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

sin2213 2.010-2 for m2 = 2.510-3 eV2

CAN BE ACHIEVED.

For larger mixing parameters the experiment is  sensible to

m2 410-4 eV2

(PART of the HLMA REGION!)

NEUTRINO OSCILLATION LIMITS

0.01 0.10 1.00

10-2

10-3

sin 2 2

m

, eV

2

2

C H O O Z

K r2 D e t (ex p ec ted )

m a tm2

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 sin2213 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 sin2213 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 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.

Positron spectrum

0 1 2 3 4 5 6 7 8Positron visible energy, MeV

0.00

0.05

0.10

0.15

0.20

REACTOR ANTINEUTRINO OSCILLA-TIONS AT ~ 1 km  FROM THE REACTOR SOURCE

 

e = Ue1 1 + Ue2 2 + Ue3 3 ( Ue3 sin13 ) 

We conservatory assume here that

 m1 < m2 < m3 and m2sol = m2

2 – m12 < 10-4 eV2

 m2atm

= m32 – m1

2 m32 – m2

2 2.510-3 eV2 >> m2sol

 The e survival probability P(e e ) is then:

  P(e e ) = 1 – sin22sin2 ,  

sin22 sin2213 4Ue32(1-Ue3

2),  

= (1.27m2 L E-1),

L(m) is the distance from the reactor

 Numerically: = 1.05 = 60   for m2 = 2.510-3 eV2, 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, AnnecyINFN&Univ. of Pisa INFN&Univ. of TriesteKurchatov Institute, MoscowUniv. 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 selectionSummary of data acquisition (April 1997 – July 1998)

Neutrino detection efficiency: 70%Total No of detected neutrinos: ~2700Neutrino detected rate at full power: 26 d-1

Background: ~1.2 d-1

Neutrino/BKG, typically ~10:1

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 (0) calculated (2.5%)

CHOOZ’99 uses (0) measured (1.4%)

CdF+LAPP+Kurchatov, 1994 y.

Kr2Det - probing Ue3 with reactor e

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 /yearS:B >>1 ~ 10:1

Kr2Det, detector scheme500 PMT EMI 9350 Diameter – 8 inches,

Coverage – 20%

V e to z o n e

tra n sp a re n t f ilm

B u ffe rlig h t g u id(m in e ra l o il)

T u b e s fo r f ill in g a n d in tro d u c in g ra d io a c tiv eso u rc e s

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 0.7851029 / ton  Detection efficiency 0.75  300 days/year  Lfar = 1000 m, Lnear = 100 m

  One finds: N far = 17103/year N near = 1.7106/year

( 1.2 / tonday ) ( 120 / tonday)

BACKGROUNDS Time correlated BKG is found by extrapolating the value measured at CHOOZ :  CHOOZ: 300 m.w.e., 0,24 / daytarget ton  Kr2Det: 600 m.w.e., 0.08/ daytarget 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, 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.

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:  

Sfar/S near = const [1 – Sin22Sin2(1.27m2 LE-

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

0 1 2 3 4 5 6 7 8 90

10000

20000

30000

C s137

annih illa tiongam m as

H (n, )D

60Co

24N a

G d(n, )

Prom pt gam m as from C f

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%.

0 1 2 3 4 5 6 7 8Positron vis ib le energy, M eV

0.01

0.03

0.07

0.14 C H O O Z lim it

m =2.5 10 eV2 -3 2

0.85

0.90

0.95

1.00

Ratio of positron spectra far/near

Lfar = 1000 m, Lnear = 100 m, Nfar = 16·103/year

Values of sin22 are shown at the curves

m2 = 2.510-3 eV2

Ratio of positron spectra far/near

Lfar = 1000 m, Lnear = 100 m, Nfar = 15·103/year

Values of sin22 are shown at the curves

0 1 2 3 4 5 6 7 8Positron vis ib le energy, M eV

0.11

C H O O Z lim it

m =3.0 10 eV2 -3 2

0.07

0.03

0.01

0.85

0.90

0.95

1.00

m2 = 310-3 eV2

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 ~ 1400-1900 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.