Neutrino-electron scattering  Neutrino Experiments at Reactors  Status of deuteron experiments

Neutrino Oscillation Search

The only channel which can be studied in a reactor experiment is the disappearance of $\bar\nu_e$. The signature of the $\bar\nu_e$ interaction is the inverse $\beta$ decay. CC reaction is forbidden for other neutrinos ( $\nu_{\mu},\nu_{\tau}$) at the energies of reactor neutrinos. As a consequence the sensitivity in mixing is limited to few $\%$ but the $\delta m^{2}$ sensitivity can be extended up to 10^-3 eV². During the last 15 years many experiments have been conducted to search for neutrino oscillation at distances ranging from 8m to 70m. No disappearance effect have been observed. From these experiments the limits on oscillation parameters are listed in table .

Table:limits on oscillation parameters 

 

$\delta m^{2}$	$sin^{2}(2\theta)$
$7*10^{-2}< \delta m^{2} < 2 eV^{2}$  $\delta m^{2} > 2 eV^{2}$  $\delta m^{2} < 10^{-2} eV^{2}$	< 10-1  < 4*10-2  full mixing

 

 

To extend the sensitivity in $\delta m^{2}$ to 10^-3 eV² detectors have to be located at a distance as large as 1km. The domain covered will allow to probe the $\nu_{e} - \nu_{\mu}$ transition to solve the atmospheric neutrino anomaly. The challenge is to compensate the neutrino flux reduction (10^-4) by reducing the cosmic ray induced background in the detector. Two experiments have proposed two different concepts to reduce the background:

• The CHOOZ experiment is located underground (300 mwe). The detector is made of a target containing 5.6 m³ of Gadolinium loaded liquid scintillator. The 8 Mev $\gamma$ from Gd de-excitation give a clean signature of the neutron capture, above the maximum energy of the natural radioactivity (2.6 MeV from Thallium). The target is surrounded by 145 m³ of unloaded scintillator. This scintillator blanket provides an additional reduction of neutron background by tagging muons going through the side of the detector and absorbing those neutrons produced in the rocks surrounding the detector. The experiment is taking data since august 1996. Preliminary results will be available in 1997.
• The Palo Verde experiment makes use of a segmented liquid scintillator detector being installed in an underground vault with a relatively small overburden of 46mwe. To reduce the cosmic ray induced neutron background, the discrimination between recoil protons and positron is made by requiring a triple coincidence between target cells as a signature of the positron and the two 511-Kev $\gamma$ rays from the positron annihilation. The sensitive volume is made of 66 cells, 9m long, containing Gd loaded liquid scintillator. This volume is surrounded by a one-meter-thick water buffer. Around the water buffer is installed an active muon system. The experiment will start taking data in the second half of 1997.

The next logical step will be to implement an experiment at a distance larger than 10km in order to search for $\nu$ oscillation down to 10^-4eV². The interest of such a search should be studied on the basis of the results of the ongoing experiments (reactor, atmospheric $\nu$, solar $\nu$). To get a neutrino event rate of 10 events per day the target size is of the order of 1 Kton. The IMB site, 13 km away from the 3600 Mwth Perry reactor, is a good candidate with an overburden of 1570-mwe. Another site is studied in Japan 60 km away from a reactor. Research and development are needed on scintillators (light transmission, stability, radio-purity) before to be able to run a 1Kton detector. Other field of physics will be accessible with such a detector: proton decay, supernova, solar neutrino. The experience from the large underground detector in preparation like Borexino will be useful in that study.