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 $\sim 10$ 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 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 $\sim 40$ keV. The scintillator signals are read out by photomultiplier tubes (PMTs). 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 $\sim 260$ cm$^{2}$ at 100 keV; (see Fig. 3.5). The energy resolution is $\sim$ 4.0 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 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 for 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 $\sim 700$ ns for BGO, and $\sim 120$ 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 PMT 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.

8.2 PIN-Si 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 geometrical area of the diode is 21.5 $\times$ 21.5 mm$^2$, while the effective area is limited to be $\sim$16.5 $\times$ 16.5 mm$^2$ by the guard ring structure. The temperature of PIN diodes are controlled to be $-15 \pm3$ $^{\circ}$C to suppress electrical noises caused by the leakage current, and almost fully depleted by giving a bias voltage of $\sim$500 V ^8.1. The PIN diodes absorb X-rays with energies below $\sim 70$ keV, but gradually become transparent to harder X-rays, which 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 actively shielded from particle events by the BGO shields, as they are placed inside the deep BGO wells. In addition, to reduce the contamination by the cosmic X-ray background, passive shields called ``fine collimators'' are inserted in the well-type BGO collimator above the PIN diodes. The fine collimator is made of 50 $\mu$m thick phosphor bronze sheet, arranged to form a 8 $\times$ 8 square meshes of 3 mm wide and 300 mm long each.

With the version 2.0 data product release (on September 2007), the HXD-team introduced a new noise-cut threshold table to improve the effectively usable lower threshold. New response and NXB database are provided. The new table will improve the reproducibility of the NXB in the lower energy band, typically below $\sim 13$ keV, by efficiently discarding the noise events sometimes observed in a few PIN diodes. On the other hand, it sacrifices the apparent effective area below $\sim 12$ keV. In total, we believe that the energy-band effectively useable to observers will improve.

8.3 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 as shown in figure 8.3. 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$^{\rm o}$.

Figure 8.3: An angular response of single fine-collimator along the satellite X-axis, obtained from offset observations on the Crab nebula.

8.4 In-Orbit background of PIN and GSO

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.

Figure 8.4: [Left] A comparison of averaged non X-ray background spectra of PIN, measured during the first six months of the mission. The Crab spectrum scaled down with two orders of magnitude are shown together. [Right] The evolution of averaged GSO-NXB spectra during the first six months of the mission.

The non X-ray background (NXB) of PIN diodes, measured in orbit, is plotted in the left panel of Fig. 8.4. The average background counting rate summed over the 64 PIN diodes is $\sim$0.6 ct s$^{-1}$, which is roughly equal to a 10 mCrab intensity. In addition, almost no long-term growth has been observed in the PIN-NXB during the first year of Suzaku, thanks to the small activation effect of silicon. On the contrary, as shown in the right panel of Fig. 8.4, a significant long-term accumulation caused by the in-orbit activation has been observed in the GSO-NXB, especially at the early phase of the mission. The background spectrum of GSO contains several activation peaks, whose intensities exponentially increased with individual half-lives. Since the longest half-life is about one year, the GSO-NXB level can be treated as to be saturated in the AO-3 phase.

Figure 8.5: A comparison of the in-orbit detector background of PIN/GSO, averaged over 2005 August to 2006 March and normalized by individual effective areas, with those of RXTE-PCA, RXTE-HEXTE, and BeppoSAX-PDS. Dotted lines indicate 1 Crab, 100 mCrab, and 10 mCrab intensities.

Figure 8.5 illustrates the comparison between detector backgrounds of hard X-ray missions. The lowest background level per effective area is achived by the HXD at an energy range of 12-70 and 150-500 keV. The in-orbit sensitivity of the experiment can be roughly estimated by comparing the background level with celestial source intensities indicated by dotted lines. Below 30 keV, the level is smaller than 10 mCrab intensity, which means a sensitivity better than 0.5 mCrab can be obtained, if an accuracy of 5% is achieved in the background modeling.

Figure 8.6: [Left] A light curve of the non X-ray background of PIN, folded with the elapsed time after the SAA passage (top), and a plot of averaged cut-off rigidity at the corresponding position (bottom). [Right] The same folded light curves of the GSO background, in 40-90, 260-440, and 440-70 keV energy band.

Since the long-term variation of both PIN-NXB and GSO-NXB can be expected to be stable, the main uncertainties of the background come from temporal and spectral short-term variations. As shown in Fig 8.6, the PIN-NXB shows a significant short-term variability, with a peak-to-peak amplitude of factor 3, anti-correlating with the cut-off rigidity (COR) in the orbit. Since the COR affects the flux of incoming primary cosmic-ray particles, most of the PIN-NXB is considered to originate in the secondary emissions produced by the interactions between cosmic-ray particles and surrounding materials of the detector. When a cut condition of COR$>$6, a standard criteria in the pipe-line processing, is applied for the event extraction, the amplitude decreases to a factor $\sim$2. During this temporal variation of the PIN-NXB, its spectral shape slightly changes; larger deviations from the average are observed at the higher energy range. In case of the GSO-NXB, as shown in the right panel of FIg 8.6, the temporal variation differs at different energy bands. In the lower energy range, a rapid decline after the SAA passage clearly appears, in addition to the similar anti-correlation with the COR. Therefore in the background modeling, all these temporal and spectral behaviors have to be properly handled.

8.5 Background modeling and current reproducibility

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 is the reproducibility in the background estimation. Since this is the first space flight of the HXD-type detector, and the reproduction of the in-orbit background is not at all easy task, the modeling accuracy evolves with the experience of in-orbit data, and the latest status of the estimation error and procedures will be regularly presented on the Suzaku websites listed in Appendix B. For the AO-3 proposal preparation, the method, limitations, and reproducibilities (as a function of the time-scale and energy range) of the current background modeling are briefly described below. When preparing the AO-3 proposal, we recommend the authors to properly judge the error, which should be taken into account in your observation, based on the following infomation.

Since there is a strong anti-correlation between the PIN-NXB and COR, the background modeling of the PIN is primarily based on the counting rate of high-energy charged particles, directly measured by the PIN diodes. Due to large energy depositions inside the silicon, penetrations of cosmic-ray particles cause large signals in the corresponding PIN diodes, and hence activate the upper discriminator (UD) in the analog electronics, and then recorded as the PIN-UD monitor count in HK data. Therefore, the PIN-UD rate directly indicates the flux of primary cosmic-ray particles, and the background counting rate at any period can be estimated based on the corresponding PIN-UD rate.

In the actual modeling procedure, the PIN-NXB rate is described by a summation of raw PIN-UD rate and integrated PIN-UD rate with a fixed decaying time constant, to take into account the small effect of the activation during SAA passages. The spectral shape of the PIN-NXB is assumed to depend on the ``estimated'' background rate, and is extracted from a database of PIN-NXB spectrum at each estimated rate, which is compiled from the Earth occultation data.

As an example, comparisons between the real data obtained during the earth occultation of a 4-day long observation and the model prediction of PIN-NXB are shown in Fig. 8.7. In the figure, light-curves and residual distributions for 15-40 and 40-70 keV energy bands sorted by 1, 4, 8, 16, 32 ks and 1 day time-scales are independently presented. In the shorter time-scales, rather large statistical errors of individual time bin makes the reproducibility worse, while better accuracies are obtained at the longer time-scales. As another example, distributions of the residuals obtained from ten long observations (not only one 4-day long observation) divided by corresponding time-scales are shown in Fig. 8.8. Note that this distribution includes statistical spread. By analyzing these data typically with a time-scale of 10 ks the systematic reproducibility error is calculated to be $\sim 3.3$% ($1 \sigma$) in addition to the statistical error, in the energy band of 15-40 keV.

Figure 8.7: The comparisons between the real data and model predictions of the PIN-NXB. Upper figures show comparisons of the light curves and residuals, while lower figures show the distributions of the residual (red) and the error of them (black).

Figure 8.8: The same plots as the residual distributions of the PIN-NXB modeling shown in figure 8.7, but extracted from ten long observations.

In the case of GSO-NXB, its temporal and spectral behaviors are complicated than those of the PIN-NXB, since the characteristics of individual radio-active isotopes produced by the in-orbit activation is not negligible. The current modeling of the GSO-NXB utilizes the fittings of light curves in 32 energy bands, which are obtained in the earth occultation periods. In individual energy bands, normalizations of model components, namely, a cosmic-ray particle term calculated from the PIN-UD rate, and a few activation terms which show exponential decays with fixed time constants after the SAA passages, are determined. Then, the background rate at a certain band during the observation of celestial sources can be predicted by adopting the same ratios. Note that the spectral binning shall be fixed in advance to the analysis.

In Fig. 8.9, comparisons between the real data obtained during the earth occultation of a 4-day long observation and the model prediction of GSO-NXB are shown, individually for 50-100 and 100-200 keV energy bands and for 1, 4, 8, 16, 32 ks and 1 day time-scales. Even in the shorter time-scales, statistical errors of individual time bin are smaller than those of the PIN-NXB. The systematic residuals also tend to be small. In any case, above 200 keV, any observation will be strongly affected by the background and how well it is understood. The distributions of the residuals, obtained from ten long observations are shown in Fig. 8.10, as well.

Figure 8.9: The comparisons between the real data and model predictions of the GSO-NXB. Upper figures show comparisons of the light curves and residuals, while lower figures show the distributions of the residual (red) and the error of them (black).

Figure 8.10: The same plots as the residual distributions of the GSO-NXB modeling shown in figure 8.9, but extracted from ten long observations.

Figure 8.11: Calculated detection limit of the HXD, for continuum (left) and for line (right) detection. Solid lines stand for statistical limit, while dashed lines for an assumed systematic errors of 3% for both the PIN- and GSO-NXB modeling.

As a reference, Fig. 8.11 presents the theoretical sensitivity calculation results, that is, expected sensitivities defined by a certain systematic error of the background modeling, and those solely determined by the statistical error at a given exposure. In the plot, an error of 3% both for the PIN and the GSO are assumed as an example. Since actual satistical and systematic errors expected in the proposed observation differ in case by case, they should be carefully verified using the Data-NXB residual distribution plots.

8.6 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.

8.7 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 every 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 from:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB.