We report the best limit on coherent elastic scattering of electron antineutrinos emitted from a nuclear reactor off germanium nuclei. The measurement was performed with the CONUS detectors ...positioned at 17.1 m from the 3.9 GW_{th} reactor core of the nuclear power plant in Brokdorf, Germany. The antineutrino energies of less than 10 MeV assure interactions in the fully coherent regime. The analyzed dataset includes 248.7 kg d with the reactor turned on and background data of 58.8 kg d with the reactor off. With a quenching parameter of k=0.18 for germanium, we determined an upper limit on the number of neutrino events of 85 in the region of interest at 90% confidence level. This new CONUS dataset disfavors quenching parameters above k=0.27, under the assumption of standard-model-like coherent scattering of the reactor antineutrinos.
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Abstract
In this article we explore the availability of radiopure tungsten and its potential as high density shield material for low background applications. For compact shield designs, ...conventionally, lead is used. Metallic tungsten and tungsten pseudo-alloys reach higher densities up to 19.3 g cm
−3
and do not exhibit a significant
210
Pb activity, which is a typical intrinsic contamination in lead. Within several
γ
-ray screening campaigns we were able to identify tungsten samples with activities similar or better than 1 mBq kg
−1
in
232
Th,
40
K,
60
Co and the second part of the
238
U decay chain. In cooperation with a manufacturer we further reduced a persisting contamination in the first part of the
238
U decay chain by a factor of ∼2.5 down to (305 ± 30) mBq kg
−1
. With Monte Carlo simulations, the construction of prototype tungsten-based setups and dedicated measurements, the shield capability of tungsten in comparison to lead was extensively studied. Specifically, the impact of cosmogenic radiation on the background at shallow depth was investigated. We showed that a 6%–40% reduction (depending on the exact shield configuration) in the muon-induced neutron fluence is achievable by replacing lead with an equivalent amount of tungsten regarding the suppression of external
γ
-radiation. Overall, many benefits using tungsten especially for low energy applications below a few 100 keV are found. The pseudo-tungsten alloy presented in this work paves the way for several applications especially regarding background suppression in particle and astroparticle physics search programs.
CONUS is a novel experiment aiming at detecting elastic neutrino–nucleus scattering in the almost fully coherent regime using high-purity germanium (Ge) detectors and a reactor as antineutrino ...source. The detector setup is installed at the commercial nuclear power plant in Brokdorf, Germany, at a short distance to the reactor core to guarantee a high antineutrino flux. A good understanding of neutron-induced backgrounds is required, as the neutron recoil signals can mimic the predicted neutrino interactions. Especially events correlated with the reactor thermal power are troublesome. On-site measurements revealed such a correlated, highly thermalized neutron field with a maximum fluence rate of
(
745
±
30
)
cm
-
2
day
-
1
. These neutrons, produced inside the reactor core, are reduced by a factor of
∼
10
20
on their way to the CONUS shield. With a high-purity Ge detector without shield the
γ
-ray background was examined including thermal power correlated
16
N
decay products and neutron capture
γ
-lines. Using the measured neutron spectrum as input, Monte Carlo simulations demonstrated that the thermal power correlated field is successfully mitigated by the CONUS shield. The reactor-induced background contribution in the region of interest is exceeded by the expected signal by at least one order of magnitude assuming a realistic ionization quenching factor.
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A
bstract
The measurements of coherent elastic neutrino-nucleus scattering (CE
ν
NS) experiments have opened up the possibility to constrain neutrino physics beyond the standard model of elementary ...particle physics. Furthermore, by considering neutrino-electron scattering in the keV-energy region, it is possible to set additional limits on new physics processes. Here, we present constraints that are derived from Conus germanium data on beyond the standard model (BSM) processes like tensor and vector non-standard interactions (NSIs) in the neutrino-quark sector, as well as light vector and scalar mediators. Thanks to the realized low background levels in the C
onus
experiment at ionization energies below 1 keV, we are able to set the world’s best limits on tensor NSIs from CE
ν
NS and constrain the scale of corresponding new physics to lie above 360 GeV. For vector NSIs, the derived limits strongly depend on the assumed ionization quenching factor within the detector material, since small quenching factors largely suppress potential signals for both, the expected standard model CE
ν
NS process and the vector NSIs. Furthermore, competitive limits on scalar and vector mediators are obtained from the CE
ν
NS channel at reactor-site which allow to probe coupling constants as low as 5 ∙ 10
−
5
of low mediator masses, assuming the currently favored quenching factor regime. The consideration of neutrino-electron scatterings allows to set even stronger constraints for mediator masses below ∼ 1 MeV and ∼ 10 MeV for scalar and vector mediators, respectively.
We report on the development and construction of the high-purity germanium spectrometer setup GIOVE (Germanium Inner Outer VEto), recently built and now operated at the shallow underground laboratory ...of the Max-Planck-Institut für Kernphysik, Heidelberg. Particular attention was paid to the design of a novel passive and active shield, aiming at efficient rejection of environmental and muon induced radiation backgrounds. The achieved sensitivity level of
≤
100
μ
Bq kg
-
1
for primordial radionuclides from U and Th in typical
γ
ray sample screening measurements is unique among instruments located at comparably shallow depths and can compete with instruments at far deeper underground sites.
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This article reports the measurement of the ionization quenching factor in germanium for nuclear recoil energies in the keV range. Precise knowledge of this factor in this energy range is highly ...relevant for coherent elastic neutrino-nucleus scattering and low mass dark matter searches with germanium-based detectors. Nuclear recoils were produced in a thin high-purity germanium target with a very low energy threshold via irradiation using monoenergetic neutron beams. The energy dependence of the ionization quenching factor was directly measured via kinematically constrained coincidences with surrounding liquid scintillator based neutron detectors. The systematic uncertainties of the measurements are discussed in detail. With measured quenching factors between 0.16 and 0.23 in the 0.4 keV
nr
to 6.3 keV
nr
energy range, the data are compatible with the Lindhard theory with a parameter
k
of 0.162
±
0.004
(stat + sys).
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We report first constraints on electromagnetic properties of neutrinos from neutrino-electron scattering using data obtained from the
CONUS
germanium detectors, i.e. an upper limit on the effective ...neutrino magnetic moment and an upper limit on the effective neutrino millicharge. The electron antineutrinos are emitted from the 3.9
GW
th
reactor core of the Brokdorf Nuclear Power Plant in Germany. The
CONUS
low-background detectors are positioned at a distance of 17.1 m from the reactor core center. The analyzed data set includes 689.1 kg d collected during reactor ON periods and 131.0 kg d collected during reactor OFF periods in the energy range of . With the current statistics, we are able to determine an upper limit on the effective neutrino magnetic moment of
μ
ν
<
7.5
·
10
-
11
μ
B
at 90% confidence level. No neutrino signal in this channel or in the CE
ν
NS channel has been observed at a nuclear power plant so far. From this first magnetic moment limit we can derive an upper bound on the neutrino millicharge of
|
q
ν
|
<
3.3
·
10
-
12
e
0
.
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Intense fluxes of reactor antineutrinos offer a unique possibility to probe the fully coherent character of elastic neutrino scattering off atomic nuclei. In this regard, detectors face the challenge ...to register tiny recoil energies of a few keV at the maximum. The
Conus
experiment was installed in 17.1 m distance from the reactor core of the nuclear power plant in Brokdorf, Germany, and was designed to detect this neutrino interaction channel by using four 1 kg-sized point contact germanium detectors with sub-keV energy thresholds. This report describes the unique specifications addressed to the design, the research and development, and the final production of these detectors. It demonstrates their excellent electronic performance obtained during commissioning under laboratory conditions as well as during the first 2 years of operation at the reactor site which started on April 1, 2018. It highlights the long-term stability of different detector parameters and the achieved background levels of the germanium detectors inside the
Conus
shield setup.
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Point-contact p-type high-purity germanium detectors (PPC HPGe) are particularly suited for detection of sub-keV nuclear recoils from coherent elastic scattering of neutrinos or light dark matter ...particles. While these particles are expected to interact homogeneously in the entire detector volume, specific classes of external background radiation preferably deposit their energy close to the semi-active detector surface, in which diffusion processes dominate that subsequently lead to slower rising pulses compared to the ones from the fully active bulk volume. Dedicated studies of their shape are therefore highly beneficial for the understanding and the rejection of these unwanted events. This article reports about the development of a data-driven pulse shape discrimination (PSD) method for the four 1 kg size PPC HPGe detectors of the
Conus
experiment in the keV and sub-keV regime down to 210 eV
ee
.
The impact of the electronic noise at such low energies is carefully examined. It is shown that for an acceptance of 90% of the faster signal-like pulses from the bulk volume, approx. 50% of the surface events can be rejected at the energy threshold and that their contribution is fully suppressed above 800 eV
ee
.
Applied to the
Conus
background data, such a PSD rejection cut allows to achieve an overall
(
15
-
25
)
%
reduction of the total background budget. The new method allows to improve the sensitivity of future
Conus
analyses and to refine the corresponding background model in the sub-keV energy region.
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The CONUS experiment is searching for coherent elastic neutrino nucleus scattering of reactor anti-neutrinos with four low-energy threshold point-contact high-purity germanium spectrometers. ...Excellent background suppression within the region of interest below 1 keV (ionization energy) is absolutely necessary to enable signal detection. The collected data also make it possible to set limits on various models regarding beyond the standard model physics. These analyses benefit as well from the low background level of
∼
10 d
-
1
kg
-
1
below 1 keV and at higher energies. The low background level is achieved by employing a compact shell-like shield that was adapted to the most relevant background sources at the shallow depth location of the experiment: environmental gamma radiation and muon-induced secondaries. Overall, the compact CONUS shield including the active anticoincidence muon-veto reduces the background by more than four orders of magnitude. The remaining background is described with validated Monte Carlo simulations which include the detector response. It is the first time that a full background decomposition in germanium operated at a reactor site has been achieved. Next to the remaining muon-induced background,
210
Pb within the shield and cryostat end caps, cosmogenic activation and airborne radon are the most relevant background sources. The reactor-correlated background is negligible within the shield. The validated background model, together with the parameterization of the noise, is used as input to the likelihood analyses of the various physics cases.
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