Last Update: October 2000

The MUNU experiment

C. Broggini, C. Cerna, F. Mattioli, G. Puglierin

INFN and Dipartimento di Fisica dell'Università, Via Marzolo 8,

I-35131 Padova, Italy

M. Avenier, D.H. Koang, J. Lamblin, D. Lebrun, A. Stutz

Institut des Sciences Nucléaires, IN2P3-UJF, 53 Avenue des Martyrs,

F-38026 Grenoble CEDEX, France

J. Busto, V. Chazal, P. Jeanneret, G. Jonkmans, J.-L. Vuilleumier

Institut de Physique, A.-L. Breguet 1,

CH-2000 Neuchâtel, Switzerland

C. Amsler, O. Link

Physik-Institut, Schönberggasse 9,

CH-8001 Zürich, Switzerland

1. Goal of the experiment

MUNU was designed to study the elastic scattering with the antineutrinos from a nuclear reactor. Its aim is to get a precise measurement of the differential cross section down to a kinetic energy of the recoiling electron of 0.3 MeV. In this way the experiment can be sensitive to a neutrino magnetic moment in the 10-11 Bohr magneton region. As a matter of fact, a non vanishing neutrino magnetic moment produces scattering events in addition to the ones due to the weak interaction.

The detector consists of a 1 m3 acrylic vessel TPC immersed into 10 m3 of liquid scintillator working as anti-Compton. The filling gas of the TPC is CF4 at 3 bar; its electrons are the target for the interaction. Since the expected event rate is low (~6/day from weak interactions) it has been necessary to minimize all possible background sources and to construct each part of the detector from selected low radioactivity material.

The detector has been mounted in a laboratory underneath a 2800 MW nuclear reactor in Bugey, at 18 m from the core.

2. Results

Since almost one year we are taking data. The detector itself is working as expected, giving a very good event reconstruction. Also the count rate of the anti-Compton has been since the beginning in agreement with the simulation: 900 Hz above 100 keV due to muons and radioactivity.

On the contrary, the electron rate was much higher than the predicted one because of the 222Rn emitted by the oxysorb. After having replaced the oxysorb by a getter with much lower Uranium contamination we decreased the event rate by 2 orders of magnitude, reaching 0.1 Hz and 1.5·10-3 Hz for electron energies above 300 and 800 keV, respectively. In spite of the reduction, the event rate was still too high and the limit we could get on the neutrino magnetic moment is similar to the existing one (2·10-10 Bohr magnetons). To further reduce the background we had to open the detector and to replace the TPC cathode, where the Radon daughters produced as positive ions were collected. The data-taking just started with reactor on and with the new cathode.

Finally we point out that the MUNU detector is the first one doing neutrino spectroscopy in the MeV region by measuring both the energy and the direction of the recoiling electrons. From this point of view it can be regarded as a low background prototype of a detector for the spectroscopy of the low energy neutrinos from the Sun (pp and 7Be).

3. INFN contribution

We had the responsibility for the design and construction of the acrylic TPC, of the system to regulate the pressure in the TPC and in the anti-Compton and of the system to store and to transfer the liquid scintillator from the anti-Compton. We also developed the slow control system and the software for the event display.

Manpower: 4 researchers, 3 technicians and the support from the Padova INFN workshops.

We had a German physicist working with us for 3 years with a non permanent INFN position, and we now have a French physicist with an INFN post-doc fellowship.

Financial support in the year 2000: 125 MLit.

4. Publications: 1

5. Talks to conferences: 7.