CALET is a new mission selected by JAXA for the Japanese Experiment Module—Exposed Facility (JEM-EF) on the International Space Station, manifested for HTV-5 (H-II Transfer Vehicle 5) in 2014. CALET will measure the high-energy spectra of electrons, nuclei, and gamma-rays to address outstanding questions including signatures of dark matter, the sources of high-energy particles and photons, and the details of particle acceleration and transport in the galaxy. The CALET project includes researchers from Japan, the U.S., Italy, and China. The CALET-U.S. team of Louisiana State University, GSFC, Washington University in St. Louis, and the University of Denver are working in CALET instrument development, testing, instrument modeling, flight operations, flight data processing and science analysis. The ASD team of John Mitchell, Thomas Hams, John Krizmanic, Alexander Moiseev, and Makoto Sasaki are responsible for the instrument simulation and performance model, technical support for instrument development, and accelerator testing and calibration.

CALET uses a deep-imaging particle calorimeter for superior energy resolution and excellent separation between hadrons and electrons and between charged particles and gamma rays. The main telescope has a field-of-view of ~45° from the zenith and a geometric acceptance of 0.12 m²-sr. The calorimeter is divided into an imaging calorimeter (IMC) section that provides tracking and accurately determines the starting point of showers, and a total absorption calorimeter (TASC) section that measures total particle energy. The IMC contains ~3 radiation lengths (X_o) of tungsten interspersed between eight x-y layers of scintillating optical fibers read out by multi-anode photomultipliers. Most electrons and photons will initiate showers in the IMC, which measures the starting point of the shower and its development until it enters the TASC. The TASC is a stack of lead tungstate (PWO) crystals arranged in x-y layers to track the axis of the shower. Each crystal is read out by two photodiodes and an avalanche photodiode. The TASC has a total thickness of 27 X_o and collects the total energy in the shower with a leakage of only a few percent for electrons. A charge detector subsystem at the top of the telescope measures the charge of incident particles and functions as an anti-coincidence detector for gamma-ray measurements.

CALET is focused on investigating the high-energy total electron spectrum into the trans-TeV energy range. These measurements have the potential to identify, for the first time, the signature of high-energy particles accelerated in a local astrophysical engine and subsequently released into the Galaxy. Electrons lose energy rapidly by synchrotron and inverse Compton processes. The distant-source spectrum is expected to be relatively featureless, falling approximately as E^-3 and softening rapidly above 1 TeV. Electrons with TeV energy must have been accelerated within about 10⁵ yrs and can have diffused at most a few hundred parsecs. The electron lifetime and the diffusion distance decrease rapidly with energy. Detection of electrons with energy significantly above 1 TeV would indicate the presence of a nearby source and the arrival directions of these electrons should also show detectable anisotropy. Individual sources might also produce features in the spectrum at lower energies. CALET will resolve discrepancies among recent results from balloon experiments (BETS, ATIC, PPB-BETS), space experiments (Fermi, PAMELA) and ground-based air Cherenkov telescope observations (HESS).

High-energy electrons and positrons may also be produced by dark-matter annihilation. CALET will search for signatures of dark-matter annihilation producing features in the electron or gamma-ray spectra. Together with measurements at the Large Hadron Collider, details of the spectra of high-energy cosmicray electrons and positrons may hold the key to revealing the nature of dark matter.

The spectra of primary cosmic-ray nuclei, and the important secondary elements such as boron, hold the key to understanding galactic particle transport at very high energy. CALET will measure the B/C ratio with precision to about a decade in energy beyond current results, and thereby test many of the models currently proposed. CALET will also extend the measurements of the spectra of cosmic ray nuclei from hydrogen to iron, with high resolution, into the region of the spectral “knee” to investigate possible structure and energy-dependent composition changes.

CALET will perform a gamma-ray all-sky survey, complementing Fermi and HESS observations, to detect intense high-energy sources, study the diffuse component, and search for new regions of emission. CALET includes a low energy (7 KeV–20 MeV) gamma-ray burst monitor. GRB measurements are also extended to high energy using the main telescope.

Construction of the instrument and spacecraft are well underway. A test of prototype detectors at the CERN (European Laboratory for Particle Physics) Super Proton Synchrotron (SPS) using proton and electron beams up to 350 GeV in energy showed the instrument to be capable of the needed discrimination between cosmic-ray electrons and the far more numerous protons, with energy resolution of a few percent for electrons. The engineering model of CALET will be calibrated in both singly-charged particle and heavy-ion beams at the SPS in Fall 2012.

The electron, nucleus, and gamma-ray measurements of CALET would be extended to higher energy and greater precision by the HEPCaT (High-Energy Particle Calorimeter Telescope) instrument studied by a team led by Mitchell and proposed as part of the OASIS (Orbiting Astrophysical Spectrometer in Space) mission. HEPCaT would measure cosmic-ray electrons to energies well above 10 TeV and nuclei to energies of 10¹⁵ eV.

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