1. Tunka Experiment: Towards 1км 2 EAS Cherenkov Array B.K.Lubsandorzhiev for TUNKA Collaboration

2. TUNKA COLLABORATION Scobeltsyn Institute of Nuclear Physics of MSU (Moscow, Russia) Institute of Applied Physics of ISU (Irkutsk, Russia) Institute for Nuclear Research RAS (Moscow, Russia) IZMIRAN (Moscow, Russia) Universita’ Torino, Italy DESY-Zeuthen, Germany

3. “…..as far as Baikal it is Siberia’s dull prose, just from Baikal Siberian delightful poetry starts….” A.P.Chekhov (Letters from Siberia)

4. History of Tunka Experiment 1991-1992 experiments with QUASAR-370 tubes on the Baikal ice 1991-1993 Development of QUASAR-370 modification for EAS Cherenkov arrays - QUASAR-370G 1993 Surface Mobile EAS Cherenkov Array (SMECA) for joint work with Lake Baikal neutrino telescope NT-200 1993 - TUNKA-4 (4 QUASAR -370G) 1996 - TUNKA-13 (13 QUASAR-370G) 2000 - TUNKA-25 (25 QUASAR-370G) 200? - TUNKA-133 (1km 2 Cherenkov EAS Array)

5. QUASAR-370G 37 cm extended bialkali low resistance hemispherical photocathode 2  acceptance YSO+BaF 2 scintillator Small 6 stages high anode current PMT

6. QUASAR-370G 2 ns TTS (FWHM) 70-80 % SER I Amax ~ 200 mkA Immunity to Earth’s magnetic field

7. TUNKA Wide Angle EAS Cherenkov Detector 675 m a.s.l. S eff ~ 1.6 10 5 m 2 E th ~ 500 TeV  ~ 0.5 o

8. The Tunka-25 „Remote detector“ 25 QUASAR-370G tubes - 37 cm diameter - integrating 4 EMI D668 tubes (AIROBICC tubes) - 20 cm diameter - fiber read-out, - FADC prototype

9. Cherenkov light lateral distribution Q(R) = Q kn ·exp((R kn -R)·(1+2/R)/R 0 ) Q(R) = Q kn ·(R kn /R) 2.2 P = Q(100)/Q(200) R 0 = 10 2.83-0.2·P [m] R kn = 200 -20·P [m] H max =10.62-0.12(P+2.73) 2 H max =X 0 /cos  - X max

10. Differential energy spectrum of CRs around the “knee” E 0 [TeV] = 370  (Q 175 ) 0.96 [photon  cm -2  eV -1 ] E knee ~ 3  10 15 eV  1 = -2.66  0.01  2 = -3.10  0.02

11. Energy spectrum of CRs in wide range Gap between direct and ground based measurements. Approximations of their data don’t coincide!

12. Chemical composition of CRs around the knee Mass composition is measured by two methods: 1. Measurements of Cherenkov light waveform at large distances from a shower core (>200 m) R ~ 300 m 2. Analysis of LDF H max =1677+1006lg(FWHM)

13. Chemical composition around the knee

14. Mean mass composition 30% p, 30% He, 20%CN, 20% Fe

15. WHAT NEXT?

16. Тunka-133 133 optical detectors covering S eff ~ 1 km 2 E th ~ 10 15 eV Expected statistics for 1 year operation ( 400 hours): > 3·10 15 eV ~ 3.010 5 events > 10 17 eV ~ 200 events > 10 18 eV ~ 1 – 3 events Study of energy spectrum and mass composition of primary cosmic rays from “classical” knee ~310 15 eV to maximum energy in SNR ~10 17 Z eV

17. Site of TUNKA experiment

18. Tunka-133: position of optical detectors Seven optical detectors form one Cluster

19. Optical detector Preamplifier HV power supply Plexi window with heating Phototube: 20 cm PMT from AIROBICC and MACRO

20. Cluster’s Electronics

21. Reconstruction of EAS parameters Shower core location ~ 6 m Primary energy measurement ~ 15% X max measurement (LDF) ~ 35 – 40 g/cm 2 X max measurement (pulse shape) ~ 25 g/cm 2

22. CONCLUSIONS TUNKA experiment operates for more than 10 years. Physics results of the experiment covers primary cosmic rays studies in the energy range 6  10 14 - 10 17 eV The «knee» of primary energy spectrum is observed around ~3 10 15 eV Primary mass composition doesn’t change significantly in the range of 10 15 - 10 16 eV gradually rising to heavier elements at higher energies. It is necessary to decrease energy threshold down to 10 14 eV to compare results with direct experiments data

23. It is very desirable to develope new version of QUASAR phototube (~50 cm in diameter): fast (1ns TTS (fwhm)) with a few ns time response. We are planning to construct new array with 133 hemispherical phototubes (20 cm in diameter) to study primary cosmic rays in the energy range of 10 15 - 10 18 eV