The LECS consists of Mirror, Detector, Electronics and Gas Units. The Mirror Unit is provided by the Italian Space Agency (ASI). The major components of the SSD provided units are shown in Fig. 1 (click here) and the overall performance is summarized in Table 1 (click here).
Figure 1: LECS instrument schematic
Table 1: LECS overall performance summary
A total of four flight Mirror Units (MUs) and one qualification model have been
produced. The LECS uses unit FM3.
Each MU consists of 30 nested, Au coated, confocal mirrors with
a double cone approximation to Wolter I geometry (Citterio et al. 1985).
The geometric area is 124 cm
and
the mirror diameters range from 16.2 to 6.8 cm. The shell thicknesses
range from 0.4 to 0.2 mm and the nominal focal length is 185 cm. The total
length of each mirror shell is 30 cm. The mirrors were produced using a
replication technique by nickel electroforming from super-polished
mandrels. The calibration and performance of the flight MUs
is described in Conti et al. (1994).
In order to deflect any plasma that might pass through the MU, a plasma protection grid is mounted on the underside of the unit. This consists of a fine pitched Au-coated W grid. In flight, the grid will be placed at +28 V potential, in order to shield the detector from ion impingement.
The Detector Unit (DU) consists of a gas cell, a
photo-multiplier tube (PMT),
Front End Electronics (FEE) and two high-voltage supplies. The
detector is protected by an Al cover on which a shutter
mechanism is mounted. The shutter will be opened once in orbit. The detector
is protected against space plasma ingress by a
protection window located just beneath the shutter
and by the MU plasma protection grid described in Sect. 2.1 (click here).
The protection window consists of two layers of
Polyimide with a total thickness of 200 nm, separated by a layer of AlN
and coated with a layer of AlN and C on the external surfaces
(Fig. 2 (click here)).
Mechanical strength is provided by a 250 
m hexagonal Polyimide
grid.
The top cover has a thin Be disk mounted on its
interior upper surface in order to minimize any X-ray
background induced by
electron emission from the gas cell top disk which is at a potential
of -20 kV.
The detector also
contains two 
Fe radioactive sources which constantly illuminate
regions of the detector outside the field of view (FOV),
allowing the position and energy gains of the instrument
to be continuously monitored. Each source produces
Mn 
and 
X-rays (5.89 and 6.49 keV, respectively)
at a rate of 20 counts s
in 1994 October, with a half-life of 2.73 years.
Figure 2: Plasma protection window cross-section
The method of operation of the detector is similar to that of a conventional GSPC (see Inoue et al. 1978; Davelaar et al. 1980; Peacock et al. 1980; Simons et al. 1985). An X-ray which is absorbed in the cell gas liberates a cloud of electrons. A uniform electric field between the entrance window kept at -20 kV and a grounded grid, causes scintillation of the Xe gas as the electrons travel towards the grid. The UV light from these scintillations is detected by the PMT mounted at the rear of the cell. The amount of light produced is proportional to the energy of the incident X-ray but, unlike a conventional GSPC, also depends on the depth at which the X-ray photon was absorbed. This depth is determined by measuring the duration of the scintillation light and is referred to as the ``Burstlength''. Thus an X-ray absorbed deep within the gas cell will have a shorter burstlength than one absorbed directly below the entrance window. The negative potential on the window and the high electric field ensure that electrons created by X-rays absorbed just beneath the entrance window have a high probability of entering the scintillation process.
The gas cell is a 11.4 cm diameter ceramic cylinder of depth 5 cm with
a metallic top disk and lower flange. The PMT is mounted 1 mm from the
quartz gas cell exit window. The cell is filled
with Xe at a nominal pressure of 1.1 bar at 
.
GSPCs are extremely
sensitive to impurities and a getter is included to passively
absorb small amounts of gas impurities.
The getter can be activated by a current
source once it has reached saturation. The gas cell has an
extremely thin multi-layer entrance window consisting
of three layers of Polyimide separated by Al/AlN multilayers. This
construction was chosen to minimize the leak rate and sensitivity
to atomic oxygen corrosion.
The nominal thickness of the window is given in Table 4 (click here)
and its cross-section is shown in Fig. 3 (click here). The Al and AlN
layers act as gas permeation barriers while the Polyimide provides
mechanical support. The strength to support a differential
pressure of over 1 bar is provided by a fine grid and a
strongback. The strongback is made of 
thick W bars
with a height of 3 mm and a pitch of 2.2 mm
(Fig. 4 (click here)).
The fine grid consists of 
thick W foil which
subdivides each strongback square into a matrix of 8 by 8 squares
(Fig. 5 (click here)).
Figure 3: Detector entrance window cross-section
Figure 4: Entrance window strongback structure
Figure 5: Entrance window support grid structure
The high voltage power supplies provide power to the gas cell and to the PMT. The cell voltage is commandable in 31 steps between -12 and -20 kV and that of the PMT between 0.8 and 1.5 kV in 255 steps. The power supplies are based on a fly-back converter followed by an 8 stage Cockcroft-Walton multiplier. The output voltages are then filtered by a triple resistive-capacitive network to produce less than 0.1% (peak-to-peak) ripple.
The PMT is a ruggedized Hamamatsu R5218 tube with a 2.8 mm thick quartz
entrance window of 76 mm diameter contained in a mu-metal housing to
shield it from external magnetic fields. Nine anodes, each
are positioned in a 
square configuration with
a separation of 1 mm as shown in Fig. 6 (click here).
The anodes are separately read out and operated
as an Anger camera (Anger 1958) to provide spatial information.
The cathode is a
special bialkali with a spectral response from 160 to 600 nm.
Amplification is provided by 15 proximity mesh dynodes.
Figure 6: PMT anode layout. The above view is looking in the direction
of the concentrators through the PMT. The arrows indicate the spacecraft
physical axes
Within the FEE, each of the signals from
the nine PMT anodes are fed to separate charge-sensitive
amplifiers with a 
time constant. The amplified signals are
then combined to provide the energy (
), X (
)
and Y (
) positions and
veto (
) signals. Defining 
to be the output of the
anode's pre-amplifier, gives:
When divided by the 
signal, the veto signal, 
, provides
a measure of the light distribution in the detector.
This signal has a high value
for events that occur close to the center of the FOV and a low one for
events that occur outside the FOV. The veto signal can also be
used to distinguish between particle events (which typically appear
extended and therefore have low veto values) and on-axis X-ray events.
The four signals are buffered and passed to
the Electronics Unit (EU) for further processing.
The DU also includes a pressure transducer for monitoring the cell pressure, a thermistor to monitor the PMT temperature, two micro-switches to determine the open and closed position of the shutter mechanism and analog monitors of the two high voltage outputs.
The EU provides the electrical interface between the instrument and the spacecraft. The main micro-processor is a radiation-tolerant 80 C86 with 64 k-Bytes of data RAM and 32 k-Bytes of program memory. An additional 32 k-Bytes of RAM is available for software updates. A second processor is dedicated to communications with the spacecraft using the ESA On-Board Data Handling protocol. Communication between the two processors is via a dual port RAM.
The EU performs many functions. The analog signals provided by the FEE
pass through optimized pulse shapers with a 
time
constant and pole-zero cancelation of the FEE's 
time constant.
Additionally, the energy
signal is fed through a 
pulse shaper for burstlength
determination. The output of the filters are sampled
at their peaks and the position, veto and burstlength signals
normalized using the energy signal. At this stage, qualification of the
signals occurs, based on programmable amplitude windows. In the case of a
qualified event, the signals are converted to digital values using
Wilkinson converters which provide good integral and
differential linearity. The measured differential non-linearity of the
energy channel is <3.5% of the least significant bit. The
energy channel, 
, is converted to a 10 bit value
and is referred to as the PHA signal.
The other channels are converted to 8 bit values and are referred to as the
RAWX, RAWY, VETO, and BL signals (coming from the 
, 
,
signals and the burstlength determination circuitry, respectively).
Between the time the EU detects an event and its
conversion, the acquisition chain is inhibited to prevent subsequent pulses
from affecting the current measurement. The time the instrument
is inhibited in this way is accumulated in a deadtime
counter. Whenever a non-qualified event is detected, the
analog-to-digital conversion process is aborted in order to minimize the
deadtime. The deadtime for each event is amplitude dependent,
and is in the range 64 
s, for non-qualified
events, to 250 
s for full-scale events.
The on-board software is written in ``C'' with assembler language used for
time critical tasks. The system runs under the control of a
real-time operating system, which schedules the tasks for
instrument monitoring and protection, command processing and execution,
data processing and packetization. Event
processing and telemetry transmission are interrupt
triggered. Whenever a qualified event is detected,
the analog electronics processes the
different signals and converts the analog signals
into digital values as described above. An on-board counter,
synchronized to the spacecraft's ultra-stable clock, time-tags the
event with 16 
s resolution. The complete data set is then
retrieved by the main processor and packetized for transmission to
ground. Alternatively,
the event data can be accumulated by the EU software into images and
spectra in order to conserve telemetry usage.
The original specification on the leak rate of gas cell entrance window
was sufficiently high to require a gas filling system.
However, the leak rate of the final design is so low
(
) that such a system is no longer
required for this purpose.
The GU was originally designed to autonomously maintain
the gas cell pressure in a programmable range between 800 and 1200 mbar.
However, this operation is no longer required and it is disabled under
normal operations.
Instead, the GU can be used to completely vent and refill the
gas cell should the gas become contaminated.
The unit consists of
two identical 0.5 liter Ti reservoirs containing
20 bar liters of Xe, input and vent valves, a pressure transducer, a
thermistor and the necessary structure and piping.
The non-latching input valve can be actuated in pulse mode from the EU
to allow gas to enter the cell. The pulse duration is
programmable to minimize the number of actuations.
The non-latching vent valve allows gas to escape from the cell into free space.
The pressure transducer monitors the gas pressure in the
reservoirs.