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 : 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 sin 2 2  13  2.0  for  m 2 = 2.5  eV 2 CAN BE ACHIEVED. For larger mixing parameters the experiment is sensible to  m 2  4  eV 2 (PART of the HLMA REGION!)

NEUTRINO OSCILLATION LIMITS

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

Philosophy: Reactors vs Accelerators Strategy: Step by step progress If sin 2 2  13 is 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 kt (!) detectors located at a few thousand km from accelerator source. See for example: S.Geer, hep-ph/ However, the first step can be done sooner (an cheaper) at reactors. It is quit possible that sin 2 2  13 is just a few times smaller than present limit. No physical principal is known why it should be so terribly small.

REACTOR ANTINEUTRINO DETECTION REACTION: e + p  e + + n Positron visible energy E e + vis : E e + vis  E (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.

Positron spectrum

REACTOR ANTINEUTRINO OSCILLA- TIONS AT ~ 1 km FROM THE REACTOR SOURCE e = U e1  1 + U e2  2 + U e3  3 ( U e3  sin  13 ) We conservatory assume here that m 1 < m 2 < m 3 and  m 2 sol = m 2 2 – m 1 2 < eV 2  m 2 atm = m 3 2 – m 1 2  m 3 2 – m 2 2  2.5  eV 2 >>  m 2 sol The e survival probability P( e  e ) is then: P( e  e ) = 1 – sin 2 2  sin 2 , sin 2 2   sin 2 2  13  4U e3 2 (1-U e3 2 ),  = (1.27  m 2 L  E -1 ), L(m) is the distance from the reactor Numerically:  = 1.05 = 60  for  m 2 = 2.5  eV 2, L = 1000 m, E = 3 MeV

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) 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 Very important to suppress  -s

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 IrvineDrexel 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

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

Kr2Det - probing U e3 with reactor e Method: Two identical detectors (BOREXINO, KamLAND design)600 mwe 100 m 1000 m Target: 50 t 50 t Rate: 1.5 ·10 6 /year 15 ·10 3 /year S:B >>1 ~ 10:1

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

NEUTRINO DETECTION RATES Assuming: Target radius 2.35 m Target volume 54 m 3 Scintillator: Density:0.85 g/cm 3 Mass:46 ton H/C ratio:1.8 # of H atoms0.785  / ton Detection efficiency days/year L far = 1000 m, L near = 100 m One finds: N far = 17  10 3 /yearN near = 1.7  10 6 /year ( 1.2 / ton  day ) ( 120 / ton  day)

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 the detector materials are much less severe than in BOREXINO and KamLAND experiments. The target scintillator can have U, Th, 40 K and 222 Rn concentrations , , g/g and 1mBq/m 3 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.

ANALYSIS The RATIO of the two MEASURED POSITRON SPECTRA in no-oscillation case is ENERGY INDEPENDENT. Small specific deviations from the constant value of this ratio: S far /S near = const  [1 – Sin 2 2  Sin 2 (1.27  m 2 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

Visible positron energy, MeV Positron spectrum

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%.

Ratio of positron spectra far/near L far = 1000 m, L near = 100 m, N far = 16·10 3 /year Values of sin 2 2  are shown at the curves  m 2 = 2.5  eV 2

Ratio of positron spectra far/near L far = 1000 m, L near = 100 m, N far = 15·10 3 /year Values of sin 2 2  are shown at the curves  m 2 = 3  eV 2

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

2) Kr2Det uses available 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 ~ m or with two far detectors at ~ 1300 and 2600 m… This however would require digging new 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.