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Design and implementation of the Photo-Detector Module electronics for the EUSO-Balloon, prototype of the JEM-EUSO telescope

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P UBLISHED BY IOP P UBLISHING FOR S ISSA M EDIALAB R ECEIVED: May 5, 2015 R EVISED: June 15, 2015 ACCEPTED: July 3, 2015 P UBLISHED: August 17, 2015

S ECOND I NTERNATIONAL S UMMER S CHOOL ON I NTELLIGENT S IGNAL P ROCESSING FOR F RONTIER R ESEARCH U NIVERSITY PARIS -D IDEROT C AMPUS , PARIS , F RANCE 14–25 J ULY 2014

AND I NDUSTRY

A. Jung on behalf of the JEM-EUSO Collaboration Laboratoire AstroParticule et Cosmologie, Universit´e Paris Diderot-Paris 7, 10, rue Alice Domon et L´eonie Duquet, 75205 Paris Cedex 13, France

E-mail: [email protected] A BSTRACT: A key feature of the JEM-EUSO observatory for Ultra High Energy Cosmic Rays (UHECRs) search is the electronics, and in particular the Photo-Detector Module (PDM) on the focal surface of an advanced diffractive optical system. As a pathfinder experiment for this spacebased mission, over the last 3 years the EUSO-Balloon project has been developed to observe the ultraviolet background from the edge of the atmosphere on board a stratospheric balloon. August 2014, the EUSO Balloon was successfully operated 8-hour balloon flight over Timmins, Ontario, Canada. The EUSO-Balloon experiment uses a detector consisting of one Photo-Detector Module, identical to the 137 modules that will be present on the JEM-EUSO focal surface. UV light generated by ultra high-energy air showers passes the optics, UV filter, and impacts the Multi Anode Photo Multiplier Tubes (MAPMTs). UV photons are converted by the MAPMT photocathode into electrons, which are multiplied by the MAPMTs dynode and fed into a Elementary Cell - Application Specific Integrated Circuit (EC-ASIC) boards. These EC-ASIC boards in turn perform the photon counting and charge estimation, by counting the single photoelectron emitted during each Gate Time Unit (of 2.5 us each). The PDM board interfaces with these ASIC boards, providing them with power and configuration parameters as well as collecting data from the external trigger. In this paper, I will describe the details of the design and the fabrication of the PDM, as well as its EUSO-Balloon flight results. K EYWORDS : Digital signal processing (DSP); Trigger concepts and systems (hardware and software); Modular electronics

c 2015 IOP Publishing Ltd and Sissa Medialab srl

doi:10.1088/1748-0221/10/08/C08014

2015 JINST 10 C08014

Design and implementation of the Photo-Detector Module electronics for the EUSO-Balloon, prototype of the JEM-EUSO telescope

Contents Introduction

1

2

Overview of the Photo-Detector Module (PDM)

3

3

Photo-Detector Module (PDM) board 3.1 Hardware 3.2 Software

3 5 5

4

Perspective

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5

Conclusions

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1

Introduction

JEM-EUSO is a space mission devoted to the exploration of the origin and propagation of Ultra High Energy Cosmic Rays (UHECRs) above the GZK-cut-off through the observations of their arrival directions and energies, hosted on-board the Japanese Experimental Module in the International Space Station (ISS). This is a new type of observing technique that will utilize an extremely large volume of the earth’s atmosphere as a detector of high energy particles. JEM-EUSO provides a very wide Field-of-View (±30 ◦ ) with a Fresnel lens based optics, recording Extensive Air Showers (EAS) with a time resolution of 2.5 microseconds [1–5]. Two pathfinder experiments were prepared. One demonstrated the capability to detect a fraction of the air showers with the EUSO telescope (EUSO-TA) [5, 6] on the ground, and the other observed the luminous background from the edge of the atmosphere and detected UV signals emitted from a laser mounted on a helicopter (EUSO-Balloon) [5, 7]. Last August, the EUSO-Balloon project successfully flew a prototype of the JEM-EUSO telescope over Timmins, Ontario, Canada, achieving a duration of approximately 8 hours. The EUSO-Balloon telescope has an optical design employing a Fresnel lenses, a diagram of which can be seen in figure 1. Figure 2 shows an overview of the JEM-EUSO readout, trigger, and control architecture. Analog signals from the Multi-Anode Photomultiplier Tube (MAPMT) photocathode count the single photon detections emitted during each Gated Time Unit (GTU) of 2.5 us and upon receiving a sufficient signal, initiate the trigger. The Photo-Detector Module (PDM) electronics receive digitized data from the front-end Application Specific Integrated Circuits (ASICs) [8] and transmit it to the Cluster Control Board (CCB) [9] on positive triggers [4]. Data from the CCB are transferred to the CPU, which controls the instrument and interfaces telemetry. The CPU manages a disk for onboard storage and the collection of housekeeping data from a dedicated Housekeeping Board [10].

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Figure 2. Overview of JEM-EUSO readout, trigger, and control architecture.

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Figure 1. The EUSO-Balloon flight illustration and the EUSO-Balloon telescope with the optical and instrument parts annotated. Note: during the first flight, the second lens, #2 was not installed.

2

Overview of the Photo-Detector Module (PDM)

The detector consists of a single PDM module similar to the 137 that will be used on the JEMEUSO focal surface. The PDM includes MAPMTs, a set of boards developed at LAL and the Spatial Photomultiplier Array Counting and Integrating Chip (SPACIROC) ASIC developed by OMEGA [11] for analog signal processing, the PDM board for digital signal processing, the Low Voltage Power Supply (LVPS) board for PDM board power, the High Voltage Power Supply (HVPS) board for MAPMT power, and the Housekeeping (HK) board for monitoring, and the support structure, as can be seen in figure 1 and 3.

2015 JINST 10 C08014 Figure 3. PDM structure includes UV filters, MAPMTs, EC-ASIC boards and PDM board.

3

Photo-Detector Module (PDM) board

Fluorescence photons produced in the EAS are focused by Fresnel lenses, and detected by the array of 36 MAPMTs. The signals from the MAPMTs are fed into the Elementary Cell - Application Specific Integrated Circuit (EC-ASIC) boards. The purposes of the EC-ASIC boards are photon counting and measurement of photon energies. The photoelectron signals from the MAPMTs are converted into digital signals using SPACIROC, which are measured with an interval of 2.5 us. The

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total charges from the MAPMTs are summed for each photon event which is proportional to the incident photon energy. This is converted to a pulse duration time via the Charge to Time (Q-to-T) conversion by measuring the signal duration over a fixed threshold. This conversion is done by a dedicated 9-channel converter termed the KI block. Figure 4 shows general architecture of the SPACIROC ASIC (see [8]).

The SPACIROC ASIC readout system is in charge of the PDM board, and is also the most challenging component of the PDM. It is made up of a compacted and highly integrated set of Printed Circuit Boards (PCBs) and Field-Programmable Gate Arrays (FPGAs). The data size from the 36 MAPMTs will be more than 331 kByte per event. The 36 SPACIROC ASICs sends digitized data to PDM board at around 8.3 Gbps. The PDM board has to be specifically designed to conduct data transfers at these high speeds. Moreover, PDM board can control SPACIROC ASICs. One of parameter of SPACIROC ASIC is width adjust which is to adjust the integrated signal from the current network and set the baseline voltage. Figure 5 shows MPAMT KI analog output (red), gate time unit clock (green), and supply voltage (blue). Altering the ASIC parameters yields significantly different KI analog output waveform shapes as shown in the difference between the two outputs shown. We are currently optimizing the ASIC parameters to provide the most useful output waveforms.

Figure 5. Measurement to alter the ASIC parameters yields significantly different KI analog output waveform shapes.

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Figure 4. SPACIROC ASIC general architecture.

3.1

Hardware

3.2

Software

Figure 6 shows the functional blocks of the PDM board FPGA, which include the automatically changing efficiency of high voltage, the trigger calculation unit, the PDM board controller, and the interfaces for the EC-ASIC, CCB, HVPS and HK boards.

Figure 6. Functional blocks of FPGA on PDM board

The SCU (System Control Unit) controls the operations of the PDM system, the ERCU (ECASIC Readout and Control Unit) provides the interface between the EC-ASIC and PDM by customized SPI communication, CIU (CCB Interface Unit) interfaces between PDM and CCB by

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The PDM board is located at the bottom of the 6 EC-ASIC boards, each consisting of 6 MAPMTs, as show in figure 3. The hardware of the PDM board performs the following functions: 1) power distribution to 6 EC-ASIC boards and PDM boards, 2) input/output connectors, 3) provides a 40 MHz Crystal Oscillator for system operations, 4) provides the FPGA and Programmable ReadOnly Memory (PROM) necessary for a programmable logic device, and 5) housekeeping for monitoring the instrument voltage and temperature. The design of the PDM board architecture is also suitable for use in the EUSO-Balloon project. Our work is focused on a Virtex 6 FPGA chip from Xilinx, which contains about 240,000 logic cells with about 14 Mbits RAM blocks.

4

Perspective

Among the recent progress in the context of the above research, a key step has been the flight of the EUSO-Balloon telescope in August 2014, launched into the stratosphere with a helium-filled balloon as shown in figure 1. The flight has been a success and allowed the JEM-EUSO collaboration to validate the instrumental concept and behavior of all subsystems. In addition data has been taken, which are currently under analysis and will be presented in forthcoming conferences. Further tests of the instrument will be performed in the near future through additional pathfinders of JEM-EUSO, notably with a long duration balloon flight and the space mission MINI-EUSO that will operate from the International Space Station.

5

Conclusions

The design of the PDM board has been finalized and constructed. It processes the readouts of the 36 MAPMTs via 6 EC boards, interfaces the PDM board to the CCB, HK and HVPS except for the Level-1 trigger. These functions are implemented in a FPGA of Virtex 6 XC6VLX-240T. This PDM board was able to accurately measure the luminous background under different cloud conditions during the EUSO-Balloon experiment.

Acknowledgments We wish to thank the CNES for the support to the EUSO-Balloon project. We also thank Korean Creative Research Initiatives (RCMST) of MEST/NRF, Basic Science Research program of MEST/NRF (2010-0025056) as well as CNRS/IN2P3 for their direct support.

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customized SPI communication, and the HKU (HK interface Unit) provides interfaces between PDM and HK. In addition, the TPU (Trigger Processing Unit) determines the best trigger between simultaneous trigger signals, and the MCU (Memory Control Unit) controls the processing of PDM data, while the ECU (Efficiency Control Unit) controls the interface between PDM and HVPS, and the RSU (Run Summary Unit) controls the creation of the run summary. It is implemented on the Virtex 6 XC6VLX-240T FPGA from Xilinx. The MAPMTs in such an experiment can suffer from an excess of false positives due to stray light. Sources of stray light include the light from cities, lightnings, meteors, airglow, and aurora, as well as scattering sources of radiance such as celestial sources in the ultraviolet including stars, planets, comets, and possible scattering from dust. In order to protect them from too much current in the last dynodes, a novel method is applied: the voltage to the cathode is decreased, while the dynodes voltage remains untouched. Then, the PMT gain stays the same (single electron gain), but the collection efficiency between cathode and dynode 1 is applied in a controlled way. This has the advantage of being very fast: < 2.5 us. The command to lower or increase the cathode voltage is given by the PDM-board, according to the KI. The L1 level trigger is a pattern recognition trigger in 3 × 3 pixel boxes: it looks for a track due to a cosmic ray shower or laser, the intensity of which is above the background [12]. The EUSO-Balloon telescope, in its first flight configuration, doesn’t include the trigger algorithm and run summary.

References [1] JEM-EUSO collaboration, Y. Takahashi, The JEM-EUSO mission, New J. Phys. 11 (2009) 065009 [arXiv:0910.4187]. [2] T. Ebisuzaki et al., The JEM-EUSO Project: Observing Extremely High Energy Cosmic Rays and Neutrinos from the International Space Station, Nucl. Phys. Proc. Sup. B 175 (2008) 237. [3] JEM-EUSO collaboration, F. Kajino, The JEM-EUSO mission to explore the extreme universe, Nucl. Instrum. Meth. A 623 (2010) 422.

[5] A. Haungs, Status of JEM-EUSO and its test experiments euso-balloon and ta-euso, EPJ Web of Conferences 52 (2013) 06005. [6] M. Casolino et al., Calibration and testing of a prototype of the JEM-EUSO telescope on telescope array site, EPJ Web of Conferences 53 (2013) 09005. [7] P. von Ballmoos et al., A balloon-borne prototype for demonstrating the concept of JEM-EUSO, Adv. Space Res. 53 (2014) 1544. [8] S. Ahmad, P. Barrillon, S. Bondil-Blin, S. Dagoret-Campagne, C. de La Taille, F. Dulucq et al., SPACIROC: A rad-hard front-end readout chip for the JEM-EUSO telescope, 2010 JINST 5 C12012. [9] JEM-EUSO collaboration, J. Bayer et al., The Cluster Control Board of the JEM-EUSO mission, in proceedings of the 32nd International Cosmic Ray Conference 3 (2011) 172. [10] J. Rojas Garces, L. Santiago Cruz, G. Medina Tanco, C. Lopez Lopez, S. Silvaran Guerrero and A. De la Cruz Mart´ınez, Housekeeping subsystem for the EUSO-Balloon project, in preparation. [11] JEM-EUSO collaboration, P. Barrillon et al., The Electronics of the EUSO-Balloon UV camera, in proceedings of the 33rd International Cosmic Ray Conference (2013) 115. [12] M. Bertaina, T. Ebisuzaki, T. Hamada et al., The trigger system of the JEM-EUSO Project, in proceedings of the 30th International Cosmic Ray Conference 5 (2008) 1049.

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[4] JEM-EUSO collaboration, I.H. Park et al., The Development of Photo-Detector Module Electronics for the JEM-EUSO Experiment, in proceedings of the 32nd International Cosmic Ray Conference 3 (2011) 305.