On Search for New Physics
in Nonequilibrium Reactor Antineutrino Energy Spectrum

V. I. Kopeikin
Russian Research Centre Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182 Russia

Abstract-The calculations of the time-dependent reactor antineutrino energy spectrum are presented. Some problems associated
with sensitive searches for neutrino magnetic moment and neutrino oscillations in reactor antineutrino flux are considered.


1. INTRODUCTION 2. REACTOR ANTINEUTRINO SPECTRUM


The previous results from e e scattering reactor Here we consider the antineutrino energy
experiments were interpreted as an upper limit of spectrum of a Light Water Reactor (LWR).
Reactors of this type are used in the most neutrino
210 10
-  
B ( B is electron Bohr magneton) on the experiments. They usually operate for 11 months,
neutrino magnetic moment  [1]. Direct followed by a shutdown of one month for one-third
measurements of the neutrino magnetic moment at of fuel elements replacement. At the beginning of
level 10 11
-  each annual reactor cycle =69% of the fissions
B would have a serious impact on 5
particle physics and astrophysics [2]. To achieve are from 235 U,
9 =21% from 239 Pu, 8 =7%
this sensitivity the low energy recoil electrons in
from 238 U, and 1 =3% from 241Pu. During
e e scattering should be detected. Preparations for
such experiments are under way [3]. As was operation 235 U burn up and 239 Pu and 241Pu are
discussed in [4], the soft part of the recoil electron accumulated from 238 U. The average ("standard")
energy spectrum may be strongly time-dependent fuel composition is
during reactor cycle. In the present study the time
5 =58%, 9 =30%, 8 =7%, 1 =5%. (3)
evolution of the e e scattering cross sections The present calculation of the time-dependent
expected in reactor experiments for the weak and reactor e spectrum during operation and
magnetic interactions are calculated. shutdown periods include:
In any neutrino reaction, the cross section a. Time-dependent e activity of the fission
observed fis is the reaction cross section for products, including possible isomeric states, effect
monoenergetic antineutrinos (E )
(cm 2 / e ) of delayed neutrons and transmutations of fission
folded with reactor products by reactor neutrons. It has been taken into
e spectrum (E , t)
( e /MeV
account the time-dependent fission contributions
fission) (t) (i=5,9,8,1) both current and two previous
i
fis (t)= (E , t) (E )
dE, [cm 2 fission 1
- ]. (1)
annual reactor cycles.
To interpret data, we, therefore, must have an b. Time-dependent activity coming from the
accurate knowledge of the antineutrino spectrum. e
neutron captures by heavy elements. A dominant
In this work the typical thermal power reactor e
contribution comes from the captures by 238 U:
spectrum and its time evolution are calculated. 238
Oscillation experiments are based on the U(n,) 239 U 239 Np 239 Pu.
reaction Relative contribution of the other antineutrino
sources in the reactor core is less than 0.5% [7].
e +p e + +n. (2) The calculation of the spectrum below 2
Any distortions of the positron energy spectrum e
MeV (to which three-fourths of all emitted
or decrease of the cross section (1), measured at antineutrinos belong) and its components is
reactor would indicate oscillations. In a recent
paper [5] we calculated corrections to the non- presented in Fig.1. The e spectrum in the energy
oscillation cross section for reaction (2) that had range of E =2 - 9 MeV has been measured at
been precisely measured near power reactor [6]. In Rovno reactor [7]:
a present study we discuss the role of the residual (
E )
=5.09exp[-( E /1.54)-( E /6.05) 2 -
e emission after reactor shutdown in measuring ( E /7.73)10 ], [MeV fission] 1
- . (4)
of the positron spectrum.






-1
(E , t= 1 6 5 d ), (M e V fis sio n ) (
E
, t )/(
E ,
3
3 )
0
1 .2
6 1
(a )
3 0
1 6 5
3 3 0
1
4 1 6 5
1

0 .8 3 0
2

2 5

3 0 .6


1
0 1 2 0 .4
A n tin e u trin o e n e rg y , M e V
0 .1 1 1 0
E , M e V
Fig. 1. Energy spectrum of antineutrinos from LWR reactor at
the middle of the 330-d operating period: 1  all antineutrinos, 1 .2 (b )
2  fission antineutrinos, 3  antineutrinos, associated with 1
neutron captures in heavy elements. 3 0

1 6 5
The spectra, both calculated (Fig.1) and
measured (4) corresponds to standard fuel 3 3 0
1
composition (3). The typical time evolution of the
antineutrino spectrum during annual reactor cycle 2 ,5 5 7 ,5 1 0
is shown in Fig. 2 and Fig. 3. A n tin e u trin o e n e rg y E , M e V

(
E , t
o f )
f /(
E ,
3 3 )
0
1
Fig. 3. Ratios of the current reactor e spectra during reactor
operation to that at the end of the 330-d reactor operating
period: lines  present calculation, circles  experiment at
1 Rovno reactor [7]. The numbers at the curves indicate days
from the beginning of the operating period.

5
0 .1 3. CROSS SECTIONS FOR SCATTERING OF
REACTOR ANTINEUTRINOS ON ELECTRONS
3 0
The calculated weak and magnetic elastic cross
sections fis (1) for the e e scattering reaction as a
function of recoil electron energy are shown in Fig.
0 .0 1 0 1 2 3 4. Corrections associated with the electron binding
A n tin e u trin o e n e rg y , M e V in atoms were considered in [8]. It should be
emphasized that, weak e e scattering plays the role
Fig.2. Ratios of the current e spectra after reactor is shut of a background, which must be exactly calculated
down to the spectrum at the end of the 330-d reactor operation and subtracted. In sensitive searches for  , the
period. The numbers on the curves indicate days after reactor measured weak recoil electron energy spectrum
shutdown.
could be used as a tool for detector check and
This evolution during reactor operation is calibration. The calculated time variations of the
mainly caused by accumulation of the nuclei beta folded cross sections (1) for weak and magnetic
activity (in the region E< 2 MeV) and changes in e e scattering during reactor cycle are presented in
the reactor fuel composition (in the region E> 2 Fig. 5.
MeV).






-4 5 4. ABOUT PRECISION MEASUREMENT OF
, 1 0 c m 2 -1 -1
M eV fiss io n
fis POSITRON SPECTRUM FOR THE REACTON
1 0 0 0
e +pe + +n
Radical improvements of the detector
7
5 characteristics including essential decrease of the
3
2 accidental and correlated backgrounds have been
1
1 0 0 achieved in the last long baseline reactor oscillation
experiments. In the present study we discuss the
third type of background which did not take earlier
w e a k into account in measuring of positron spectrum. It
is a reactor correlated positron background,
associated with residual emission after reactor
1 0 e
shutdown. This positron background is about 3% in
1 1 0 1 0 0 1 0 0 0 the energy range T + < 1.2 MeV, as regards to
e
E le c tro n re c o il T , k e V positron rate when reactor in operation, see Fig. 6.

Fig. 4. Cross sections for weak and magnetic scattering of
reactor antineutrinos on free electrons at the middle of the 330-
d operating period. The numbers on the curves indicate the S (T , t ) / S (T , 3 3 0 )
off
11
- 0 .0 4
values of the moment  in 10 B . 1
5
(T, t ) / (T , 330)
o n 3 0
fis fis

1 0 0 0 k e V
1 .0 0 .0 1

3 0 0 k e V re a c to r O F F
(a ) 0 .0 0 5
1 0 0 k e V


1 0 k e V
0 1 2
0 .8 P o sitro n k in e tic e n e rg y T , M e V
1 1 0 1 0 0
Fig. 6. Ratios of the current positron spectra for the reaction
(T, t ) /
(T, 330) +
fis off fis e +pe +n, associated with residual antineutrino emission
0 .2 after reactor is shut down, to the spectrum at the end of 330-d
1 0 k e V (b ) reactor operation period. The numbers at the curves indicate
days after reactor shutdown.


1 0 0 k e V This effect may be more significant if
oscillation experiment is implemented by one
3 0 0 k e V detector positioned from near (distance r) and fare
1 0 0 0 k e V (distance R) reactors. Such an experimental setting
0 .0 was implemented, for example, in Bugey, r=15 m
1 1 0 1 0 0 and R=95 m [9]. In this situation the positron
O F F
< > background signal from the near stopped reactor
t , d a y s
S OFF
near for T + < 1.2 MeV is approximately equal
e
Fig. 5. Ratios of the current e e scattering cross sections positron signal from the operating fare reactor
expected in experiment to those at the end of the 330-d reactor S ON , that is:
operation period for the reactor (a) operating and (b) shutdown far
(OFF) periods. Data are presented for the four groups of recoil S OFF 1 for T + =0 - 1.2 MeV and
electron energies. The solid (dashed) lines represent magnetic near / S ON
far e

(weak) scattering. 0.15 for T + =1.2 - 1.7 MeV.
e






5. CONCLUSION JETP 64, 446 (1986)]; P. Vogel, J. Engel, Phys.
Searches for new physics in neutrino Rev. D 39, 3378 (1989).
experiments at nuclear reactors require refining our 3. L. A. Mikaelyan, Yad. Fiz. 65, (2002),
knowledge of the reactor e spectrum. At the submitted.
present level of experimental accuracy and 4. V. I. Kopeikin, L. A. Mikaelyan, V. V. Sinev ,
sensitivity a detailed and profound analysis of Yad. Fiz. 63, 1087 (2000) [Phys. At. Nucl. 63,
reactor 1012 (2000)].
e spectrum for each particular experiment 5. V. I. Kopeikin, L. A. Mikaelyan, V. V. Sinev ,
should be carried out. Yad. Fiz. 64, 914 (2001) [Phys. At. Nucl. 64,
849 (2001)].
ACKNOWLEDGMENTS 6. V. N. Vyrodov, Y. Declais, H. de Kerret et. al.,
The author would like to thank L. A. Mikaelyan Pis'ma Zh. Eksp. Teor. Fiz. 61, 161 (1995)
and V. V. Sinev for helpful discussions. This work [JETP Lett. 61, 163 (1995)].
was supported by the Russian Foundation for Basic 7. V. I. Kopeikin, L. A. Mikaelyan, V. V. Sinev ,
Research (project nos. 00-15-06708 and 00-02- Yad. Fiz. 60, 230 (1997) [Phys. At. Nucl. 60,
16035.) 172 (1997)].
8. V. I. Kopeikin, L. A. Mikaelyan, V. V. Sinev
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