The Mini-Calorimeter for the AGILE satellite

Transcription

1 The Mini-Calorimeter for the AGILE satellite Claudio Labanti, Andrea Argan, Andrea Bulgarelli, Guido di Cocco, Marcello Galli, Fulvio Gianotti, Martino Marisaldi, Alessandro Mauri, Elio Rossi, Marco Tavani, Alessandro Traci and Massimo Trifoglio Abstract-- AGILE is a small space mission of the Italian Space Agency (ASI) devoted to observations for astrophysics in the gamma-ray energy range 30 MeV 50 GeV with a simultaneous window in the X-ray band 10keV 40keV. AGILE payload is made of a Tungsten-Silicon Tracker, a CsI Mini-Calorimeter for the high energy band and a Silicon based X-ray detector (Super- Agile); an anticoincidence system carries out background rejection. In the gamma-ray s band the satellite will have a field of view of about 1/5 of the sky, with angular resolution of a few arc-minutes and good timing resolution. For the high energy AGILE detection principle is based on the pair production process that arises from the interaction between photons and the Tungsten layers above the Silicon Tracker. The Silicon Tracker is designed to determine the direction of the incoming radiation, while the Mini-Calorimeter evaluates the energy of the interacting photons and particles. For the detection of transients and gamma-ray burst events, the Mini-Calorimeter will also work as a stand-alone gamma-ray detector, with no imaging capabilities, covering the energy range 250 kev 250 MeV. The Mini-Calorimeter is made of an array of position sensitive CsI(Tl) scintillator bars with Photodiode read-out. The characteristics of its detector elements and the front end electronics signal processing allow a moderate position reconstruction of the detected event. In this paper a description of the Mini- Calorimeter is reported as well as a summary of performances reached with the pre-flight instrument models already built and tested. A I. INTRODUCTION GILE Astrorivelatore Gamma ad Immagini LEggero is a small Italian Space Agency (ASI) mission[1], aimed to the observation of the high energy gamma ray sky in the energy range 30 Mev 50 GeV [2], [3]. AGILE will have a large field of view (~3 sr), combined with good angular resolution (a few arc minutes of source pointing accuracy), good sensitivity and an unprecedented low dead time for gamma rays (lower than 200 µs). Furthermore AGILE will This work was supported in part by the Italian Space Agency. C. Labanti, is with the IASF CNR INAF Sezione di Bologna, Italy (telephone: , labanti@bo.iasf.cnr.it) The same address is for A Bulgarelli, G. di Cocco, F. Gianotti, M. Marisaldi, A. Mauri, E. Rossi, A. Traci and M. Trifoglio. A. Argan is with the IASF CNR INAF Sezione di Milano, Italy ( argan@mi.iasf.cnr.it). M. Galli is with the ENEA Bologna, Italy ( argan@mi.iasf.cnr.it). M. Tavani is with the IASF CNR INAF Sezione di Roma, Italy ( tavani@rm.iasf.cnr.it). give simultaneous information in the X-ray and gamma ray bands thanks to the SuperAgile [4] detector. AGILE is planned to be operational during the years The Agile payload, Fig. 1, weights about 130 kg, is composed of three scientific instruments: the Silicon Tracker [5], that is made of 12 layers of Silicon strip detectors (with a pitch of 120 µm between strips and thickness of 410 µm) interleaved with 10 plates of tungsten (0.07 radiation length X0 each) for gamma ray interaction. The Mini-Calorimeter (MCAL) is placed below the Tracker and is made of two planes of 15 CsI(Tl) bars each. Fig. 1. AGILE payload. The three active instruments, SuperAGILE, the Tracker and the MiniCalorimeter are surrounded by an active anti-coincidence shield not shown in the picture. Due to payload weight constraints, MCAL is just 1.5 X0; it will collect energy information on particles converted in the Silicon Tracker (with energy between 1MeV and 500MeV), and will also monitor the gamma-ray sky for transients (in the energy range 250 kev 250 MeV). Above the Tracker is placed a silicon X-ray detector (sensitive to X-rays with energy between 10 kev and 40 kev) which will give further information on gamma ray sources observed, and will look for X-ray transients as well. Surrounding the payload is an anticoincidence system, aimed at charged particle background rejection; it is made of plastic scintillators (with thickness of 0.5 to 0.6 cm) with photomultiplier readout. A Data Handling system (DH) will process the detected events both for

2 impulsive events detection and for effective particle background rejection. II. THE MCAL DETECTOR The Agile Mini-Calorimeter, Fig. 2, is composed of a detection plane and of a Front End Electronics (FEE), all contained in an unique mechanical structure. The MCAL functions are the following: i) To obtain additional information on the energy of particles converted in the Tracker and interacting on MCAL. In this case MCAL works in GRID (Gamma Ray Imaging Detector) mode, and it is a slave of the Silicon Tracker. ii) To detect gamma ray impulsive events in the energy range 250 kev 250 MeV. In this case MCAL works in BURST mode, as an independent burst monitor. MCAL detector consists of 30 CsI(Tl) bars arranged in two planes. The bars exhibit a low light attenuation combined with high light output. The readout of the scintillation light is accomplished by 2 custom PIN Photodiodes (PD) coupled one at each of the small sides of the bar. The bars are polished and then wrapped with a reflective coating. Each bar, complete with its own 2 PDs, is hosted in a dedicated carbon fiber housing 1 mm thick, that provides rigidity and modularity to the MCAL detection plane. An aluminum frame allows the positioning of 15 bars in the upper plane and 15 bars in the lower plane orthogonal to the previous ones. The detection elements are hosted in the upper part of MCAL main frame; the preamplifiers collecting the PD signals are arranged in four box on each side of the detector and at its same level. Below the detector plane is placed an aluminum box that has the same area and that contains the two FEE electronic boards. The overall mechanical envelope of MCAL constitutes the lower part of the whole AGILE payload. For each bar the Photodiode s signals are collected by means of low noise charge preamplifiers, and then conditioned in the FEE; the circuits have been optimized for best noise performance, fast response, combined with low power consumption and a wide dynamic range. The Front End Electronics is made of 60 identical analogue electronic channels, one for each PD-preamplifier; the limited number of channels and the availability of enough physical space below the detector for the electronics boards allowed the used of standard components for the circuits. Each preamplifier is connected to a shaping amplifier with shaping time of about 3 µs; the noise figure of the whole branch is about 800 e - rms. To allow contemporary GRID and BURST operations the amplified signal is then fed to two independent blocks. The GRID circuit of each electronic channel includes a sample and hold whose operations are governed by the DH system. When a GRID event is detected by DH all the PD signals are sampled, converted in digital form and sent to DH, in this case all the bars of MCAL acts as part of an unique position sensitive detector. The BURST chain of each bar includes an autonomous discriminator on the sum of the signals coming from the two sides of a bar; this discriminator triggers the operations of sample and hold, amplitude conversion and event time tagging of the two PD signal of the bar. In this case each bar of MCAL acts as an independent detector. Fig. 2. Exploded view of the MCAL elements. The event detected in a bar is combined, in the FEE, with those coming from other bars and then sent to DH system; here a Burst event is formed adding a time tag. In case of coincident events in more than one bar a sparse read-out logic of triggered bars will minimize dead time of the unique converter. The time resolution for BURST events is determined by the

3 discriminator jitter characteristics and is lower than 2 µs. The DH uses the BURST events generated in the MCAL to increment a set of rate-meters on various time scale, these are continuously examinated to detect a burst. The range of MCAL GRID branch of the bars can be selected by Telecommand and can go up to 500 MeV, the range of MCAL BURST branch can be modified up to 250 MeV. MCAL FEE has also the task of managing all the other ancillary functions as Telecommands implementation and HouseKeeping preparation and transmission to DH. I. THE MCAL HARDWARE Many prototypes of the elements that constitute the MCAL instrument have been built and tested. The aims of this activity spanned from verification of detector architecture and construction processes to tuning of the chosen circuits to test of the performances achieved. A. The Scintillator Bar Detectors 30 CsI(Tl) scintillator bars are the active core of AGILE MCAL; each one is 15x23x375 mm in size, with two PIN PD readout. The PDs have the same footprint of the bar smallest side i.e 15x23 mm and an active area of 256 mm 2, the capacitance of each PD is about 130 pf while its leakage current, at room temperature, is about 1.5 na. A permanent optical coupling between PDs and CsI is made by means of a clear siliconic glue; this material has been chosen to realize an elastic bond between two components that exhibits a quite different coefficient for thermal expansion. To maximize the light output and to keep the light attenuation inside the bar within an optimal range of values, the bars surfaces are polished and the bars are first wrapped with a reflective coating and by a thin layer of adhesive Mylar; the whole system is then arranged inside a carbon fiber structure; Fig. 3 shows two bars completely assembled. The characteristics of a CsI(Tl) bar as a detector depend on: - the quantity of scintillation light that can be converted in signal by the PD, this will be referred to as light output seen by one PD for interaction at a fixed distance from it, for example1 cm. - the relation governing the quantity of light seen by one PD for scintillation occurring at different distances from it; this will be referred to as light attenuation law The MCAL bars are prepared in such a way to exhibit an exponential relation for the light attenuation law; this is achieved by proper selection of the CsI(Tl) ingots with Fig. 3. MCAL detector bars inside their housing. constant doping and with proper detector surface treatment and wrapping. The exponential attenuation law of the light is a quite good representation along all the bar but the first few cm near the PD, where border effects are responsible for a deviation of the above law of up to 5 7 %. Due to attenuation of the scintillation light, the signals seen by the two PDs of a bar contain information related to the position of interaction and energy of the detected gamma-ray, that can be reconstructed by proper weighting of the PD signals. The precision of these evaluation depends on the signal amplitude, and on both the statistical and the electronic noise. The same parameters also affect the level of the minimum detectable energy. The scintillator behavior can be verified exposing different positions along the bar detector to a 22 Na source collimated in such a way to illuminate the bar with a spot few mm wide. An example of the raw data collected when the source is collimated at the bar center is depicted in Fig. 4 representing the density of the couple of signals seen by the two PD of the bar. The peaks at 511 and 1275 kev are clearly identifiable as well as the structure induced by Compton events. From these data the following spectra can be derived: - The distributions of events seen by each PD. - The distributions of energy values calculated for each couple of PD signal - The distribution of positions values calculated for each couple of PD value Using the parameter of the two energy peaks in the different spectra evaluated when the source is collimated in different positions all the parameter characterizing the bar can be derived. In the detector tested, the exponential function describes quite well the light attenuation law in the bar; the signal at 1 cm from the PD is always ~22 e-/kev and light attenuation coefficient of the exponential is ~ cm-1. Tl

4 Fig. 4. Representation of the data collected by an MCAL bar exposed to a 22 Na source collimated at the centre of the bar. The coordinate of a point in the plane is obtained using the digital values of the signal on the two bar s PD. On this representation the source is confined around the diagonal of the plane. The two peaks at 511 and 1275 are clearly identifiable as well as the Compton structure of the detected events. From reconstructed energy spectra achieved combining the two PD signals the position of the source peaks is almost constant along all the bar. On the reconstructed energy spectra the energy resolution is about 7.5 % 1275 kev for interactions along all the bar. At the same energy the position resolution, derived from the position spectra, is about 2 cm (1 σ). The same detectors have been extensively tested in the temperature range from -20 to + 40 C operating, during the test, temperature gradients up to 3 C/hr. Energy and position resolutions are almost constant in the temperature range between -20 and + 20 C. At 30 C the energy resolution 1275 kev raises to about 8.5 % while at 40 C the same measured value is about 13 %. An analogue behavior can be measured for the position resolution of reconstructed events that become 2.2 cm (1 σ) at 30 C and 2.9 cm (1 σ) at 40 C. This behavior is the evidence of two concurring effects: the reduction of the PD-preamp electronic noise with the lowering of the temperature and an increase of the scintillation light output with the increase of the temperature. At 40 C the performances of the detector bars are slightly degraded by the great increase of the electronic noise not compensated enough by the scintillator light output. The detectors were further tested with repeated thermal cycles ranging from -20 to + 40 C. After each cycle the main parameters of the bars were measured; the variations in light output and light attenuation coefficient stay inside the statistical variations indicating that the assembly of the detector is strong and suitable for space application. B. The MCAL Front End Electronics A Simplified Engineering Model (SEM) prototype of the MCAL FEE has been built and extensively tested with the aim of verifying the whole system performances. This prototype includes all the functions foreseen in MCAL design, it can handle events both in GRID and BURST mode and performs signal conditioning and analogue to digital conversion of the analogue signals. The number of bar detectors SEM can handle is reduced to 8 instead of the 30 that will be built in the final flight model. Each one of the 16 analogue chains serving the two PD of each bar, deals with the charge delivered by a PD with the following stages: - A low noise charge preamplifier. - A signal shaper amplifier, with shaping time of about 3 µs; this stage is tailored to achieve the best signal to noise performance with the typical CsI(Tl) signal timing and is common to both GRID and BURST chains. The GRID chain is made of the stages: - A signal stretch and hold circuit, the hold command for this stage is generated inside AGILE DH system accordingly with the condition of the other AGILE detectors. In this way the GRID chain of MCAL is slave of Tracker events and conditioned by the anti-coincidence status. When an hold command is issued by AGILE DH all the PD signals are stretched, and will be then A/D converted. - A fast trigger circuit. This stage has been added to deliver, in less than 1 µsec, to DH a digital signal when MCAL detects an event with a total energy greater than 50 MeV. Such an amount of energy released in the AGILE MCAL, actually could cause a backsplash of particles triggering the anticoincidence system, that will thus inhibits data acquisition. In case of a pulse from the MCAL fast trigger circuit, DH will ignore every other signal and will force all the AGILE detectors to data collection. - A multiplexer with the inputs connected to all the stretched PD signals and the output connected to a 12 bit ADC. - Digitized data are stored in a de-randomizing FIFO followed by dedicated GRID interface towards the DH. The BURST chain is made of the stages: - A baseline restorer circuit; - A programmable threshold discriminator circuit. The signal discrimination is made in two steps, a simple threshold level circuit is used to enable a zero crossing discriminator circuitry. This latter stage has been added to increase the timing precision of the burst events that are time tagged on the logical signal from the discriminator. In the BURST chain the detectors act independently from each other and from the other systems. The data produced in this way are continuously processed in the DH for detection of sudden increase of events. The triggers generated by signals above threshold are sent to an FPGA, that verifies the logical conditions for processing the signal, for example, looking at the anti-coincidence status. Only bars with valid triggers will be converted

5 - A multiplexer with the inputs connected to all the stretched PD signals and the output connected to a 12 bit ADC. In the BURST branch just the signal coming from the bars whose discriminator triggered are converted. - Digitized data are stored in a de-randomizing FIFOs followed by dedicated BURST interface towards the DH. MCAL SEM includes also ancillary function as - Dedicated fast interfaces managing the transmission of stored data to AGILE DH. - Handshaking with DH for the complete generation of BURST data, as it is inside the DH that the timing part of these data is created. - A series of circuits for HouseKeeping data generation; this data includes voltages and temperature monitoring and many rate-meters to keep track of the trigger rate of the various discriminators. - A series of circuits for the execution of Telecommands received from DH for FEE setting. - A slow I/O interface that manages the I/O of Telecommands from DH and the transmission to DH of HouseKeeping data. C. The MCAL Test Equipments Dedicated Test Equipment (TE) and science console have been realized for testing and characterizing the bar detectors and SEM FEE prototypes. The bar detector TE is made in such a way to stimulate the detectors with a collimated source whose spot can illuminate different areas of the crystal. The acquisition system collects and stores the PD signal with an electronic chain functionally similar to the BURST chain of the MCAL FEE. A suitable software allows to extract all the detector parameters, from a complete series of measurements achieved exposing the bar to the collimated source in many, usually eleven, positions. The MCAL FEE TE has been build to test the SEM FEE. It accomplishes all the functions made by AGILE DH system to interface MCAL FEE. This TE is composed of a host computer, running Linux, connected to a rack containing several boards installed on a VME bus. The main function of the TE is to control the MCAL instrument configuration and to acquire the instrument data, through the custom made VME boards. A graphical user interface (GUI) provides the operator with the current status of the instrument and graphical widgets for the generation of the TCs. Three VME boards simulate the AGILE data handling unit, providing the acquisition of the events, the on-board time clock for events time tag, the digital house-keepings acquisition and the generation of the TCs which write/read the status and the FEE configuration registers. The host computer is able to archive data on local disc and to forward all the telemetry and TC data packets to the science console through a TCP/ IP connection. In near real time, the raw data packets are archived by the science console, which extracts the events list and archives it in FITS format. The FITS archive is accessed by the quick-look and on-line analysis software, in order to produce the events list histograms for each electronic chain (GRID and BURST) and the count rate time profile, with user adjustable parameters. II. TESTS OF A COMPLETE MCAL PROTOTYPE The MCAL elements as bar detectors and FEE once tested individually were assembled together to form a reduced model of the instrument. This MCAL prototype were used together with those of the AGILE Silicon Tracker and of the Anticoincidence system in repeated test session carried out at CERN laboratories in summer The system was the placed in a beam consisting of high energy charged particles (up to 3 GeV/c) and photons, produced by bremsstrahlung. Between the many purposes of these tests there were to evaluate the response that can be achieved for energy and position reconstruction at these energy. For example, the verification of position resolution in MCAL bars, was measured comparing the value achieved directly with MCAL data with the projection on MCAL of a track detected in Silicon Tracker. Furthermore, from data collected in the BURST MCAL chain, energy responses has been verified for example from spectra where it appears the Landau shaped behavior of energy deposits on the bars due to interaction with high energy charged particles. III. CONCLUSIONS The MCAL instrument is going to carry out in the AGILE satellite the function of the energy estimation of GRID events originating from the interaction of high energy photons in the Silicon Tracker, and the detection of gamma-ray transients coming from the whole sky. Its main characteristics are good signal to noise ratio and low dead time, combined with good timing resolution for BURST events. IV. REFERENCES [1] Tavani, M. et al., The AGILE instrument, in X-Ray and Gamma-Ray Telescopes and Instruments for Astronomy. Edited by Joachim E. Truemper, Harvey D. Tananbaum. Proc. SPIE, Volume 4851, pp (2003) see also [2] Cocco, V. et al, The Science of AGILE: part I Nuclear Physics B Proceedings Supplements, v. 113, iss. 1000, p (2002) [3] Pittori, C. et al.; The science of AGILE: part II, Nuclear Physics B Proceedings Supplements, v. 113, iss. 1000, p (2002) [4] Feroci, M. et al., Super-Agile: the X-ray monitor of the AGILE gammaray mission. In: Proceedings of the IV INTEGRAL Workshop, Alicante (Spain), September 4 8, 09/2001. [5] Barbiellini, G. et al., The next generation of high-energy gamma-ray detectors for satellites: the AGILE Silicon Tracker. In: AIP Conference Proceedings, October 7, 587, pp