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

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

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

History of Tunka Experiment experiments with QUASAR-370 tubes on the Baikal ice 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 TUNKA-4 (4 QUASAR -370G) TUNKA-13 (13 QUASAR-370G) TUNKA-25 (25 QUASAR-370G) 200? - TUNKA-133 (1km 2 Cherenkov EAS Array)

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

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

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

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

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 = ·P [m] R kn = ·P [m] H max = (P+2.73) 2 H max =X 0 /cos  - X max

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

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

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 = lg(FWHM)

Chemical composition around the knee

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

WHAT NEXT?

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

Site of TUNKA experiment

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

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

Cluster’s Electronics

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

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

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 eV