. This temperature is 200. The result of our experiments matched the transformation temperature at which the coil motion of protein is frozen and a glass structure is formed. On the other hand, myoglobin does not exhibit such a phenomenon.
"We are sure that the nature of electron transport in protein is deeply related to its coil motion."
Dr. Nagamine and his staff have measured DNA electron transport using muons. Various methods have shown various results for DNA electron transport speeds. There are 5-digit differences.
"We already have the data for DNA electron transport. The data shows unexpectedly high electron conductivity. It is just conceivable that DNA could flow along electric wires."
Regardless of the status or environment of the biomolecules being investigated, including crystals and underwater substances, the "muon electron labeling" method is a strong candidate for use in examining the electron transport of living protein. Therefore, preparation has been underway to meet such a demand.
Dr. Nagamine says, "In the near future, it might be feasible to explore the relationship between thought and electron transport in real time by implanting muons into a human brain. (Figure 3)." His vision is expanding all the time.

Probing the Inside structure of a Volcano

In Japan, which is a volcanic island country, many regions are endangered by volcanic eruptions from craters such as Fugendake, Usuzan, and Miyakejima. If you could take an X-ray photograph of the inside of a volcano, you could predict the likelihood of an eruption. One day, Dr. Nagamine discovered that the muon has the potential to be used to predict volcanic eruptions. (Figure 4)
Muons are entering the earth's atmosphere as cosmic rays, striking your open hand at the rate of one every second. When protons, which are the universe's primary form of cosmic rays, hit the earth's atmosphere, they generate mesons (also referred to as Yukawa mesons) or K mesons. Muons are also generated. Muons are unstable, and only cause electromagnetic interaction with other particles while completing their path to the earth. The majority of muons falls from overhead. However, there are also high-energy muons with energies of more than 100 giga electron volts that come in from the horizontal direction in large quantities.
"The muons' ability to penetrate matter depends on their energy. Muons with energies of 100 giga (10⁹) electron volts can pass through 100 meters of rock, and muons with energies of 1 tera (10¹²) electron volts can penetrate 1000 meters into the earth."
The width and shape of a mountain can be measured by positioning multiple detectors to detect the number and direction of the muons that pass through. The first experiment was conducted on Mt. Tsukuba. Three detectors were placed at a horizontal distance of two kilometers from the top of Otokoyama and at an altitude of 150 meters above sea level. The experiment resulted in the accurate probing of the shapes of Otokoyama and Onnayama of the Mt.Tsukuba, obtaining the density length (the product of the length of the muon route and the mean density) for the internal structure of each mountain.
Next, detectors were placed in the Onioshidashi Asama area at a horizontal distance of 3.75km from Mt. Asama. The test focused on the feasibility of identifying a magma channel 300 meters long in a mountain height of two kilometers using muons. If there is a cavity in the magma channel inside the mountain, even more low-energy muons will be able to pass through it.
The test results helped to identify an internal area where muons penetrate easily, and its shape matched the inverse conical shape that had been geographically predicted (Figure 7). Based on this data, the magma can be visualized as it rises up the pipe. Continuing observations have been conducted since the beginning of this year, focusing on Mt. Iwate. A phreatic explosion is one that occurs inside the ridge of the mountain.
"This mountain is suitable for muon measurement because it is not too thick."
Furthermore, magma does not always flow in an existing magma channel in a volcano. In such a case, a magma channel database based on muon measurement is not enough to make accurate predictions. Before a phreatic explosion occurs, ground water comes up together with magma. If one could keep track of the water distribution inside the mountain by the hour, one might be able to construct a more accurate database.
"In cooperation with volcanologists, we are searching for what we should focus on and how we should refine the muon measurement method in order to construct a much better database."
Dr. Nagamine says that gathering informations from weather satellites and building a number of more accurate databases have improved the accuracy of weather forecasting. He and his staff have been pushing ahead with a grand project to use the muon detection system to construct a database that will provide X-ray images of the inside of mountains and the earth's crust.