CONUS (COherent Neutrino nUcleus Scattering) Experiment is a research project at the commercial nuclear power plant in

Brokdorf Brokdorf is a municipality in the district of Steinburg, in Schleswig-Holstein, Germany. It is located on the bank of the Elbe river, approx. 20 km east before the river flows into the North Sea. As of December 2019, the total population of B ...

, Germany (see Figure 1). The CONUS project is sponsored by the

Max-Planck-Institut für Kernphysik The Max-Planck-Institut für Kernphysik ("MPI for Nuclear Physics" or MPIK for short) is a research institute in Heidelberg, Germany. The institute is one of the 80 institutes of the Max-Planck-Gesellschaft (Max Planck Society), an independent, n ...

and Preussen Elektra GmbH. The CONUS project is searching for the fundamental process of coherent elastic scattering of

neutrinos A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...

off

atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in ...

. The primary goal is to confirm the existence of this process and to use this interaction type to investigate further neutrino properties within and beyond the standard model of elementary particle physics.

Science Principles and Motivation - Coherent Elastic Neutrino Nucleus Scattering

As electrically neutral

leptons In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neut ...

, neutrinos only interact via the

weak force Weak may refer to: Songs * "Weak" (AJR song), 2016 * "Weak" (Melanie C song), 2011 * "Weak" (SWV song), 1993 * "Weak" (Skunk Anansie song), 1995 * "Weak", a song by Seether from '' Seether: 2002-2013'' Television episodes * "Weak" (''Fear t ...

with other particles. Due to this fact, neutrino detectors are generally very large and filled with several (kilo)tons of target material. There are basically two possibilities to detect neutrinos: First, they can interact with the electrons in the

atomic shell In chemistry and atomic physics, an electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" (also called the "K shell"), followed by the "2 shell" (o ...

of a target atom, and second they can interact with the

protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron m ...

and

neutrons The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beha ...

of an atomic nucleus. Interactions between neutrinos and electrons as well as neutrinos and nuclear constituents have already been well studied. However, at low energies up to a maximum of a few tens of mega-electronvolt (MeV), neutrinos can interact coherently with the

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

as a whole (see Figure 2). This process was predicted in 1974 and is known as coherent elastic neutrino nucleus scattering (CEυNS, pronounced "sevens"). Although its

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Ab ...

is several magnitudes larger than the cross section of the conventionally used interaction channels (see Figure 3), the tiny recoil of the struck nucleus leads to a very low energy release, making the process very hard to detect. Therefore, experiments investigating this process need detectors with an extremely low energy threshold, i.e., below 1 kilo-electronvolt (keV). On the other hand, since the CEυNS interaction cross sections is enhanced, a few kilogram of detector material can already be enough to detect the interaction. As the first experiment worldwide, the COHERENT experiment was able to experimentally prove the existence of coherent elastic neutrino nucleus scattering in 2017. Herein, it used a relatively high energy neutrino beam in comparison with reactor neutrinos. Further complementary studies at lower energies in the fully coherent regime are yet to come. Examining this low energy neutrino region is the main goal of the CONUS project.

Detector

Site

The detection as well as detailed investigations of the properties of CEυNS utilizing neutrinos from a

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...

require the detector to be located as close as possible to the

reactor core A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place and the heat is generated. Typically, the fuel will be low-enriched uranium contained in thousands of indi ...

to guarantee a maximized neutrino flux. To achieve this, the CONUS detector is located at a distance of 17 m from the reactor core inside the nuclear reactor facility at Brokdorf,J. Hakenmüller et al.: "Neutron-induced background in the CONUS experiment"
arXiv:1903.09269
/ref>H. Bonet et al. (CONUS Collaboration): "Constraints on Elastic Neutrino Nucleus Scattering in the Fully Coherent Regime from the CONUS Experiment", Phys. Rev. Lett. 126, 041804 (2021), see Figure 4. This is only possible due to the selected detector technology H. Bonet et al.: "Large-size sub-keV Sensitive Germanium Detectors for the CONUS Experiment", https://arxiv.org/abs/2010.11241 such that it can be placed inside the facility without interfering with the operation of the reactor. The Brokdoft reactor runs at a maximum thermal power of 3.9 GW, making it one of the most powerful in the world. On average, about 7.2 neutrinos are produced per

nuclear fission Nuclear fission is a nuclear reaction, reaction in which the atomic nucleus, nucleus of an atom splits into two or more smaller atomic nucleus, nuclei. The fission process often produces gamma ray, gamma photons, and releases a very large ...

(6 from fission products and 1.2 due to decays after neutron captures on

Uranium-238 Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However ...

). At the detector site this results in a flux of about 23 trillion neutrinos per second and square centimeter.J. Lindhard, M. Scharff, and H.E. Schiøtt: "Range Concepts and Heavy Ion Ranges (Notes on Atomic Collisions, II", Kgl. Danske Videnskab. Selskab. Mat. Fys. Medd. 33, 14 (1963)

Detectors and Measurement

The CONUS Collaboration is using four highly pure germanium

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

detectors, each weighing 1 kg (see Figure 5). If a neutrino originating from the reactor core scatters off a

germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbo ...

nucleus, the small recoil energy of the nucleus is partially converted into

ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged a ...

energy and partially into dissipation heat. Only the first energy part contributes to electrical signal formation in the ionisation detectors as used in CONUS. The dissipation phenomenon is known as

quenching In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as ...

and is typically described by the

Lindhard theory In condensed matter physics, Lindhard theoryN. W. Ashcroft and N. D. Mermin, ''Solid State Physics'' (Thomson Learning, Toronto, 1976) is a method of calculating the effects of electric field screening by electrons in a solid. It is based on quant ...

. Thus, a precise knowledge of this quenching factor is crucial, since its uncertainty is one of the main systematics of the experiment. To detect coherent elastic neutrino nucleus scattering, CONUS is collecting reactor-on and reactor-off data. By comparing these data, an excess of events in the expected energy window during reactor-on time can reveal the existence of CEυNS. In addition, measurements during reactor-off times allow for a precise determination of the background rate and its components. CONUS started collecting data on April 1, 2018, and has been continuously operated since then.

Shield

Although CEυNS is the neutrino interaction with the highest cross section, it still is a rare process. Moreover, since it comes with a very small energy and

momentum transfer In particle physics, wave mechanics and optics, momentum transfer is the amount of momentum that one particle gives to another particle. It is also called the scattering vector as it describes the transfer of wavevector in wave mechanics. In the ...

(<1 keV), a suitable detector needs to be shielded from any additional background. The three main background types and their mitigation strategies applied in CONUS are summarized here:
The relevant backgrounds can be put into 3 different categories: * Cosmic Radiation: Cosmic

muons A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wit ...

and muon-induced showers can interact with the target material of the detector in large quantities. Thus,

cosmic radiation Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ow ...

is one of the most relevant backgrounds. To suppress this type of background, many low background experiments are located deep underground. This is however not possible for CONUS; here the reactor building offers a modest overburden leading to a reduction of the muon flux by a factor of 2-3 only. To achieve an even better suppression of the muonic background, the CONUS detector is surrounded by an active muon veto system (see Figure 5). It consists of

scintillator A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbe ...

layers that can detect incoming muons crossing the detector setup. This way, the muon-induced background can be reduced approximately by a factor of 100. * Local Background: Besides cosmic radiation, there is also background coming from the direct surroundings. The most important contributions to the local background are naturally occurring radioactivity in the surroundings and neutrons radiated from the reactor core. To shield the detectors against this background, they are covered with several layers of

lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...

(25 cm in total) as well as boron-doped polyethylene plates (see Figure 5). Another important background source that experiments (especially those located in a closed environment) have to deal with is the radioactive decay of airborne

radon Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through ...

. Radon is an

inert gas An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...

and can therefore leak through tiniest gaps of the shield layers and decay close to the detectors. To solve this problem, the detector chamber is continuously flushed with radon-free air from compressed air bottles. * Intrinsic Radioactivity: The detectors also contain small concentrations of radioactive isotopes. Hence, it is necessary to minimize the amount of radioactive impurities inside the detector. To achieve this goal, the materials used to build the detector were carefully analyzed with the help of the GIOVE detector G. Heusser et al.: "GIOVE - A New Detector Setup for High Sensitivity Germanium Spectroscopy At Shallow Depth", Eur. Phys. J. C 75, 531 (2015), at the underground laboratory of the Max-Planck-Institut für Kernphysik and selected correspondingly. Despite the small volume of the CONUS setup of 1.6m^3, the massive high-density shield leads to a total mass of 11 tons.

Results (January 2021)

In 2020, The CONUS project published first results on CEυNS using 3.73 kg of active detector material after almost 70 days of effective measurement time with the reactor turned on and about 16 days with the reactor turned off. With these data the until now most precise upper limit for the existence of the CEυNS process in the fully coherent regime could be determined. This limit constitutes valuable information for basic neutrino research, since it allows one to test predictions for the strength of CEυNS in the standard model theory or in variations of it. The unique performance of the CONUS detectors with their very low energy thresholds, ultra-low background levels and long-term stability is highlighted in Ref. With additional data collected until and beyond the end of the reactor operation in late 2021, additional improvements of the

data acquisition Data acquisition is the process of sampling signals that measure real-world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems, abbreviated by the acro ...

systems and a better understanding of the quenching factor in germanium, the sensitivity of the experiment is expected to improve significantly in the next few years.

Applications

Understanding the process of coherent elastic neutrino nucleus scattering may offer some possibilities in other areas, too.
It is generally expected that the coherent elastic neutrino nucleus scattering process plays a significant role in the dynamics of Core-Collapse Supernovae. Investigating this process will therefore help to better understand the dynamics of such stellar explosions. Furthermore a detailed study of coherent elastic neutrino nucleus scattering could potentially reveal some new physics beyond the standard model of

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and ...

. For instance, it could be used to study electromagnetic properties of neutrinos (e.g. neutrino

magnetic moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...

), to investigate potential deviations of the

weak mixing angle The weak mixing angle or Weinberg angle is a parameter in the Weinberg– Salam theory of the electroweak interaction, part of the Standard Model of particle physics, and is usually denoted as . It is the angle by which spontaneous symmetry bre ...

at low energies or to study possible non-standard interactions in the neutrino-

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All common ...

sector. Besides its fundamental importance for

basic science Basic research, also called pure research or fundamental research, is a type of scientific research with the aim of improving scientific theories for better understanding and prediction of natural or other phenomena. In contrast, applied resea ...

, the detection of neutrinos via CEυNS offers some practical applications. One example is the possibility to use CONUS-like detectors for reactor monitoring.A. Bernstein et al.: "Colloquium: Neutrino Detectors as Tools for Nuclear Security" Rev. Mod. Phys. 92, 011003 (2020),

External links

MPIK Division Lindner: CONUS

References

{{coords , 53.850731, 9.345844, display=title Particle physics facilities Experimental particle physics Research institutes in Germany