A central issue in particle physics, astrophysics and cosmology is the question of whether or not the rest mass of the neutrino is exactly zero. In the minimal standard electro-weak model, all neutrinos are massless and the lepton number is exactly conserved. Despite a long-standing success, however, the standard model is incomplete and is generally agreed to need extensions.

Experimental searches for the neutrino mass are therefore important. Many such experiments are under way, near particle accelerators (at CERN but also at the Fermi Laboratory in USA) and near nuclear reactors, which are an intense, well-understood source of low energy Ve's.

Chooz is an incisive new experiment combining and extending proven methods of particle detection and background rejection. The experimental goal is to probe Dm2 values down to 10-3 eV 2 for large values of sin22q and mixing angles to 0.08 for favourable regions of Dm2. The neutrino source will be a pair of reactors at the Chooz B nuclear power station in the Ardennes region of northeastern France; each reactor will have a thermal power of 4.2 GW and is scheduled for start-up next year. An essential feature of the experimental site is the availability of a tunnel with an overburden of 115 m of rock, at a distance of >> 1 km from the source. The neutrino detector will be installed in this tunnel; the experiment will therefore be the first long baseline search for neutrino oscillations at a nuclear reactor. The rock overburden strongly reduces (by a factor of 300) the cosmic ray background and it will therefore compensate for the neutrino flux reduction (by a factor of 100) due to the large source/detector distance. The neutrino target is made of 5 tons of gadolinium-loaded liquid scintillator.

Neutrinos will be detected by the reaction average nue+p going to n+e{+}. Following thermalisation of the recoil neutron, an additional energy of 8 MeV will be released as a result of the capture of the neutron by a gadolinium nucleus (time constant 28ms). The electromagnetic energy associated with an event will be fully contained and detected in the target and in a second shell of pure hydrocarbon scintillator (17 tons) surrounding the target body. A third concentric scintillator shell (90 tons) will shield against the residual cosmic ray by a total of 200 eight-inch photo-multiplier tubes.