The GRAINE (Gamma-Ray Astro-Imager Nuclear Emulsion) collaboration, primarily composed of members from Kobe University and Nagoya University and lead by Prof. AOKI Shigeki from the Graduate School of Human Development and Environment of Kobe University, conducted a demonstration experiment on a gamma-ray telescope system using balloon-borne nuclear emulsion plates (emulsion films).

Micrograph of the emulsion film developed at the University of Sydney. The width of the picture is 100 micrometers (0.1 millimeter). The row of silver grains, which precipitated during the development process (the grain size is about 1 micron), represent the track of charged particles recorded in the upper sky.

Gamma rays are generated when high-energy cosmic rays such as protons, electrons, and nuclei traveling in space at a high speed collide with materials such as interstellar gases. Because gamma rays travel in straight lines through space without being affected by magnetic fields, high-energy astronomical phenomena can be indirectly observed by measuring the energy and radiant directions of gamma rays.

Although the Fermi Gamma-ray Space Telescope, which became operational in 2008, has greatly advanced observational research of radiations in the gamma-ray region, gamma-ray observations are rare compared to those of infrared or visible rays. However, high performance gamma-ray observation devices should be developed considering that gamma-ray radiators, including unknown celestial objects, black holes, and neutron stars (pulsars), have been found.

The GRAINE collaboration has developed an emulsion telescope using nuclear emulsion plates (emulsion films) to observe gamma rays traveling from space at a high resolution. Such emulsion films can record the tracks of charged particles on the order of one micrometer (one thousandth of a millimeter), which realizes 100 times better resolution than that of a state-of-the-art gamma ray astronomical satellite.

In 2014, the collaboration started manufacturing emulsion films at a facility located in Nagoya University. In March of 2015, emulsion films with a total area of 50 square meters were manufactured to be fabricated into an emulsion telescope. This telescope was mounted on a balloon for observations operated by the Japan Aerospace Exploration Agency (JAXA).

A balloon experiment was conducted at the Balloon Launching Station of the University of New South Wales located at Alice Springs in Australia on May 12 –13. The Vela pulsar, a neutron star emitting bright gamma rays, was observed at an altitude of 36 kilometers for more than 6 hours. The emulsion films were subsequently recovered and underwent a development process at the University of Sydney, confirming that the films were kept in sound conditions during the observations.

The recovered films are currently being analyzed using the Hyper Track Selector (HTS), which is a nuclear emulsion read-out apparatus developed by Nagoya University that is capable of scanning emulsion films at the highest speed in the world. The analysis work will take about one year, and the performance of the telescope system will be assessed accordingly.

Prof. AOKI Shigeki has stated, “We would like to successfully complete the world’s first astronomical observation using an emulsion telescope and demonstrate the high resolution of our apparatus. Additionally, We would like to lead the way to full-scale scientific observations using such an apparatus with a larger area and longer time duration.”

The balloon was released from the Balloon Launching Station located
at Alice Springs in Australia on May 12 at 6:03 a.m. JST.
The balloon rose to an altitude of 36 kilometers, and observations were
conducted for 11 hours, and for 6 hours of them,
gamma rays from the Vela pulsar were in the field of view of the telescope.

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(Graduate School of Human Development and Environment, Communications Division)