IT
Elena Aprile, Alessandro Curioni, Karl-Ludwig Giboni, Uwe Oberlack, and Sandro Ventura
AbstractLXeGRIT is a balloon-borne Compton telescope for
MeV
-ray astrophysics, based on a liquid xenon time projection
chamber (LXeTPC) with charge and light readout. The first bal-
loon flights in 1997 revealed limitations of the trigger electronics
and the data-acquisition (DAQ) system, leading to their upgrade.
New electronics was developed to handle the xenon scintillation
light trigger. The original processor module was replaced by a
commercial VME processor. The telemetry rate was doubled
to 2
500 kbps and onboard data storage on hard disks was
implemented. Relying on a robust real-time operating system, the
new DAQ software adopts an object-oriented design to implement
the diverse tasks of trigger handling, data selection, transmission,
and storage, as well as DAQ control and monitor functions. The
new systems performed well during two flights in spring 1999
and fall 2000. In the 2000 flight, the DAQ system was able to
handle 300350 triggers/s out of a total of about 650 Hz, including
charged particles.
Index TermsCompton telescope, data-acquisition system, time
projection chamber (TPC), trigger.
I. I
NTRODUCTION
L
XeGRIT is the first liquid xenon time projection chamber
(LXeTPC) used outside a laboratory, on a balloon-borne
platform. For details, we refer the reader to [1] and [2]. Here,
we summarize its main features. Fig. 1 shows a schematic of
the LXeTPC. It consists of a 20
20
7 cm sensitive volume
filled with high-purity liquified xenon, which is an efficient
scintillation and ionization medium. The fast scintillation light
is viewed by four ultraviolet (UV)-sensitive photomuliplier
tubes (PMTs) from below and defines the interaction time.
Electrons are drifted in a 1-kV/cm field, applied between a solid
ceramics cathode, and a wire mesh is used as a Frisch grid. The
electrical field is doubled in the collection region below the
grid to focus the drifting charge clouds through the mesh and a
structure of 2
62
- and
-wires, which sense the induction
signals. Each charge cloud is collected on one of four separate
anodes, made of wire meshes, which distinguish the energy
deposits of individual
-ray interactions. The
-coordinate
is derived from the drift time with respect to the light trigger
Manuscript received November 27, 2000; revised February 22, 2001 and
April 25, 2001. This work was supported by NASA under Grant NAG5-5108
to the Columbia Astrophysics Laboratory.
E. Aprile, A. Curioni, K.-L. Giboni, and U. Oberlack are with the Columbia
Astrophysics Laboratory, Columbia University, New York, NY 10027 USA
(e-mail: age@astro.columbia.edu).
S. Ventura is with INFN/University of Padua, Padova 35131 Italy.
Publisher Item Identifier S 0018-9499(01)07037-X.
and from the known drift velocity of
2 mm/ s. The TPC is
enclosed in a cylindrical vessel and is thermally insulated by
a vacuum cryostat, which also encompasses the PMTs. In the
1999 flight, the chamber was surrounded by an active
-ray
anticoincidence shield. A thin plastic scintillator on top of the
chamber provided veto signals for charged particles.
The advantages of a large homogeneous detector as a
Compton telescope for astrophysics justify the complex readout
system needed to acquire the complete spatial, temporal, and
energy information of any ionizing event. Compared with other
balloon-borne scientific instruments, the TPC generates an
enormous data rate, which after acquisition has to be processed
for background rejection, partial analysis as high-level trigger,
as well as packaging for either on-board storage or transfer via
telemetry to ground. The front-end electronics, acquiring the
data, was custom built, as was the original readout processor.
The data-acquisition system showed some severe shortcomings
during the first engineering flights in 1997. The analog and
digital front-end electronics were designed to fit the exact
specifications and particular requirements of both the detector
and the application. Most limitations were introduced by the
custom-built data processing system. Recognizing the advances
and the availability of powerful computer systems, it was
decided to replace the existing unit with a commercial device.
Additionally, a new unit was introduced to handle the trigger
signals, since the original circuit did not allow sufficient control
over the trigger decisions, and also did not provide all the rates
necessary to derive flux values.
The advantages of the new data-acquisition (DAQ) system
are especially obvious during the development phase. The ar-
chitecture of the data paths on the computer boards is opti-
mized for efficient information transfer between the processor
and the various communication interfaces. The computer archi-
tecture is backed by a powerful operating system. The system is
also equipped with a fast-Ethernet port, which provides a high
throughput link for control and data taking in the laboratory.
Two high-speed RS-485 serial ports serve to transmit data on
fast telemetry channels, whereas a small computer system inter-
face (SCSI) allows connection of hard disks for onboard storage
of large amounts of data. Most importantly, a VME port makes
it possible to easily interface a variety of different data sources.
Although adequate for the present instrument, the processor
system is used close to its capacity. Larger detectors, or even
higher data rates, would require multiple processor systems with
more online computer power for online reconstruction of the
00189499/01$10.00 © 2001 IEEE 1300
IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 48, NO. 4, AUGUST 2001
Fig. 1.
Schematic of the LXeTPC.
events. The present system, besides providing valuable scien-
tific data for -ray astrophysics, also shows the way for the de-
velopment of future more complex systems.
II. S
YSTEM
H
ARDWARE
A. Front-End Electronics and Flash-ADC (FADC) System
The front-end and FADC system of the LXeGRIT instrument
is described in [3]. Here we recall its main features before fo-
cusing on the system upgrades. The front-end electronics con-
verts the charge signals from the 124 induction wires (62
-
and 62
-wires) and the four anodes into voltage pulses. Each
channel has a charge-sensitive preamplifier, which drives the
twisted pair line to the digitizer system. The digitizers convert
the analog signals into a digital history of the ionizing event.
The FADC system consists of 17 printed circuit boards housed
in a standard VME-crate: 16 -ray (
) induction signal pro-
cessor (GRISP) boards with eight channels each to handle the
124 wire signals and one -ray anode signal processor (GRASP)
board to handle the four anode signals.
The wire signals are digitized with 8-bit precision at a
rate of 5 MHz. The information is stored in a dual-port random-
access memory (DRAM). The depth of this buffer is 256 sam-
ples, corresponding to 51.2 s, which covers the maximum drift
time in the TPC of about 40 s for a drift velocity of
2 mm/ s.
The charge-collection signals from the four anode channels are
digitized at the same rate with 10-bit precision, for better energy
determination with a large dynamic range.
For each channel, the digital signal is passed through a
comparator to record the sample number when a software-set
threshold is exceeded. The recording of the threshold crossing
point facilitates locating useful information and can be used
to reduce the data amount and to accelerate the data readout
process. Each GRISP board with at least one channel above
threshold issues a signal that sets a flag in a 16-bit register,
which was located on the microprocessor board in the original
design and is now located on a separate board (latch card)
within the crate.
The GRASP board can send three different interrupt requests
to the processor:
STARTADC
and
SAVEDATA
signal the start and
the completion of event digitizing, while F
LUSHDATA
signals
that the process was interrupted by a second trigger, the system
aborted the data recording, and is ready to accept a new event.
In the new design, these interrupts are registered on the latch
card mentioned above and read out by the external processor. APRILE et al.: DATA-ACQUISITION SYSTEM FOR LXeGRIT
1301
The GRASP board can also start an event digitizing process on
command from the readout processor, independent of an ex-
ternal trigger. These test triggers are used to determine baselines
and noise conditions on anodes and wires.
The front-end and FADC system of the LXeGRIT instrument
has remained unchanged from the original design, with the ex-
ception of the trigger electronics. The circuitry amplifying and
discriminating the signals from the four UV-sensitive photomul-
tiplier tubes, originally on the GRASP board, has been replaced
by new electronics. Event recording is now triggered by a fast
TTL pulse signaling the start of the event. The recording is pre-
triggered but will be stopped if a second trigger pulse signals the
occurrence of a second event within 40 s, while the charges of
the first event are still drifting in the sensitive volume of the
TPC. In this case, both events are rejected.
B. Readout Processor
Since the connections to the GRISP/GRASP boards followed
the VME standard to a large extent, it was natural to choose a
VME processor board. The final choice was a Motorola MVME
2700 coupled to a communication interface MVME761 transi-
tion module. Not all connections in a standard VME bus are used
by the GRISP/GRASP FADC system, and some bus lines were
assigned a different meaning. The processor could therefore n