- Email: [email protected]
Tunka-HiSCORE – A new array for multi-TeV γ-ray astronomy and cosmic-ray physics
Nuclear Instruments and Methods in Physics Research A 732 (2013) 290–294
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Tunka-HiSCORE – A new array for multi-TeV γray astronomy and cosmic-ray physics O. Gress c,n, M. Brüeckner g, S. Berezhnev b, N. Budnev c, M. Buker a, O. Chvalaev c, A. Dyachok c, U. Einhaus a, S. Epimakhov a, D. Hampf a, D. Horns a, N. Kalmykov b, N. Karpov b, E. Konstantinov c, E. Korosteleva b, M. Kunnas a, V. Kozhin b, L. Kuzmichev b, B. Lubsandorzhiev d, N. Lubsandorzhiev d, R. Mirgazov c, R. Monkhoev c, R. Nachtigall a, A. Pakharukov c, M. Panasyuk b, L. Pankov c, E. Popova b, A. Porelli f, V. Prosin b, V. Ptuskin e, G. Rowell h, Yu Semeney c, B. Shaibonov d, A. Silaev b, A. Silaev(ju) b, A. Skurikhin b, C. Spiering f, D. Spitschan a, L. Sveshnikova b, M. Tluczykont a, R. Wischnewski f, I. Yashin b, A. Zagorodnikov c, V. Zirakashvili e a
Institut fur Experimental Physik, University of Hamburg, Germany Skobeltsyn Institute of Nuclear Physics MSU, Moscow, Russia Institute of Applied Physics ISU, Irkutsk, Russia d Institute for Nuclear Research of RAS, Moscow, Russia e IZMIRAN, Troitsk, Moscow Region, Russia f DESY, Zeuthen, Germany g Institute for Computer Science, Humboldt-University Berlin, Germany h School of Chemistry and Physics, University of Adelaide, Australia b c
art ic l e i nf o
a b s t r a c t
Available online 21 June 2013
Tunka-HiSCORE (HiSCORE: Hundred Square-km Cosmic Origin Explorer) is a new Cherenkov EAS array for multi-TeV γray astronomy and cosmic ray studies. The array will consist of wide-angle (0.6 sr) optical stations with 150 m distance between individual stations. The array will be constructed in several stages: beginning with a 1 km2 array at the first stage up to 100 km2 at the last stage. The first three stations have been installed during 2012 in Tunka Valley (50 km from the Southern tip of Lake Baikal) at the site of the Tunka-133 array. We describe the construction of the optical stations and the array DAQ, and present the first results of common operation of the new optical stations with the Tunka-133 array. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cosmic ray γray astronomy Cherenkov light Data acquisition system
1. Introduction We propose to explore the so far poorly measured cosmic ray and γray sky in the energy region from 20 TeV to 100 PeV with the large area wide-angle cosmic ray and γray detector Tunka-HiSCORE [1,2]. Tunka-HiSCORE will be a non-imaging Cherenkov light-front sampling array with large sensitive area (0.5 m2) detector modules, distributed over an array covering a total area of 3.3 km2. The Tunka-HiSCORE array is being developed for the study of fundamental questions of astroparticle and particle physics [3,4], including the origin of charged Galactic cosmic rays (γray observations), spectral and composition measurements of charged Galactic cosmic rays, diffuse γray emission (e.g. Galactic plane, local super-cluster), attenuation by Galactic interstellar radiation
n
Corresponding author. Tel.: +7 839 523 32170. E-mail address: [email protected] (O. Gress).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.06.034
fields and the cosmic microwave background. Existing and planned experiments do not cover the cutoff energy region of Galactic PeVatrons. So the main area of our interests is cosmic ray physics from 100 TeV to 1 EeV, an ultra-high energy γray survey (PeVatron search) and particle physics beyond the LHC energy range.
2. Tunka-HiSCORE project The principle of Tunka-HiSCORE detector is based on the method of air shower front sampling technique using Cherenkov light. The detector stations measure the light amplitudes and full development using the shower front arrival time distribution (at distances from the shower core ≳100 m). These measurements allow detailed measurements of the spectrum and composition, as well as the γ=hadrons separation through reconstruction of the shower maximum depth.
O. Gress et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 290–294
291
to its large effective area. Today's existing γray telescopes can effectively detect γphotons with energies up to 10 TeV, so most of this energy region remains unexplored. Up to now no photon with energy above 100 TeV has been detected. The high effective area of the Tunka-HiSCORE observatory will open a new and previously unexplored energy range allowing us to test new models of particle physics and astrophysics.
3. Optical stations
Fig. 1. Position of optical stations (squares) for the future Tunka-HiSCORE array relative to the Tunka-133 detectors (circles) [5,6]. Possible arrangement of optical stations is limited by the Irkut river flowing in the Tunka Valley. Each station consists of a box with four PMTs.
The Tunka-HiSCORE project assumes the creation of the TunkaHiSCORE observatory consisting of a large number of optical distributed over an array covering a total area of 1 up to 10 km2 at the site of the Tunka-133 installation [5,6]. As an example, a possible layout of optical stations is shown in Fig. 1, for an assumed area of 3.3 km2. The distance between individual stations will be 150 m. Each station will consist of four photomultipliers and each PMT will have a Winston cone of 0.4 m in diameter so that the light-collection area will increase to 0.5 m2. The specific location of the optical detectors is currently under discussion. For the first stage, 64 stations will be placed on one square kilometer. In a second stage we plan adding 49 stations with large-diameter PMT photocathodes. In addition, each station will include: slow control electronics, environment sensors, readout and data transmission electronics, and a nanosecond-precision clock. The station mechanics consists of a metal box with a mechanism to open or close the lid. The stations will need to tilt for the night star sky survey. The PMT anode signals will be added together and limited in amplitude. Amplitude limitation of the summed-up signal (or clipped sum trigger) is used for reducing afterpulsing effects in photomultipliers. The field of view of each station's optics will be about 0.6 sr and the angular resolution is 0:10 at an air shower energy of no less than 50 TeV. Synchronization of events between stations will be achieved with sub-nanosecond precision by the local clock in each station using optical fiber connections (White Rabbit system [7]). According to the array simulation [2] the flux sensitivity will be about 10−12 erg/(cm2 s) for a γphoton detection threshold ≳20 TeV. The count rate of air showers from one station should be ∼15 Hz. The air shower detection by the optical stations in the cosmic ray energy range from 100 TeV to 1 EeV will be performed on the principle of “non-imaging Cherenkov technique”. The array will simultaneously scan a large fraction of the night star sky (survey mode). We expect 680 γray photons during 110 h of observation from the Crab source (SN1054). For other γray sources, in particular Tycho (SN1572), Mrk421, Cas A, G119.5+10.2 and G106.3+2.7, the expected number of events is estimated at 10–200 γray photons during ∼200 h or more of observation time [8]. The Tunka-HiSCORE observatory will be sensitive to the low γphoton flux in the energy range from 20 TeV to 100 PeV due
In autumn 2012 three optical stations consisting of metal boxes with a size of 100 100 100 cm3 were installed. The stations are located at the corners of a right triangle in the center of the Tunka133 installation (see Fig. 2). Two stations have four PMTs while the third one has two PMTs. The nearest distance between stations is 150 m. Each PMT is equipped with a Winston cone (10 segments of ALANOD 4300UP foil with a reflectivity of 80%) that increases the light collection area by a factor of 4. A Plexiglass pane on top of the Winston cones protects the PMTs against dust and humidity. Heating prevents the formation of white frost during the cold season. The high-voltage supply source for the PMT, the voltage divider and the preamplifiers for the dynode and anode signals from the photomultipliers are made as one device. We use a six-stage 8 in. PMT (EMI 9350 KB) with a nominal gain of 104 at 1.4 kV supply. A controller unit is used to control the high-voltage supply and the anode currents from the photomultipliers (see Fig. 3).
4. Data acquisition system The signals from the photomultiplier anodes go via coaxial cable to the FADCs (based on 12bit AD9430 ADC chip and XILINX Spartan-3 FPGA) and through a splitter to the analog summing circuit (Σ), the summator (see Fig. 3).
Fig. 2. Location of the first optical stations (squares) of the Tunka-HiSCORE γdetector prototype among the Tunka-133 Cherenkov detectors (circles) in 2012/2013. On the top right there is a photo of an electronics box (left) and an optical station's box (right).
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O. Gress et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 290–294
Fig. 3. Main components of the γstation detector prototype: photodetector (4 PMT EMI 9352 KB; HV div., H.V. divider; PreAmp., preamplifier; Controller, H.V. supply and heating), Station DAQ (Master, coincidence circuit; Σ, summator), Central DAQ (PC, host electronics and software).
Fig. 4. An example of the Cherenkov pulse amplitudes from an extensive air shower. 1,2,3,4 – signals from PMTs installed in the optical station, 5 – resulting output signal from analog summator.
The sum signal is also supplied to the FADC. Alignment of both the photomultiplier amplification of the signal amplitude and the propagation delay is required for the correct operation of the summing circuit. With this scheme it is possible to derive the event trigger from both the summed signal and an individual
signal of each photomultiplier. Signals are sampled by a 12-bit FADC in steps of 5 ns (1024 samples). The trigger condition for the Master scheme was to set the amplitude threshold for the pulse sum as low as possible and thus to cut the night sky light background fluctuations. In case of a
O. Gress et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 290–294
trigger (air shower event) the data are read from the FADC and transferred through the optical cable to the central DAQ. A VME Heating Controller provides thermostatic control of the station DAQ electronics. We plan using a DRS4 chip [9] (or a similar scheme) as basic element of the DAQ system. The DRS4 chip allows 1024 bin sampling with 0.5-5 Gigasamples/s at a readout rate of up to 300 Hz.
5. Measurement results The first prototype γstation containing two 8 in. PMTs (EMI 9350 KB) and two Winston cones was deployed in April 2012. The first Cherenkov pulses and first air shower data were obtained from this γstation. Three γstations were commissioned in October 2012. Tests of data taking and optical γstation prototype DAQ components were carried out from October 2012 to April 2013. The main goals of the test measurements were to estimate the detection threshold for air showers and to study possible ways of threshold reduction. In particular, we investigated electronic noise effects and the efficiency of the night sky background suppression in the summator, as well as the effect of a sensitivity improvement of the photomultiplier cathode by using a special chemical coating. The Tunka-HiSCORE prototype stations measure the faint Cherenkov light emitted by extensive air showers which are initiated by ultra-high-energy cosmic rays and γrays. Air shower Cherenkov light pulse shapes from the PMTs of an optical station and the resulting pulse from the summator circuit are shown in Fig. 4. Pulse amplitudes from the photomultipliers were aligned using both LED signals and amplitude spectra from air shower Cherenkov light pulses (off-line processing). Good suppression of electronic noise is required for the correct operation of the summator.
293
The integral amplitude spectrum of Cherenkov light flashes from air showers is shown in Fig. 5. The spectrum of the night sky light background fluctuation from summator is also given. The value of the sky background fluctuations in RMS units is 6.13, which corresponds to a voltage of 9.2 mV. Cherenkov light flash amplitude spectra were obtained from two different optical stations, one of them with the plexiglass cover on, the other one without the plexiglass cover. The measurements were performed at the same time. The slope of the amplitude spectra yields in both cases γ ¼ −1:6. Photomultipliers in the second station (without plexiglass) were coated with a special chemical substance to increase the photocathode's sensitivity to the air shower Cherenkov light. The absence of Cherenkov light absorption in the plexiglass in the ultraviolet spectrum range leads to a decrease (by a factor of 1.6) in the air shower detection threshold (accordingly, the count rate increases by a factor of 2.6). As a result, the air shower count rate is ∼5 Hz. For comparison, the air shower count rate for the Tunka-133 installation is ∼2 Hz whereas the individual photodetector count rate is ∼0:1 Hz. The air shower detection threshold for the optical γstation is estimated at ∼50 TeV.
6. Conclusion The first Tunka-HiSCORE prototype γstation containing two PMTs and two Winston cones was deployed on the Tunka-133 EAS Cherenkov array in April 2012. Three γstations were commissioned in October 2012, two of which consist of four PMTs with cones. Data taking, testing of γstation components and data analysis were carried out during the 2012/2013 data-taking period. Ways to further decrease the air shower registration threshold for the optical γstations have been investigated. In particular, high-voltage electronics with low noise level and faster preamplifiers for reducing the photomultiplier pulse duration are required to improve the efficiency of the analog summator. We plan using a PMT with bigger photocathode area and therefore also a cone with bigger input area to lower the photon registration threshold. The energy detection threshold will decrease due to the improvement of the photomultiplier quantum sensitivity (new types of PMT: ET 9352 KB and R5912) and the reduction of the anode pulse length from 20 to 10 ns. The DAQ system will be modified and adapted to a higher event rate and the event synchronization will be improved to reach sub-nanosecond accuracy. An international collaboration has been set up and has resulted in the Tunka-HiSCORE prototype deployment in the Tunka Valley.
Acknowledgments This work has been supported by the Russian Federation's Ministry of Science and Education (G/C 14.518.11.7046, agreements N 14.B37.21.0785 and N 14.37.21.1294), the Russian Foundation of Basic Research (Grant nos. 11-02-00409, 13-02-00214, 13-0210001), the President of the Russian Federation (Grant MK1170.2013.2), the Helmholtz association (Grant HRJRG-303) and the Deutsche Forschungsgemeinschaft (Grant TL 51-3).
Fig. 5. Integral amplitude spectra of the night sky and Cherenkov light flashes from extensive air showers. Triangles: night sky light background fluctuation spectrum from summator, RMS is 6.13 units or 9.2 mV. Squares: spectrum from the optical station covered with plexiglass, shower count rate is 1.8 Hz. Circles: spectrum from the optical station without plexiglass, shower count rate is 4.7 Hz. For this station (“Blue PMTs”) the photocathode sensitivity is shifted into the blue spectral region by a special chemical coating.
References [1] M. Tluczykont, et al., Gamma-ray and Cosmic Ray Astrophysics from 10 TeV to 2 1 EeV with the Large-Area ( 4 10 km ) Air-Shower Detector SCORE, arXiv:astroph.IM/0909.0445v1. [2] M. Tluczykont, et al., The Ground-Based Large-Area Wide-Angle γray and Cosmic-ray Experiment HiSCORE, arXiv:astro-ph.IM/1108.5880v1.
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[3] A. De Angelis, et al., Very High Energy Gamma Astrophysics, arXiv:astro-ph/ 0712.0315v6. [4] E. Lorenz, R. Wagner, Very-high Energy Gamma-Ray Astronomy, arXiv:physics. hist-ph/1207.6003v1. [5] B.V. Antokhonov, et al., Nuclear Instrumentation and Methods in Physics Research A 628 (2011) 124.
[6] S.F. Berezhnev, et al., The Tunka-133 EAS Cherenkov Light Array: Status of 2011, arXiv:astro-ph.HE/1201.2122v1. [7] 〈http://www.ohwr.org/projects/white-rabbit〉. [8] 〈http://tevcat.uchicago.edu/〉. [9] 〈http://drs.web.psi.ch/〉.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Tunka-HiSCORE – A new array for multi-TeV γray astronomy and cosmic-ray physics O. Gress c,n, M. Brüeckner g, S. Berezhnev b, N. Budnev c, M. Buker a, O. Chvalaev c, A. Dyachok c, U. Einhaus a, S. Epimakhov a, D. Hampf a, D. Horns a, N. Kalmykov b, N. Karpov b, E. Konstantinov c, E. Korosteleva b, M. Kunnas a, V. Kozhin b, L. Kuzmichev b, B. Lubsandorzhiev d, N. Lubsandorzhiev d, R. Mirgazov c, R. Monkhoev c, R. Nachtigall a, A. Pakharukov c, M. Panasyuk b, L. Pankov c, E. Popova b, A. Porelli f, V. Prosin b, V. Ptuskin e, G. Rowell h, Yu Semeney c, B. Shaibonov d, A. Silaev b, A. Silaev(ju) b, A. Skurikhin b, C. Spiering f, D. Spitschan a, L. Sveshnikova b, M. Tluczykont a, R. Wischnewski f, I. Yashin b, A. Zagorodnikov c, V. Zirakashvili e a
Institut fur Experimental Physik, University of Hamburg, Germany Skobeltsyn Institute of Nuclear Physics MSU, Moscow, Russia Institute of Applied Physics ISU, Irkutsk, Russia d Institute for Nuclear Research of RAS, Moscow, Russia e IZMIRAN, Troitsk, Moscow Region, Russia f DESY, Zeuthen, Germany g Institute for Computer Science, Humboldt-University Berlin, Germany h School of Chemistry and Physics, University of Adelaide, Australia b c
art ic l e i nf o
a b s t r a c t
Available online 21 June 2013
Tunka-HiSCORE (HiSCORE: Hundred Square-km Cosmic Origin Explorer) is a new Cherenkov EAS array for multi-TeV γray astronomy and cosmic ray studies. The array will consist of wide-angle (0.6 sr) optical stations with 150 m distance between individual stations. The array will be constructed in several stages: beginning with a 1 km2 array at the first stage up to 100 km2 at the last stage. The first three stations have been installed during 2012 in Tunka Valley (50 km from the Southern tip of Lake Baikal) at the site of the Tunka-133 array. We describe the construction of the optical stations and the array DAQ, and present the first results of common operation of the new optical stations with the Tunka-133 array. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cosmic ray γray astronomy Cherenkov light Data acquisition system
1. Introduction We propose to explore the so far poorly measured cosmic ray and γray sky in the energy region from 20 TeV to 100 PeV with the large area wide-angle cosmic ray and γray detector Tunka-HiSCORE [1,2]. Tunka-HiSCORE will be a non-imaging Cherenkov light-front sampling array with large sensitive area (0.5 m2) detector modules, distributed over an array covering a total area of 3.3 km2. The Tunka-HiSCORE array is being developed for the study of fundamental questions of astroparticle and particle physics [3,4], including the origin of charged Galactic cosmic rays (γray observations), spectral and composition measurements of charged Galactic cosmic rays, diffuse γray emission (e.g. Galactic plane, local super-cluster), attenuation by Galactic interstellar radiation
n
Corresponding author. Tel.: +7 839 523 32170. E-mail address: [email protected] (O. Gress).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.06.034
fields and the cosmic microwave background. Existing and planned experiments do not cover the cutoff energy region of Galactic PeVatrons. So the main area of our interests is cosmic ray physics from 100 TeV to 1 EeV, an ultra-high energy γray survey (PeVatron search) and particle physics beyond the LHC energy range.
2. Tunka-HiSCORE project The principle of Tunka-HiSCORE detector is based on the method of air shower front sampling technique using Cherenkov light. The detector stations measure the light amplitudes and full development using the shower front arrival time distribution (at distances from the shower core ≳100 m). These measurements allow detailed measurements of the spectrum and composition, as well as the γ=hadrons separation through reconstruction of the shower maximum depth.
O. Gress et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 290–294
291
to its large effective area. Today's existing γray telescopes can effectively detect γphotons with energies up to 10 TeV, so most of this energy region remains unexplored. Up to now no photon with energy above 100 TeV has been detected. The high effective area of the Tunka-HiSCORE observatory will open a new and previously unexplored energy range allowing us to test new models of particle physics and astrophysics.
3. Optical stations
Fig. 1. Position of optical stations (squares) for the future Tunka-HiSCORE array relative to the Tunka-133 detectors (circles) [5,6]. Possible arrangement of optical stations is limited by the Irkut river flowing in the Tunka Valley. Each station consists of a box with four PMTs.
The Tunka-HiSCORE project assumes the creation of the TunkaHiSCORE observatory consisting of a large number of optical distributed over an array covering a total area of 1 up to 10 km2 at the site of the Tunka-133 installation [5,6]. As an example, a possible layout of optical stations is shown in Fig. 1, for an assumed area of 3.3 km2. The distance between individual stations will be 150 m. Each station will consist of four photomultipliers and each PMT will have a Winston cone of 0.4 m in diameter so that the light-collection area will increase to 0.5 m2. The specific location of the optical detectors is currently under discussion. For the first stage, 64 stations will be placed on one square kilometer. In a second stage we plan adding 49 stations with large-diameter PMT photocathodes. In addition, each station will include: slow control electronics, environment sensors, readout and data transmission electronics, and a nanosecond-precision clock. The station mechanics consists of a metal box with a mechanism to open or close the lid. The stations will need to tilt for the night star sky survey. The PMT anode signals will be added together and limited in amplitude. Amplitude limitation of the summed-up signal (or clipped sum trigger) is used for reducing afterpulsing effects in photomultipliers. The field of view of each station's optics will be about 0.6 sr and the angular resolution is 0:10 at an air shower energy of no less than 50 TeV. Synchronization of events between stations will be achieved with sub-nanosecond precision by the local clock in each station using optical fiber connections (White Rabbit system [7]). According to the array simulation [2] the flux sensitivity will be about 10−12 erg/(cm2 s) for a γphoton detection threshold ≳20 TeV. The count rate of air showers from one station should be ∼15 Hz. The air shower detection by the optical stations in the cosmic ray energy range from 100 TeV to 1 EeV will be performed on the principle of “non-imaging Cherenkov technique”. The array will simultaneously scan a large fraction of the night star sky (survey mode). We expect 680 γray photons during 110 h of observation from the Crab source (SN1054). For other γray sources, in particular Tycho (SN1572), Mrk421, Cas A, G119.5+10.2 and G106.3+2.7, the expected number of events is estimated at 10–200 γray photons during ∼200 h or more of observation time [8]. The Tunka-HiSCORE observatory will be sensitive to the low γphoton flux in the energy range from 20 TeV to 100 PeV due
In autumn 2012 three optical stations consisting of metal boxes with a size of 100 100 100 cm3 were installed. The stations are located at the corners of a right triangle in the center of the Tunka133 installation (see Fig. 2). Two stations have four PMTs while the third one has two PMTs. The nearest distance between stations is 150 m. Each PMT is equipped with a Winston cone (10 segments of ALANOD 4300UP foil with a reflectivity of 80%) that increases the light collection area by a factor of 4. A Plexiglass pane on top of the Winston cones protects the PMTs against dust and humidity. Heating prevents the formation of white frost during the cold season. The high-voltage supply source for the PMT, the voltage divider and the preamplifiers for the dynode and anode signals from the photomultipliers are made as one device. We use a six-stage 8 in. PMT (EMI 9350 KB) with a nominal gain of 104 at 1.4 kV supply. A controller unit is used to control the high-voltage supply and the anode currents from the photomultipliers (see Fig. 3).
4. Data acquisition system The signals from the photomultiplier anodes go via coaxial cable to the FADCs (based on 12bit AD9430 ADC chip and XILINX Spartan-3 FPGA) and through a splitter to the analog summing circuit (Σ), the summator (see Fig. 3).
Fig. 2. Location of the first optical stations (squares) of the Tunka-HiSCORE γdetector prototype among the Tunka-133 Cherenkov detectors (circles) in 2012/2013. On the top right there is a photo of an electronics box (left) and an optical station's box (right).
292
O. Gress et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 290–294
Fig. 3. Main components of the γstation detector prototype: photodetector (4 PMT EMI 9352 KB; HV div., H.V. divider; PreAmp., preamplifier; Controller, H.V. supply and heating), Station DAQ (Master, coincidence circuit; Σ, summator), Central DAQ (PC, host electronics and software).
Fig. 4. An example of the Cherenkov pulse amplitudes from an extensive air shower. 1,2,3,4 – signals from PMTs installed in the optical station, 5 – resulting output signal from analog summator.
The sum signal is also supplied to the FADC. Alignment of both the photomultiplier amplification of the signal amplitude and the propagation delay is required for the correct operation of the summing circuit. With this scheme it is possible to derive the event trigger from both the summed signal and an individual
signal of each photomultiplier. Signals are sampled by a 12-bit FADC in steps of 5 ns (1024 samples). The trigger condition for the Master scheme was to set the amplitude threshold for the pulse sum as low as possible and thus to cut the night sky light background fluctuations. In case of a
O. Gress et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 290–294
trigger (air shower event) the data are read from the FADC and transferred through the optical cable to the central DAQ. A VME Heating Controller provides thermostatic control of the station DAQ electronics. We plan using a DRS4 chip [9] (or a similar scheme) as basic element of the DAQ system. The DRS4 chip allows 1024 bin sampling with 0.5-5 Gigasamples/s at a readout rate of up to 300 Hz.
5. Measurement results The first prototype γstation containing two 8 in. PMTs (EMI 9350 KB) and two Winston cones was deployed in April 2012. The first Cherenkov pulses and first air shower data were obtained from this γstation. Three γstations were commissioned in October 2012. Tests of data taking and optical γstation prototype DAQ components were carried out from October 2012 to April 2013. The main goals of the test measurements were to estimate the detection threshold for air showers and to study possible ways of threshold reduction. In particular, we investigated electronic noise effects and the efficiency of the night sky background suppression in the summator, as well as the effect of a sensitivity improvement of the photomultiplier cathode by using a special chemical coating. The Tunka-HiSCORE prototype stations measure the faint Cherenkov light emitted by extensive air showers which are initiated by ultra-high-energy cosmic rays and γrays. Air shower Cherenkov light pulse shapes from the PMTs of an optical station and the resulting pulse from the summator circuit are shown in Fig. 4. Pulse amplitudes from the photomultipliers were aligned using both LED signals and amplitude spectra from air shower Cherenkov light pulses (off-line processing). Good suppression of electronic noise is required for the correct operation of the summator.
293
The integral amplitude spectrum of Cherenkov light flashes from air showers is shown in Fig. 5. The spectrum of the night sky light background fluctuation from summator is also given. The value of the sky background fluctuations in RMS units is 6.13, which corresponds to a voltage of 9.2 mV. Cherenkov light flash amplitude spectra were obtained from two different optical stations, one of them with the plexiglass cover on, the other one without the plexiglass cover. The measurements were performed at the same time. The slope of the amplitude spectra yields in both cases γ ¼ −1:6. Photomultipliers in the second station (without plexiglass) were coated with a special chemical substance to increase the photocathode's sensitivity to the air shower Cherenkov light. The absence of Cherenkov light absorption in the plexiglass in the ultraviolet spectrum range leads to a decrease (by a factor of 1.6) in the air shower detection threshold (accordingly, the count rate increases by a factor of 2.6). As a result, the air shower count rate is ∼5 Hz. For comparison, the air shower count rate for the Tunka-133 installation is ∼2 Hz whereas the individual photodetector count rate is ∼0:1 Hz. The air shower detection threshold for the optical γstation is estimated at ∼50 TeV.
6. Conclusion The first Tunka-HiSCORE prototype γstation containing two PMTs and two Winston cones was deployed on the Tunka-133 EAS Cherenkov array in April 2012. Three γstations were commissioned in October 2012, two of which consist of four PMTs with cones. Data taking, testing of γstation components and data analysis were carried out during the 2012/2013 data-taking period. Ways to further decrease the air shower registration threshold for the optical γstations have been investigated. In particular, high-voltage electronics with low noise level and faster preamplifiers for reducing the photomultiplier pulse duration are required to improve the efficiency of the analog summator. We plan using a PMT with bigger photocathode area and therefore also a cone with bigger input area to lower the photon registration threshold. The energy detection threshold will decrease due to the improvement of the photomultiplier quantum sensitivity (new types of PMT: ET 9352 KB and R5912) and the reduction of the anode pulse length from 20 to 10 ns. The DAQ system will be modified and adapted to a higher event rate and the event synchronization will be improved to reach sub-nanosecond accuracy. An international collaboration has been set up and has resulted in the Tunka-HiSCORE prototype deployment in the Tunka Valley.
Acknowledgments This work has been supported by the Russian Federation's Ministry of Science and Education (G/C 14.518.11.7046, agreements N 14.B37.21.0785 and N 14.37.21.1294), the Russian Foundation of Basic Research (Grant nos. 11-02-00409, 13-02-00214, 13-0210001), the President of the Russian Federation (Grant MK1170.2013.2), the Helmholtz association (Grant HRJRG-303) and the Deutsche Forschungsgemeinschaft (Grant TL 51-3).
Fig. 5. Integral amplitude spectra of the night sky and Cherenkov light flashes from extensive air showers. Triangles: night sky light background fluctuation spectrum from summator, RMS is 6.13 units or 9.2 mV. Squares: spectrum from the optical station covered with plexiglass, shower count rate is 1.8 Hz. Circles: spectrum from the optical station without plexiglass, shower count rate is 4.7 Hz. For this station (“Blue PMTs”) the photocathode sensitivity is shifted into the blue spectral region by a special chemical coating.
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