The XGS project aims to develop a multi-channel broadband X and gamma -ray spectrometer for space applications. The experiment envisages the use of solid-state Silicon Drift Detectors coupled to inorganic scintillator bars. A prototype is under development in the framework of an INAF funded project, in which the detector signal will be digitized by a fast ADC and further digitally processed. An overview of the system architecture and the test equipment currently under development based on low-cost commercial system-on-chip FPGA boards will be given

AMATI, LORENZO

Baldazzi, G.

CAMPANA, RICCARDO

FIORINI, MAURO

FUSCHINO, FABIO

LABANTI, CLAUDIO

Marisaldi, M.

Rignanese, L. P.

USLENGHI, MICHELA

OA@INAF - Istituto Nazionale di Astrofisica

2017Publication Year2020-08-25T13:53:55ZAcceptance in OA@INAFA FPGA-based digital readout system for a multi-channel X and gamma-ray spectrometerTitleCAMPANA, RICCARDO; FUSCHINO, FABIO; LABANTI, CLAUDIO; Marisaldi, M.; AMATI, LORENZO; et al.Authorshttp://hdl.handle.net/20.500.12386/26806HandleMEMORIE DELLA SOCIETA ASTRONOMICA ITALIANAJournal88NumberMem. S.A.It. Vol. 88, 149c© SAIt 2017 Memorie dellaA FPGA-based digital readout system for amulti-channel X and gamma-ray spectrometerR. Campana1,2, F. Fuschino1,2,3, C. Labanti1,2, M. Marisaldi1,2, L. Amati1,G. Baldazzi2,3, M. Fiorini4, L. P. Rignanese2,3, and M. Uslenghi41 INAF/IASF-Bologna, Via Gobetti 101, I-40129 Bologna, Italye-mail: campana@iasfbo.inaf.it2 INFN - Sezione di Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy3 Department of Physics, University of Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy4 INAF/IASF-Milano, Via Bassini 15, I-20133 Milano, ItalyAbstract. The XGS project aims to develop a multi-channel broadband X and γ-ray spec-trometer for space applications. The experiment envisages the use of solid-state Silicon DriftDetectors coupled to inorganic scintillator bars. A prototype is under development in the frame-work of an INAF funded project, in which the detector signal will be digitized by a fast ADCand further digitally processed. An overview of the system architecture and the test equipmentcurrently under development based on low-cost commercial system-on-chip FPGA boards willbe given.Key words. High energy astrophysics – X-ray detectors – γ-ray detectors – Silicon DriftDetectors1. IntroductionThe study of Gamma-Ray Bursts (GRB) isone of the most interesting and challenging formodern science. The huge luminosities, red-shift distribution, and association with peculiarcore-collapse supernovae and compact objectmergers, make the study of these object highlyinteresting for several fields of astrophysicsand fundamental physics, e.g. the physicsof matter in extreme conditions and plasmaphysics, black hole physics, general relativityand generation of gravitational waves. Severalopen issues still affect our comprehension ofthese phenomena, and their exploitation.In order to tackle these questions, we needtime resolved spectroscopy (and possibly po-larimetry) of the GRB prompt emission overa broad energy range from ∼1-2 keV, i.e. wellbelow the range of past, present and near futureGRB detectors, to several MeV.We are therefore working towards a proto-type for a monolithic system (X and GammaSpectrometer, XGS) which would allow de-tection, spectroscopy and timing of GRBs andother high energy transients over an unprece-dented broad energy band. The unique capa-bilities of such an instrument are the combi-nation of a low energy threshold (1-2 keV)and energy resolution, up to ∼30 keV, sig-nificantly better than that of any other GRBdetection system (e.g. based on scintillatorsor CdTe/CZT semiconductors), besides timing150 Campana: A FPGA-based digital readout system for XGSFig. 1. The siswich architecture.capabilities down to a few micro-second reso-lution over the whole energy band.In the framework of an INAF-fundedproject, we are developing a discrete front-endelectronics prototype aiming to demonstratesuch an architecture. The analog signal at thepreamplifier output of the various channels willbe digitized and processed offline. In this pa-per the overall architecture will be briefly de-scribed (Section 2), and the digital signal pro-cessing philosophy envisaged for the prototypewill be summarized in Section 3. Finally, inSection 4 the FPGA-based digital back-end ar-chitecture will be introduced.2. The XGS projectThe working principle of the XGS detectoris shown in Figure 1, the so-called siswichconcept (Marisaldi et al., 2004, 2005; Labantiet al., 2008). A scintillator crystal bar, madeof CsI(Tl), is coupled at both ends with asingle-cell Silicon Drift Detector (SDD, Gatti& Rehak, 1984). Low-energy X-rays (from 1–2 keV to a few tens of keV) are directly ab-sorbed by the solid-state detector, leading to avery sharp (∼100–200 ns) rise-time analog sig-nal on the detector preamplifier. Higher energyX-rays and γ-rays are instead absorbed in theCsI(Tl), thus producing a large number of op-tical scintillation photons. These photons arecollected and detected by the SDDs, giving riseto a much slower (∼few µs, dominated by theFig. 2. The XGS concept. Left panel: basic unitof the module, composed by 4 CsI(Tl) bars andtwo 2×2 single-cell SDD arrays. Right panel:the complete XGS module, showing also afield of view collimator for X-rays.Fig. 3. Digital trapezoidal filter. An exampledigitized waveform from a ∼10 keV X-rayphoton directly absorbed in the SDD is shown,together with two possible realizations of thetrapezoidal filter, with different rise and flat-toptimes.scintillator crystal characteristic light time con-stants) output signal.The different rise times of the signals thusallow to discriminate between the two types ofevents, and the detector is thus collectively sen-sitive to a very large energy band.For the overall XGS detector, a large num-ber of scintillator bars are coupled to twoSDDs each, resulting in a compact modular in-strument. In the left panel of Figure 2, fourCsI(Tl) bars are coupled to two 2×2 single-cell SDD arrays, and then the whole instrument(Figure 2, right panel) is built from a large andCampana: A FPGA-based digital readout system for XGS 151customizable number of these basic units. Acollimator could also be added to limit the in-coming directions for detectable X-rays, whilethe instrument has a roughly isotropic sensi-tivity for high energy γ-rays. For the proto-type being developed, each scintillator bar has4.5×4.5×45 mm3 dimensions, and a completeXGS module is composed by at least 64 ofthese bars.3. Digital signal processingThe preamplifier output from the SDDs is astep-like signal, with an exponential decay. Inγ-ray spectroscopy, usually, an analog shapingchain is employed, consisting of several logi-cal blocks that will filter and shape the signal inorder to boost its signal-to-noise ratio. The am-plitude of the shaped signal, proportional to theenergy deposited in the detector, is then digi-tized and stored.In digital signal processing the preamplifiersignal is directly digitized by a fast ADC andfurther processed on- or offline by software-implemented algorithms. Digital algorithmshave several advantages with respect to the tra-ditional analog chain:i. they closely approximate the “ideal” filter;ii. there is no dead time associated with thepeak detection and digitization, thus al-lowing a higher throughput;iii. an analog system allows only a fewfixed shaping choices (shaping times andshapes), while a digital system allowsmultiple and unlimited shapings on samedata;iv. digital filters allow much higher stabilityand reproducibility.A classical digital filter for the applicationsconsidered is the trapezoidal filter (Jordanov &Knoll, 1994; Guzik & Krakowski, 2013), eas-ily implementable as a cascade of finite andinfinite impulse response filters. In Figure 3a representative preamplifier waveform, digi-tized at 30 MS/s by a 12-bit commercial digi-tizer, is shown together with two possible real-izations of the trapezoidal filter for this signal,having different rise and flat-top times.4. Back-end digital architecture4.1. Digital signal processing withRedPitayaFor the complete XGS prototype, a digi-tal back-end architecture able to acquire thepreamplifier output of several tens of chan-nels, with a suitable trigger logic, is required.The digitized waveforms should be then digi-tally processed in real-time or offline. The useof a digital, FPGA-based back-end will allowto drive the design of a low-power, low-noiseintegrated mixed-signal electronics optimizedfor the future space experiment.An attractive and low-cost solution envis-ages the use of a commercial ADC board, theRedPitaya1. This board is based on a XilinxZynq 7010 system-on-chip, in which a FPGAis coupled to an ARM Cortex 9 CPU, al-lowing a great flexibility in the deploymentand configuration. Each board houses two ana-log input channels, with a 14-bit, 125 MS/sADC, and can be connected to a network via aGigabit Ethernet port. A Linux operating sys-tem, loaded via a microSD drive, runs on theARM CPU. An interesting feature is the avail-ability of the source code for both the operat-ing system and the FPGA core2, allowing foran easy customization, e.g. by adding desiredfeatures to the default configuration.A first scalable prototype, using two boardsand thus able to read out 4 analog channels, hasbeen developed (Figure 4, left panel).4.2. Trigger logic and boardsynchronizationIn order to acquire simultaneously the sig-nal from all the bars when one channel isover a given threshold, a modification of thedefault FPGA configuration has been imple-mented (Figure 4, right panel), exploiting theon-board availability of several GPIO pins. Adigital signal is sent to an external pin whenone or both channels in a board crosses thethreshold (internal OR). This signal is mixed in1 www.redpitaya.com2 https://github.com/RedPitaya/RedPitaya152 Campana: A FPGA-based digital readout system for XGSFig. 4. Left panel: a RedPitaya board. Right panel: sketch of the multi-board setup architecture.Fig. 5. Screenshot of the graphical user interface for the acquisition and quicklook of the digitalback-end architecture.a multi-port digital OR using an external trig-ger synchronization board, and then feed to theexternal trigger pin of all boards. In this way,the effective trigger logic is that of a “tripleOR trigger” (internal on either channel or ex-ternal), allowing for the simultaneous acquisi-tion of the waveform across N boards and 2Nchannels. This allows to determine the amountof optical cross-talk between the bars but alsoto characterize Compton-scattering events (in-volving multiple scintillator bars) and to inves-tigate the polarimetric performance of the sys-tem.In Figure 5 is shown the GUI acquisitionand quicklook software, developed at IASF-Bologna in Python using the Qt libraries. Whenlaunching an acquisition, a server-side pro-gram is started on the CPU of each board,configuring the acquisition parameters and thetrigger logic and opening a socket for sendingCampana: A FPGA-based digital readout system for XGS 153the waveforms as Ethernet packets. The client-side program, therefore, acquires these pack-ets and performs digital filtering and spectrumquicklook, beside saving the digitized wave-forms in a data file.5. ConclusionsThe good overall performance of the sys-tem (e.g. energy resolution, optical couplingbetween SDD and scintillator) has been de-mostrated (Campana et al., 2016). We are cur-rently working towards the implementation ofthe complete XGS prototype, using an inno-vative, low-cost digital back-end (Section 4).The aim is to demonstrate the feasibility ofthis compact and modular architecture concept(and to drive the development of a dedicatedanalog front-end electronic circuit) for a space-borne, wide energy range, sensitive photon de-tector for the future missions for high energyastrophysics.Acknowledgements. This project is funded by INAFthrough a TecnoPRIN 2014 grant. The Silicon DriftDetectors have been developed in the framework ofthe ReDSoX collaboration.ReferencesCampana, R., Fuschino, F., Labanti, C., et al.2016, Proc. SPIE, 9905, 99056IGatti, E. & Rehak, P. 1984, NuclearInstruments and Methods in PhysicsResearch, 225, 608Guzik, Z. & Krakowski, T. 2013, Nukleonika,58, 333Jordanov, V. T. & Knoll, G. F. 1994,Nuclear Instruments and Methods in PhysicsResearch A, 345, 337Labanti, C., Marisaldi, M., Fuschino, F., et al.2008, Proc. SPIE, 7021, 702116Marisaldi, M., Labanti, C., & Soltau, H. 2004,IEEE Transactions on Nuclear Science, 51,1916Marisaldi, M., Labanti, C., Soltau, H., et al.2005, IEEE Transactions on NuclearScience, 52, 1842

A FPGA-based digital readout system for a multi-channel X and gamma-ray spectrometer

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A FPGA-based digital readout system for a multi-channel X and gamma-ray spectrometer

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