J. Haser

Investigating the spectral anomaly with different reactor antineutrino experiments

Abstract The spectral shape of reactor antineutrinos measured in recent experiments shows anomalies in comparison to neutrino reference spectra. New precision measurements of the reactor neutrino spectra as well as more complete input in nuclear data bases are needed to resolve the observed discrepancies between models and experimental results. This article proposes the combination of experiments at reactors which are highly enriched in U 235 with commercial reactors with typically lower enrichment to gain new insights into the origin of the anomalous neutrino spectrum. The presented method clarifies, if the spectral anomaly is either solely or not at all related to the predicted U 235 spectrum. Considering the current improvements of the energy scale uncertainty of present-day experiments, a significance of three sigma and above can be reached. As an example, we discuss the option of a direct comparison of the measured shape in the currently running Double Chooz near detector and the upcoming Stereo experiment. A quantitative feasibility study emphasizes that a precise understanding of the energy scale systematics is a crucial prerequisite in recent and next generation experiments investigating the spectral anomaly.

Afterpulse measurements of R7081 photomultipliers for the Double Chooz experiment

We present the results of afterpulse measurements performed as qualification test for 473 inner detector photomultipliers of the Double Chooz experiment. The measurements include the determination of a total afterpulse occurrence probability as well as an average time distribution of these pulses. Additionally, more detailed measurements with different light sources and simultaneous charge and timing measurements were performed with a few photomultipliers to allow a more detailed understanding of the effect. The results of all measurements are presented and discussed.

PMT Test Facility at MPIK Heidelberg and Double Chooz Super Vertical Slice

The two inner detectors of Double Chooz (DC) [1] will both be observed by 390 photomultiplier tubes (PMTs) to record light pulses produced by neutrino induced reactions inside the scintillator volumes. The task of calibrating the PMTs was shared among DC Japan and MPIK/ RWTH Aachen. At Heidelberg a test facility was built which allows to calibrate 30 PMTs simultaneously. The VME based data acquisition (DAQ) includes devices for charge spectrum recording, for time measurements and scalers for dark rate counting. The PMT calibration campaign encompassed the determination of the single photo electron (SPE) gain and resolution, transit time spread and linearity for multi-PE events. Two trigger boards developed by RWTH Aachen were integrated to test their performance and to examine afterpulse probabilities and dark rate counting. The derived results were translated into time and charge probability density functions (see fig. 1), which in turn were implemented into the Double Chooz Monte Carlo simulation. The Super Vertical Slice (SVS) is an upgrade of the PMT test facility. It consists of the complete and original Double Chooz electronics plus an acrylic vessel of 30 liters filled with the Gadolinium doped ν-Target scintillator. The electronics consists of PMTs, high voltage (HV) power supply, HV splitter, frontend electronics (FEE), trigger system (TS), waveform digitizers (WFD) and DAQ. It has been successfully used to finalize the interface of FEE and TS, such as signal gains, and to control the quality of the final FEE production. The electronics response in case of very high energy depositions has been studied. The obtained results hint at the feasibility of using muon-induced isotopes, such as 12B or Michel electrons, for detector calibration. The SVS has been first to record pulses from the original ν-

Neutron Detection Uncertainties in the θ13 Analysis of the Double Chooz Experiment

The reactor antineutrino experiment Double Chooz aims to provide a precise measurement of the neutrino mixing angle θ₁₃. In the analysis with one detector, accuracy in the predicted neutrino spectrum from simulation is a necessity with regard to normalization and energy shape. The detection efficiency of neutron events, which are part of the coincidence signal created by neutrinos, introduce the largest uncertainty contribution of the normalization of the experiment related to the signal detection. In order to accomplish a matching of the efficiencies observed in data and simulation, a correction of the Monte Carlo normalization and an associated systematic uncertainty are inputs in the θ₁₃ analysis. Calibration source deployments in the inner two detector volumes allow for a measurement of the neutron detection efficiency using ²⁵²Cf fission neutrons. New methods enable to compute the correction integrated over the whole volume and the corresponding uncertainty of the selection cut related efficiency. With these revised approaches a factor two improvement in the detection efficiency uncertainty was achieved. The correction of the neutron capture fraction – the capture fraction quantifies the proportion of captures on a particular element – is evaluated and tested for its robustness. Furthermore, a crosscheck of this quantity is discussed using neutrons produced by cosmic muon spallation. Finally, the uncertainty on border effects, emerging from neutron migration at the fiducial volume boundaries, is estimated by means of different Monte Carlo configurations with varying parameters and neutron physics modelings.