Abstract
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 (745pm 30),hbox {cm}^{-2},hbox {day}^{-1}. These neutrons, produced inside the reactor core, are reduced by a factor of sim 10^{20} on their way to the CONUS shield. With a high-purity Ge detector without shield the gamma -ray background was examined including thermal power correlated ^{16}hbox {N} decay products and neutron capture gamma -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.
Highlights
CONUS is a novel experiment which aims at detecting CEνNS signals using reactor antineutrinos
All potential neutron sources at the KBR reactor site had to be inquired first: cosmogenic neutrons induced by muons in the reactor building and in the CONUS shield; neutrons from the spent fuel storage pond above the experiment; (α,n) reactions from natural radioactivity in the surrounding concrete walls and basements; neutrons from the reactor core; and γ radiation from neutron-induced isotopes decaying along the primary coolant of the pressurized water reactor
The overall fluence of neutrons from the outer fuel assembly ring hitting the reactor pressure vessel (RPV) wall is reduced by a factor of 1.4 · 10−4, while for the second outer ring it is 1 · 10−5, as these neutrons have to pass the additional layer of fuel assemblies where they can be absorbed or induce fission
Summary
CONUS is a novel experiment which aims at detecting CEνNS signals using reactor antineutrinos. Whereas the first three classes are steady-state sources, the latter two are potentially troublesome Both can mimic CEνNS signals, since they are correlated with the thermal power and. C (2019) 79:699 can contribute counts to the region of interest Quantifying these backgrounds via independent measurements and determining their impact on the CONUS HPGe detector energy spectra are of fundamental importance. The MC simulations were used to support and interpret the neutron measurement results in detail They were used to predict the impact of the measured thermal power correlated neutrons and γ -ray flux on the CONUS HPGe detectors. 7 describes the MC simulation of the measured neutron field and γ -ray background passing through the CONUS shield to investigate the impact of the neutron-induced signals on the energy spectra of the CONUS detectors
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