The Silicon Array
The early collaboration of the Pisa/Siena group with CALET resulted in a proposal to extend the electron measurements (one of the main science goals of the mission) to the hadronic sector with a dedicated particle identifier. A two-layered Silicon Array (SIA) was designed to be equipped with high resistivity pixelated silicon detectors (manufactured from 500μm thick 6" wafers). Each sensor, segmented into 64 square pixels (each with 1.28 cm2 sensitive area), was readout by dedicated low-noise, low-power, large dynamic range front-end electronics developed by the Italian group [more...]. The SIA allowed for high precision multiple dE/dx measurements for elements from proton to Fe and beyond. The development of the front-end ASIC, readout electronics and mechanical structure went along with a number of prototypes that were tested at CERN with relativistic ion beams, featuring an excellent linearity and charge resolution up and above Fe (Z=26) [more...].
The picture below shows a conceptual view of the building block of the array consisting of a pair of vertically aligned sensors readout by a single FE board for a total of 128 pixels.
 
SIA
 
A conceptual drawing of the lower layer of the Silicon Array is shown in the next picture. The geometry of the upper layer is such that the two layers together form a seamless arrangement of detectors that completely eliminates dead areas.
 
SIA-layer
 
The Charge Detector
Later on, with the downscaling of CALET from a 0.75 m2sr instrument to the present 0.12 m2sr, the Silicon Array was replaced by a more cost-effective Charge Detector (CHD): a two-layered hodoscope of scintillator paddles readout by photomultipliers. The Italian group contributed to the design and prototyping of the detector. A picture of an early detector concept is shown below.
 
CHD-IC-concept
 
CHD prototypes were built in Italy and in Japan and tested at CERN and HIMAC.  The picture below shows an EM prototype (white arrow) at CERN-SPS H8 beam line. 
 
CHD-IC-beam-test
 
The Beam Tracker  
In order to determine the charge resolution of the CHD, a dedicated Beam Tracker was built by the group of Pisa/Siena, to allow for a precise reconstruction of the beam track and a very accurate identification of the charge of the incoming nucleus by means of 14 independent measurements of dE/dx. The Beam Tracker was subdivided into two sections: the Upper Tracker (UT) with 4 layers of Si strip detectors and 4 layers of Si pixel arrays, and the Bottom Tracker (BT) with 6 layers of Si strip detectors. The track coordinates were provided by silicon strip sensors of the same thickness with 183 μm-pitch strips ganged in parallel, four by four. The detectors were positioned with the strips perpendicular to the beam and alternately oriented in orthogonal directions. The FE and readout electronics were derived from the modules developed for the SIA.
 
Tracker-beam-test
 
Measurements with prototypes of the CHD scintillators were performed in a dedicated beam test that took place at CERN in January 2013. Relativistic ions were extracted as secondary products from the interactions of a primary Pb beam of the SPS impinging on an internal Be target. Fully ionized nuclear fragments with A/Z= 2, ranging from deuterium to heavy nuclei with atomic number Z> 26, were steered along the H8 beam line of the SPS, where the CALET test apparatus was preceded by the Beam Tracker. More than 15 million triggers were collected in two sets of runs with beam energies of 13 and 30 GeV/amu, respectively.
CHD-IC-tagging
 
Example of charge tagging with multiple dE/dx measurements by the Beam Tracker in a 30 GeV/amu fragmented ion beam with A/Z=2 for elements with 1 ≤ Z ≤ 27 at CERN-SPS.
 
The HV System of CALET
All crystals of the TASC calorimeter are readout by APD/PD photosensors except the ones in the top layer that are equipped with photomultipliers and used by the trigger system. The IMC calorimeter is read-out by multi-anode PMTs. The High Voltage System (HVPS) of CALET (picture below) has been developed in Italy under the coordination of IFAC/CNR (Florence) and the flight hardware has been delivered to JAXA by ASI and commissioned under the supervision of the same group. The HV system for the CGBM has also been provided by ASI.
 
HV-system

Photomultipliers (PMTs)  HV

- 80 channels (redundant DC/DC with linear regulator stage)

- range: 0 to -870 V with 10-bit resolution (minimum step 850 mV)

- 30 outputs @500 μA, 50 outputs @300 μA

 

Avalance Photodiodes (APDs)  HV

- 22 channels (redundant DC/DC stage)

- range: 0 to -450 V with 10-bit resolution (minimum step 440 mV)

- ripple voltage ≈ 150 mVpp at maximum current for all channels