Low Energy Telescope and Si Detector
The Low Energy Telescope (LET) consists of 32 sub-telescopes, each with 16x16 deg non-overlapping fully coded field of view. LET employ tungsten masks, a 2x5 array of Si detector modules, and panels of graded-Z (Pb/Sn/Cu) passive shield. For the key parameters of LET, see the table in the HET section.
Similar to CZT detector arrays in HET, in order to build a large arrays of Si detectors for EXIST, we take the following modularization scheme. First, we start with 2x2x0.1 cm2 Si crystal bonded onto an 128x128 readout ASIC. This basic building block is called a Detector-Crystal-Unit (DCU). Second, we combine a 2x5 array of DCUs into a Detector-Crystal-Array (DCA), which is 4x10 cm2. Third, we put 5 DCAs into a Detector Module (DM), which is 10x20 cm2. Then, 2x5 array of DMs make a sub-telescope detector plane (20x20 cm2). Finally 32 sub-telescopes complete 1.3 m2 of Si detectors.
Modularization Scheme for Si detectors in EXIST.
In order to cover the wide energy band from 3 to 30 keV, we need relatively thick (1-1.5 mm) Si detectors. The baseline approach is hybrid detectors, where Si and readout part are separated. Hybrid detectors use CMOS readout chips similar to those in IR cameras, but modified for detection of single X-ray photons. Prototype silicon sensors (1 mm thick) are fabricated. Transstor level pixel design and simulation has been completed at U. Iowa.
Schematic of Hybrid detector for LET
The following picture shows 32x32 array of 160 µm pixels on 1 mm thick Si sensor, fabricated by Oxford Instruments for U. Iowa. The figure on the right shows the energy resolution obtained at 5.9 keV (200 eV FWHM), with discrete preamplifier.
1 mm thick Si sensor and spectral resolution (200 eV)