CALET is an all-calorimetric instrument preceded by a particle identification system. The energy measurement relies on two independent calorimetric sections: a fine grained pre-shower, known as IMaging Calorimeter (IMC) is followed by a Total Absorption Calorimeter (TASC) for a total thickness (at normal incidence) of 30 radiation length (X0) and about 1.5 proton interaction length. The effective geometrical factor of CALET for high energy electrons is ≃1,200 cm2 sr.  The total weight is approximately 650 kg.





The charge of the incoming cosmic ray is measured, via the Z2 dependence of specific ionization loss, in a double layered, segmented, plastic scintillator array, known as Charge Detector (CHD), positioned above the IMC.  Both layers are segmented into 14 scintillator paddles, each scintillator (32mm wide, 450mm long and 10mm thick) being readout individually by a PMT. The system can operate over a large dynamic range for charges from Z=1 to Z=40 with sufficient charge resolution to resolve individual elements.



The IMC is able to image the early development of the shower thanks to its fine granularity, provided by 1 mm2 scintillating fibers (SciFi) readout individually. It consists of 7 layers of tungsten plates, each sandwiched by 2 layers of scintillating fibers with square cross section, arranged in belts along the x and y directions and is capped by an additional pair of orthogonal SciFi layers. The readout of the SciFi layers consists of multianode photomultiplier tubes (MAPMT) with 64 anodes. The transverse dimensions of the IMC are approximately 45 cm by 45 cm and its total thickness is equivalent to 3 X0. The first 5 tungsten-SciFi layers sample the shower every 0.2 X0 while the following 2 layers provide 1.0 X0 sampling. The IMC is designed to: (i) reconstruct the incident particle trajectory; (ii) separate the incident track from backscattered particles; (iii) determine the starting point of the shower; (iv) provide a redundant measurement of the particle’s charge via multiple dE/dx sampling.






The TASC is a homogeneous calorimeter equipped with 192 Lead Tungstate (PWO) ”logs” arranged in 12 layers.  Each crystal has dimensions 20mm(H) x 19mm(W) x 326mm(L). The top PWO layer is readout by PMTs and a dual photodiode / avalanche-photodiode package (PD/APD) is used for the readout of the remaining layers (16 logs per layer). Alternate layers are oriented perpendicular to each other to provide x,y coordinates of the shower core. The total area of the TASC is about 1,024 cm2 and the total thickness is close to 27 X0 at normal incidence.


The excellent proton rejection capability of the instrument (better than 105) takes advantage of a detailed imaging of the first 3 radiation lenghts and of the full containement of e.m. showers up to energies corresponding to the high end of the electron spectrum (in the TeV region). The imaging power of the IMC allows also to identify photons not interacting in the first layers while generating an e.m. shower in the downstream layers. This is shown as an example in the  picture below, where MC simulated showers induced (from left to right) by one 10 GeV gamma-ray, 1 TeV electron and 10 TeV proton are visualized.



Charged particles and gamma rays > 10 GeV will be triggered above a 15 MIP threshold from the sum of the last dynodes of the MAPMTs of the last layer of the IMC and a 55 MIP threshold from the sum of the PMTs of the first layer of the TASC. Electrons in the 1 GeV - 10 GeV range will be observed only for a limited exposure by reducing the IMC trigger threshold. The trigger rate above 10 GeV is estimated around 13 Hz.