The Double Chooz Experiment presents an indication of reactor electron antineutrino disappearance consistent with neutrino oscillations. An observed-to-predicted ratio of events of 0.944 ± 0.016 (stat) ± 0.040 (syst) was obtained in 101 days of running at the Chooz Nuclear Power Plant in France, with two 4.25 GW th reactors. The results were obtained from a single 10 m 3 fiducial volume detector located 1050 m from the two reactor cores. The reactor antineutrino flux prediction used the Bugey4 flux measurement after correction for differences in core composition. The deficit can be interpreted as an indication of a non-zero value of the still unmeasured neutrino mixing parameter sin 2 2θ13. Analyzing both the rate of the prompt positrons and their energy spectrum we find sin 2 2θ13= 0.086 ± 0.041 (stat) ±0.030 (syst), or, at 90% CL, 0.017 < sin 2 2θ13 < 0.16. We report first results of a search for a non-zero neutrino oscillation [1] mixing angle, θ 13 , based on reactor antineutrino disappearance. This is the last of the three neutrino oscillation mixing angles [2,3] for which only upper limits [4,5] are available. The size of θ 13 sets the required sensitivity of long-baseline oscillation experiments attempting to measure CP violation in the neutrino sector or the mass hierarchy.In reactor experiments [6,7] addressing the disappearance ofν e , θ 13 determines the survival probability of electron antineutrinos at the "atmospheric" squaredmass difference, ∆m 2 atm . This probability is given by:where L is the distance from reactor to detector in meters and E the energy of the antineutrino in MeV. The full formula can be found in Ref.[1]. Eq. 1 provides a direct way to measure θ 13 since the only additional input is the well measured value of |∆m 2 atm | = (2.32Other running reactor experiments [9,10] are using the same technique.Electron antineutrinos of < 9 MeV are produced by reactors and detected through inverse beta decay (IBD): ν e + p → e + + n. Detectors based on hydrocarbon liquid scintillators provide the free proton targets. The IBD signature is a coincidence of a prompt positron signal followed by a delayed neutron capture. We present here our first results with a detector located ∼ 1050 m from the two 4.25 GW th thermal power reactors of the Chooz Nuclear Power Plant and under a 300 MWE rock overburden. The analysis is based on 101 days of data including 16 days with one reactor off and one day with both reactors off.The antineutrino flux of each reactor depends on its thermal power and, for the four main fissioning isotopes, 235 U, 239 Pu, 238 U, 241 Pu, their fraction of the total fuel content, their energy released per fission, and their fission and capture cross-sections. The fission rates and associated errors were evaluated using two predictive and complementary reactor simulation codes: MURE [17,18] and DRAGON [19]. This allowed a study of the sensitivity to the important reactor parameters (e.g.. thermal power, boron concentration, temperatures and densities). The quality of these simulations...

The Double Chooz experiment has observed 8,249 candidate electron antineutrino events in 227.93 live days with 33.71 GW-ton-years (reactor power × detector mass × livetime) exposure using a 10.3 m 3 fiducial volume detector located at 1050 m from the reactor cores of the Chooz nuclear power plant in France. The expectation in case of θ13= 0 is 8,937 events. The deficit is interpreted as evidence of electron antineutrino disappearance. From a rate plus spectral shape analysis we find sin 2 2θ13 = 0.109 ± 0.030(stat) ± 0.025(syst). The data exclude the no-oscillation hypothesis at 99.8% CL (2.9σ).

Over the course of several decades, organic liquid scintillators have formed the basis for successful neutrino detectors. Gadolinium-loaded liquid scintillators provide efficient background suppression for electron antineutrino detection at nuclear reactor plants. In the Double Chooz reactor antineutrino experiment, a newly developed beta-diketonate gadolinium-loaded scintillator is utilized for the first time. Its large scale production and characterization are described. A new, light yield matched metal-free companion scintillator is presented. Both organic liquids comprise the target and "Gamma Catcher" of the Double Chooz detectors. In the Double Chooz (DC) experiment [3] two new types of Gd-LS have been studied and further tested. These are Gd-BDK TM (for Gd beta-diketonate) and Gd-CBX TM (for Gd carboxylate) [4,5]. These two systems meet the basic requirements for the DC scintillator: chemical stability of the Gd molecules and the other LS components, compatibility with detector materials, transparency, intrinsic light yield, radiopurity, Gd solubility and the stability of these properties over several years of data taking. Optimization of the optical properties includes the need to tune the light yield while maintaining a constant density and matching the light emission to the spectral response of the photomultiplier tubes (PMTs).The Gd-LS that is currently loaded in the DC Far Detector is based on beta-diketone chemistry, the first use in a large scale antineutrino detector. The chemistry of this new Gd-BDK scintillator [6] is based in part on knowledge obtained from studies of the similar Indium (In-BDK) system [7] developed at MPIK, Heidelberg and used in the LENS (Low Energy Neutrino Spectroscopy) Prototype at Gran Sasso [8]. The specific Gd organic scintillator candidate, that we initially considered, was based on the use of the simplest five carbon BDK anion -ACAC (acetylacetonate). We found this BDK compound difficult to sublime and thus it lacked a potential productive route to achieving the level of optical and radiochemical purity needed in the DC experiment. Consequently, in the final version, the BDK was selected to be THD (2,2,6,6-tetramethyl-heptane-3,5-dionate) based on extensive research experience in producing solid, liquid and gaseous Gd-THD for testing in the Ho-163 neutrino mass experiments [9]. The more effective shielding of the metal ions by the

Large liquid-scintillator-based detectors have proven to be exceptionally effective for low energy neutrino measurements due to their good energy resolution and scalability to large volumes. The addition of directional information using Cherenkov light and fast timing would enhance the scientific reach of these detectors, especially for searches for neutrino-less double-beta decay. In this paper, we develop a technique for extracting particle direction using the difference in arrival times for Cherenkov and scintillation light, and evaluate several detector advances in timing, photodetector spectral response, and scintillator emission spectra that could be used to make direction reconstruction a reality in a kiloton-scale detector.

The energy dependent light output of liquid scintillators used in the Double Chooz experiment was measured for electrons up to 140 keV energy. A new Compton scattering coincidence apparatus was built for this purpose. A detailed study on possible systematic errors was made. We report the experimental results of our investigations and tested them for concordance with the predictions of various models. All models reasonably fit the experimental data after adjusting the respective free parameters. The results were also used to tune the Geant4-based Monte Carlo simulation software which is used in Double Chooz. The experimental data can be described by the simulation choosing an effective value for the Birks parameter.KEYWORDS: Scintillators, scintillation and light emission processes (solid, gas and liquid scintillators); Liquid detectors; Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc); Large detector systems for particle and astroparticle physics 1 Current address: Hartmann Scientific,