CALET
CALorimetric Electron Telescope (CALET) on JEM-EF
Mission Descriptions
Overview
CALET (Calorimetric Electron Telescope) is a mission aimed at observing cosmic rays and gamma rays. It primarily focuses on high-energy electron and gamma-ray observations to investigate the origins and acceleration mechanisms of cosmic rays, explore dark matter, and solve the mysteries of the cosmic X-ray background (CXB) radiation, among other objectives. CALET was installed on the Japanese Experiment Module "Kibo" of the International Space Station (ISS) on August 19, 2015. This mission is led mainly by the Japan Aerospace Exploration Agency (JAXA).
CALET consists of two main observation instruments: the CAL (Calorimeter) and the CGBM (Gamma-ray Burst Monitor). CALET has a field of view of about 45 degrees from the zenith, and the effective geometric factor for electrons above 10 GeV is nearly constant at approximately 1040 cm² sr.
Observation Instruments
The main observation instruments of CALET and their components are as follows:
CAL (Calorimeter)
The CAL is the primary observation instrument of CALET, designed to measure the energy of cosmic rays with high precision. CAL consists of three main components:
CHD (CHarge Detector)
The CHD is a detector for measuring the charge of particles, allowing the identification of the atomic number of individual particles. The CHD uses silicon strip sensors to detect the charge generated as particles pass through.
IMC (IMaging Calorimeter)
The IMC is an imaging calorimeter designed to visualize initial particle showers with high resolution. It has a structure with alternating layers of scintillator fibers and tungsten plates. When particles collide with the tungsten plates, showers are generated and detected by the scintillator fibers, allowing the precise reconstruction of particle trajectories.
TASC (Total AbSorption Calorimeter)
The TASC is a total absorption calorimeter designed to measure the energy of particles with high precision. The TASC uses lead tungstate (PbWO4) scintillator crystals, detecting the light generated when particles collide with the crystals. This allows accurate measurement of particle energy.
CGBM (CALET Gamma-ray Burst Monitor)
The CGBM is designed to observe gamma-ray bursts (GRBs) and gravitational wave sources, covering a wide energy range from approximately 7 keV to 20 MeV. The CGBM consists of two types of detectors: the Hard X-ray Monitor (HXM) and the Soft Gamma-ray Monitor (SGM).
HXM (Hard X-ray Monitor)
The HXM employs scintillators using LaBr3(Ce) crystals, observing an energy range from 7 keV to 1 MeV. LaBr3(Ce) is superior in terms of light output, energy resolution, and time response compared to NaI(Tl). The HXM consists of two cylindrical LaBr3(Ce) crystals, with the front cylinder measuring 66.0 mm in diameter and 6.35 mm in thickness, and the rear cylinder measuring 78.7 mm in diameter and 6.35 mm in thickness, making it sensitive to soft X-rays below 10 keV.
SGM (Soft Gamma-ray Monitor)
The SGM employs BGO crystals as scintillators, covering an energy range from 100 keV to 20 MeV. BGO crystals have a high density (ρ=7.13 g/cm³) and high effective atomic number (Zeff=74), providing excellent gamma-ray stopping power. The SGM's BGO crystals are cylindrical, with a diameter of 102 mm and a thickness of 76 mm.
Each sensor of the CGBM primarily includes a scintillation crystal, photomultiplier tube (PMT), high-voltage divider, and charge-sensitive amplifier (CSA), allowing the high-sensitivity detection of gamma-ray bursts.
Achievements
CALET has achieved high-precision direct observations of cosmic rays, covering an energy spectrum of total electrons from 11 GeV to 4.8 TeV. These observations have suggested spectral structures in the highest energy regions, and further analysis and increased statistical data are expected to advance observations in the high-energy regions of nuclear components.
CALET has also detected components of gamma-ray bursts above 1 GeV, aiming for ultra-wideband observations from 7 keV to the TeV range. This observational capability plays a unique role in exploring gravitational wave counterparts, with CALET having already reported upper limits on X-ray and gamma-ray emissions for the gravitational wave event GW151226.
Moreover, CALET is advancing research on the Sun-Earth magnetosphere through observations of solar modulation of electron components related to solar activity. Specifically, it has detected high-energy oscillations and MeV electrons massively emitted from the Van Allen belts. Observing the Relativistic Electron Precipitation (REP) phenomenon is expected to help elucidate the Electromagnetic Ion Cyclotron (EMIC) waves, crucial for improving the accuracy of space weather forecasts.