The CALET "pallet" for the mission, illustrated in Figure 1, contains a star tracker (ASC) a gamma-ray burst monitor (GBM: SGM and HXM), the mission data controller (MDC) and the main telescope (CAL). The star tracker will provide fine pointing knowledge for observed sources or for events seen by the GRB monitor. This mission equipment fills the standard pallet which has been successfully tested in an HTV launch as well as being in use for current experiments on the JEM-EF. CALET will employ the ISS cooling loop and the heat exchange piping can be seen on the southwest side of Figure 2.
Figure 1 The CALET "pallet" installments for the mission. It consists "Main detector" (CAL and GBM) and "Support equipments" (GPSR, ASC etc.).
Figure.2 The heat exchange pipes are installed on CAL(green)
The heart of the mission is the main telescope , shown in detail in Figure 3, which has a field-of-view of ~45 degrees from the zenith. The "footprint" of the telescope is ~0.7 m x 0.7 m and fits on the standard pallet. The original version had a "footprint" = 1.2 m x 1.2 m which required the development of a non-standard pallet which would have increased the overall technical risk. The geometrical factor of the re-scoped CALET telescope is 0.12 m²-sr compared to the preliminary design of 0.7 m²-sr, a reduction of a factor of 5.8.
Figure 3 The architecture of calorimeter (CAL) The central region is example image of cascade shower of incident particle.
The original design was large in order to achieve the science goals in a mission as short as two years, dictated by the 2015 launch and the then announced plan to de-orbit the ISS in the 2015-16 time-frame. The re-scoped CALET with a mission life of 5 years, therefore has a reduced exposure, compared to the original, of 5.8 x 2/5 = 2.3. Thus, the effect upon the statistical significance of the measurements to be made (as demonstrated in the next section) is only evident for points at the high end of the energy range, and the effects are marginal. The re-scoped CALET is fully capable of meeting its science goals with the (big) benefit of reduced risk (technical, cost and schedule). The mechanical design for the main telescope has been completed; the front-end circuit (FEC) have been tested; the readout system interfaces are defined and the thermal loop for instrument heat rejection is under development.
CALET uses well-developed techniques to measure the incident charged particles or gamma rays. The event is first measured by the CHD and the top layers of the Imaging Calorimeter (IMC). This determines if it is a photon or a charged particle. The IMC contains ~3 radiation lengths (X0) of Tungsten interspersed between eight x-y layers of scintillating optical fibers. Most electrons and photons will initiate showers in the IMC which then provides measurements of the lateral development of the shower until the shower enters the Total Absorption Calorimeter (TASC) at the bottom. The TASC collects the total energy in the shower with a leakage of only a few percent. Protons, however, do not readily interact in the IMC, rather starting their cascades in the TASC. There is, in general, considerable energy leakage out the bottom for hadronic cascades. Tracking the shower core in the TASC + IMC, and the individual particle in the IMC + CHD, provides the event trajectory which can be mapped onto the sky.
The top of the instrument contains the charge measuring subsystem consisting of scintillator strips arranged in x and y and read out by photomultiplier
tubes using the system developed for the IMC. This version of the CHD will be fabricated in Japan. An earlier design envisioned using Silicon detectors that were to be provided by the CALET-IT team. Such a Silicon detector sub-system has now been ruled-out, due to scheduling constraints, design/development is proceeding on the scintillator variant as the prime system.
The IMC is based upon 1 mm x 1 mm scintillating optical fibers with eight x-y layers of fibers interspersed with Tungsten plates for a thickness of 3 X0. The fibers are fed to Multi-Anode Photomultiplier Tubes (MAPMT). The IMC provides charge identification, fast triggering, trajectory measurement, back-scatter rejection and analysis of the early stages of shower development. The IMC design has not changed during the evolution of CALET, and its performance has been evaluated both on high altitude balloon flights and at accelerators.
The Total Absorbing Scintillator Calorimeter (TASC), shown at the bottom, is a stack of 20 mm x 20 mm x 32 cm logs of the crystal scintillator Lead Tungstenate (PWOC) pathlength of = 30 X0. Reducing the path-length requirement can increase the geometric factor at a cost of increased energy leakage from the sides of the TASC. Conversely, requiring more central events (e.g. no trajectory in the outer 2 cm of the TASC) reduces the effective geometric factor. Optimizing the event selection criteria for different data analysis tasks remains one of the important jobs for the pre-launch period. Of prime importance in Fig. 3 is the fact that the acceptance is essentially independent of energy above a few 10's of GeV) arranged in x-y layers to track the axis of the shower. Each log is read out by two regular photodiodes plus an avalanche photodiode, the latter providing sufficient gain to observe singly ionizing particles (mip) for laboratory calibration with muons and accelerator calibration/testing with electron and proton beams at CERN (or elsewhere). The TASC has a total thickness of 27 X0. The change from Bismuth Germanate to PWO allowed CALET to take advantage of the higher density of PWO to decrease the physical height of the TASC, while maintaining its 27 X0, depth, and thereby increase the solid angle of the acceptance.
Prototypes of the different sub-systems have been tested on a number of balloon flights plus at the Super Proton Synchrotron (SPS) at CERN (Sept. 09 and Sept. 10). The performance matched that anticipated from the instrument Monte-Carlo simulations, and no major problems were encountered. Moreover, the scintillation CHD concept was exposed in October, 2010 to heavy ion beams at GSI in Darmstadt, Germany. The performance demonstrated sufficient resolution to allow the separation of B from C and the separation of major element species up to Iron.
Figures 3, 4, and 5 show, respectively, the geometry factor, the detection efficiency and the energy resolution expected for the CALET telescope for electron observations based upon Monte-Carlo simulations of the instrument performance using the EPICS/COSMOS simulation system.  For these calculations the DPMJET3 hadronic interaction model was employed. CALET will have a geometric factor of 1,200 cm²-sr (Fig. 3) for events that pass through the IMC and have a combined (IMC + TASC) pathlength of = 30 X0. Reducing the path-length requirement can increase the geometric factor at a cost of increased energy leakage from the sides of the TASC. Conversely, requiring more central events (e.g. no trajectory in the outer 2 cm of the TASC) reduces the effective geometric factor. Optimizing the event selection criteria for different data analysis tasks remains one of the important jobs for the pre-launch period. Of prime importance in Fig. 3 is the fact that the acceptance is essentially independent of energy above a few 10's of GeV.
Similarly, the overall efficiency, shown in Fig. 4 for a low energy (~ 1 GeV) and the high energy (~10 GeV) trigger, also displays energy independence. This has been verified up to several hundred GeV using the accelerator beams at CERN. Figure 5 illustrates the energy resolution (rms) that will be achieved by CALET. This is comparable to the resolutions achieved by ATIC and projected for the AMS instrument. Again, accelerator calibrations have verified these predictions at the energies available at CERN.
CALET measures the trajectory of each event. This allows the arrival direction to be determined using the ISS ephemeris plus the star tracker. The anticipated angular resolution is ~0.1° for gamma rays and ~0.05 ° for electron events above few hundred GeV, as shown in Fig. 6. The fact that gamma rays are not observed in the first few layers of the IMC results in a less precise trajectory determination which reduces the angular resolution, slightly, compared to electrons.
The combination of the high spatial resolution in the IMC (where most showers are likely to start) and the 27 radiation length TASC calorimeter, gives CALET an exceptional proton rejection, well above any experiment flown to date. In ca calorimetric instrument such as CALET (c.f. ATIC, Fermi), proton-electron separation relies on the shower development profile. Electromagnetic cascades are more compact than hadronic cascades. Neat the beginning of the shower, the width of the cascade is smaller for electrons than for protons. In addition, the electron showers are almost totally contained whereas a proton shower of the same total deposited energy shows a significant energy leakage from the bottom of the calorimeter. Thus, analyzing the width of the cascade and the fractional energy deposit (energy deposited in layer N / total energy deposited in the calorimeter), layer by layer, provides the p-e separation. To illustrate the method, we define FE = Energy Fraction as the fractional energy deposited in the final layer of the PWO calorimeter. We also define the energy weighted spread of the shwer. Starting from the shower axis (determined in the IMC and TASC), the width of the shower core is determined and weighted by the fractional energy deposit in each layer. These are then averaged for all of the layers in the calorimeter.
Using these parameters, the p-e separation has been studied at trans-TeV energies in a detailed Monte-Carlo evaluation. The results are shown in Figure 7. For this study 1.6 x 106 proton events (blue) were generated from an E-2.7 spectrum above 4 TeV. These are compared to 1.1 x 104 electron events generated at 4 TeV (red). There are only a few proton events (x) within the electron region. From a fuller analysis of these simulations there few protons can be eliminated by applying appropriate layer cuts, biving a proton rejection factor of ~105 for a retention of 95% of the electrons. This demonstrates that CALET will not be background limited in the trans-TeV energy regime.
In summary, CALET will measure:
Total Electrons: ~1 GeV to ~20 TeV (1-10 GeV for solar modulation studies)
Gamma Rays: ~10 GeV to ~10 TeV
Protons, Helium and Heavy Nuclei: ~50 GeV to 1000 TeV
Gamma Ray Bursts: 7 keV - 20 MeV (plus high energy associated photons in main telescope).
As a comparison, Table 1 shows the characteristics of existing and planned experiments that measure electrons. The first two columns are the balloon experiments; the next two are space experiments now returning data; and the final two columns give anticipated performance for AMS (to be launched in early 2011) and CALET (2015). In terms of energy range (?E) CALET has the largest reach, but in terms of exposure, Fermi is the clear leader. The critical parameters are the energy resolution and the e-p separation for which AMS and CALET have the best performance (AMS's e-p separation is 104 using the magnet/tracker, but can be as high as 106 if the TRD is factored in). The AMS energy range is based upon the expected MDR of the magnet and tracking system. Working to 80% of MDR still allows AMS to provide excellent data to just below a TeV, depending upon the background levels. For the higher energy measurements, only CALET can provide the data.