- Email: [email protected]
The extreme universe space observatory (EUSO) instrument
PROCEEDINGS SUPPLEMENTS Nuclear Physics B (Proc. Suppl.) 113 (2002) 329-336
ELSEVIER
www.elsevier.com/locate/npe
The Extreme Universe Space Observatory (EUSO) Instrument A. Petrolini” a Dipartimento di Fisica dell’Universit& on-behalf of the EUSO Collaboration.
di Genova & INFN, Via Dodecaneso
33, I-16146 Genova, Italy;
The EUSO experiment is designed to study the Extensive Air Showers produced in the atmosphere by Extreme High Energy Cosmic Rays from an Observatory installed on the International Space Station. The project is currently under phase A study by ESA. The basic facts of current design of the EUSO Instrument are described.
figure 1)
1. EUSO
The interpretation of the fenomenology of the Extreme High Energy Cosmic Rays (EHECR) is one of the most interesting and challenging topics of modern astro-particle physics [l]. EHECR reach the Earth with a low flux, of the order of T = 0.01 particle year-l km-‘, and therefore a sophisticated detector is required. The EUSO (Extreme Universe Space Observatory) experiment, proposed to the European Space Agency (ESA) for installation on the International Space Station (ISS), has just started The experiment goal is its phase A study. the study of the Extreme High Energy part (E ;s 3.10 rg eV) of the EHECR energy spectrum, by observing from Space the Extensive Air Showers (EAS) produced by the interaction of the primary EHECR with the atmosphere. This is accomplished by installing EUSO on the ESA Columbus External Payload Facility (CEPF), looking down to nadir during night-time. A large mass of atmosphere can be observed from Space. An EAS can be detected by observing the air scintillation light, isotropically produced during the EAS development, as well as the forward beamed Cherenkov light, diffusely reflected by the Earth. The scientific objectives of EZJSO are described in another contribution to this conference [2].
2. The observational
technique
EUSO is based on the AirWatch concept, originally proposed by John Linsley more than twenty years ago [3,4]: to observe EAS from Space (see 0920~5632/02/$
- see
front matter 0
PII SO920-5632(02)01860-l
2002
Elsevier Science B.V
Figure
1. The EUSO observational
approach.
The observation from Space of the EAS produced air scintillation light and of the Cherenkov light diffusely reflected by the Earth by means of a fast and high-granularity photo-detector allows to record the EAS development in the atmosphere. In fact the scintillation light is proportional, at any point along the EAS development, to the All rights
reserved.
A. Petrolini/Nuclenr-
330
Physics B (Proc. Suppl.)
number of charged particles in the EAS. The additional observation of the diffusely reflected Cherenkov light (reflected either by land, sea or clouds) provides additional information, useful to improve the EAS reconstruction. It is therefore possible to estimate the energy and arrival direction of the primary EHECR, and to gather information about its nature. EUSO will observe the Earth atmosphere at night by looking down to nadir with a large aperture and large Field of View (FoV) optics, focusing the image onto the Focal Surface (FS) photodetector. The latter must have a good sensitivity to be able to detect the faint signal from the less energetic EAS, allowing to connect the observed energy spectrum with the observations from ground-based experiments. A sufficiently fast detector allows one to determine the direction of the primary EHECR by means of one single observatory. The required information can nevertheless be obtained with a system having only a limited spatial resolution. The peculiar characteristics of the EAS, including the kinematical ones, allow one to distinguish them from the various backgrounds, because those have a typically different space-time development. Key points of the observational technique are the following. l
l
l
l
A large geometrical aperture can be obthanks to the large distance, tained, depending on the Instrument FoV. A large mass of atmosphere (the calorimeter medium) can therefore be observed. Observation of deeply penetrating EAS, from primary particles interacting deeply in the atmosphere, is possibile, by the direct observation of the EAS development and starting point. All sky coverage is possible with one single apparatus. The approach is complementary to the ground-based one. In fact the Space experiments are best suited for the observation of higher energy cosmic rays compared to ground-based experiments. However an overlap of the observed energy spectrum with the one known from ground-based ex-
I13 (2002) 329-336
periments is required for a better comparison. Moreover the systematic effects are different in the two approaches. 2.1. The scintillation light The scintillation light is isotropic and proportional, at any point, to the number of charged particles in the EAS, largely dominated by electrons and positrons. The total amount of light produced is proportional to the primary particle energy and the shape of the EAS profile (in particular the atmospheric depth of the EAS maximum) contains information about the primary particle identity. The scintillation yield in air, Ya+., in the 330 nm + 400 nm wavelength range, is about Ya+ = 4.5 photons per charged particle per meter at any height h 5 20 km, depending, in a known way, from altitude and air composition. The main emission lines are located at the three wavelenghts 337 nm, 357 nm and 391 nm. 2.2. The Cherenkov light The possible observation of the Cherenkov light diffusely reflected by the Earth (by land, sea or clouds) will help the EAS parameters determination While the amount of observed Cherenkov photons depends on the reflectance and geometry of the impact surface, the directionality of the Cherenkov beam provides a precise extrapolation of the EAS to the first reflecting surface. 2.3. The atmosphere The atmosphere is relatively transparent down to x M 330 nm, where the ozone absorption becomes strong. Preliminay simulations, based on the LOWTRAN7 code [5], show that for typical cloud-less atmosphere models the vertical transmission coefficient from ground to EUSO is larger than t = 0.4, in all the interesting wavelenght range, and losses are dominated by Rayleigh scattering. The main atmospheric components affecting the signal transmission are Rayleigh and Mie scattering, ozone absorption (severe up to X 2: 330 nm), clouds (affecting either signal transmission and EAS characterization). Real time measurements of these components might improve the EUSO performance (section 3.5).
A. Petrolini/Nuclear
2.4.
Physics B (Proc. Suppl.) 113 (2002) 329-336
The background
Many different kind of backgrounds are expected including: man-made lights, auroras, natural photo-chemical effects (in atmosphere, sea and land), low-energy cosmic rays, reflected The observation of moon-light and star-light. some atmospheric phenomena, such as meteors, is interesting in itself and is currently under study. A selective and efficient trigger is required to distinguish the EAS from the background. However the typical characteristics of EAS are quite different from the background ones, in particular the kinematic characteristics. The backgrounds, for instance, typically have a time-scale of the order of ms. Another type of background is the random night-glow which has been measured, most recently, by the BABY experiment [6]. The resulting estimate for EUSO is B x 3.1O’r photons mm2 s-l str-‘, in the wavelenght range 330 nm 5 X 5 400 nm at h M 400 km heigth. The observation
2.5.
duty cycle
EUSO will spend a large part of its time above the seas, which appear to be the a good site for observation due to the little man-made light, maximum possible atmospheric depth and rather uniform surface properties. The duty cycle has been estimated taking into account: the ISS night time; ground locations with significant light output, natural or anthropomorphic; the lunar cycle; clouds in the FoV affecting the detection or interpretation of the EAS; ISS activities or contingencies that do not allow normal operation. The preliminarly estimated duty cycle is about n N 0.1. Observation under some moon-light is possible, even if this will imply an energy detection threshold higher than normal due to the increased background rate. 2.6. On-going support measurements A series of activities have been started, or are planned, to improve the knowledge of significant parameters affecting the EUSO performance. l l
Night-glow background measurements [6]. Measurement of the Cherenkov light diffuse reflection coefficient, from land, clouds and
l
331
sea, in different conditions, including lowenergy EAS observation [7]. Measurement of the air scintillation yield in different conditions, including the study of the effect of humidity.
3. The EUSO
Instrument
The design of an Instrument to look from Space to the EAS produced in the atmosphere by EHECR is a challenging task mainly because the EHECR flux reaching the Earth is very small, the observable signal is vary faint and the apparatus has to operate in Space. The main Instrument requirements are the following. The Instrument must collect as many photons as possible, in order to be able to detect the faint signal from the less energetic EAS and discriminate it from the background. As a consequence a large aperture is required, as well as a good transmission of the optical elements and good photon detection efficiency in the 330 nm + 400 nm wavelength range. In fact a sufficiently low lower energy threshold is mandatory to connect the energy spectrum observed by EUSO with the observations by groundbased experiments, operating at a lower energy range. A large FoV is required to be able to observe a mass of atmosphere as large as possible, thus increasing the expected event rates. The physics requirements can be obtained with a system having an angular granularity sufficient to ensure an angular resolution on the EAS direction of Acu = 1”. A single-photon detector is required, fast enough to be able to follow the space-time development of the EAS and reconstruct the EAS kinematical parameters from one single observation point. A low noise and good signal to noise ratio are required to detect the faint signal produced by the less energetic EAS and discriminate it from the background. Small cross-talk and after-pulse rate are required
332
A. Petrolini/Nuclear Physics B (Proc. Suppl.) 113 (2002) 329-336
l
l
l
to avoid degradation of the energy and angular resolution. An efficient and reliable trigger system, capable of a good background rejection, is required to cope with the limited data storage, data transfer and computing capabilities available on-board. All the constraints and requirements related to the Space mission have to be accounted for. Mandatory characteristics are therefore: a compact and robust system with low mass, volume and power consumption, good reliability and time stability, radiation hardness and low sensitivity to magnetic fields of the order of the gauss. A system is required to protect the Instrument from possibly dangerous environmental factors, including intense light.
3.1. The main Instrument parameters The present provisional EUSO parameters, the ones most relevant to the photo-detector design, are summarized in table 3.1. 3.2. The optics 3.2.1. Optics requirements The main requirements for the primary optics, deriving from and making explicit the general requirements in section 3. follow. Large aperture, D 2 2 m diameter. Large FoV, y 2 30” (half-angle). Good transparency in the wavelength range 330 nm 2 X < 400 nm. Angular resolution Ao = 0.1”. Compatibility with Space mission requirements, including low mass, radiation hardness, suitable structural properties, high reliability and operating stability over a period of a few years. These requirements point to the use of a Fresnel lens system made of light-weight polymers. 3.2.2. Optics baseline The preliminary baseline design, consistent with the parameters in table 3.1, is based on two double-sided, curved, 2.5 m diameter thin Fresnel lenses, made of smaller pieces and kept together
by a light supporting structure. The resulting spot size diameter on the FS is in the range 3 mm t 6 mm, depending on the position [8]. The filter might be either an interference or absorption filter, the latter possibly located at the FS. 3.3. The photo-detector The photo-detector has to be able to detect the EAS by observing the air scintillation light produced during the EAS development and the Cherenkov light diffusely reflected by the Earth. It must be able to determine the position of the arriving photons as a function of time, to be able to follow the space-time development of the EAS. 3.3.1. The photo-detector architecture The photo-detector includes the following subsystems: the sensors; the Light Collection System (LCS), possibly incorporating the filter; the front-end electronics; the ancillary systems required for the correct operation, survival and control of the photo-detector as well as for data acquisition (e.g. power supplies and thermal control); the FS supporting structure. 3.3.2. Photo-detector requirements The main requirements for the EUSO photodetector deriving from and making explicit the general requirements in section 3, follow. The photo-detector must have single photon sensitivity in the 330 t 400 nm wavelength range, with a good and uniform detection efficiency. The photo-detector must have a fast response (well below At x 0.1 ps) to follow the space-time development of the EAS. On the other hand only a modest spatial resolution (of the order of a few mm) is required. The photo-detector must have a low noise and good signal to noise ratio. Small crosstalk and after-pulse rate are also required. It must have a large area (of the order of a few square meters), due to the large re-
A. Petrolini/Nuclear
Physics B (Proc. Suppl.) 113 (2002) 329-336
General Desired pixel size at the Earth surface ISS average orbit height Observation duty cycle Orbital period Operational lifetime Geometrical aperture Optics Optics maximum diameter Optics aperture (entrance pupil diameter) Optics f# Optics field of view (half-angle) Optics spot size diameter on the FS Average transmission of the optics Photo-Detector Provisional geometry of the FS Overall photo-detector detection efficiency Pixel dimensions Number of channels Environment Overall atmospheric transmission (330 nm 5 X 5 400 nm) Background (330 nm 5 X < 400 nm at x 400 km height)
333
A N 0.5 + 1.0 km H ~~380 km n N 0.10 i 0.15 Te N 90 min three full years N 4.5.105 km2str DM N 2.5 m
D-2m
f# = 1.25 FoV
= 30’ z y
3mm+6mm Kept 21 0.6
paraboloid:
z N or2 with (sy= 0.290 m-i M 0.1 Edet =3+6mm M 1~105+4~105 K atm 2 0.4
B x 3.1011 photons
rnb2 s-l str-i
Table 1 The provisional EUSO parameters, at the start of phase A.
l
l
quired FoV, and it must cover the FS with a sensitive area as large as possible, reducing dead or inefficient areas. The curved FS can be more easily covered by a mosaic of small units. The large FoV and the desired spatial granularity require a large number of pixels (hundreds of thousands) and therefore a sophisticated electronics system, capable to handle such a large number of channels, is required. Compatibility with Space mission requirements, including low mass, low power consumption, suitable structural properties, compactness, radiation hardness, low sensitivity to magnetic fields of the order of the gauss and high reliability and operating stability over a period of a few years.
3.3.3. Sensors for EUSO A number of possible sensors for use in the EUSO photodetector were evaluated during the past years [9,10]. The use of commercial MultiAnode Photo-Multipliers Tubes (MAPMT) [ll] was finally proposed as the most viable option, based on existing suitable and reliable devices. MAPMT produced by Hamamatsu (R7600 series) fulfill most of the requirements and they are a well-established technology with characteristics which are easily quantified. The availability of devices with different pixel sizes (and, correspondingly, different number of channels, namely 16 or 64) proves to be extremely useful to tune the photo-detector design to the requirements. And R&D program was carried on to validate this choice and to find possible solutions to a few open items which might limit the usefulness of the device in EUSO, the most important of which is the low overall geometrical acceptance of the device. Operational issues, such as power consump-
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Physics
tion, were also preliminarily investigated. All the characteristics were found to be compatible with the presently known constraints and requirements [9,12]. These devices have been extensively tested, recently, by many groups. For instance they have been used in the readout of a cluster of nine MAPMT to detect Cherenkov light in a testbeam setup by the RICH-LHCb Collaboration at CERN [13]. They have also successfully flown with the AMS detector on the Space Shuttle [14]. Other interesting possibilities, namely the Flat Panel PMT [15], do not seem to be compatible with the presently accepted EUSO time-schedule. Recently a modified version of the classical R7600 series was developed by Hamamatsu. It has a weak electrostatic focusing in front of the first dynode, allowing to increase the geometrical acceptance of the device. This device is very attractive and it is currently under study to quantify its performance, especially its behaviour in a magnetic field [16]. The use of this device would allow to use a simpler LCS, requiring a smaller demagnification. The geometrical acceptance can be improved by means of a suitable LCS [9], to be placed in front of each device, and performing the required demagnification onto the MAPMT sensitive area. This system might consist of a lens system, a system made of a bundle of tapered light pipes (working either by normal reflection or by total internal reflection) or a fiber optic taper. A discussion of different LCS can be found elsewhere [9,12,16]. 3.3.4. FS architecture and design As the FS is curved the packing of the sensors has to be optimised to reduce losses in the geometrical acceptance, due to dead regions between the close packed devices, and defocusing effects, due to the approximate fit to the ideal FS. A modular structure is preferred because it has many useful implications including: independence of the different modules, fault propagation limitation, easier spare modules management and procurement; moreover it makes the development, design, integration and testing phases easier. The overall structure should consist of
B (Proc. Suppl.)
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small functional units (elementary cells) assembled in super-modules. The elementary cell consists of a limited number of MAPMT, together with their associated front-end electronics, sharing some common resources such as being installed on the same Printed Circuit Board (PCB) base-board, having common HV/LV power supplies (and possibly voltage dividers), common connection cables and common heat dissipation facilities. The elementary cells can be built around a thick multi-layer PCB. A number of these units, each one making an essentially autonomous system, are then assembled to make a super-module. These are independent structures tied to each other by the main FS support structure and having a shape determined by the layout of the FS. A base-board was designed and prototyped for 2 x 2 MAPMT [12]. This constitutes the elementary unit, with one single HV connection and one resistive bleeder circuit per MAPMT. It was attempted to close-pack the MAPMT with a p = 1 mm clearance between adjacent MAPMT. It was avoided to use any additional space along the edges of the base-board to allow close packing of different base-boards, with pc = 1 mm clearance. The base-board allows for mounting of the front-end chip on the same base-board as the MAPMT, on the opposite side with respect to it. In this way the front-end electronics is close to the sensor, in a compact structure minimizing cabling. The removal of the heat produced by the bleeder circuit and front-end electronics is accomplished by inserting a copper layer inside the PCB, thermally connected to the heat-bridges. The support structure of the whole FS has to carry the individual super-modules, transferring to the elementary-cells loads compatible with the components requirements. Power, uniformly dissipated on the FS, has to be transferred towards the FS perimeter to the external radiators. This can be accomplished by heat conduction on the support itself, helped by dedicated heat pipes. 3.3.5. The front-end electronics The front-end electronics [17] is needed to preamplify the signals from the sensors, to discriminate these signals with a programmable
A. Petrolini/Nuclear
threshold, to mask noisy channels, to provide preprocessed information to the trigger system, and to store the information until readout is done. A highly integrated custom front-end chip (ASIC) is under development. Required features are a very compact design with minimal distance between the MAPMT and the front-end electronics, a completely modular system with minimal cabling and self-triggering capabilities. 3.3.6.
The trigger and read-out
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Physics B (Proc. Suppl.) 113 (2002) 329-336
electronics
The trigger system [18] is a critical point of EUSO. It has to be able to provide a fast trigger with hundreds of thousands of channels. It has to be selective in order to tag the EAS produced signal rejecting the background in an efficient way. The system is modular, based on the logical concept of the macro-cell, which is an assembly of sensors, logically working as a single entity as far as triggering purposing are concerned. The trigger module has been studied to provide different kind of triggers including a normal mode for EAS detection, a slow mode for atmospheric phenomena and a fast mode for detector calibration. The read-out electronics has been designed to obtain an effective reduction of channels and data to read-out, developing a method that reduces the number of the channels without penalizing the performance of the detection system. Rows wired-OR and columns wired-OR connections have been adopted inside every single macro-cell for diminishing the number of channels to read-out while keeping the triggering capabilities. 3.3.7. The control electronics The control Electronics is in charge of managing the operations of the Instrument. In particular its main functions are: the collection of scientific data consisting of the position and arrival time of detected photons; the collection of the housekeeping monitors to check the correct configuration and operation of the instrument; the preparation of the telemetry source packets (scientific, housekeeping, . ..) and their transmission to the ISS: the reception, validation and distribution of telecommands; the control of the Instru-
ment operative modes during observations, diagnostic and calibration intervals (these modes also include the autonomous maintenance of the detector in safe conditions); the provision of data patches and dump capability for on-board software programming; the management of time information. 3.4. Instrument calibration A full, absolute calibration of the whole Instrument is foreseen before launch. Methods for relative and absolute calibration of the Instrument on-orbit, to be carried on periodically, are under study. 3.5. The auxiliary instrumentation A detailed knowledge of the atmospheric prop erties on a event by event basis, instead of average values, is useful to improve the systematic errors on the measurements. The possibility to complement EWSO with auxiliary instrumentation (e.g. a LIDAR), capable to provide information on the local atmospheric properties affecting any specific EAS, is under study. The decision has to be a trade-off between the improved EUSO data quality and the unavoidable use of resources of the auxiliary instrumentation. 3.6. EUSO structure and envelope The EUSO structure, in its preliminary conceptual form, is composed of rings supporting the Instrument major masses and by rods connecting the rings. Multi-layers system will cover the Instrument, ensuring thermal, micro-meteoroids and orbiting debris protection, contamination protection and shadowing of the FS in all flight conditions. The thermal control is completed by heaters, heat pipes and external radiators. A baffle is required to prevent undesired light from entering EUSO during data-taking. A shutter is required to protect the Instrument from intense light. 3.7.
Budgets
and resources
utilization
The EUSO Instrument has to comply with the strict budget limits imposed by the installation on the ISS Columbus module. Preliminary estimates tend to require a mass not larger than M = 1.5 ton, an average power consumption not larger
A. Petrolini/Nuclear Physics B (Proc. SuppI,) I13 (2002) 329-336
336
than P = 0.8 kW and maximum allowed dimensions of the order of V = 2.5 x 2.5 x 4.5 m3. 4. EUSO
performance
The expected EUSO performance has been preliminarily evaluated by means of a dedicated simulation program. Physical processes and Instrument response were treated in a parameterized form with simplifying, but conservative, hypotheses, based on the present assumptions and knowledge.
4.1. Trigger efficiency The trigger efficiency, as a function of the EAS energy, reaches 0.5 at about E N 5.10rg eV; full efficiency is reached at about E N 9.101’ eV. 4.2. Angular resolution The expected angular resolution on the EHECR direction depends on the EAS zenith angle (19,). Assuming observation of the diffusely reflected Cherenkov flash, and, conservatively, a 0 = 0.2’ optics resolution, the angular resolution for the current baseline design ranges from Aa N lo to Ao N 2O, assuming to select, respectively, EAS with 8, 2 65” and BZ > 35’. 4.3. Expected number of events By extrapolating the observed AGASA spectrum (with spectral index y = -2.8) the expectations for EUSO are more than one thousand events per year with energy above 5.101’ eV (assumed duty cycle 7 = 0.1). 5. Conclusions The EUSO project has just started its phase A study. The baseline preliminary design developed in the pre-phase A studies, will be subject to an extensive review and a much more detailed study to assess its validity. The phase A is expected to fully assess the EUSO feasibility. 6. Acknowledgements The support from ESA and from the many national funding agencies (in particular from Italy, France, Portugal, US and Japan) is gratefully acknowledged.
REFERENCES
2. 3.
P. Blasi, these proceedings. P. Galeotti, these proceedings. J. Linsley, Call for Projects and Ideas in High
4.
Energy Astrophysics, 1979. 0. Catalano, AirWatch from Space, IFCAI-
1.
CNR/Palermo Internal Note, 24/09/1999. F. X. Kneizys et al., Users Guide to LOWTRAN 7, AFGL-TR-88-0177, 1988, Air Force Geophysics Laboratory, Hanscom, MA, 01731. 6. 0. Catalan0 et al., Nucl. Instr. and Meth. A, 480 (2002) 547. 7. 0. Catalan0 et al., ULTRA: UV Light Transmission and Reflection in the Atmosphere, EUSO Internal Note. 8. J. Adams et al., proposal to the NASA MIDEX AO, 2001. Collaboration, Workshop on 9. The AirWatch Observing Giant Cosmic Ray Air Showers, AIP Conf. Proc., 433, 353-357, (1998); The AirWatch Collaboration, Proc. SPIE 3445, 486, (1998); R.Stalio et al., Proc. 26th ICRC 2, 403 (1999); M. Ameri et al., Study report on the EUSO photo-detector design, Report INFN/AE-01/04, April 2001. Optimization of 0 WLArisaka, 10. K. 5.
Air Watch Detectors,
Optics
28/12/1999, http://www.physics.ucla.edu/
and
Photo-
available at arise&a/.
series, Hamamatsu 11. MAPMT R5900/R7600 Photonics. 12. M. Ameri et al., Proc. 27th ICRC (2001), Hamburg, August 2001. 13. E. Albrecht et al., Performance of a cluster of multi-anode photo-multipliers equipped with lenses for use in a prototype RICH detector, accepted for publication on Nucl. Instr.
and Meth. A. 14. B. Alpat, Nucl. Phys. Proc. Suppl. 85, 15 (2000). Photonics. 15. Flat Panel PMT, Hamamatsu 16. H. Shimizu, RIKEN, private communication. 17. P. Music0 et al., these proceedings. 18. 0. Catalano, 11 Nuovo Cimento 24C, n. 3, (2001), pag. 445.
ELSEVIER
www.elsevier.com/locate/npe
The Extreme Universe Space Observatory (EUSO) Instrument A. Petrolini” a Dipartimento di Fisica dell’Universit& on-behalf of the EUSO Collaboration.
di Genova & INFN, Via Dodecaneso
33, I-16146 Genova, Italy;
The EUSO experiment is designed to study the Extensive Air Showers produced in the atmosphere by Extreme High Energy Cosmic Rays from an Observatory installed on the International Space Station. The project is currently under phase A study by ESA. The basic facts of current design of the EUSO Instrument are described.
figure 1)
1. EUSO
The interpretation of the fenomenology of the Extreme High Energy Cosmic Rays (EHECR) is one of the most interesting and challenging topics of modern astro-particle physics [l]. EHECR reach the Earth with a low flux, of the order of T = 0.01 particle year-l km-‘, and therefore a sophisticated detector is required. The EUSO (Extreme Universe Space Observatory) experiment, proposed to the European Space Agency (ESA) for installation on the International Space Station (ISS), has just started The experiment goal is its phase A study. the study of the Extreme High Energy part (E ;s 3.10 rg eV) of the EHECR energy spectrum, by observing from Space the Extensive Air Showers (EAS) produced by the interaction of the primary EHECR with the atmosphere. This is accomplished by installing EUSO on the ESA Columbus External Payload Facility (CEPF), looking down to nadir during night-time. A large mass of atmosphere can be observed from Space. An EAS can be detected by observing the air scintillation light, isotropically produced during the EAS development, as well as the forward beamed Cherenkov light, diffusely reflected by the Earth. The scientific objectives of EZJSO are described in another contribution to this conference [2].
2. The observational
technique
EUSO is based on the AirWatch concept, originally proposed by John Linsley more than twenty years ago [3,4]: to observe EAS from Space (see 0920~5632/02/$
- see
front matter 0
PII SO920-5632(02)01860-l
2002
Elsevier Science B.V
Figure
1. The EUSO observational
approach.
The observation from Space of the EAS produced air scintillation light and of the Cherenkov light diffusely reflected by the Earth by means of a fast and high-granularity photo-detector allows to record the EAS development in the atmosphere. In fact the scintillation light is proportional, at any point along the EAS development, to the All rights
reserved.
A. Petrolini/Nuclenr-
330
Physics B (Proc. Suppl.)
number of charged particles in the EAS. The additional observation of the diffusely reflected Cherenkov light (reflected either by land, sea or clouds) provides additional information, useful to improve the EAS reconstruction. It is therefore possible to estimate the energy and arrival direction of the primary EHECR, and to gather information about its nature. EUSO will observe the Earth atmosphere at night by looking down to nadir with a large aperture and large Field of View (FoV) optics, focusing the image onto the Focal Surface (FS) photodetector. The latter must have a good sensitivity to be able to detect the faint signal from the less energetic EAS, allowing to connect the observed energy spectrum with the observations from ground-based experiments. A sufficiently fast detector allows one to determine the direction of the primary EHECR by means of one single observatory. The required information can nevertheless be obtained with a system having only a limited spatial resolution. The peculiar characteristics of the EAS, including the kinematical ones, allow one to distinguish them from the various backgrounds, because those have a typically different space-time development. Key points of the observational technique are the following. l
l
l
l
A large geometrical aperture can be obthanks to the large distance, tained, depending on the Instrument FoV. A large mass of atmosphere (the calorimeter medium) can therefore be observed. Observation of deeply penetrating EAS, from primary particles interacting deeply in the atmosphere, is possibile, by the direct observation of the EAS development and starting point. All sky coverage is possible with one single apparatus. The approach is complementary to the ground-based one. In fact the Space experiments are best suited for the observation of higher energy cosmic rays compared to ground-based experiments. However an overlap of the observed energy spectrum with the one known from ground-based ex-
I13 (2002) 329-336
periments is required for a better comparison. Moreover the systematic effects are different in the two approaches. 2.1. The scintillation light The scintillation light is isotropic and proportional, at any point, to the number of charged particles in the EAS, largely dominated by electrons and positrons. The total amount of light produced is proportional to the primary particle energy and the shape of the EAS profile (in particular the atmospheric depth of the EAS maximum) contains information about the primary particle identity. The scintillation yield in air, Ya+., in the 330 nm + 400 nm wavelength range, is about Ya+ = 4.5 photons per charged particle per meter at any height h 5 20 km, depending, in a known way, from altitude and air composition. The main emission lines are located at the three wavelenghts 337 nm, 357 nm and 391 nm. 2.2. The Cherenkov light The possible observation of the Cherenkov light diffusely reflected by the Earth (by land, sea or clouds) will help the EAS parameters determination While the amount of observed Cherenkov photons depends on the reflectance and geometry of the impact surface, the directionality of the Cherenkov beam provides a precise extrapolation of the EAS to the first reflecting surface. 2.3. The atmosphere The atmosphere is relatively transparent down to x M 330 nm, where the ozone absorption becomes strong. Preliminay simulations, based on the LOWTRAN7 code [5], show that for typical cloud-less atmosphere models the vertical transmission coefficient from ground to EUSO is larger than t = 0.4, in all the interesting wavelenght range, and losses are dominated by Rayleigh scattering. The main atmospheric components affecting the signal transmission are Rayleigh and Mie scattering, ozone absorption (severe up to X 2: 330 nm), clouds (affecting either signal transmission and EAS characterization). Real time measurements of these components might improve the EUSO performance (section 3.5).
A. Petrolini/Nuclear
2.4.
Physics B (Proc. Suppl.) 113 (2002) 329-336
The background
Many different kind of backgrounds are expected including: man-made lights, auroras, natural photo-chemical effects (in atmosphere, sea and land), low-energy cosmic rays, reflected The observation of moon-light and star-light. some atmospheric phenomena, such as meteors, is interesting in itself and is currently under study. A selective and efficient trigger is required to distinguish the EAS from the background. However the typical characteristics of EAS are quite different from the background ones, in particular the kinematic characteristics. The backgrounds, for instance, typically have a time-scale of the order of ms. Another type of background is the random night-glow which has been measured, most recently, by the BABY experiment [6]. The resulting estimate for EUSO is B x 3.1O’r photons mm2 s-l str-‘, in the wavelenght range 330 nm 5 X 5 400 nm at h M 400 km heigth. The observation
2.5.
duty cycle
EUSO will spend a large part of its time above the seas, which appear to be the a good site for observation due to the little man-made light, maximum possible atmospheric depth and rather uniform surface properties. The duty cycle has been estimated taking into account: the ISS night time; ground locations with significant light output, natural or anthropomorphic; the lunar cycle; clouds in the FoV affecting the detection or interpretation of the EAS; ISS activities or contingencies that do not allow normal operation. The preliminarly estimated duty cycle is about n N 0.1. Observation under some moon-light is possible, even if this will imply an energy detection threshold higher than normal due to the increased background rate. 2.6. On-going support measurements A series of activities have been started, or are planned, to improve the knowledge of significant parameters affecting the EUSO performance. l l
Night-glow background measurements [6]. Measurement of the Cherenkov light diffuse reflection coefficient, from land, clouds and
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sea, in different conditions, including lowenergy EAS observation [7]. Measurement of the air scintillation yield in different conditions, including the study of the effect of humidity.
3. The EUSO
Instrument
The design of an Instrument to look from Space to the EAS produced in the atmosphere by EHECR is a challenging task mainly because the EHECR flux reaching the Earth is very small, the observable signal is vary faint and the apparatus has to operate in Space. The main Instrument requirements are the following. The Instrument must collect as many photons as possible, in order to be able to detect the faint signal from the less energetic EAS and discriminate it from the background. As a consequence a large aperture is required, as well as a good transmission of the optical elements and good photon detection efficiency in the 330 nm + 400 nm wavelength range. In fact a sufficiently low lower energy threshold is mandatory to connect the energy spectrum observed by EUSO with the observations by groundbased experiments, operating at a lower energy range. A large FoV is required to be able to observe a mass of atmosphere as large as possible, thus increasing the expected event rates. The physics requirements can be obtained with a system having an angular granularity sufficient to ensure an angular resolution on the EAS direction of Acu = 1”. A single-photon detector is required, fast enough to be able to follow the space-time development of the EAS and reconstruct the EAS kinematical parameters from one single observation point. A low noise and good signal to noise ratio are required to detect the faint signal produced by the less energetic EAS and discriminate it from the background. Small cross-talk and after-pulse rate are required
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l
l
l
to avoid degradation of the energy and angular resolution. An efficient and reliable trigger system, capable of a good background rejection, is required to cope with the limited data storage, data transfer and computing capabilities available on-board. All the constraints and requirements related to the Space mission have to be accounted for. Mandatory characteristics are therefore: a compact and robust system with low mass, volume and power consumption, good reliability and time stability, radiation hardness and low sensitivity to magnetic fields of the order of the gauss. A system is required to protect the Instrument from possibly dangerous environmental factors, including intense light.
3.1. The main Instrument parameters The present provisional EUSO parameters, the ones most relevant to the photo-detector design, are summarized in table 3.1. 3.2. The optics 3.2.1. Optics requirements The main requirements for the primary optics, deriving from and making explicit the general requirements in section 3. follow. Large aperture, D 2 2 m diameter. Large FoV, y 2 30” (half-angle). Good transparency in the wavelength range 330 nm 2 X < 400 nm. Angular resolution Ao = 0.1”. Compatibility with Space mission requirements, including low mass, radiation hardness, suitable structural properties, high reliability and operating stability over a period of a few years. These requirements point to the use of a Fresnel lens system made of light-weight polymers. 3.2.2. Optics baseline The preliminary baseline design, consistent with the parameters in table 3.1, is based on two double-sided, curved, 2.5 m diameter thin Fresnel lenses, made of smaller pieces and kept together
by a light supporting structure. The resulting spot size diameter on the FS is in the range 3 mm t 6 mm, depending on the position [8]. The filter might be either an interference or absorption filter, the latter possibly located at the FS. 3.3. The photo-detector The photo-detector has to be able to detect the EAS by observing the air scintillation light produced during the EAS development and the Cherenkov light diffusely reflected by the Earth. It must be able to determine the position of the arriving photons as a function of time, to be able to follow the space-time development of the EAS. 3.3.1. The photo-detector architecture The photo-detector includes the following subsystems: the sensors; the Light Collection System (LCS), possibly incorporating the filter; the front-end electronics; the ancillary systems required for the correct operation, survival and control of the photo-detector as well as for data acquisition (e.g. power supplies and thermal control); the FS supporting structure. 3.3.2. Photo-detector requirements The main requirements for the EUSO photodetector deriving from and making explicit the general requirements in section 3, follow. The photo-detector must have single photon sensitivity in the 330 t 400 nm wavelength range, with a good and uniform detection efficiency. The photo-detector must have a fast response (well below At x 0.1 ps) to follow the space-time development of the EAS. On the other hand only a modest spatial resolution (of the order of a few mm) is required. The photo-detector must have a low noise and good signal to noise ratio. Small crosstalk and after-pulse rate are also required. It must have a large area (of the order of a few square meters), due to the large re-
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General Desired pixel size at the Earth surface ISS average orbit height Observation duty cycle Orbital period Operational lifetime Geometrical aperture Optics Optics maximum diameter Optics aperture (entrance pupil diameter) Optics f# Optics field of view (half-angle) Optics spot size diameter on the FS Average transmission of the optics Photo-Detector Provisional geometry of the FS Overall photo-detector detection efficiency Pixel dimensions Number of channels Environment Overall atmospheric transmission (330 nm 5 X 5 400 nm) Background (330 nm 5 X < 400 nm at x 400 km height)
333
A N 0.5 + 1.0 km H ~~380 km n N 0.10 i 0.15 Te N 90 min three full years N 4.5.105 km2str DM N 2.5 m
D-2m
f# = 1.25 FoV
= 30’ z y
3mm+6mm Kept 21 0.6
paraboloid:
z N or2 with (sy= 0.290 m-i M 0.1 Edet =3+6mm M 1~105+4~105 K atm 2 0.4
B x 3.1011 photons
rnb2 s-l str-i
Table 1 The provisional EUSO parameters, at the start of phase A.
l
l
quired FoV, and it must cover the FS with a sensitive area as large as possible, reducing dead or inefficient areas. The curved FS can be more easily covered by a mosaic of small units. The large FoV and the desired spatial granularity require a large number of pixels (hundreds of thousands) and therefore a sophisticated electronics system, capable to handle such a large number of channels, is required. Compatibility with Space mission requirements, including low mass, low power consumption, suitable structural properties, compactness, radiation hardness, low sensitivity to magnetic fields of the order of the gauss and high reliability and operating stability over a period of a few years.
3.3.3. Sensors for EUSO A number of possible sensors for use in the EUSO photodetector were evaluated during the past years [9,10]. The use of commercial MultiAnode Photo-Multipliers Tubes (MAPMT) [ll] was finally proposed as the most viable option, based on existing suitable and reliable devices. MAPMT produced by Hamamatsu (R7600 series) fulfill most of the requirements and they are a well-established technology with characteristics which are easily quantified. The availability of devices with different pixel sizes (and, correspondingly, different number of channels, namely 16 or 64) proves to be extremely useful to tune the photo-detector design to the requirements. And R&D program was carried on to validate this choice and to find possible solutions to a few open items which might limit the usefulness of the device in EUSO, the most important of which is the low overall geometrical acceptance of the device. Operational issues, such as power consump-
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tion, were also preliminarily investigated. All the characteristics were found to be compatible with the presently known constraints and requirements [9,12]. These devices have been extensively tested, recently, by many groups. For instance they have been used in the readout of a cluster of nine MAPMT to detect Cherenkov light in a testbeam setup by the RICH-LHCb Collaboration at CERN [13]. They have also successfully flown with the AMS detector on the Space Shuttle [14]. Other interesting possibilities, namely the Flat Panel PMT [15], do not seem to be compatible with the presently accepted EUSO time-schedule. Recently a modified version of the classical R7600 series was developed by Hamamatsu. It has a weak electrostatic focusing in front of the first dynode, allowing to increase the geometrical acceptance of the device. This device is very attractive and it is currently under study to quantify its performance, especially its behaviour in a magnetic field [16]. The use of this device would allow to use a simpler LCS, requiring a smaller demagnification. The geometrical acceptance can be improved by means of a suitable LCS [9], to be placed in front of each device, and performing the required demagnification onto the MAPMT sensitive area. This system might consist of a lens system, a system made of a bundle of tapered light pipes (working either by normal reflection or by total internal reflection) or a fiber optic taper. A discussion of different LCS can be found elsewhere [9,12,16]. 3.3.4. FS architecture and design As the FS is curved the packing of the sensors has to be optimised to reduce losses in the geometrical acceptance, due to dead regions between the close packed devices, and defocusing effects, due to the approximate fit to the ideal FS. A modular structure is preferred because it has many useful implications including: independence of the different modules, fault propagation limitation, easier spare modules management and procurement; moreover it makes the development, design, integration and testing phases easier. The overall structure should consist of
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small functional units (elementary cells) assembled in super-modules. The elementary cell consists of a limited number of MAPMT, together with their associated front-end electronics, sharing some common resources such as being installed on the same Printed Circuit Board (PCB) base-board, having common HV/LV power supplies (and possibly voltage dividers), common connection cables and common heat dissipation facilities. The elementary cells can be built around a thick multi-layer PCB. A number of these units, each one making an essentially autonomous system, are then assembled to make a super-module. These are independent structures tied to each other by the main FS support structure and having a shape determined by the layout of the FS. A base-board was designed and prototyped for 2 x 2 MAPMT [12]. This constitutes the elementary unit, with one single HV connection and one resistive bleeder circuit per MAPMT. It was attempted to close-pack the MAPMT with a p = 1 mm clearance between adjacent MAPMT. It was avoided to use any additional space along the edges of the base-board to allow close packing of different base-boards, with pc = 1 mm clearance. The base-board allows for mounting of the front-end chip on the same base-board as the MAPMT, on the opposite side with respect to it. In this way the front-end electronics is close to the sensor, in a compact structure minimizing cabling. The removal of the heat produced by the bleeder circuit and front-end electronics is accomplished by inserting a copper layer inside the PCB, thermally connected to the heat-bridges. The support structure of the whole FS has to carry the individual super-modules, transferring to the elementary-cells loads compatible with the components requirements. Power, uniformly dissipated on the FS, has to be transferred towards the FS perimeter to the external radiators. This can be accomplished by heat conduction on the support itself, helped by dedicated heat pipes. 3.3.5. The front-end electronics The front-end electronics [17] is needed to preamplify the signals from the sensors, to discriminate these signals with a programmable
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threshold, to mask noisy channels, to provide preprocessed information to the trigger system, and to store the information until readout is done. A highly integrated custom front-end chip (ASIC) is under development. Required features are a very compact design with minimal distance between the MAPMT and the front-end electronics, a completely modular system with minimal cabling and self-triggering capabilities. 3.3.6.
The trigger and read-out
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electronics
The trigger system [18] is a critical point of EUSO. It has to be able to provide a fast trigger with hundreds of thousands of channels. It has to be selective in order to tag the EAS produced signal rejecting the background in an efficient way. The system is modular, based on the logical concept of the macro-cell, which is an assembly of sensors, logically working as a single entity as far as triggering purposing are concerned. The trigger module has been studied to provide different kind of triggers including a normal mode for EAS detection, a slow mode for atmospheric phenomena and a fast mode for detector calibration. The read-out electronics has been designed to obtain an effective reduction of channels and data to read-out, developing a method that reduces the number of the channels without penalizing the performance of the detection system. Rows wired-OR and columns wired-OR connections have been adopted inside every single macro-cell for diminishing the number of channels to read-out while keeping the triggering capabilities. 3.3.7. The control electronics The control Electronics is in charge of managing the operations of the Instrument. In particular its main functions are: the collection of scientific data consisting of the position and arrival time of detected photons; the collection of the housekeeping monitors to check the correct configuration and operation of the instrument; the preparation of the telemetry source packets (scientific, housekeeping, . ..) and their transmission to the ISS: the reception, validation and distribution of telecommands; the control of the Instru-
ment operative modes during observations, diagnostic and calibration intervals (these modes also include the autonomous maintenance of the detector in safe conditions); the provision of data patches and dump capability for on-board software programming; the management of time information. 3.4. Instrument calibration A full, absolute calibration of the whole Instrument is foreseen before launch. Methods for relative and absolute calibration of the Instrument on-orbit, to be carried on periodically, are under study. 3.5. The auxiliary instrumentation A detailed knowledge of the atmospheric prop erties on a event by event basis, instead of average values, is useful to improve the systematic errors on the measurements. The possibility to complement EWSO with auxiliary instrumentation (e.g. a LIDAR), capable to provide information on the local atmospheric properties affecting any specific EAS, is under study. The decision has to be a trade-off between the improved EUSO data quality and the unavoidable use of resources of the auxiliary instrumentation. 3.6. EUSO structure and envelope The EUSO structure, in its preliminary conceptual form, is composed of rings supporting the Instrument major masses and by rods connecting the rings. Multi-layers system will cover the Instrument, ensuring thermal, micro-meteoroids and orbiting debris protection, contamination protection and shadowing of the FS in all flight conditions. The thermal control is completed by heaters, heat pipes and external radiators. A baffle is required to prevent undesired light from entering EUSO during data-taking. A shutter is required to protect the Instrument from intense light. 3.7.
Budgets
and resources
utilization
The EUSO Instrument has to comply with the strict budget limits imposed by the installation on the ISS Columbus module. Preliminary estimates tend to require a mass not larger than M = 1.5 ton, an average power consumption not larger
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than P = 0.8 kW and maximum allowed dimensions of the order of V = 2.5 x 2.5 x 4.5 m3. 4. EUSO
performance
The expected EUSO performance has been preliminarily evaluated by means of a dedicated simulation program. Physical processes and Instrument response were treated in a parameterized form with simplifying, but conservative, hypotheses, based on the present assumptions and knowledge.
4.1. Trigger efficiency The trigger efficiency, as a function of the EAS energy, reaches 0.5 at about E N 5.10rg eV; full efficiency is reached at about E N 9.101’ eV. 4.2. Angular resolution The expected angular resolution on the EHECR direction depends on the EAS zenith angle (19,). Assuming observation of the diffusely reflected Cherenkov flash, and, conservatively, a 0 = 0.2’ optics resolution, the angular resolution for the current baseline design ranges from Aa N lo to Ao N 2O, assuming to select, respectively, EAS with 8, 2 65” and BZ > 35’. 4.3. Expected number of events By extrapolating the observed AGASA spectrum (with spectral index y = -2.8) the expectations for EUSO are more than one thousand events per year with energy above 5.101’ eV (assumed duty cycle 7 = 0.1). 5. Conclusions The EUSO project has just started its phase A study. The baseline preliminary design developed in the pre-phase A studies, will be subject to an extensive review and a much more detailed study to assess its validity. The phase A is expected to fully assess the EUSO feasibility. 6. Acknowledgements The support from ESA and from the many national funding agencies (in particular from Italy, France, Portugal, US and Japan) is gratefully acknowledged.
REFERENCES
2. 3.
P. Blasi, these proceedings. P. Galeotti, these proceedings. J. Linsley, Call for Projects and Ideas in High
4.
Energy Astrophysics, 1979. 0. Catalano, AirWatch from Space, IFCAI-
1.
CNR/Palermo Internal Note, 24/09/1999. F. X. Kneizys et al., Users Guide to LOWTRAN 7, AFGL-TR-88-0177, 1988, Air Force Geophysics Laboratory, Hanscom, MA, 01731. 6. 0. Catalan0 et al., Nucl. Instr. and Meth. A, 480 (2002) 547. 7. 0. Catalan0 et al., ULTRA: UV Light Transmission and Reflection in the Atmosphere, EUSO Internal Note. 8. J. Adams et al., proposal to the NASA MIDEX AO, 2001. Collaboration, Workshop on 9. The AirWatch Observing Giant Cosmic Ray Air Showers, AIP Conf. Proc., 433, 353-357, (1998); The AirWatch Collaboration, Proc. SPIE 3445, 486, (1998); R.Stalio et al., Proc. 26th ICRC 2, 403 (1999); M. Ameri et al., Study report on the EUSO photo-detector design, Report INFN/AE-01/04, April 2001. Optimization of 0 WLArisaka, 10. K. 5.
Air Watch Detectors,
Optics
28/12/1999, http://www.physics.ucla.edu/
and
Photo-
available at arise&a/.
series, Hamamatsu 11. MAPMT R5900/R7600 Photonics. 12. M. Ameri et al., Proc. 27th ICRC (2001), Hamburg, August 2001. 13. E. Albrecht et al., Performance of a cluster of multi-anode photo-multipliers equipped with lenses for use in a prototype RICH detector, accepted for publication on Nucl. Instr.
and Meth. A. 14. B. Alpat, Nucl. Phys. Proc. Suppl. 85, 15 (2000). Photonics. 15. Flat Panel PMT, Hamamatsu 16. H. Shimizu, RIKEN, private communication. 17. P. Music0 et al., these proceedings. 18. 0. Catalano, 11 Nuovo Cimento 24C, n. 3, (2001), pag. 445.