discovered_01_2016

WWW.HZDR.DE 26 27 RESEARCH // THE HZDR RESEARCH MAGAZINE Neutrons or WIMPs – that is the question When the xenon detector registers an event, it is most likely not a WIMP, but rather a fast neutron. "This is why Uwe Oberlack’s group from Mainz has built a small xenon detector for experiments at our neutron source nELBE. We want to figure out how the detector can separate the wheat from the chaff," says HZDR physicist Andreas Wagner. The true-to-scale detector from Mainz looks like a steel thermos bottle, but it works in exactly the same way as its big brother in Italy. Xenon, which is liquid at a temperature of about -170 degrees Celsius emits light as soon as a WIMP or another particle recoils off one of the xenon atoms. To be more precise, an impacting particle energetically excites the xenon atom, which scintillates as it returns to its original state. Several light sensors register and amplify this light. With the help of fast neutrons from the nELBE neutron source, reactions that interfere with the proof of WIMPS in the underground lab can now be closely studied. Flight duration, energy and angle nELBE is driven by the ELBE accelerator's intensive electron beam. In the neutron lab, this "beam", which actually consists of a multitude of individual electron packets, impacts with an arrangement of liquid lead. The deceleration of the electrons generates about 100 billions of neutrons per second. The scientist in charge, Arnd Junghans, explains what is special about the nELBE neutron source: "Thanks to the exact timing of the electron pulses, we can precisely measure the flight time of the neutrons, and hence, their speed and energy. Since the parameters of the beam, and thus the neutron flow, can be set as desired, the HZDR neutron source is the perfect tool for detector research." Fellow physicist Andreas Wagner says: "So as a result, we know the energy-dependent sensitivity of the detector." This knowledge helps the scientists involved in the XENON1T experiment distinguish potential candidates for Dark Matter from fast neutrons. But there is even more nuclear physics involved here. The light neutrons can make direct impact with the xenon cores, which are about 130 times heavier. When it experiences such a central impact, the neutron transfers a great deal of energy to the nucleus of the atom. When it bounces off to the side, however, the neutron only loses a small amount of energy and flies straight ahead, exiting the detector at a certain angle. "That is why we specifically built ten neutron detectors and arranged them all around the detector with the liquid xenon for the experiment," says Wagner. "This way, we are able to analyze two parameters, the flight time and the angle, which tells us how much energy was transferred to the nucleus." Three doctoral students from Mainz came specially for this multi-week series of experiments. Many indicators suggest that a major proportion of the matter that surrounds us is Dark Matter. "Let’s take the rotational speed of a galaxy, for example. How fast do the arms of the galaxy rotate around its center? Without the existence of Dark Matter, we could not reconcile the calculated speed with the distribution of mass we observe," Andreas Wagner explains. Thanks to Rossendorf’s expertise with regard to particles in general and neutrons in particular, the scientists involved in the XENON1T experiment are now one crucial step nearer to tracking down Dark Matter. Should the theory of weakly interacting particles, WIMPs, be confirmed, the scientists ought to be on the safe side if they erupt in cheers one day, when they detect the first WIMP signals. But since experts only expect two or three such events per year, it could still take a while. _Institute of Radiation Physics at HZDR Dr. Andreas Wagner | Dr. Arnd Junghans a.wagner@hzdr.de | a.junghans@hzdr.de _Johannes Gutenberg-Universität Mainz Prof. Uwe Gerd Oberlack oberlack@uni-mainz.de www.xenon1t.org CONTACT TOGETHER: Seeking to track down WIMPs (from left to right): Andreas Wagner and Toni Kögler (HZDR), Melanie Scheibelhut (Uni Mainz), Arnd Junghans (HZDR), Uwe Oberlack and Pierre Sissol (Uni Mainz). Photo: Oliver Killig

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