Tae Niita

The event trigger system for CALET

The CALorimetric Electron Telescope, CALET, is a mission to study high energy phenomena in the universe by observing high energy cosmic rays (electrons, gamma rays, and nuclei) on the International Space Station. The instrument consists of a segmented plastic scintillator charge-measuring module, an imaging calorimeter consisting of 8 scintillating fiber planes interleaved with tungsten plates of 3 radiation length, and a total absorption calorimeter consisting of orthogonal PWO logs of 27 radiation length. It is necessary to eliminate the background events, mostly low energy protons that prevent efficient observation of high energy cosmic rays. Therefore, CALET has an on-board trigger system to select events which are 1) high energy showers, 2) low energy showers and 3) non-interacting protons or heavy nuclei. These triggers are generated by a combination of the signals from the charge detector, the imaging calorimeter, and the top layer of PWO in the total absorption calorimeter. A CERN-SPS beam test of the CALET prototype detector was carried out by using muons, electrons, and hadrons. We introduce the CALET trigger system and present its performance verified during the beam test.

The balloon-bone CALET prototype detector (bCALET)

The CALET payload will be installed in the Japanese Experiment Module Exposed Facility (JEM-EF) of the International Space Station (ISS). We developed a balloon-borne payload to evaluate the performance of CALET by carrying out precursor flights for the electron and gamma-ray observations. The first flight of bCALET-1 (balloon-borne CALET prototype) was carried out in 2006, and the enhanced version, bCALET-2, was successfully flown in August 2009. The bCALET-2 is composed of IMaging Calorimeter (IMC) and Total AbSorption Calorimeter (TASC). The IMC has an area of 256 mm × 256 mm, and is consisted of 8 layers of scintillating fiber belts with a total 3.6 radiation lengths of tungsten plates interleaved within the fiber planes for imaging the pre-shower development. TASC is consisted of crossed BGO logs (25 mm × 25 mm × 300 mm in each) with a total of 13.4 radiation lengths depth, for measuring the total energy deposit of incoming shower particles. The geometry factor is nearly 320 cm 2 sr over 10 GeV. We succeeded the observation of the electron energy spectrum in 1 GeV ∼ several 10 GeV electron and the atmospheric gamma-rays in 1G eV∼ a few 10 GeV, which are consistent with previous observations by BETS. The results are compared with simulations for confirming the detector performance.

The CALET CHD for determination of nuclear charge

Calorimetric Electron Telescope (CALET) will be a high energy cosmic ray observatory on the Japanese Experimental Module – Exposed Facility of the International Space Station. In addition to electrons and gamma-rays, CALET has an excellent detection capability of cosmic ray nuclei. In order to determine the atomic number of measured nuclei, the CHarge Detector (CHD) is placed on the top of the calorimeter. The CALET-CHD consists of two orthogonal layers of plastic scintillator charge-measuring modules. Each layer is segmented into 14 scintillator paddles (45 cm×3.2 cm×1 cm) for the reduction of back scattering effects. We evaluated the charge resolution of the plastic scintillators with heavy ion accelerators. In this presentation, we will report the design of the CALET-CHD and its nuclei identification capability as inferred from heavy ion beam tests.