Instruments

Low energy coverage will be provided by Silicon Drift Detectors (SDDs) with an active thickness of 450mm to cover the low energy range from 1-30 keV. We use SDD modules with 1.5 cm² collecting area, and a thin Beryllium window to block lower energy photons. Low electronic noise is achieved by means of a built-in thermoelectric cooler. These devices have an energy resolution of 2.5% (at 5.9 keV) and a timing resolution of 10μs. These modules have been used in the APXS and XSM payloads on Chandrayaan-2 (Shanmugam et al. 2019; Vadawale et al. 2014).

Each Daksha LE package consists of five SDDs, giving a physical area of 7.5 cm². The sun is very bright at low energies, and direct sunlight shining on a detector can swamp the readout, washing out any other astrophysical signal. Hence, no Low Energy Detectors are placed on the sun-ward side of Daksha.

The workhorse for Daksha are sensitive pixelated Cadmium Zinc Telluride (CZT) detectors. We use 5 mm thick detectors manufactured by GE Medical, sensitive to an energy range from 20 keV to 200 keV with an energy resolution of 12% at 60 keV. Each 3.9 cm x 3.9 cm detector is divided into a 16x16 pixel array, and integrated electronics provide a digital readout. An ME Box would consist of 20 such detectors. These detectors have been used before in several space experiments including RT2 (Nandi et al. 2009), High Energy X-ray Spectrometer on Chandrayaan-1 (Vadawale et al. 2009), and CZT Imager on AstroSat (Rao et al. 2017; Bhalerao et al. 2017) where they have proven to be space-worthy and highly effective for this energy range.

At high energies, the coverage of CZT detectors is complemented by Thallium activated Sodium Iodide scintillators, NaI(Tl). Our configuration for these high energy (HE) detectors consists of a 20cm x 20cm x 2cm NaI(Tl) crystal, sensitive to photons from 100 keV to >1 MeV. The detector works on a sparse array of Si-PM with uniform spatial sensitivity operating on low voltage (40 V). This arrangement ensures uniform light collection, and also gives high position sensitivity with spatial resolution of ~cm. The shell, covered with LE and ME packages, will be largely transparent to higher energy photons. Hence, we place the HE scintillators on the base plate. We opt for four HE detector packages mounted parallel to the four inclined square faces of the shell. All the power electronics and processing electronics will be mounted on the base plate.