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L. Mikaelyan for KURCHATOV INSTITUTE NEUTRINO GROUP

Kr2Det : TWO - DETECTOR REACTOR  NEUTRINO OSCILLATION EXPERIMENT AT KRASNOYARSK UNDERGROUND SITE. L. Mikaelyan for KURCHATOV INSTITUTE NEUTRINO GROUP.  TOWARDS VERY SMALL MIXING PARAMETERS IN THE ATMOSPHERIC NEUTRINO MASS REGION. THE MAIN GOAL OF THE PROJECT IS :

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L. Mikaelyan for KURCHATOV INSTITUTE NEUTRINO GROUP

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  1. Kr2Det:TWO - DETECTOR REACTOR  NEUTRINO OSCILLATION EXPERIMENT AT KRASNOYARSK UNDERGROUND SITE L. Mikaelyan forKURCHATOV INSTITUTE NEUTRINO GROUP

  2.  TOWARDS VERY SMALL MIXING PARAMETERS IN THE ATMOSPHERIC NEUTRINO MASS REGION THE MAIN GOAL OF THE PROJECT IS : TO FIND THE CONTRIBUTION of m-3 MASS STATE to the ELECTRON NEUTRINO FLAVOR STATE or to SET NEW MORE STRINGENT CONSTRAINTS WE HOPE THAT at 1km from REACTOR the SENSITIVITY sin2213 2.010-2 for m2 = 2.510-3 eV2 CAN BE ACHIEVED. For larger mixing parameters the experiment is  sensible to m2 410-4 eV2 (PART of the HLMA REGION!)

  3. NEUTRINO OSCILLATION LIMITS

  4. Plan of the report The CHOOZ oscillation limits Kr2Det: • Idea of experiment, • Detectors, • Neutrino detection rates, backgrounds, • Method of analysis, Systematics, • Expected sensitivity, • Other applications, • Conclusions

  5. Philosophy: Reactors vs Accelerators Strategy: Step by step progress If sin22q13 is 100-1000 times smaller than present CHOOZ limits then necessary sensitivity can be achieved (in distant future and in several steps) with neutrino super beams and neutrino factories using 25-1000 kt (!) detectors located at a few thousand km from accelerator source. See for example: S.Geer, hep-ph/0210113 However, the first step can be done sooner (an cheaper) at reactors. It is quit possible that sin22q13 is just a few times smaller than present limit. No physical principal is known why it should be so terribly small.

  6. REACTOR ANTINEUTRINO DETECTION REACTION: e + p  e+ + n Positron visible energy Ee+vis : Ee+vis E - 0.8 (MeV) (Eis the energy of incidente) The neutron is captured in H (or in Gd).  In the experiment spatial- and time correlated (e+ , n )  pairs are detected  and positron energy spectrum measured.

  7. Positron spectrum

  8. REACTOR ANTINEUTRINO OSCILLA-TIONS AT ~ 1 km  FROM THE REACTOR SOURCE e = Ue11 + Ue22 + Ue33 ( Ue3 sin13 ) We conservatory assume here that  m1 < m2 < m3 and m2sol = m22 – m12 < 10-4 eV2 m2atm = m32 – m12 m32 – m22 2.510-3 eV2 >> m2sol  The e survival probability P(e e ) is then: P(e e ) = 1 – sin22sin2, sin22 sin2213 4Ue32(1-Ue32),    = (1.27m2 L E-1), L(m) is the distance from the reactor Numerically: = 1.05 = 60  form2 = 2.510-3 eV2, L = 1000 m, E= 3 MeV

  9. CHOOZ detector (300 mwe) The detector is contained in 5.5 m diameter cylindrical steel tank shielded by low radioactivity sand (75 cm) and cast iron (14 cm) Very important to suppress g-s Gd • Inside the tank there are three concentric regions: • Central 5-ton target in a transparent Plexiglas container filled with a 0.09% Gd loaded scintillator, • Intermediate 17-ton volume filled with unloaded scintillator, • Outer 90-ton optically separated veto counter filled with unloaded scintillator

  10. CHOOZ final results France-Italy-Russia-US Collaboration College de France LAPP, Annecy INFN&Univ. of Pisa INFN&Univ. of Trieste Kurchatov Institute, Moscow Univ. of Irvine Drexel Univ. Univ. of New Mexico Two reactors of summed rated power W=8.5 GW (thermal) Detector located at a depth of 300 mwe, at ~1000 m from the reactors. Neutrino events selection Summary of data acquisition (April 1997 – July 1998) Neutrino detection efficiency: 70% Total No of detected neutrinos: ~2700 Neutrino detected rate at full power: 26 d-1 Background: ~1.2 d-1 Neutrino/BKG, typically ~10:1

  11. CHOOZ final results Neutrino detection rates Xrate = measured/expected Positron spectrum shape: measured vs expected Number neutrino separately measured from two reactors CHOOZ’97 Xrate = 0.98  4%(stat)  4%(syst) CHOOZ’99 Xrate = 1.01  2.8%(stat)  2.7%(syst) CHOOZ’97 used s(0) calculated (2.5%) CHOOZ’99 uses s(0) measured (1.4%) CdF+LAPP+Kurchatov, 1994 y.

  12. Kr2Det - probing Ue3with reactor ne Method: Two identical detectors (BOREXINO, KamLAND design) 600 mwe 100 m 1000 m Target: 50 t 50 t Rate: 1.5 ·106 /year 15 ·103 /year S:B >>1 ~ 10:1

  13. Kr2Det, detector scheme 500 PMT EMI 9350Diameter – 8 inches, Coverage – 20%

  14. NEUTRINO DETECTION RATES Assuming: Target radius 2.35 m   Target volume 54 m3 Scintillator:  Density: 0.85 g/cm3   Mass: 46 ton   H/C ratio: 1.8 # of H atoms 0.7851029 / ton   Detection efficiency 0.75 300 days/year  Lfar = 1000 m, Lnear = 100 m One finds: N far = 17103/yearN near= 1.7106/year ( 1.2 / tonday ) ( 120 / tonday)

  15. BACKGROUNDS Time correlated BKG is found by extrapolating the value measured at CHOOZ : CHOOZ: 300 m.w.e., 0,24 / daytarget ton  Kr2Det: 600 m.w.e., 0.08/ daytarget ton, which is less than 10% of the far detector e detection rate Accidentals Due to high neutrino rates requirements to radiopurity of thedetector materials are much less severe than in BOREXINOand KamLANDexperiments. The target scintillator can have U, Th, 40K and 222Rn concentrations 10-13, 10-13 , 10-13 g/g and 1mBq/m3 respectively. In no Gd case the surrounding rock is expected to be the main source of the BKG… Additional passive shielding around the detector may be required.

  16. ANALYSIS The RATIO of the two MEASUREDPOSITRON SPECTRA  in no-oscillation case is ENERGY INDEPENDENT. Small specific deviations from the constant value of this ratio: Sfar/S near = const [1 – Sin22Sin2(1.27m2 LE-1)] are searched for oscillations. The analysis is independent of the exact knowledge of the neutrino flux and their energy spectrum, burn up effects, numbers of target protons… HOWEVERE RELATIVE DIFFERENCE of the DETECTOR RESPONCE FUNCTIONS MUST BE STRICTLY CONTROLLED. CALIBRATION OF THE DETECTORS IS THE KEY PROBLEM OF THIS EXPERIMENT

  17. Positron spectrum Visible positron energy, MeV

  18. DIFFERENCE of the DETECTOR’s RESPONSE CAN BE MEASURED AND CORRECTED FOR WE HOPE THAT WITH CALIBRATION PROCEDURES DETECTOR SPECTROMETRIC DIFFERENCE CAN BE CONTROLLED DOWN TO 0.5%.

  19. Ratio of positron spectra far/near Lfar = 1000 m, Lnear = 100 m, Nnfar = 16·103/year Dm2 = 2.510-3 eV2 Values of sin22q are shown at the curves

  20. Ratio of positron spectra far/near Lfar = 1000 m, Lnear = 100 m, Nnfar = 15·103/year Dm2 = 310-3 eV2 Values of sin22q are shown at the curves

  21. CONCLUSIONS & DISCUSSIONS 1) In the one reactor – two detector scheme systematic errors are minimized since NO ACCURATE KNOWLEDGE is  needed of  - Reactor power and its fuel composition,  - Reactor e energy spectrum, its variation during the operational run  - Hydrogen atom concentrations - Target volumes With ONE reactor backgrounds can periodically be measured. We conclude that Kr2Det can sensitively search for neutrino oscillations in the atmo-spheric mass region

  22. 2) Kr2Det usesavailable underground halls for the Far (1000m) and Near (115m) 50 ton detectors  The oscillation signal could be some-what increased with the Far detector at ~ 1400-1900 m or with two far detectors at ~ 1300 and 2600 m… This however would require diggingnew caverns and using detectors with larger target masses… * * * The Kr2Det project is still in R&D phase All parameters of the experiment are open for discussions…  Use of a good Gd scintillator would help to suppress accidentals from U/Th in the rock Effective collaborators are welcome to join.

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