8. Hard X-ray Detector

Figure 8.1: The Hard X-ray Detector before installation.

The Hard X-ray Detector (HXD; see Figure 8.1) is a non-imaging, collimated hard X-ray scintillating instrument sensitive in the band of $\sim 10$ keV to $\sim 600$ keV.

It has been developed jointly by the University of Tokyo, Aoyama Gakuin University, Hiroshima University, ISAS/JAXA, Kanazawa University, Osaka University, Saitama University, SLAC, and RIKEN. Its main purpose is to extend the bandpass of the Astro-E2 observatory to the highest feasible energies, thus allowing broad-band studies of celestial objects.

Figure 8.2: Schematic picture of the HXD instrument, which consists of two types of detectors: the PIN diodes located in the front of the GSO scintillator, and the scintillator itself.

The HXD sensor (HXD-S) is a compound-eye detector instrument, consisting of 16 main detectors (arranged as a 4 $\times$ 4 array) and the surrounding 20 crystal scintillators for active shielding. Each unit actually consists of two types of detectors: a GSO/BGO phoswich counter, and 2 mm-thick PIN silicon diodes located inside the well, but in front of the GSO scintillator. The PIN diodes are mainly sensitive below $\sim 60$ keV, while the GSO/BGO phoswich counter (scintillator) is sensitive above $\sim 30$ keV. The scintillator signals are read out by photomultiplier tubes. The schematic drawing of the HXD is given in Fig. 8.2. The HXD features an effective area of $\sim 160$ cm$^{2}$ at 20 keV, and $\sim 330$ cm$^{2}$ at 100 keV (see Fig. 8.2). The energy resolution is $\sim$ 3.0 keV (FWHM) for the PIN diodes, and $7.6 / \sqrt{E}$ % (FWHM) for the scintillators where $E$ is energy in MeV. The HXD time resolution is 61 $\mu$s.

8.1 GSO/BGO Counter Units

Each main detector unit is of a well-type design with active anti-coincidence shields. The shields and the coarse collimator itself are made of Bismuth Germanate (BGO; Bi$_{4}$Ge$_{3}$O$_{12}$) crystals, while the X-ray sensing material ``inside the well'' is GSO (Gadolinium Silicate, or Gd$_{2}$SiO$_{5}$(Ce)) crystal. The aspect ratio of the coarse collimators yields an acceptance angle for the GSO of 4.5$^{\rm o}$ (FWHM). Each unit thus forms a 2 $\times$ 2 matrix, containing four 24 mm $\times$ 24 mm, 5 mm thick GSO crystals, each placed behind the PIN diode. The reason for the choice of the two different crystals for the sensor and the shield is dictated by the large stopping ability of both, yet the very different rise/decay times, of $\sim 706$ ns for BGO, and $\sim 122$ ns for GSO, at a working temperature of $-20^{\rm o}$C. This allows for an easy discrimination of the shield vs. X-ray sensor signals, where a single PM tube can discriminate between the two types of scintillators in which an event may have occurred. Any particle events or Compton events that are registered by both the BGO and GSO can be rejected by this phoswich technique, utilizing custom-made pulse-shaping LSI circuits. The BGO crystals are also placed underneath of the GSO sensors, and thus each well is a five-sided anti-coincidence system.

8.2 PIN Diodes

The low energy response of the HXD is provided by 2 mm thick PIN silicon diodes, placed in front of each GSO crystal. The diodes absorb X-rays with energies below $\sim 40$ keV, but gradually become transparent to harder X-rays, which in turn reach and are registered by the GSO detectors. The X-rays are photoelectrically absorbed in the PIN diodes, and the signal is amplified, converted to a digital form, and read out by the associated electronics. The PIN diodes are of course also shielded from particle events by the BGO shields, as they are placed inside the deep BGO wells. The field of view of the PIN diodes is collimated to $34' \times 34'$ FWHM by the passive fine collimators. Above $\sim 100$ keV, the GSO field of view approaches the actively collimated 4.5$^{\rm o}$ (FWHM), because the fine collimators become gradually transparent.

8.3 HXD Background, Background Rejection, and its Sensitivity

Figure 8.3: The HXD background, including both intrinsic and cosmic components for both the PIN and GSO detectors.

Figure 8.4: [Left] The sensitivity of the HXD to continuum emission, taking into account the expected background. [Right] Same, for line emission.

Although the HXD is a non-imaging instrument, its instantaneous background may be reproduced through modeling, without requiring separate off-source observations. This is because the HXD has been designed to achieve an extremely low in-orbit background ( $< 1 \times 10^{-4}$ c s$^{-1}$ cm$^{-2}$ keV$^{-1}$), based on a combination of novel techniques: (1) the five-sided tight BGO shielding as mentioned above; (2) the use of the 20 shielding counters made of thick BGO crystals which surround the 16 main GSO/BGO counters; (3) sophisticated onboard signal processing and onboard event selection, employing both high-speed parallel hardware circuits in the Analog Electronics, and CPU-based signal handling in the Digital Electronics; and (4) the careful choice of materials that do not get strongly radio-activated under in-orbit particle bombardment. Finally, the fine collimator effectively reduces both the CXB contribution and the source confusion.

The current best estimate of the detector background is plotted in Fig. 8.3; this plot includes the intrinsic background component as measured on the ground, and the component expected due to detector activation. Figure 8.4[Left] illustrates the sensitivity of the detector for the measurement of the continuum, while Fig. 8.4[Right] gives the sensitivity to line emission. As is the case for every non-imaging instrument (and in particular, for those sensitive in the hard X-ray range), the limiting factor for the sensitivity of the HXD will be the error in estimation of background. Current estimates of this error are about 10% at 1 $\sigma$, and the final goal is about 5%. For sources fainter than the background estimation error, background subtraction has to be performed carefully. Since this is the first space flight of the HXD-type detector, the details will depend on the experience with in-orbit data, and the status of background estimation error and procedures for background subtraction will be presented on the Astro-E2 websites. For any pre-flight estimates, the background files are available at the Astro-E2 proposal websites listed in Appendix C.

8.4 Data analysis procedure

HXD data are accumulated by event by event basis. After on-board data selection, event data are further screened by the ground pipe-line analysis process. By referring to the trigger and flag information (including the inter-unit anti-coincidence hit patterns), the pipe-line assigns specific grades to the HXD events such as pure PIN events and pure GSO events. Detector responses and background files that match the particular grade of the events will be provided by the HXD team. Note that currently there are no user-specified parameters for the HXD.