The analysis and results are presented from the complete data set recorded at Palo Verde between September 1998 and July 2000. In the experiment, the ¯e interaction rate has been measured at a distance of 750 and 890 m from the reactors of the Palo Verde Nuclear Generating Station for a total of 350 days, including 108 days with one of the three reactors off for refueling. Backgrounds were determined by ͑a͒ the swap technique based on the difference between signal and background under reversal of the positron and neutron parts of the correlated event, and ͑b͒ making use of the conventional reactor-on and reactor-off cycles. There is no evidence for neutrino oscillation and the mode ¯e →¯x was excluded at 90% C.L. for ⌬m 2 Ͼ1.1ϫ10 Ϫ3 eV 2 at full mixing, and sin 2 2 Ͼ0.17 at large ⌬m 2 .

We report on the initial results from a measurement of the antineutrino flux and spectrum at a distance of about 800 m from the three reactors of the Palo Verde Nuclear Generating Station using a segmented gadolinium-loaded scintillation detector. We find that the antineutrino flux agrees with that predicted in the absence of oscillations excluding at 90% C.L.n e -n x oscillations with Dm 2 . 1.12 3 10 23 eV 2 for maximal mixing and sin 2 2u . 0.21 for large Dm 2 . Our results support the conclusion that the atmospheric neutrino oscillations observed by Super-Kamiokande do not involve n e .PACS numbers: 14.60. Pq, 13.15. + g, 14.60.Lm, 25.30.Pt Nuclear reactors have been used as intense sources of n e in experiments searching for neutrino oscillations [1]. These experiments usually detectn e by the processn e 1 p ! n 1 e 1 , where the cross-section-weighted energy spectrum ofn e , peaking at about 4 MeV, can be deduced from the measured e 1 spectrum. Anyn e flux deficit or distortions of then e energy spectrum would indicate oscillations. The low energy of reactorn e allows these experiments to reach very small mass parameters, albeit with modest mixing-angle sensitivity. Past experiments [2] with detectors at 50-100 m from a reactor have explored the mass-parameter range down to 10 22 eV 2 . The work described here and a similar experiment elsewhere [3] are the first long baseline (ϳ1 km) searches, designed to explore the parameter range down to 10 23 eV 2 as suggested by the early Kamiokande atmospheric neutrino anomaly [4]. Although later results from Super-Kamiokande [5] (appeared while this work was in progress) seem to disfavor the n m -n e channel, a direct experimental exploration amply motivated this work.The Palo Verde neutrino oscillation experiment is located at the Palo Verde Nuclear Generating Station near Phoenix, Arizona. The total thermal power from three identical pressurized water reactors is 11.6 GW. Two of the reactors are 890 m from the detector, while the third is at 750 m. Our detector is placed in a shallow underground site (32-m-water-equivalent overburden), thus eliminating the hadronic component of cosmic radiation and reducing the muon flux by a factor of ϳ5. The fiducial mass, segmented to reject the remaining background, consists of 11.3 tons of 0.1% Gd-loaded liquid scintillator contained in a 6 3 11 array of 9-m-long acrylic cells, as shown in Fig. 1. Each cell is viewed by two 5-in. photomultiplier tubes, one at each end. An e is identified by space-and time-correlated e 1 and n signals. Positrons deposit their energies in the scintillator and annihilate, yielding two 511-keV g's, giving a triple coincidence. Neutrons thermalize and are captured in Gd, giving a g-ray shower of 8 MeV total energy.The Gd loading of the scintillator has two advantages: it reduces the neutron capture time from 170 (on protons) to 30 ms and provides a high energy g shower to tag the neutron capture, resulting in a substantial background reduction. Both the positron and the neutron are triggered by...

The e flux and spectrum have been measured at a distance of about 800 m from the reactors of the Palo Verde Nuclear Generating Station using a segmented Gd-loaded liquid scintillator detector. Correlated positron-neutron events from the reaction e p→e ϩ n were recorded for a period of 200 d including 55 d with one of the three reactors off for refueling. Backgrounds were accounted for by making use of the reactor-on and reactor-off cycles, and also with a novel technique based on the difference between signal and background under reversal of the e ϩ and n portions of the events. A detailed description of the detector calibration, background subtraction, and data analysis is presented here. Results from the experiment show no evidence for neutrino oscillations. e → x oscillations were excluded at 90% C.L. for ⌬m 2 Ͼ1.12ϫ10 Ϫ3 eV 2 for full mixing and sin 2 2Ͼ0.21 for large ⌬m 2 . These results support the conclusion that the observed atmospheric neutrino oscillations do not involve e .

The yield of neutrons produced by cosmic ray muons at a shallow depth of 32 meters of water equivalent has been measured. The Palo Verde neutrino detector, containing 11.3 tons of Gd loaded liquid scintillator and 3.5 tons of acrylic served as a target. The rate of one and two neutron captures was determined. Modeling the neutron capture efficiency allowed us to deduce the total yield of neutrons $ Y_{tot} = (3.60 \pm 0.09 \pm 0.31) \times 10^{-5}$ neutrons per muon and g/cm$^2$. This yield is consistent with previous measurements at similar depths.Comment: 12 pages, 3 figure

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