Large-size sub-keV sensitive germanium detectors for the CONUS experiment

Abstract

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.

Direct measurement of the ionization quenching factor of nuclear recoils in germanium in the keV energy range

“…This quantity has been extensively measured for nuclear recoils in the 10-100 keV range [1][2][3][4][5] and it follows the energy dependence predicted by Lindhard et al [6]. For experiments aiming at detecting reactor antineutrinos via CEνNS in HPGe detectors [7][8][9][10], recoils of a few keV nr (nuclear recoil energy) are expected, producing ionization signals in the sub-keV ee (electron equivalent energy) range. At these energies, precise experimental measurements of the ionization quenching factor are still lacking and the overall validity of the Lindhard theory has been questioned [11][12][13][14].…”

“…A reference signal for time-of-flight (TOF) measurements was derived from an inductive beam pick-up located close to the neutron production target. For proton energies between the threshold of 1.88 and 2.37 MeV, the 7 Li(p,n) 7 Be reaction produces only monoenergetic neutrons. For higher proton energies, transitions to the first excited state in the residual 7 Be nucleus result in a second neutron contribution of lower energy.…”

“…For proton energies between the threshold of 1.88 and 2.37 MeV, the 7 Li(p,n) 7 Be reaction produces only monoenergetic neutrons. For higher proton energies, transitions to the first excited state in the residual 7 Be nucleus result in a second neutron contribution of lower energy. The energy of the proton beam was measured using a 90 o analyzing magnet, which is regularly calibrated using a set of resonances and reaction thresholds.…”

“…The neutron response of these detectors was characterized in detail in a dedicated commissioning campaign to ensure that all the detectors were equivalent and their data can be combined. The measurements were carried out in several monoenergetic neutron fields produced in open geometry using the 7 Li(p,n) 7 Be reaction and a 70 µg/cm 2 metallic lithium target and ns-pulsed proton beams (cf. Tab.…”

“…One of the experiments looking for the yet not observed CEνNS signal from reactor antineutrinos is CONUS. It consists of four 1kg-sized HPGe detectors [7] deployed at the nuclear power plant of Brokdorf (Germany). For CONUS, the quenching factor is not only crucial for the interpretation of the Standard Model neutrino data [23], but it is also the current main systematic uncertainty in the search for new physics [24].…”