Characterization of mini-CryoCube detectors from the RICOCHET experiment commissioning at the Institut Laue-Langevin

The experiment aims to measure the coherent elastic neutrino-nucleus scattering process from antineutrinos emitted by a research nuclear reactor operated by the Institut Laue-Langevin (Grenoble, France). This article presents a description of the experimental installation and the detector performance achieved during its commissioning with a mini-CryoCube module consisting of three 42-g germanium cryogenic calorimeters. The baseline resolutions and background levels are reported both during reactor-on and reactor-off periods and as noise mitigation techniques were improved. A baseline resolution of 40 eV electron equivalent was achieved for the ionization channel after setup improvements, and the phonon channel resolutions ranged from 50 to 80 eV of total phonon energy. In the energy region from 2 to 7 keV, a nuclear recoil rate of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mn>14</a:mn> <a:mo stretchy="false">(</a:mo> <a:mn>3</a:mn> <a:mo stretchy="false">)</a:mo> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi>events</a:mi> <a:mo>/</a:mo> <a:mo stretchy="false">(</a:mo> <a:mi>kg</a:mi> <a:mtext> </a:mtext> <a:mi>day</a:mi> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi>keV</a:mi> <a:mo stretchy="false">)</a:mo> </a:mrow> </a:math> is measured during the reactor-off period selecting events in coincidence with muon veto signals. This rate is in agreement with the cosmogenic neutron rate calculated from GEANT4 simulations. After the rejection of events in coincidence with signals in the muon veto detectors, a combined 90% confidence level limit on the nuclear recoil background of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mrow> <g:mo form="prefix">&lt;</g:mo> <g:mn>9</g:mn> <g:mtext> </g:mtext> <g:mtext> </g:mtext> <g:mi>events</g:mi> <g:mo>/</g:mo> <g:mo stretchy="false">(</g:mo> <g:mi>kg</g:mi> <g:mtext> </g:mtext> <g:mi>day</g:mi> <g:mtext> </g:mtext> <g:mi>keV</g:mi> <g:mo stretchy="false">)</g:mo> </g:mrow> </g:math> is obtained in that energy region during the reactor-on period, which is compatible with our GEANT4 model calculation corresponding to a total rate of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mrow> <l:mn>5</l:mn> <l:mtext> </l:mtext> <l:mtext> </l:mtext> <l:mi>events</l:mi> <l:mo>/</l:mo> <l:mo stretchy="false">(</l:mo> <l:mi>kg</l:mi> <l:mtext> </l:mtext> <l:mi>day</l:mi> <l:mtext> </l:mtext> <l:mi>keV</l:mi> <l:mo stretchy="false">)</l:mo> </l:mrow> </l:math> . The sensitivity of this analysis was however found to be limited by a surface event contamination which is currently being addressed by the Collaboration with upgraded detectors.