2.2.1.

Overview

As shown in Fig. 3, HXI-S consists of the HXI camera part and the active shield part. The former includes four layers of DSSDs and a CdTe-DSD imagers, whereas the latter is composed of nine active shield units made of large ${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ (BGO) scintillator crystals individually coupled to avalanche photo diodes (APD),¹³ and the housing structure made of carbon-fiber-reinforced plastic (CFRP) to mechanically hold them. The DSSD and APD are purchased from Hamamatsu Photonics, the CdTe-DSD is from Acrorad, and the BGO is from Nikolaev Institute of Inorganic Chemistry.

Fig. 3

Overview of HXI-S, composed of a camera, nine active shields, and the housing structure, mounted on the base plate.

Right above the imager at the edge of the FoV, a calibration source is located, which is made of an ${}^{241}\mathrm{Am}$-doped plastic scintillator coupled to an APD. Another APD without a scintillator is located near the ${}^{241}\mathrm{Am}$ source, to monitor the cosmic-ray (CR) flux within the active shield. CSA and high-voltage (HV) supplies for the APD are placed in two APD-CSA boxes attached to the housing. An APD-CSA box covers seven channels (four side and one bottom BGO units, and the ${}^{241}\mathrm{Am}$ and the CR-monitor APDs) and the other covers four channels (four top BGO units). Another box containing two HV suppliers, one for the DSSDs and the other for the CdTe-DSD, called the HV box is also attached. All the HV power supplies are provided from a company SITAEL S.P.A.

HXI-S has a weight of 41.8 kg and a height of 65 cm (see Fig. 3). Structurally, the HXI camera, the bottom BGO unit, and the CFRP housing holding the other eight BGO units are mounted on the base plate made of Al-alloy. In other words, the three components are mechanically and thermally almost isolated to make the development work easy.

Because the HXI utilizes semiconductor imagers (DSSD and CdTe-DSD) and APD, the HXI cold plate is cooled down to $-{25}^{°}\mathrm{C}$ in orbit and controlled within $\pm 1^{°}\hspace{0.17em}\mathrm{C}$ using attached heaters. Even though the temperature at the bottom of the HXI-S is thus controlled well, it is exposed to the Sun when the satellite is in the day side, and the heat transfer along the housing structure causes internal temperature gradients. It can be a problem especially in the top BGO units, the most distant from the cold plate because gain of the APDs is sensitive to temperature (typically $-3\%/°\mathrm{C}$ gain variation). To relax this issue, graphite sheets are taped on the surface of the CFRP housing to enhance its thermal conductivity. Outside the HXI-S housing, there are 14 CFRP plates almost completely covering HXI-S to reduce thermal coupling from outside. The plates are held with arms made of glass-fiber-reinforced plastic that has low thermal conductivity. Both inside and outside of the plates are covered with multilayer insulators (MLI) to suppress the radiation heat transfer. Outermost layer of the outside MLI is made of an Ag back-coated teflon sheet to effectively reflect the sunlight as well as to radiate the heat into the sky. Overall, all the APD and camera temperature were controlled within $\pm 2°\mathrm{C}$ in orbit, consistent with preflight estimation. Careful thermal design was also performed in the camera part, to keep the imagers’ temperature lower than $-15°\mathrm{C}$ when the HXI cold plate is controlled at $-25°\mathrm{C}$, see Noda et al.¹⁴ for detail. There is a 15.5-cm-tall baffle structure at the top of HXI-S, which is internally covered with 0.3-mm Pb sheet to stop the CXB coming directly (not through the HXT FoV) into the imager. The Pb sheet was originally designed to be plated on the outside surface of the baffle for easy manufacturing. Monte Carlo simulation performed in the design phase showed that CXB photons coming from the opening window and reaching the inside of the baffle structure are scattered off by the internal CFRP, which is mostly made of elements lighter than carbon, and become a major contributor to the imager background. Thus, the Pb sheet was relocated inside.

The opening of the CXB baffle is the optical axis of the instrument and is covered with the entrance window made of two layers of $30\text{-}\mu \mathrm{m}$-thick poly-carbonate sheets, with Al sputtered to the both sides (i.e., four layers of Al in total). The thickness of an Al layer is designed to be 150-nm each. As the window is directly illuminated with sunlight, the light leakage was carefully measured. Originally, it was a single-layer sheet with both sides of Al sputtered, but we found the leak is not negligible, and thus decided to make it double. After the acoustic shock and vibration tests, as well as the light leak test, the double sheet design was finally adopted.

2.2.2.

Camera part

Design of the camera part is roughly shown in the left panel of Fig. 3 and well reported in Sato et al.¹⁵ The top four layers, namely layers 0, 1, 2, and 3, are made of 0.5-mm-thick DSSDs covering from 5 to about 30 keV, and the last one, layer 4, is made up of 0.75-mm-thick CdTe-DSD covering from about 30 to 80 keV. Both detectors have $250\text{-}\mu \mathrm{m}$ pitch strips in both surfaces, orthogonally oriented to provide two-dimensional position sensitivity. Each side has 128 strips, covering $32\text{-}\times 32\text{-}{\mathrm{mm}}^2$ area. Each strip electrode of the DSSD, in both sides, has a width of $150\mu \mathrm{m}$ with a $100\text{-}\mu \mathrm{m}$-wide gap. Some fraction of photons absorbed in the gap is detected as a “split event,” with signals recorded in two adjacent strips (see Hagino et al.¹¹ for detail). The strip electrodes of the CdTe-DSD have a width of $230\mu \mathrm{m}$ with a $20\text{-}\mu \mathrm{m}$-wide gap in both sides. The die sizes of both detectors are $34.0\times 34.0{\mathrm{mm}}^2$. The top side (toward the entrance window) of a DSSD is the $p$-electrode biased to the ground level, and the bottom side is the $n$-electrode biased to $\sim +250\mathrm{V}$. Similarly, the top side of a CdTe-DSD is the Pt-electrode, and the other side is the Al-electrode biased to $+250$ to 350 V.

The DSSDs and the CdTe-DSD are mounted on $100\text{-}\times 100\text{-}{\mathrm{mm}}^2$ trays, stacked in 4-mm distance individually. Focal point of the HXT is set at the top layer DSSD (layer 0), located $\sim 16.5\mathrm{mm}$ above the CdTe-DSD. The 12-m-long focal length and 45-cm diameter of the HXT make the defocusing effect in the last CdTe layer $<0^{\prime }.17$, which is small compared with the $1^{\prime }.7\mathrm{HPD}$.

Each tray is electrically divided into two parts, for top- and bottom-side readout. Each side has four 32-ch low-noise analog ASICs with ADC. The ASIC is named VATA-HXI and was developed in collaboration with IDEAS.⁷ ASIC inputs are directly connected to the detector strip electrodes using either wire bonding (DSSD) or combining wire bonding and bump bonding (CdTe-DSD). The bias voltage is applied by floating the common level of the whole circuits handling $n$-side (or Al-side) strips by $+250$ to 350 V. All digital signals are decoupled using magnetic couplers. The power-lines are also generated via isolated DC/DC converters.

Logical “or” of the trigger outputs of four ASICs in $p$- and $n$-sides of each tray individually are sent to HXI-DPU that sends back sample-and-hold control signal and ADC control signals after specified time intervals. The control signals are shared within all five trays (=10 sides) to synchronize the data acquisition activities. All 1280-ch analog outputs are converted simultaneously by Wilkinson-type ADCs following the individual triggers, whereas only channels with value exceeding the predefined digital value “DTHR” are read out (so-called, a sparse data scan). In addition, the median value (16th highest among the 32 channels each) of the corresponding ASIC is attached to be used to monitor the common mode noise, which is subtracted from individual signals in the on-ground analysis procedure. This approach is optimized for low-noise readout and not for high speed. Nonetheless, the system still is capable to handle trigger rate largely exceeding 1000 Hz, meeting the requirement to observe a source “brighter than the Crab nebula.”

The five-layer imager design is adopted to provide “depth sensing” information to further reduce the detector background.¹⁶ For example, photons with energy below $\sim 15\mathrm{keV}$ are almost completely absorbed in layer 0 (the top layer DSSD), and any signals generated in the other four layers with energy below $\sim 15\mathrm{keV}$ are background. Therefore, selecting only layer 0 for the lowest energy end is the best approach to achieve high sensitivity. In contrast, above $\sim 30\mathrm{keV}$, most of the photons penetrate all the four layers of DSSDs and are absorbed in the CdTe-DSD, and thus we need to use it. The energy band of each layer is optimized after obtaining the in-orbit background data, as reported in Sec. 3.2 and also in Hagino et al.¹¹

The signal information of x-ray photons is recorded event by event. The data packet is composed of the header part with trigger time, event sequence number, trigger patterns, flags (including those from active shields), and other miscellaneous information, followed by the ASIC part containing information from selected ADCs (including the channel IDs and the common mode noise level). In normal operation, only triggers from p-side of DSSDs and Pt-side of CdTe-DSD are utilized. A typical x-ray event has two signals from one imager, one in the $p$-side (or Pt-side) and the other in the $n$-side (or Al-side), representing $Y$- and $X$-positions in raw detector coordinate (RAWY and $X$), respectively. Compton scattering, interstrip charge sharing, CR penetration, and other secondary interactions within the detector generate more signals. Pulse heights of both sides are used to identify position and qualify the validity of the event (i.e., the two values shall be the same). However, for spectroscopy we only use the pulse height of the $p$-side in the DSSDs because that of $n$-side has larger noise level caused by the difference in the strip electrode implementation. In CdTe-DSD, slow mobility and short lifetime of the holes degrade the Pt-side spectra, and therefore we use those obtained from the Al-side.

Timing information of each event has a resolution of $25.6\mu \mathrm{s}$. Design and verification of the time-tagging system, not only for the HXI but also including the whole satellite mission systems are summarized in a separate paper.¹⁷

2.2.3.

Active-shield and related parts

The background shielding of the HXI combines both passive and active measures. First of all, the flux of the CXB and the Earth albedo gamma-rays (above $\sim 200\mathrm{keV}$) are strong and shall be baffled as much as possible. CR particles both trapped ($E\sim 100\mathrm{MeV}$) in the South Atlantic anomaly (SAA) and directly coming from the outer space ($E\gtrsim 4\mathrm{GeV}$) generate signals in the imager not only when they directly hit it but also from secondary particles and x-rays generated in the baffling/shielding structures. Both prompt and delayed (activation) signals are generated from these particles. The well-shaped active shield design adopted in the HXI is based on the successful heritage from Suzaku HXD,^8,18 to suppress background signals from all these sources.

The active shield made of BGO crystals is forming a well-like shape and is designed to stop or tag gamma-rays (e.g., by identifying Compton scattered events) and CR particles from almost $4\pi$ direction. In Fig. 4, the geometry of the nine BGO units is presented. There are four top BGO units (ID 1 to 4 in the telemetry data), one bottom unit [ID 7 (ID 5 and 6 are used for the ${}^{241}\mathrm{Am}$ source and the CR-monitor.)] and four side units (ID 8 to 11), respectively. Each unit has a typical thickness of $\sim 3\mathrm{cm}$ to passively stop protons with energy $<\sim 100\mathrm{MeV}$, mostly in the SAA, so that the semiconductor imagers, especially CdTe-DSD, experience less activation. The thickness is also important to provide good interaction probability for activation-induced gamma rays, in particular 511-keV photons from $\beta +$ decays.

Fig. 4

BGO units and mechanical structures of HXI-S.

Each BGO scintillator is glued by an elastic adhesive to a dedicated fiber-reinforced plastic (FRP) plate (using both carbon and glass fibers) through a white paint made of ${\mathrm{BaSO}}_4$ reflector. The FRP plate is specifically designed to have a thermal expansion rate similar to that of the BGO. The other five surfaces of the scintillator are covered with reflector sheets, the enhanced specular reflector (ESR) from 3M, and a GoreTex sheet. The APD is glued on the BGO’s smallest-area surface to keep the light accumulation efficiency as high as possible¹⁹ in the top and side units. As the bottom BGO is almost cubic, the APD is glued on the top surface for simplicity. Each BGO unit is fixed to the CFRP housing via the mechanical interface of the FRP plate. Small 3- to 4-mm gaps between the BGO crystals are unavoidable for manufacturing. Orientation of these gaps is carefully designed to avoid direct leak of the CXB into the imagers, but the Monte Carlo simulation revealed that the CXB photons scattered within HXI-S will result in non-negligible increase in detector background. Thus, we covered those gaps by taping 0.3-mm-thick Pb sheets on the housing structure.

Each APD has an entrance window with a size of $10\times 10{\mathrm{mm}}^2$, optically connected to the BGO crystal via a silicone adhesive. Signals from the 11 APDs are fed into the two APD-CSA boxes, which are also providing the bias voltage of the APDs. Outputs from the APD-CSA boxes are processed by the APD processing and management unit (APMU) in the HXI-AE box. Detail of the readout electronics is summarized in Ohno et al.²⁰ The APMU output is connected to HXI-DPU and provides four signal lines to the camera control logic. One line is used to provide a signal within $5\mu \mathrm{s}$ from the detection of the BGO signal, which can either be used to veto AD conversion of the camera to reduce dead time or just be recorded as “fast BGO” flag in the imager event data. Another one tags high-energy deposit events (upper discriminators) representing the time interval in which the CSA of the APD is saturated (the second bit of the “fast BGO” flag). The third one is similar to the “fast BGO” flag, but providing the signal 37 to $42\mu \mathrm{s}$ later, using sophisticated filter with much lower energy threshold and is recorded as “HITPAT BGO” flags in the event data.

The final (fourth) line is used for the ${}^{241}\mathrm{Am}$ source. Output signals in the APD, originating from $\alpha$ particles from ${}^{241}\mathrm{Am}$ captured in the plastic scintillator are sent to the camera control logic and recorded as “${}^{241}\mathrm{Am}$” flag, as the second bit of the “HITPAT BGO” flags. Long-term stability of the imager gain is monitored using the x-ray and gamma-ray lines from this isotope. The ${}^{241}\mathrm{Am}$ source is provided by CEA-Sakley and has a decay rate as low as $\sim 4\mathrm{Bq}$, which is designed to provide better than 1% statistical accuracy of gain monitoring in individual strips every 14 days. As the decay rate is low, the HXI sensitivity will not significantly degrade even if the APD does not work properly in orbit.

The APMU is counting the trigger rate and accumulating the spectra of all 11 APDs; the nine BGO units, the ${}^{241}\mathrm{Am}$ source, and the CR-monitor. Because of the limit in the satellite data recorder size, the APD spectra are recorded only 10 min after every SAA passage, and used to calibrate the APD signal gains.