The HPGSPC instrument is a High Pressure Gas Scintillation Proportional Counter filled with a high purity gas mixture of Xenon (90%) and Helium (10%) at 5 atmospheres.
The basic mechanism of Gas Scintillation Proportional Counters is quite well known and has been extensively described by different authors (Gedanken et al. 1972; Manzo et al. 1980).
After penetrating the beryllium window, the X-ray photon is
absorbed in the gas cell by a Xenon atom via the photoelectric
effect. The primary photoelectron emitted gives rise to a localised
cloud of secondary electrons at the position of X-ray absorption.
The secondary electron cloud, driven by a relatively moderate
electric field, after drifting in the so called Drift Region,
enters a high field region (Scintillation Region) in which the
electrons acquire sufficient energy to excite the Xenon atoms. As a
result of collisions, the excited atoms form excited diatomic Xenon
molecules which then deexcite by the emission of VUV photons in the
1500-1950 angstrom range. Typical duration of scintillation,
measured as the time interval between the 10% and 90% of the
signal, is 3.5 
sec and does not depend on the energy. In the
case of the HPGSPC VUV light is collected by an array of seven
photomultipliers in an Anger Camera configuration. The number of
electrons emitted per keV by the cathode of central photomultiplier
is about 250.
Figure 1: Schematic view of the Flight model of the HPGSPC
A schematic view of the main components of the Flight Model of the HPGSPC is shown in Fig. 1 (click here). Table 1 (click here) summarises the main characteristics. The HPGSPC instrument consists, primarily, of a Detector Unit (DU) and an Electronic Unit (EU).
The construction of the sealed cylindrical gas cell, whose cross
section is shown in Fig. 2 (click here), was carried out by AEG, Ulm,
following Ultra High Vacuum standards. This is imposed by the
requirements on leak tightness (
He/s) and
cleanliness of the filling gas (at a few part per million level) for
an operational life lasting more than four years. It consists,
primarily, of a titanium body (3 mm thick, with a diameter of 360 mm
and a depth of 184.5 mm) covered with an entrance window (30 cm
diameter) made of beryllium. The beryllium window, shown in
Fig. 3 (click here), was one of the most technically challenging
parts of the instrument. The design must meet two contradictory
requirements: transparency to X-rays down to 3 keV and the
mechanical stiffness required to withstand the 5 atmospheres
differential pressure on a diameter of 30 cm. The window is made of
two beryllium foils, 0.5 and 0.8 mm thick supported by an egg crate
structure of orthogonal beryllium ribs. The core self supporting
structure is welded to the titanium body via an aluminium/titanium
transition ring aimed at compensating for mechanical stresses
generated during the bake-out at 
degrees.
Table 1: HPGSPC overall characteristics
Figure 2: Gas cell cross section
Figure 3: Collimator and window
An absorption region (Drift Region) of 10 cm and a scintillation
gap (Scintillation Region) of 1 cm are defined by two circular mesh
grids with a diameter of 300 mm, made with orthogonal titanium wires
with a spacing of 3 mm and a diameter of 
. To maintain the
uniformity of the electric field in the Drift Region five field
shaping rings made of NCR alloy are placed every 16.0 mm. The two
mesh grids can be set up to a nominal value of 10 and 25 kVolt. In
the Flight Model, however, because of microdischarge occurring in a
localised point in one of the high voltage feedthroughs, the reduced
field applied in the Scintillation Region was only 2 kVolt/cm/atm
with a reduction of VUV light production. Due to the high electrical
field, the two grids are terminated by guard electrodes whose shape
is critical in terms of higher field values generated. Each of the
two grids is held in place by six ceramic insulators which are
welded to the outer wall of the cell. For each grid one of the
insulators is also used as a high voltage feedthrough. The high
voltage applied, the constraints on the dimension and shape, the
breakdown field strength of the gas have made the design of the
grids rather critical. In the end, a compromise between the best
theoretical (Rogovsky profile) and the most feasible shape has been
used. This is a circular profile with a variable radius.
Figure 4: The PMTs in an Anger camera configuration: the central PMT
is surrounded by six lateral PMTs
At the bottom of the detector seven ruggidized EMI D319Q
photomultipliers (PMTs) in an Anger camera configuration, as in
Fig. 4 (click here), detect the VUV light produced in the
Scintillation Region. As an interface between the PMTs and the
pressurized gas cell a titanium flange, welded to the titanium body,
supports seven suprasil 1 quartz windows each 5 mm thick. The 6.3 cm
between the scintillation grid and the quartz windows define the so
called ``Back Region". On top of the detector a 10 cm high
collimator (manufactured by Officine Galileo, Florence) limits the
field of view to 
degree FWHM. The
collimator consists of hexagonal cells made with 
of
aluminium plated with 
of lead. Four highly collimated
calibration sources are mounted in the collimator in the position
shown in Fig. 3 (click here). Each calibration source consists of a
mixture of 
and 
radioactive sources, with
calibration lines at 6, 22, 25 and 88 keV. The sources are used for
a continuous monitoring of the gain and for a real time equalization
of the relative gain of the photomultipliers performed by the
Automatic Gain Control (AGC) chain. To reduce the residual
background, the Detector Unit is shielded with 1 mm of lead and 2 mm
of tin around the sides and bottom.
The HPGSPC High Voltage supplies, the Front End Electronic (FEE), the Electronic Unit and the data conditioning system have been designed and produced by Laben, Milan.
The seven signals coming out from the PMTs are slightly preamplified, shaped and then transmitted, via a Current Loop transmitter, from the FEE to the Electronic Unit that constitutes the main electronic part of the Instrument. The Electronic Unit interfaces the Detector Unit to the On Board Data Handling (OBDH) bus. In the EU, the PMT signals received from the DU are processed, A/D converted and finally formatted for transmission to the OBDH. In addition, Housekeeping signals (High Voltage monitoring and temperature monitoring) are converted and formatted, jointly with other Digital Housekeeping, for transmission to the OBDH.
The signals coming out from the DU are slightly delayed (this is
needed to allow the event qualification board to send the
"integrate" signal if the current pulse exceeds a programmable
threshold) and then integrated by seven gated integrators each with
8 
sec integration time constant. A Sum signal is then obtained
with an analogue sum of the seven integrated signals. The pulse
duration is also measured by the burst length chain that measures
the interval time between the 20% and the 80% of the integrated
sum signal. As shown below, information on burst length is used to
discriminate genuine X-rays against background events or anomalous
events such as those absorbed in the Scintillation Region and/or
Back Region.
Event qualification and processing is performed both by the Analogue Processor and the Digital Processor. While analysis of energy and of pulses shape along with burst length qualification and A/D conversion is performed by the Analogue Processor, two main tasks are performed by Digital Processor: single and double event management and the position reconstruction and energy correction of the events. These tasks, as discussed in the next paragraphs, are crucial for the correct scientific working of the Instrument.
A real time equalisation of the 7 PMTs is performed on-board by
the Automatic Gain Control System. For each event that is qualified
as a 22 keV photon from the central 
calibration source
the channel value measured by each Photomultiplier is compared with
the correspondent reference channel experimentally determined during
on-ground calibration. If the measured channel is lower than the
reference channel the gain of the PMT is increased by a step 
acting on the voltage of the phototube. On the contrary if the
measured channel is greater than the reference channel the gain of
the phototube is changed decreasing by a single step the voltage
applied to the phototube. For each PMT a gain variation of 0.1%,
which corresponds to 100 mVolt, has been chosen to maintain overall
fluctuations in 
peak position reconstruction around
0.5% FWHM. This choice, given the actual calibration source count
rate, allows equalisation of the system in less than a minute
without degrading the energy resolution.
The HPGSPC instrument can be operated in three different telemetry modes:
The total amount of telemetry allocated for the HPGSPC is 50 kBit/sec and can go up to 100 kBit/sec if all the other instruments are in a standby position. Due to telemetry constraints and depending on the source count rate not all the information generated by the EU for every event can be transmitted. The main observing mode is, therefore, the Dir001 mode while the Dir004 is considered the diagnostic operative mode. The latter has been extensively used during the on-ground calibration and during the Commissioning and Science Verification Phase to configure the instrument optimally and to determine its response characteristic in flight. It will, also, be used during periodic calibration of the instrument or to verify the correct performances of the PMTs.
Whatever Operative Mode is selected, other information is telemetered to the ground to monitor the detector status and performance:
Table 2: HPGSPC telemetry of direct modes