New at PRISMA+: Hans Steiger


Dr. Hans Steiger began conducting research in the detector laboratory at the PRISMA+ Cluster of Excellence in July. As holder of a doctorate in Physics, he was awarded the “Detector Innovation Fellowship” and – as requested in the call for tenders – brings a new exciting project to Mainz entitled “TAO”. TAO stands for “Taishan Antineutrino Observatory” and is part of the planned JUNO neutrino experiment in southern China. The aim of JUNO is to determine the neutrino mass ordering. The ultimate question behind this is which of the three neutrinos from the Standard Model is the lightest and which is the heaviest? The answer is of great significance to researchers around the world, because, among other things, it could provide valuable clues to “new physics” beyond the Standard Model.

Neutrinos are elementary particles that have next to no mass and that are emitted during processes such as fusion in the sun and radioactive decay in nuclear reactors. They come in three different types: electron, muon and tau neutrinos, and can change from one type to another, a phenomenon known as neutrino oscillation. It is possible to determine the mass of the particles from observations of the patterns of oscillation.

JUNO uses neutrinos from two neighboring reactor complexes on the South China coast, each around 50 kilometers away, in order to analyze their oscillation patterns with unprecedented accuracy. TAO has an important role to play in helping to achieve this level of precision. The detector should provide the most accurate reference spectrum of reactor neutrinos, while at the same time helping to answer fundamental nuclear physics questions in reactor physics.

To do this, TAO will be built directly outside the new Taishan 1 reactor, one of the most powerful reactors in the world. It is expected that due to the small distance (less than 30 meters), the vast majority of the electron-antineutrinos which are formed during the fission reaction in the reactor have not yet been converted into other types of neutrinos. Therefore, by comparing the spectra measured at JUNO and TAO, it should be possible to filter out the oscillation pattern with an exceptional degree of accuracy.

TAO will consist of a sphere two meters in diameter filled with 2.6 tons of liquid scintillator. Inside it, flashes of light generated by incoming neutrinos or their reaction products are detected using extremely sensitive silicon photomultipliers. Eventually, TAO is expected to achieve a record high resolution of 1.5 – 2% for an energy deposition in the liquid scintillator of 1MeV. This requires an entirely new concept regarding both the liquid scintillator and the photomultipliers.

To ensure that the silicon photomultipliers – which have never been used in a scintillation detector of this size before – are as sensitive as possible and free from disruptive background signals, the entire detector system is cooled to -50 degrees Celsius. “My research focuses on the development of an innovative liquid scintillator that meets many different requirements, but above all it must be chemically stable at these low temperatures whilst at the same time produce large amounts of scintillating light,” Hans Steiger explains. “To do this, it is necessary to reduce the freezing point of individual components using special additives, for example.” The scintillator also contains Gadolinium. This addition allows the interaction between the electron-antineutrinos and the scintillator to be distinguished much more easily from ambient backgrounds.

A lot of development work has already gone into the TAO project and in a test at the end of 2019, the prototype was successfully cooled to -50 degrees for the first time. “We have also been able to significantly improve the solubility of a crucial component, known as a wavelength shifter, at low temperatures,” continues Hans Steiger. “However, further research is needed to further optimize the liquid scintillator for low temperatures and to accurately characterize its optical properties.”

In future, Hans Steiger will be able to use the Laboratory for Scintillation and Fluorescence Detectors (LSFD) at PRISMA+, which is currently under construction and will offer ideal conditions for the development and characterization of scintillation and fluorescence materials. The detector laboratory is one of the key initiatives of the Mainz cluster in general. It provides the infrastructure and the necessary expertise to develop, build and test state-of-the-art particle detectors. Special emphasis is placed on tracking detectors and time-projection-chambers, photosensors and fast electronics, as well as scintillator and fluorescence detectors. “This specialized infrastructure, which is explicitly tailored to detector development, is enormously exciting and valuable for me,” says Hans Steiger. “Together with the independence and flexibility offered by the Detector Innovation Fellowship, these are ideal conditions to push on with the TAO project in Mainz, together with colleagues around the world.”

It is also highly beneficial that a group from Mainz, led by Prof. Michael Wurm, is already heavily involved with the central JUNO detector. Hans Steiger represents the first involvement of a German institution in the TAO project, it is therefore a significant new feather in the cap of the detector laboratory and PRISMA+. At the same time, there is a great deal of overlap with existing PRISMA+ research topics, including neutrino physics and detector development. Both JUNO and TAO are scheduled to start data taking in 2022.

About Hans Steiger
Hans Steiger initially studied for a B.Ed. in Physics and Mathematics in Bavaria, and subsequently obtained an M.Sc. from the Technical University of Munich. At the graduate school there, he completed his doctoral thesis in Experimental Astroparticle Physics. By this time, he was already involved in the JUNO Experiment, dealing with high-purity liquid scintillators which included the construction and operation of a pilot plant for scintillator purification at the Daya Bay Experiment in Shenzhen, China. Together with Prof. Dr. Michael Wurm, he also participated in the OSIRIS project within JUNO, which involved a detector for monitoring scintillator purity.