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Author's personal copy Nuclear Instruments and Methods in Physics Research A 732 (2013) 320–324
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Euso-Balloon: A pathfinder mission for the JEM-EUSO experiment Giuseppe Osteria a,n, Valentina Scotti a,b a b
Istituto Nazionale di Fisica Nucleare Sezione di Napoli, Naples, Italy Università di Napoli Federico II, Dipartimento di Fisica, Naples, Italy
on behalf of the JEM-EUSO Collaboration1 art ic l e i nf o
a b s t r a c t
Available online 29 May 2013
EUSO-Balloon is a pathfinder mission for JEM-EUSO, the near-UV telescope proposed to be installed on board the ISS in 2017. The main objective of this pathfinder mission is to perform a full scale end-to-end test of all the key technologies and instrumentation of JEM-EUSO detectors and to prove the entire detection chain. EUSO-Balloon will measure the atmospheric and terrestrial UV background components, in different observational modes, fundamental for the development of the simulations. Through a series of flights performed by the French Space Agency CNES, EUSO-Balloon also has the potential to detect Extensive Air Showers (EAS) from above. EUSO-Balloon will be mounted in an unpressurized gondola of a stratospheric balloon. We will describe the instrument and the electronic system which performs instrument control and data management in such a critical environment. & 2013 Elsevier B.V. All rights reserved.
Keywords: Astroparticle detectors Cosmic rays Balloon born experiment Electronics
1. Introduction
Electronics: Focal surface electronics, trigger, data acquisition and controls.
The Extreme Universe Space Observatory on board the International Space Station's (ISS) Japanese Experiment Module (JEMEUSO) is a new type of observatory which observes transient luminous phenomena occurring in the Earth's atmosphere [1]. The main objective of JEM-EUSO is to investigate the nature and origin of Ultra High Energy Cosmic Rays, UHECRs ðE 4 5 1019 eVÞ, which constitute the most energetic component of the cosmic radiation. The instrument is planned to be attached to JEM Exposed Facility of ISS during the first half of 2017 for a 3 years long mission. The JEM-EUSO instrument is a wide-angle refractive telescope in the wavelength of near Ultra Violet (UV) observing fluorescence and Cherenkov photons emitted by air showers created by UHECRs in the Earth's atmosphere. It consists basically of four parts:
Optics: three high transmittance optical Fresnel lenses (dia
n
meter of 2.35 m) focusing the arriving UV photons onto the Focal Surface (FS). Focal surface detector: 4936 Multi-Anode Photo-Multiplier Tubes (MAPMTs) of 64 pixels each.
Corresponding author. Tel.: +39 81676167. E-mail address:
[email protected] (G. Osteria). 1 http://jemeuso.riken.jp.
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.05.124
Mechanical structure. An Atmosphere Monitoring System (Infra Red (IR) camera and Lidar) and a calibration system complete the apparatus. The mission consortium includes 17 countries and is lead by RIKEN (Japan), in coordination with the Japanese Space Agency (JAXA). EUSO-Balloon is developed by the JEM-EUSO consortium as a demonstrator for technologies and methods featured in the space instrument. The EUSO-Balloon mission has been proposed by a collaboration of three French laboratories (APC, IRAP and LAL) involved in the international JEM-EUSO consortium. The instrument is being built by various institutes of the JEM-EUSO collaboration. Balloon flights will be performed by the balloon division of the French Space Agency CNES. The first flight is scheduled in 2014.
2. Objectives for EUSO-Balloon EUSO-Balloon (see Fig. 1) is a technology demonstrator for the JEM-EUSO mission, so its main goal is to demonstrate the operation of its crucial components like High Voltage system, trigger, etc. “in (worst) space environment”. The instrument can perform measurement of the background, which includes star light, airglow, light from artificial sources, in the near UV region from a float altitude of about 40 km. Since JEM-EUSO is sensitive to the variation of the background sources in this UV range, this
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Fig. 1. Concept of the EUSO-Balloon instrument with crash pads. Fig. 2. A schematic view of the EUSO-Balloon telescope.
measurement is an important step for successful operation and optimization of the working mode of the JEM-EUSO mission. Finally EUSO-Balloon will be able to detect for the first time airshowers and laser induced events by looking downward from the edge of space. According to our estimation we expect to get 2–3 events/flight at energy above 1018 eV.
3. The EUSO-Balloon instrument EUSO-Balloon will consist of a Fresnel optics made from 3 PMMA square lenses (UV transmitting Polymethyl-Methacrylate), a focal plane detector made from a single PDM (PhotoDetector Module) and a Data Processor (see Fig. 2). The balloon-borne instrument points toward the Nadir monitoring a 121 121 wide Field Of View (FOV) in a wavelength range between 290 and 430 nm, at a rate of 4 105 frames/s. The FOV will be resolved into 2304 pixels. The pixel size in the FOV is 0.251 0.251 corresponding to 175 m 175 m on the ground, for a float level of 40 km. In order to monitor the actual cloud covers, a co-aligned IR camera will observe the FOV of the main instrument [2]. The UV light collected by the optical system is detected by the MAPMTs of the PDM. The PDM is mounted on a translation stage inside the telescope to adjust its position along the z-axis. Besides monitoring the UV background, the PDM detects candidate shower events by the first-level trigger implemented in the PDM-board. Data acquired by the PDM are transferred via the PDM-board to the DP component which performs data management and storage, instrument control and commanding. The telescope is composed of a self-contained, watertight instrument booth containing PDM, electronics, telemetry and an optics module serving as optical bench for the lenses. The structure of both the instrument booth and the optics module is made of 10 mm “Fibrelam” aerospace panels. The total weight of structure, instrument, dedicated electronics, telemetry system and batteries is around 250 kg.
4. Instruments sub-systems The three main components of the instrument, the Optical System, the PDM and the DP are, in turn, divided into several subsystems. A block diagram of the instrument summarizing the subsystems, and the main sub-assembly items are shown in Fig. 3.
In what follows we will describe the three main components and their sub-assembly items in more detail. 4.1. Optical system The optical system focuses the incoming UV photon toward a pixel of the detector on the optical focal surface. The FOV of the optics is 121 121. A 1 m long baffle defines an Entrance Pupil Diameter, the aperture of the optical system is 1 m 1 m. The distance between the front surface of the front lens and the focal surface is 1620.7 mm. Each lens has a 1 m2 square surface, a thickness of 8 mm and a weight of 9.6 kg. The 1st and 3rd lenses are Fresnel lenses, the 2nd lens is a diffractive lens. 4.2. Photo-Detector Module The PDM is composed of 36 MAPMTs, containing 64 anodes each, equipped with UV filters and their associated electronic chain. A schematic view of the PDM is shown in Fig. 4. On the mechanical structure the MAPMTs are grouped into 9 Elementary Cells (EC) which supply the 14 voltages needed for each MAPMT and collect the signals from their anodes. These signals are transmitted to the SPACIROC ASIC [3] for processing. The output signals of all the ASICs are delivered to the PDM board [4]. The PDM is equipped with two High Voltage Power Supplies (HVPS) (based on a Cockroft–Walton design) providing the necessary voltages to the MAPMT dynodes. To cope with strong light signals (the dynamic scale of the signal ranges from 1 to 106), the HVPS are provided with switches that can modify the voltage between the cathode and the first dynode reducing the electron flux reaching the anodes of the MAPMTs. 4.2.1. Photomultipliers and filters MAPMTs are the photo-detectors that sense the UV photons arriving through the lenses. A color glass filter “SCHOTT BG3”, which transmits the UV photons and absorbs the visible light, is glued on each MAPMT. The MAPMTs are manufactured by Hamamatsu and have been developed specifically for the JEM-EUSO mission. The MAPMT has a maximum sensitive area of 23.04 mm 23.04 mm, a quantum efficiency greater than 35% and a cross talk of about 1%. 4.2.2. ASICs The ASIC (Spaciroc) has been designed by Omega group at LAL in AMS 0:35 μm SiGe technology. It reads out the MAPMT anode
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Fig. 3. EUSO-Balloon functional block diagram.
Fig. 4. The schematic view of the Photo-Detector Module.
signals. It has two main purposes: counting the number of photoelectrons reaching each pixel of the MAPMTs and measuring the intensity of the photon flux by performing charge-to-time conversion. The ASIC digitizes the analog signal inputs and sends the data to the PDM board via dedicated digital connections. The measured power consumption (static) is around 1.17 mW/channel. 4.3. PDM board The PDM board handles all the EC units. It performs the 1st level trigger looking for pixels firing for several contiguous Gate Time Units (GTU) on the PDM. It manages the interfaces with the EC boards, CCB board as well as the HV system. The average power consumption of the PDM board is less than 8 W. 4.4. The data processor The DP system is the component of the electronics subsystem which includes most of the digital electronics of the instrument. It controls the front-end electronics, performs the 2nd level trigger filtering, tags events with arrival time and payload position, manages the Mass Memory for data storage, measures operating
and dead time of the instrument, provides signals for time synchronization of the event, performs housekeeping monitoring and handles the interface to telecommands and to the telemetry system. The DP functionalities are obtained by connecting different specialized items, which form a complex system. The main sub-assembly items are:
Control Cluster Board (CCB) CPU Data Storage (DST) House-Keeping System (HK) Clock Board (CLKB) GPS Receiver (GPSR) Data Processor Power Supply (DP-LVPS)
The CCB [5] has a direct connection to the PDM through a 40MHz parallel bus. It processes and classifies the received data performing the 2nd level trigger. The circuit is developed around a Xilinx Virtex-4 FX-60 with extended industrial temperature range on a eurocard board. The CPU receives science data from the CCB and from the CLKB [6]. According to the classification operated by the CCB, the CPU
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will decide for each event if data will be stored on board, transmitted to ground via telemetry system or both. For each trigger roughly 330 kB of data are transmitted via SpaceWire protocol from CCB to the CPU. The interface which transforms SpaceWire data packets into a data stream compatible with the port/expansion available on the CPU boards is a commercial offthe-shelf device, the “SpaceWire to PCI MK2” board manufactured by Star Dundee Ltd. The CPU Motherboard is the iTX-i2705 model, manufactured by Arbor. It is based on an Atom N270 1.6 GHz processor. The power consumption of the board is less than 12 W. The mass storage is composed of an array of two Solid-State Drives (SSD) operating in disks fault-tolerant mode RAID-1 (Redundant Array of Independent Disks). The selected SSD devices are the 1 TB CZ Octane SATA II 2.5 in. SSD drive with a SATA II interface. The CLKB [7] is the part of the data processor that ensures the time synchronization of the events. It generates the system clock and the synchronization signal and distributes them to all the devices of the FS electronics. The board has an interface with a GPS receiver which allows to collect, for each event, information on the position of the instrument and the UTC time with a precision of a few microseconds. The Clock Board receives the 2nd level trigger signal from the CCB and a release-busy signal from CPU. These two signals drive the logic implemented on the board to measure the live time and dead time of the apparatus. The CLKB receives commands from CPU and transmits data to the CPU by using the SpaceWire communication protocol. An FPGA Xilinx Virtex5 XC5VLX50T (Industrial grade) implements all the required functionalities of the CLKB. The House-Keeping system collects telemetry from several subsystems of the instrument in slow control mode. It is responsible for monitoring voltages and currents of the Low Voltage Power Supply, has a serial bus to convey telemetry and telecommands through the CPU interface and to other sub-systems. The HK is implemented around an off-the-shelf microcontroller board (Arduino Mega 2560), combined with 5 custom-made protocol interface boards to pre-process the various signals. The DP is hosted in a Eurocard 6U subrack with 3U dividers. Each sub-assembly, in turn, is hosted in a metallic frame-type plug-in unit which accepts boards in eurocard format. The DP mass and power budgets are 15 kg and 50 W, respectively.
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used for JEM-EUSO has been modified by scaling for the altitude of the instrument, changing the surface parametrization and introducing the new optical system and field of view. According to the results of the simulation, the instrument is able to detect and image EAS with energies above 1018 eV that might develop in its FOV. Fig. 5 shows an example of a signal track reconstructed on the PDM for a proton of 1018 eV and an inclination relative to the nadir of 301 landing inside the FOV of the instrument. In Fig. 6 the black, red and blue curves give preliminary results on the performance for a 10 h duration flight for different assumptions of background and cosmic ray flux. Due to the low cosmic ray flux the detection of a couple of events will require several days exposure time; therefore the detection of the first air shower from the edge of the space in UV will most probably require more than one flight.
Fig. 5. Track reconstructed on PDM for a proton of 1018 eV and an inclination relative to the nadir of 301.
5. Status The systems, sub-systems as well as sub-assemblies of the instrument have been realized as prototypes in different laboratories of most of the countries composing the JEM-EUSO collaboration. Since the EUSO-Balloon components are identical (PDM, trigger) or very similar (lenses) to the JEM-EUSO components, the relevant institutions within the JEM-EUSO collaboration have contributed to the instrument according to their corresponding tasks and responsibilities in JEM-EUSO. The prototypes of the components of EUSO-Balloon have been then integrated at Riken obtaining a full prototype of the instrument in perfect working order. This prototype will be installed during Spring 2013 in the Telescope Array (TA) site. On this site it will be possible to cross-calibrate the instrument with the TA Fluorescence Detector through noise background comparison in various situations. An absolute calibration of the instrument will be performed acquiring data during Lidar or electron beam shots. Finally, by using the TA trigger signal it will be possible to acquire showers in coincidence with TA. 6. Simulation The EUSO Simulation and Analysis Framework has been used to simulate the response of the instrument [8]. The configuration
Fig. 6. Preliminary results on the expected number of events for a 10 h duration flight for different assumptions of background and cosmic ray flux. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
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7. Conclusion
References
EUSO-Balloon will image the UV sky background (star light, airglow, artificial lights) in the bandwidth used by the JEM-EUSO mission. All the key components and the relative sub-assembly items will be tested according to the configuration foreseen for the JEM-EUSO mission, in particular, the trigger scheme and its capability to cope with the variable sky conditions. Simulation studies have been performed to understand the structure of the expected signal and the effective energy threshold of EUSO-Balloon and its possibility to detect Extensive Air Showers. Results confirm the capability of the instrument for detecting primary cosmic rays of energy E 41018 eV. Everything is on schedule for the first flight from Timmins (Canada) in Spring 2014.
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