Tibet AS-gamma Experiment

In collaboration with China, an air shower observation array is built at Yangbajing
(4300m above sea level, 90.53 deg E,30.11 deg N) in Tibet, China to observe high-energy cosmic rays.

Fig. 1

Fig. 1 Our air shower array constructed at Yangbajing (4300m above sea level) in Tibet.

Our research subjects are:
Our air shower array consists of 697 scintillation counters which are placed at a lattice with 7.5 m spacing and 36 scintillation counters which are placed at a lattice with 15 m spacing. Each counter has a plate of plastic scintillator, 0.5 m2 in area and 3 cm in thickness, equipped with a 2-inch-in-diameter photomultiplier tube (PMT). The time and charge information of each PMT hit by an air shower event is recorded to determine its direction and energy. The detection threshold energy is approximately 3 TeV, which is the lowest one achieved by an air shower array in the world. The event trigger rate is currently 1.5 kHz and the data size becomes 26 GB / day. The angular resolution of the air shower array is estimated by the Moon's shadow to be less than 1 degree, which is also the world best performance (see Fig. 1,3).
Thanks to the good angular resolution and high statistics, we succeeded in clearly observing the "Sun's shadow" and " Moon's shadow" in the galactic cosmic rays as they have finite diameter (0.5 deg) and shield cosmic rays coming from their directions (Fig. 2, 14). The Moon's shadow position displacement in the north-south direction demonstrates our pointing accuracy, while the west-east displacement gives us the energy scale calibration in the TeV region by means of cosmic-ray bending effect in the geomagnetic field.

Fig. 2 Fig. 2 Deficit in galactic cosmic rays around the Moon direction
("Moon's shadow"). Contour map of the weights of deficit event
densities around the Moon centered in the figure. The contour lines
are drawn with a step of 2 sigma. Angular distance is measured from
the direction of the Moon along the right ascension (abscissa) and the
declination (ordinate).

At the center of our air shower array, burst detectors and emusion chambers were set up to closely observe the core region of an air shower event. The total area of them is 80 m2. Each burst detector is composed of a plate of plastic scintillator 160 cm (length) x 50 cm (width) x 2 cm (thikness) and 4 photodiodes attached to each corner of the plate. On each burst detecter, placed were 6 layers of emulsion chambers (x-ray film interleaved with Pb plate). This hybrid experiment incorporating the air shower array, burst detectors emulsion chambers enables us to select and measure the proton component in primary cosmic rays in the "knee" region (10 15-1016 eV).
Besides, we set up a solar neutron telescope, 9 m2 in area and 40 cm in thickness, composed of plastic scintillator plates with photomultiplier tubes, surrounded by proportional tubes. The solar acivities have 11-year period and were in a very active state around the year 2000. The solar telescope aims at detecting high-energy solar neutrons accompanied with solar flares at an active phase. Systematic study of such solar neutrons will provide us with important hints to ion acceleration mechanism in a solar flare as well as to cosmic-ray acceleration mechanism.

Fig. 3 Fig. 4 Fig. 5
Fig. 3 Scintillator counter employed
in our air shower array.
Fig. 4 One unit of emulsion chambers
and burst detector.
Fig. 5 Solar neutron telescope.

eV: electron volt. Energy obtained by an electron accelerated between 1 volt potential difference. 1 eV = 1.6x10-19J, 1 TeV = 1012 eV.