JEM-EUSO—Investigating a perplexing mystery
Ultrahigh-energy cosmic particles are an intriguing puzzle in high-energy physics, and RIKEN is involved in a project to solve it. On May 16, the RIKEN proposal for the second utilization plan of the Japanese Experiment Module (JEM) on board the International Space Station was accepted, and its kickoff meeting was held on June 6 to 8 at RIKEN, Wako.
The particles have extremely high kinetic energies, as much as 1020 electronvolts, far greater than other cosmic-ray particles. They are also extremely rare—only 11 have been observed in 13 years of searching with the Akeno Giant Air Shower Array (AGASA), which has an effective area as large as the area enclosed by the Yamanote line in the Tokyo Metro. No one knows where they come from, or how they could have that much energy left over after the long journey through intergalactic and interstellar space.
To observe these rare phenomena, a telescope with an extremely wide field of view is needed, and it is being provided by the Extreme Universe Space Observatory (EUSO), to be installed on the JEM on the International Space Station, after JEM is launched in 2008.
Instead of looking out into space like a conventional telescope, EUSO will look down at the Earth from space, searching for streaks of ultraviolet fluorescence and Cerenkov radiation, which cosmic particles produce when they interact with the Earth's atmosphere.
EUSO incorporates a Fresnel lens, a very thin, wide-aperture lens made up of concentric rings, which provides a 60 ° field of view. The lens was made using ultraprecise grinding technology at the RIKEN Ohmori laboratory.
To detect the faint streaks of ultraviolet fluorescence and Cerenkov radiation that the particles produce as they enter the Earth’s atmosphere, EUSO contains 6,000 photomultiplier tubes, which were developed and built in Japan.
EUSO is said to be launched by a Japanese H-IIB rocket and transferred by H-II transfer vehicle (HTV), and it is expected to detect about 1,000 ultrahigh-energy cosmic particles in the five years of its operation. From this data researchers hope to determine whether the particles originated from a single source or occur throughout the universe.
