NEVOD-DECOR experiment: results and future

1. 23rd ECRS, Moscow, July 3-7, 2012 NEVOD-DECOR experiment: results and future O. Saavedra* for the DECOR Collaboration National Research Nuclear University MEPhI, Moscow, Russia * Dipartimento di Fisica dell’ Universita di Torino, Italy Istituto di Fisica dello Spazio Interplanetario, INAF, Torino, Italy

2. Brief history 20 years ago (1 June 1992), the first cooperation agreement between MEPhI and Torino group (ICGF, TU) was signed. The main purpose was the creation of the coordinate detector DECOR around the Cherenkov water calorimeter NEVOD for investigations of multi-particle events in cosmic rays at large zenith angles. Streamer tube chambers formerly used in the NUSEX experiment, front-end electronics and other equipment were delivered in Moscow. After R&D works, the first supermodules of the detector were put into operation in 1998. In 2001, the construction of DECOR was completed. Long-term experiments (about 20,000 net operation time) on detection of muon bundles with NEVOD-DECOR complex were conducted in 2002-2007.

3. General view of NEVOD-DECOR complex Coordinate-tracking detector DECOR (~115 m2) Cherenkov water detector NEVOD (2000 m3)

4. DECOR supermodules (SMs) Side DECOR: Eight vertically suspended supermodules, each with area 8.4 m2, consisting of 8 planes of streamer tube chambers with two-dimensional external strip readout. Spatial and angular accuracy of muon track location is better than 1 cm and 1 degree, respectively. Top DECOR: Four horizontal SMs with area 11.5 m2 each. On the basis of top DECOR, after replacement of the DAQ system, the muon hodoscope URAGAN was created (2005) which is now being used for muon flux variation studies (see talk I.I.Yashin, SH 580; posters N.S.Barbashina, SH 574; I.I.Astapov, SH 605; V.V.Shutenko, SH 584 at this Symposium).

5. Side DECOR supermodules (SMs)in the galleries around NEVOD water tank

6. High-multiplicity muon bundle event in side DECOR (~ 130 muons, θ = 80°)

7. Event geometry reconstruction

8. Multiplicity and angular distributions Over 40 thousand muon bundles with various multiplicities and zenith angles were selected from experimental data accumulated in 2002-2007. NB: 6 – 7 orders of magnitude in the event intensity !!!

9. Novel approach to the analysis of muon bundles: method of Local Muon Density Spectra (LMDS) In an individual muon bundle event, local muon density D (at the observation point) is measured. Distribution of events in estimated muon density D forms the LMDS.

10. EAS collection area in LMDS method EAS transverse section in muons θ = 35º θ = 80º(with EMF)

11. Accessible energy range in LMDS method Different zenith angles correspond to different EAS energies: very wide range of primary particle energies simultaneously. Statistics is determined not by the detector size, but by EAS dimensions (at large zenith angles, few km2); sufficient to reach primary energies ~1018 eV.

12. Measured LMDS at different zenith angles

13. Comparison of UHECR energy spectrum with other data At large zenith angles and high multiplicities, the measured muon bundle intensity is not compatible with fluorescence data for any interaction model, even under assumption of a heavy (iron) primary composition. More muons in simulation is needed.

14. Results of LMDS studies in NEVOD-DECOR experiment LMDS measurements gave possibility to obtain information on primary flux and interaction characteristics in a record wide energy range from 1015 to more than 1018 eV and to reveal the following basic features: - Increase of LMDS slope at PeV energies (the first knee); - Relative increase of muon bundle intensity which may be interpreted as a trend to a heavier mass composition above the knee; - Further increase of LMDS slope near 1017 eV (indication for the second – iron? – knee); - Excess of muon bundles in comparison with expectation based on independent estimates of UHE primary spectrum and widely used hadronic interaction models.

15. Further direction of the experiments: studies of muon component energy characteristics with modernized NEVOD measuring system 91 quasi-spherical optical modules (QSM), each consisting of 6 PMT with flat photocathodes; 546 new low-noise photomultipliers FEU-200; Signal readout from two dynodes (1092 amplitude channels, total dynamic range from 1 to 105 photoelectrons); New triggering and DAQ systems; Modernized calibration telescope system (40+40 scintillation counters with amplitude analysis). Continuous measurements started in December 2011

16. Energy spectrum of cascade showers from muons: first results (poster S.S. Khokhlov, MN 532) “Narrow geometry” (reconstruction DECOR+NEVOD): 10 – 1000 GeV “Wide geometry” (reconstruction by NEVOD QSMs): 100 – 10000 GeV

17. Energy deposit of muon bundles in the Cherenkov water calorimeter NEVOD Analysis of energy deposit of muon bundles in NEVOD versus muon density D and zenith angle estimated with DECOR. Data accumulation and event selection have been started.

18. Scintillation EAS array around NEVOD-DECOR complex Poster I.A.Shulzhenko, PCR_2, 582 The use of scintillation detectors from KASCADE-Grande is supposed. Main tasks: Cross-calibration of the traditional EAS detection technique and LMDS method in the energy range 1015 -1017 eV; Study of energy characteristics of hadrons and EAS cores with NEVOD.

19. Thank you for your attention!