

Two Detector Reactor Neutrino Oscillation Experiment Kr2Det at
Krasnoyarsk. Status Report.

Yu. Kozlov, L. Mikaelyan, V. Sinev

Russian Research Center Kurchatov Institute, Kurchatov Square 1, Moscow, 123182 Russia


Abstract. We consider status of the Kr2Det project aimed at sensitive searches for neutrino oscillations
in the atmospheric neutrino mass parameter region around m2 ~ 3x10-3 eV2and at obtaining new
information on the electron neutrino mass structure (U ).
e3


uncertainties associated with the reactor
neutrino flux and spectrum and (ii) by a
considerable increase of the number of
I. INTRODUCTION detected neutrinos.

The CHOOZ experiment [1] conclusively
showed that oscillations of electron neutrinos 2. The Kr2Det PROJECT
do not play the dominant role in the
atmospheric neutrino anomaly: 2.1. Detectors

sin22 Two identical liquid scintillation
CHOOZ 0.1 (at m2= 3x10-3 eV2) (1)
spectrometers stationed at distances R
In the three active neutrino mixing scheme 1 = 1100
m (far position) and R2 = 150 m (near position)
sin22CHOOZ = 4 U2e3 (1 - U2e3) where Ue3 is the from the Krasnoyarsk underground (~ 600
contribution of the mass-3 eigenstate to the e mwe) reactor detect (e+, n) pairs, produced in
flavor state. Thus the CHOOZ result indicates the e + p n + e+ reaction. A miniature
that this contribution is not large: version of the KamLAND and BOREXINO [4]
U2 three concentric zone detector composition is
e3 2.4 x 10-2.
chosen for the design of the spectrometers
The main physical goals of the Kr2Det (Fig. 1.) The neutrino targets have 46 t of
project [2] are: (i) to look for much smaller liquid scintillator enclosed in transparent
mixing angels, find the element Ue3 or set balloons. The target is viewed by ~500 8-inch
stronger constraints on its value, (ii) to provide EMI- 9350 PMT's [5] through a ~ 90 cm layer
normalization for future long baseline of the isoparaffin of the zone-2. PMT's of the
experiments at accelerations and (iii) to reach same type were successfully used in the
better understanding of the role e can play in CHOOZ experiment and are used now in the
the atmospheric neutrino anomaly. We SNO and BOREXINO detectors. A ~120
mention also that new information on Ue3 can photoelectron signal is expected for 1 MeV
help to choose between possible solar neutrino energy deposit in the detector center. The
oscillation solutions [3]. PMT's are mounted on a stainless steel screen,
The main practical goal of the project is to which optically separates the external zone-3
decrease, relative to the CHOOZ, the from two central zones. The 75 cm thick zone-
systematic and statistic errors as much as 3 is filled with mineral oil and serves as active
possible. This can be achieved (i) by using two (muons) and passive shielding from the
(far and near) detector scheme of the external radioactivity.
experiment, which eliminates systematic






pulse shape analysis is planned. Under these
assumptions the neutrino detection efficiency
= 77% was found and neutrino detection rate
N= 46 day-1 calculated for the detector at 1100
m position.

The expected rate of the correlated
background in the Kr2Det project (see Ref.
[2]), 0.1 per day per ton of the target mass, is ~
3 times lower than was measured at CHOOZ,
which reasonably agrees with the deeper
position (600 vs 300 mwe) of the Kr2Det
detectors.

While correlated background originates
from cosmic muons, the accidentals come from
Fig. 1. The detector the natural radioactivity of materials. In Tab. 1
are summarized radioactivity levels of the
scintillator and of other detector materials and
2.2. Neutrino rates and backgrounds calculated single rates, which define the rate of
The neutrino event satisfies the following accidental coincidences. Backgrounds coming
requirements: (i) a time window on the delay from U and Th contained in the surrounding
between e+ and neutron 2 - 600 s, (ii) energy rock is still under consideration.
window for the neutron candidate 1.7 - 3.0
MeV and for e+ 1.2 - 8.0 MeV, (iii) distance
between e+ and neutron less than 100 cm. No

Table 1. Single background rates due to natural radioactivity of detector materials.

Material Isotope Purity Background, Hz
mass, ton G/g .>1.0 MeV >1.2 MeV >1.7 MeV
238U 10-13 0.078 0.051 0.045
Scintillator, 232Th 10-13 0.033 0.022 0 017
40K 10-13 0.020 0.005 0.001
45 ton 222Rn 1mBq/m3 0.050 0.040 0.040
Scintillator 0.181 0.118 0.103
238U 10-13 0.0035 0.0019 0.0012
Zone-2 oil, 232Th 10-13 0.0014 0.0008 0.0007
40K 10-13 0.0097 0.0033 0.0007
76 ton 222Rn 10mBq/m3 0.033 0.018 0.0116
zone-2 oil 0.0476 0.0240 0.0142
238U 1x10-8 0.028 0.013 0.008
PMT, 500 kg 232Th 2x10-8 0.037 0.026 0.022
40K 7x10-9 0.024 0.002 < 0.001
PMT 0.089 0.041 0.030
Screen 238U 5x10-9 0.056 0.27 0.016
stainless steel 232Th 1x10-8 0.074 0.52 0.045
2 ton 40K 2x10-9 0.027 0.002 < 0.001
screen 0.157 0.081 0.061
238U 10-12
Zone-3 oil, 232Th 10-12 < 0.001
40K 10-12
218 ton 222Rn 10mBq/m3
zone-2 oil 0.0027 0.0016 0.0014
Total BKG rates 0.477 0.257 0.210




level of radioactive impurities than level
The calculated accidental background rate sufficient for present project.
is N
Acc 0.1 day-1,which is hundred times
smaller than neutrino signal N. We note that Calculated neutrino detection rates and
BOREXINO and KamLAND detectors aimed backgrounds are summarized in Tab. 2.
to detect solar neutrinos in subMeV range will
use scintillators with 3  4 of magnitude lower



Table 2. Neutrino detection rates and backgrounds

Parameter Distance Depth Target mass Neutrino detection Backgrounds, d-1
m mwe ton rates N, d-1 correlated accidental
Far detector 1100 600 46 46 5 0.1
Near detector 150 600 46 2500 5 0.1


2.3. Data analysis and expected results

In no oscillation case the ratio of two
simultaneously measured positron energy
spectra SFAR/SNEAR is energy independent.
Small deviations from constant value of this
ratio

SFAR/SNEAR = C(1 - sin22sin2F)x

(1 - sin22sin2N)-1 (2)

are searched for oscillations (F,N stands for
1.27m2 L F,N /E and L F,N are the distances
between the reactor and detectors) The results
of the analysis do not depend on the exact
knowledge of the neutrino flux and their
energy spectrum, burn up effects, the numbers
of target protons.... However possible relative
difference of the detector energy scales should
be strictly controlled. This can be done by a
combination of different methods briefly Fig.2. Reactor neutrino 90 % CL disappearance
mentioned in Ref. [2]; limits. Shaded is the atmospheric  allowed
region.
Expected 90% CL oscillation limits are
presented in Fig.2. It was assumed that 40000 www sites. An effective international
neutrinos are detected in the far detector and cooperation would be highly desirable to
that detector response functions differences are complete the project and start the experiment.
controlled down to 0.5%. We appreciate valuable discussions with E.
Akhmedov, S. Bilenky and A. Smirnov on the
physics of neutrino oscillations. Discussions
3. CONCLUSIONS with our colleagues of Kurchatov Institute'
Mass structure of the electron neutrino can neutrino groups are of great help in developing
sensitively be explored using two detector this project. We are thankful to Yu.
techniques in underground laboratory. Kamyshkov and A. Piepke for valuable
information on the KamLAND
The project is relatively inexpensive when experiment.This study is supported by RFBR
compared with modern neutrino experiments grants N 00-02-16035, 00-15-98708.
listed in the Neutrino Oscillation Industry



REFERENCES 3. E. Akhmedov, G. Bianco, M. Rebelo, Phys.
Rev. Lett. 84, 3535 (2000)
1. M. Apollonio et al. CHOOZ Collaboration,
Phys. Lett. B 446, 415 (1999). 4. A.. Piepke, for the KamLAND
Collaboration, Nucl. Phys. B (Proc. Suppl.) 91,
2. L. Mikaelyan, Nucl. Phys. B (Proc. Suppl.) 99 (2001)
91, 120 (2001);
5. A. Baldini, C. Bemporad et al., NIM A 372,
L. Mikaelyan, V. Sinev, Yad. Fiz. 63, 1077 207 (1996).
(2000).




