Detecting coherent neutrino-nucleus scattering
The coherent scattering of neutrinos on atomic nuclei is an interaction predicted from theory and is of major relevance for fundamental research. The CONUS experiment at the nuclear power plant in Brokdorf operated by PreussenElektra GmbH aims to characterize this interaction channel. Neutrinos, which are created in large amounts by nuclear fission products and not usable for power generation, are studied in this experiment on a novel way.
Neutrinos - Interaction with matter
Figure 1: Coherent scattering of a neutrino on an atomic nucleus.
Since neutrinos only interact weakly with matter, the devices for neutrino detection are typically very large up to masses of hundreds of tons material with a still increasing trend. In principle, neutrinos can interact with matter in two ways: either on electrons in the atomic shell or on protons/neutrons in the nucleus. In the latter case, there is the possibility for the neutrino to ‘coherently’ scatter on the nucleus as a whole increasing substantially the probability for the scattering process. On the other hand, the energy transfer on the nucleus is tiny for the coherent scattering, similar to the situation of a table tennis ball hitting a basketball. It is quite simple
to strike, but hard to move the target. Therefore, dedicated detectors with very low energy thresh-
old are needed for this detection, but only few kilograms of material might be sufficient to meet the goal! The challenges of such an experiment made the experimental confirmation of the theoretical predictions in the 1970s impossible for more than 40 years. In 2017 the coherent scattering of neutrinos on nuclei was detected for the first time by the COHERENT experiment. The higher-energy neutrinos in this experiment were produced atomic nucleus using a neutron beam. Complementary measurements with lower-energy neutrinos at nuclear reactors are still pending and will further improve the understanding of models in particle physics.
The CONUS experiment
Figure 2: Inner view of the safety containment of the nuclear power plant in Brokdorf. The location of the CONUS setup ist marked with a red star.
The detection and characterization of the coherent neutrino-nucleus scattering require an experimental setup very close to a strong and well controlled neutrino source. In competition to other international efforts, this idea is realized in the CONUS (COherent Neutrino nUcleus Scattering) experiment, which is operated in collaboration with PreussenElektra GmbH in Brokdorf. The measurement of the reactor neutrinos does not influence the reactor or make any demands on the operation of the power plant. The distance of the experimental setup to the reactor core is 17 meters only. Therefore, from one of the worldwide strongest reactors, an extremely high flux of 24 trillions of neutrinos per second and square centimeter is available for measurements. The combination with the purpose-built shielding and the advanced detectors makes the experiment a leading project.
The CONUS experiment uses high-purity germanium semiconductor detectors sensitive to ionizing radiation. Several layers of ultrapure lead and boron-doped polyethylene shielding with a total mass of 11 tons protect the detectors against external radioactivity. Furthermore, concrete and a water pool at the reactor site provide an extra shield reducing cosmic muon radiation. The remaining fraction of this background radiation is rejected using an extra cosmic-ray detector. The shielding design is based on the world-leading expertise established at MPIK over many decades.
Status of the experiment
Early April 2018, data collection with the CONUS setup started. First the reactor was turned off for about one month and then restarted. A comparison of the reactor ON and the reactor OFF data yielded a hint for the rare neutrino interaction process already after two months of data acquisition only. With the additional statistics of more data and refined analyses methods, the CONUS collaboration will be able to scrutinize the coherent neutrino-nucleus scattering with reactor neutrinos in the upcoming years. After the foreseen shutdown of the reactor end of 2021, CONUS plans to continue data collection for one more year to study, characterize and discriminate background events for an improved signal to background ratio.
Detection and precise measurement of coherent neutrino nucleus scattering are of fundamental importance for basic research, since they give insights in various microscopic processes. Moreover, neutrinos play a crucial role in several eminent astrophysical and cosmological events in the Universe. This includes star collapses (supernovae) emitting inconceivable amounts of neutrinos undergoing coherent scattering processes with nuclei during their propagation through the imploding star layers. The CONUS experiment at the nuclear power plant in Brokdorf offers the unique opportunity to measure for the first time coherent neutrinonucleus scattering in the energy range of reactor neutrinos applying most up-to-date germanium detector technology. A precise measurement of the neutrino flux at nuclear reactors could in principle be used in the context of reactor monitoring, safeguard applications or for thermal power determination.
Prof. Dr. Dr. h.c. Manfred Lindner:
Tel: +49 6221 516800
E-Mail: manfred.lindner [at] mpi-hd.mpg.de
Dr. Werner Maneschg:
Tel: +49 6221 516287
E-Mail: werner.maneschg [at] mpi-hd.mpg.de
Dr. Christian Buck:
Tel: +49 6221 516829
E-Mail: christian.buck [at] mpi-hd.mpg.de