HREPHAH 0H3HKA, 2004, moM 67, № 11, c. 2079-2082
RARE PROCESSES AND ASTROPHYSICS
KLYPVE/TUS SPACE EXPERIMENTS FOR STUDY OF ULTRAHIGH
ENERGY COSMIC RAYS
© 2004 B. A. Khrenov*, V. V. Alexandrov, D. I. Bugrov, G. K. Garipov, N. N. Kalmykov, M. I. Panasyuk, S. A. Sharakin, A. A. Silaev, I. V. Yashin, V. M. Grebenyuk1), D. V. Naumov1), A. G. Olshevsky1), B. M. Sabirov1), R. N. Semenov1), M. Slunechka1), I. I. Skryl1), L. G. Tkatchev1), O. A. Saprykin2), V. S. Syromyatnikov2), V. E. Bitkin3), S. A. Eremin3), A. I. Matyushkin3), F. F. Urmantsev3), V. Abrashin4), V. Koval4),
Y. Arakcheev4), A. Cordero5), O. Martinez5), E. Morena5), C. Robledo5), H. Salazar5), L. Villasenor6), A. Zepeda7), I. Park8), M. Shonsky9), J. Zicha10)
Skobeltsyn Institute of Nuclear Physics, Moscow State University, Russia
Received January 20, 2004
The KLYPVE space experiment has been proposed to study the energy spectrum, composition and arrival direction of the UltraHigh Energy Cosmic Rays (UHECR) by detecting from satellites the atmosphere fluorescence and scattered Cherenkov light produced by EAS, initiated by UHECR particles. The TUS setup is a prototype KLYPVE instrument. The aim of the TUS experiment is to detect dozens of UHECR events in the energy region of the GZK cutoff, to measure the light background, to test the atmosphere control methods, to study stability of the optic materials, PMTs, and other instrumental parts in the space environment.
1. INTRODUCTION
Among many important astrophysical problems the nature, energy spectrum and sources of UltraHigh Energy Cosmic Rays (UHECR) are of paramount importance. It is likely that the Galactic Center can be a source of cosmic rays with energies (1—2) x 1018 eV [1]. The change of the UHECR energy spectrum at (3—5) x 1018 eV may be explained by the change in the origin of the cosmic rays (from the galactic to the extragalac-tic one). For extragalactic protons with energies above 5 x 1019 eV the Greisen—Zatsepin—Kuzmin (GZK) cutoff is expected due to interaction with relic
!)Joint Institute for Nuclear Research, Dubna, Russia.
2)Rocket Space Corporation "Energia", Consortium "Space Regatta", Korolev, Russia.
3)Special Construction Bureau "Luch", Syzran, Russia.
4)State Research and Production Space Center, Samara, Russia.
5)Benemerita Universidad Autonomia de Puebla, Puebla, Mexico.
6)Universidad Michoacana, Morelia, Mexico.
7)Depto de Fisica, Cinvestav-IPN, Mexico City, Mexico.
8)Department of Physics, UWHA Woman University, Seoul, Korea.
9)NIO KOMPAS, Turnov, Czech Republic.
10)Technical University, Prague, Czech Republic.
E-mail: khrenov@eas.sinp.msu.ru
photons. The flux of UHECR with energies above 1020 eV, measured by the AGASA array [2] is against the GZK cutoff and this result is stimulating various theoretical speculations [3]. However, in the last years the data from the HiRes detector are in favor of the cutoff [4] and certainly new more precise and conclusive data are needed for clarifying the UHECR phenomena. At present a few new projects to study UHECR have been proposed, including experiments in space, among them the KLYPVE project [5, 6]. The TUS setup accommodated on the Russian RESURS satellite was proposed [7, 8] for study of all aspects of the KLYPVE operation in space. In this paper the 2003 status of the KLYPVE/TUS projects is presented.
2. REGISTRATION METHOD AND THE MAIN PARTS OF THE INSTRUMENT
The KLYPVE/TUS detector on board a satellite has to register the fluorescent and scattered Cherenkov light generated by Ultrahigh Energy EAS in the atmosphere. The height of the satellite orbit and the field of view (FOV) of the detector determine the area of the atmosphere available for the UHECR event registration. The rate of UHECR events depends on the detector energy threshold for registering
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Parameter KLYPVE TUS
Orbit height [km] 400 350-600
Mirror area [m2] 10 1.4
Focal distance [m] 3 1.5
Pixel angular size [mrad] 5 10
FOV [rad] 0.24 0.16
Number of pixels 2304 256
Time sample of FADC [¿ts] 0.4 0.8
Fig. 1. TUS segmented mirror-concentrator. The central part is used only as a mechanicalbase ofthe mirror.
UHECR events in this area and on the range of zenith angles in which the primary UHECR particle parameters (energy, arrival direction) are determined from the registered data. The available zenith angle range is determined by the pixel resolution of the photoreceiver. It was shown in the first KLYPVE and TUS papers [5—8] that simple optics of the mirror-concentrator and the PMT pixel grid in its focal plane may give a high rate of registered Extreme High Energy events (energies more than 5 x 1019 eV) when operating at orbit heights of 400—500 km: for the TUS setup the expected (if the AGASA data are valid) rate is about 50 events per year (10 times higher than in the AGASA experiment) and for the KLYPVE detector it will be higher and accuracy in measurement of primary particle parameters will be higher. The KLYPVE detector will start measure-
Table
Fig. 2. Steel mold for production of mirror segments.
ments with the energy threshold of 1019 eV but the TUS (with smaller mirror-concentrator) will start to operate near the energy 5 x 1019 eV. The program of the KLYPVE experiment in comparison with other UHECR projects is presented in [9].
The space KLYPVE (or TUS) detector has two main parts: the segmented Fresnel mirror-concentrator and the grid of pixels (PMTs) with the corresponding electronics in its focal plane. The main parameters of the KLYPVE and TUS detectors are presented in the table.
At present the TUS detector is under construction with the mirror of 6 hexagonal segments (Fig. 1) that has the operating area of 1.4 m2 and the focal distance 150 cm. Production of the samples of the mirror segment is organized as pressing the carbon plastic replicas off the steel mold (Fig. 2). The plastic mirror segments are placed on the frames — parts of the mechanical construction for the mirror development in space. The mirror and photoreceiver will be transported to space in a packed mode [5]. In the operating mode the mirror segments on the corresponding frames will be fixed to one plane with the angular accuracy of 1 mrad.
The TUS photoreceiver is designed as an orthogonal network of 16 x 16 = 256 pixels. The pixel is a circular PMT with a square 1.5 x 1.5-cm window light guide. All pixels are covered by the UV filter, transparent in the fluorescence wavelength band 310—420 nm. The choice of the PMT is a compromise between a good time resolution, high sensitivity in the fluorescence wavelength range, stable performance in the presence of high light noise, fast recovering after exposing to the scattered day atmosphere light, and slow aging. The Hamamatsu PMT type R1463P with a multialcali cathode and a linear dynode system was selected. It was shown that the highest energy events (energies more than 1020 eV) could be detected even at the background light of the Moon if the gain of PMTs is adjusted to the background [6]. For this aim the PMT voltage is made variable and a special circuit
aflEPHAa OH3HKA tom 67 № 11 2004
KLYPVE/TUS SPACE EXPERIMENTS
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was suggested for keeping gain of all tubes uniform in the range of gain varying by 10 times [10].
Pixels are organized in clusters: 16 pixels in line is a cluster having a common HV supply, a FADC channel, a FIFO memory. Data are sampled in time bins ts from every pixel in the cluster with the help of a multiplexer. Triggering and DAS are common for the whole retina of pixels. The FPGA technique is used for the digital part of the electronics.
The UHECR finding algorithm operates in two trigger levels: at the 1st level the pixels with the signal above the first threshold q in integration time ti are selected. At the 2nd level the group of triggered pixels making an EAS track are selected. At the onboard TUS computer an additional separation of real UHECR events is made, the data are compressed and prepared for sending to the mission center.
The prototype TUS "telescope" is planned for testing at the Cerra La Negra mountains in Mexico [11]. The mountain TUS telescope will allow recording EAS tracks at distances 25—100 km from the detector. In the telescope field of view atmosphere transparency monitoring will be provided by a special control device using the xenon lamp flashes.
3. THE UHECR EVENT SIMULATION
The UHECR events were simulated with the aim of testing the efficiency of the selection system and estimating the measurement accuracy. Development of EAS initiated by primary UHECR particles was considered within the framework of the CORSIKA/QGSJET model. A special UHECR event simulation program package SLAST was elaborated [12]. With the calculated EAS signals performance of the electronics designed for the KLYPVE/TUS detectors was simulated. It includes two lines of pixel signal analysis — performance of the digital oscilloscope with time sampling ts and performance of the triggering system.
In the TUS detector digital oscilloscope the signal is recorded as a PM tube anode potential in time samples ts = 0.8 f s. Integration of the signal in time t = 12 f s, needed for the selection of useful EAS events, is done in a digital form. Time constant of the RC charge integration circuit at the anode is chosen equal to ts. It was shown in the simulation of the digital oscilloscope performance that fluctuations of the signal sampled in time intervals ts = RC are close to fluctuations in the number of photoelectrons at the tube cathode for the EAS of energies E > 30 EeV. It confirms that the digital oscilloscope operating with the selected time sample gives adequate data on the event.
Operation of the triggering system was simulated for various EAS energies and zenith angles. For a
Triggering efficiency, % 100
80 60 40
20 -
40
60 80 100 Primary energy, EeV
Fig. 3. The TUS triggering efficiency as a function of primary energy and zenith angles (values on the cur
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