Abstract:
The particularity of the cosmic environment provides us with an unparalleled experimental
laboratory for studying the state of matter under extreme conditions such as extremely
high densities, extremely strong gravities, and extremely strong magnetic fields. Under
these extreme conditions, various complex physical processes occur, accompanied by the
generation of high-energy radiation. This radiation has become an important medium for
exploring the state of matter and its evolution mechanism under extreme conditions. Xrays
(electromagnetic radiation with energy from around 100 eV to around 100 keV)
are an important component of cosmic radiation and are widely present in various highenergy
celestial environments. At the endpoints of stellar evolution, compact objects are
formed. Depending on the initial mass, a star may evolve into a white dwarf, a neutron
star, or a black hole. These compact celestial bodies and the systems they form (such as
binary star systems) or their surrounding environments are characterized by extremely
high material densities, extremely strong gravities, and magnetic fields, where intense
energy exchange and physical processes occur, usually accompanied by strong X-ray
radiation. In these extreme environments, some physical phenomena and interactions
may not only become the source of X-rays but also affect the propagation characteristics
of X-rays. By accurately detecting the energy spectrum, time characteristics, position
information, and polarization information of X-rays, physical mechanisms under
extreme conditions can be inferred and inverted, thereby gaining a deeper understanding
of high-energy astrophysical processes, enabling us to understand more about the origin,
features, and evolution of the Universe. The Earth’s atmosphere absorbs most of the incident
X-rays, so the detection of X-rays must rely on the space observatory. This makes
X-ray astronomical observations an important field in astronomical research.
The enhanced X-ray Timing and Polarimetry (eXTP) mission is a new generation of
X-ray observatories, led by the Institute of High Energy Physics of the Chinese Academy
of Sciences and promoted by Chinese and European scientists. In its original configuration,
four scientific payloads provide unique characteristics compared to other operating
X-ray observatories. The Spectroscopic Focusing Array (SFA) and the Large Area Detector
(LAD) together constitute an X-ray timing spectrometer from 0.5 keV to 30 keV,
of which the LAD can be extended to an energy dynamic range of up to 80 keV. The Polarimetry
Focusing Array (PFA) is designed for X-ray polarization measurement, while
the Wide Field Monitor (WFM), a survey payload with a wide field of view, is used to
discover observation sources of interest and provide trigger signals for subsequent observations
of the above three payloads.
As a timing spectrometer covering the energy from 2 to 30 keV X-rays, the LAD has the characteristics of a large effective detection area, a good energy resolution, and a
good time resolution. The realization of these performances depends on the coordinated
work of 640 large area multi-channel silicon drift detectors, each of which contains 224
anodes. In order to efficiently manage such a large number of detectors and read out the
signals from so many anode channels in an orderly manner, the LAD adopts a modular
detection concept and a hierarchical digital circuit architecture to improve the stability
and operability of the system. Each detection module contains 16 large area multichannel
silicon drift detectors, and each detector is integrated with a dedicated Front
End Electronics (FEE). Two identical FPGA-based digital circuits—Module Back-End
Electronics (MBEEs), are assembled in the detector module as the first-level digital circuit.
Every 10 detection modules are connected to a Panel Back-End Electronics (PBEE),
which is the second-level digital circuit. The LAD consists of 4 PBEEs and their corresponding
40 detection modules and is uniformly managed and controlled by the Instrument
Control Unit (ICU).
The topic of this thesis is to present the development process of the MBEE, including
the hardware and FPGA program design and implementation. In terms of hardware
design, it includes circuit structure design, chip selection, Printed Circuit Board (PCB)
topology planning, layout and routing strategy, and manufacturing. As the hub of the
module electronics, the MBEE is responsible for controlling, configuring, and monitoring
the detection module, recording the arrival time of the X-ray, and reconstructing the
energy of the X-ray photon based on the data of the digitized signal. The realization of
these functions is inseparable from the design of the FPGA program. The MBEE’s program
is developed for task requirements in specific operation modes. The operation of
the MBEE depends on the decoding of the telecommand sent by the PBEE to complete
tasks such as switching operation modes, obtaining housekeeping data, and adjusting the
parameters of the module electronics. This thesis describes the implementation process
of these operations in detail. In observation mode, the MBEE is responsible for processing
the digitized signals from the FEEs to record and reconstruct the X-ray event.
Based on the concept of the pipeline, it performs pedestal subtraction, common mode
noise subtraction, energy reconstruction, energy discrimination, and science data packet
generation in sequence to get the X-ray arrival time and energy information.
In order to better measure and verify the circuit interfaces of the MBEE in the early
phase of development, the PBEE simulator, MPSU simulator, and FEE simulator were
designed to preliminarily build a complete detection chain. Among them, the FEE simulator
is particularly critical, not only for interface testing but also to provide support for
the development of the data processing pipeline in this early phase. The FEE simulator
with a Graphical User Interface can simulate the distribution of the energy on the anode
channels after a single detector half receives an X-ray photon with a certain energy
in a certain incident position, and present the digital output after the energy deposition,
charge conversion on the detector, the amplification and analog-to-digital conversion on
the FEE. Based on the interface and output of the FEE simulator, raw data for the development
of the data processing pipeline are provided. The thesis describes the preliminary functional tests of the MBEE and the test results of the detection chain built based on the
MBEE and these simulators. The tests show that the MBEE can realize the controlling,
configuration, and status monitoring of the module electronics, and successfully extract
the time and reconstruct the energy of the X-ray event generated by the FEE simulator,
verifying the effectiveness of the MBEE core functions.
eXTP LAD