7. Hard X-ray Detector

Figure 7.1: The Hard X-ray Detector before installation.

The Hard X-ray Detector (HXD; see Figure 7.1) is a non-imaging, collimated hard X-ray scintillating instrument sensitive in the $\sim 10$ keV to $\sim 600$ keV band. 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 Suzaku observatory to the highest feasible energies, thus allowing broad-band studies of celestial objects.

Figure 7.2: 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. 7.2. The HXD features an effective area of $\sim 160$ cm$^{2}$ at 20 keV, and $\sim 260$ cm$^{2}$ at 100 keV; see Fig. 2.5). 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.

7.1 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 BGO crystals are also placed underneath of the GSO sensors, and thus each well is a five-sided anti-coincidence system. The effective thickness of the BGO active shield is thus about 6 cm for any direction from the PIN and GSO, except to the pointing direction.

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.

7.2 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 $\sim 60$ 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 four PIN diodes and the PM tube comprises one unit in view of the signal processing.

7.3 HXD field of view

The field of view of the HXD changes with incoming energy. Below $\sim 100$ keV, the passive fine collimators define a $34' \times 34'$ FWHM square opening. The narrow field of view compared to Beppo-SAX-PDS and RXTE-HEXTE experiments is one of the key issues with HXD observations. Above $\sim 100$ keV, the fine collimators become transparent and the BGO active collimator defines a 4.5 $^{\rm o}\times$ 4.5 $^{\rm o}$ FWHM square opening. In summary, all the PIN energy range and the lower quarter of the GSO range has a field of view of $34'$, while the GSO events above $\sim 100$ keV have wider field of view, up to 4.5$^{\rm o}$.

7.4 HXD Background and Sensitivity

Figure 7.3: Example of the observed HXD background on orbit. Plots normalized both with effective (solid) and geometric (dashed) area are presented.

Figure 7.4: [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 can be reproduced through modeling, without requiring separate off-source observations. The HXD has been designed to achieve an extremely low in-orbit background ( $\sim 10^{\rm -4}$ c  s$^{\rm -1}$ cm$^{\rm -2}$ keV$^{\rm -1}$), 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 become strongly radio-activated under in-orbit particle bombardment. Finally, (5) the narrow field of view below $\sim 100$ keV defined by the fine collimator effectively reduces both the CXB contribution and the source confusion.

The HXD detector background measured in orbit is plotted in Fig. 7.3. Figure 7.4 [Left] illustrates the sensitivity of the detector for the measurement of the continuum, while Fig. 7.4 [Right] gives the sensitivity to line emission. The HXD background is currently known to correlate with cut-off-rigidity and time-after-SAA, and at least in the GSO band a small growing effect due to radio isotope buildup is detected. In modeling the background, all these effects must be taken into account. 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. For one-day averaged background, current (Oct. 2005) estimates of this error are about 5% (at 90 % confidence) for the PIN and about 10% for the GSO. Because the background estimation is in some sense a difficult science, the modeling accuracy will be supported on "best effort" basis. 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 Suzaku websites listed in Appendix C. The background estimation systematics are expected to be less than 3% and 5% within a year for the PIN and the GSO, respectively. The final goal is to achieve 1% and 3%, respectively.

For sources fainter than the background estimation error, background subtraction has to be performed carefully. Typical source flux level to be handled with care, based on current understandings of background estimation error, is listed in table 7.1. In general, for analysis in the energy range up to 40 keV, $\sim 1$ mCrab is the level to take special care, and for those up to 150 keV, it is $\sim 100$ mCrab. Above 200 keV, any observation will be strongly affected by background and how it is understood. Please refer to the website at Appendix C for updated information.

Table 7.1: Typical limiting source flux due to the background estimation error.

sys. err	10-30 keV	30-90 keV	90-270 keV	270-600 keV
5%	8*	80	480	4800
3%	5	48	300	3000
1%	1.6	16	100	1000

*flux in $10^{-12}$ erg s$^{-1}$ cm$^{-2}$.
Below 100 keV, 1 mCrab is $\sim 16 \times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$ for a band width of a factor of 3.

7.5 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. With progress of background modeling, these background files will be updated for all existing observations to date. Note that currently there are no user-specified parameters for the HXD.

7.6 The Anti-coincidence counters as a Wide-band All-sky Monitor (WAM)

Tight active shielding of HXD results in a large arrays of guard counters surrounding the main detector parts. These anti coincidence counters, made of $\sim 4$ cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. With limited angular ( $\sim 5^{\rm o}$) and energy ($\sim 30$% at 662 keV) resolutions, they works as a Wide-band All-sky Monitor (WAM).

Analog signals from normally four counters in each side of HXD sensor are summed up and a pulse height histogram is recorded each second. If a transient event such as gamma-ray burst (GRB) is detected, four energy band light curve with finer (31.25 ms) timing resolution is also recorded. The energy coverage of WAM is from $\sim 50$ keV to $\sim 5$ MeV, and its effective area is $\sim 800$ cm$^2$ at 100 keV and 400 cm$^2$ at 1 MeV. These data are shared with PI and the HXD team; the PI can utilize the full set of WAM data. Because these transient events, especially GRBs, need spontaneous contribution to the community, the HXD team will make the analysis products, such as light curves and spectra, public as soon as possible.