JEM-EUSO: the Extreme Universe Space Observatory

JEM-EUSO will be second cosmic ray instrument on board of ISS, after the Alpha Magnetic Spectrometer 02 (AMS-02, aimed at detection of low energy cosmic rays). Contrary to the AMS-02, JEM-EUSO will not be a particle detector in a proper sense, because it will not detect the high-energy particles themselves. Instead, it will use the Earth’s atmosphere as a calorimeter of particle physics detector. It will detect ultraviolet fluorescence light produced by the air molecules excited by the Ultra High Energy Cosmic Rays (UHECR) induced Extensive Air Shower (EAS). JEM-EUSO will be a refractive ultraviolet telescope with aperture 2.5 m and field of view of 60° operating in the 300-400 nm wavelength range. The light will be focused by a system of three Fresnel lenses made from UV transparent polymers. UV photons reaching the focal plane will be detected by an array of multi-anode photomultipliers (MAPMT) with 3×105 pixels.

Overview of the JEM-EUSO mission. JEM-EUSO is currently designed to be attached to the Japanese Experiment Module/Exposure Facility (JEM/EF) of the International Space Station (ISS), orbiting around the earth every 90 minutes at the altitude of approximately 400km.

The cascade of high-energy particles, EAS, propagates with the speed close to the speed of light through the air, in the direction, which coincides with the direction of the primary UHECR initiating the EAS. The fluorescence emission appears for a short (about 100 μs from the top of the atmosphere to the ground) period of time along the path of the EAS. JEM-EUSO will continuously take images of a large region of the atmosphere of the size about 400 km across on the time scale of 2.5 μs. Angular resolution of the images will be 0.1° which corresponds to ~700 m linear distance on the ground assuming the 400 km altitude of the ISS. An EAS arriving at a non-negligible zenith angle will cross many pixels until it reaches the ground. Thus, an UHECR induced EAS will appear during ~100 μs as a fast-moving source on the focal surface of the telescope. A combined measurement of the direction of motion and of the velocity of the source will provide a measurement of the arrival direction of the primary UHECR particle.

The intensity of the fluorescence emission is proportional to the total energy of high-energy particles in EAS deposited in a given volume of the atmosphere. Since all the high-energy particles in EAS are produced in result of interaction of single primary particle, UHECR, in the atmosphere, the total UV flux from the EAS is proportional to the energy of the primary UHECR. Measurement of the total flux in the EAS image on the focal plane of JEM-EUSO will provide a measurement of the UHECR energy with 30% precision at the energy 1020 eV.