For nearly each one of the 37 elementary subatomic particles, there is a corresponding antiparticle. When space, time and matter were created in the Big Bang around 13.8 billion years ago, physicists calculate, matter and antimatter should have been created in pretty much equal amounts. But the universe today is full of stars, planets and other cosmic bodies made of matter – and the search for antimatter has so far been unsuccessful. Why is that?
An explanation might be forthcoming if physicists detect the theoretically predicted “neutrinoless double-beta decay (0nββ)” of the radioisotope germanium-76 (76Ge) into selenium-76. This is a process where two neutrons decay into two electrons, two protons and two neutrinos. “However those neutrinos should not be detectable because – if the theory that neutrinos are their own antiparticles is correct – they annihilate each other as soon as they are created,” says PD Dr. Radhakrishnan Sumathi, Head of the IKZ’s Semiconductors Section. “At the same time, this would explain the lightness of the neutrinos and deliver clues as to why matter in the present universe is much more abundant than antimatter.”
Of course, it’s not that simple. The half-life of 76 Ge is extremely long, in excess of 1 x 1026 years (more than a billion million times the age of the universe!), and therefore the forerunner experiment GERDA – which ran from 2015 to 2020 – could only observe its decay with relatively low sensitivity (yet still the world’s highest sensitivity at that time) using 40 kilograms of highly pure germanium (Ge) crystal detectors. The probability of detecting this decay will be higher in the next experiment, which will use a larger amount of this Ge semiconductor material. The project LEGEND (Large Enriched Germanium Experiment for Neutrinoless Double beta decay) at the Gran Sasso National Laboratory (Italy), involving 50 research institutes from Europe, the USA and Canada, will use 200 kilograms. Because the more the Ge atoms, the greater the probability of the decay process and the discovery potential.
This will require single crystals of exceptionally high purity. And producing these is a real challenge, even for IKZ. “For silicon semiconductor crystals in microelectronic applications, we can have a purity of 99.9999 percent; for this experiment, however, we will need Ge of 99.9999999999 percent purity! (That’s parts per trillion level purity),” says Radhakrishnan Sumathi. She is the IKZ project leader of the same-named LEGEND project of the German Ministry of Education and Research, in which the Technical University of Munich, Technische Universität Dresden and the University of Tübingen are involved.
The path towards such extreme, “ultra-high” purity is long and rocky. Natural Ge occurs as five isotopes, 7.8 percent of which is 76Ge. Therefore, it has to be enriched in an elaborate process. “The raw material, with which the Russian company ECP will supply us in the form of germanium oxide powder (GeO2), is accordingly expensive,” Sumathi relates. This oxide will first be reduced with hydrogen into elemental Ge metal. At this point it will still contain impurities – in tiny amounts of elements like aluminium, boron and phosphorus.
“In a zone-refining process lasting several weeks, the impurities will segregate themselves off at the two ends of the metal bar, and we will then cut those parts off,” Sumathi describes the approach. The part in the middle will then be used to grow a single crystal by the Czochralski method. The 76Ge serves as both a ββ-source and detector – and for the latter purpose, the single crystals will not only have to be highly pure, but will also have to satisfy other additional specific criteria. “We need to get only 100 to 10,000 dislocations per square centimeter. Otherwise, the crystal would have too many point defects, or dislocations, that could amplify the background noise,” Sumathi explains. In that case, the crystal would have to be melted down again and regrown.
All steps – from oxide powder reduction to single crystal growth – will be performed in the cleanroom under a hydrogen atmosphere. And under the strictest safety precautions because, if a spark were to set off the hydrogen in the air, it would cause an explosion.
Neutrinos are electrically neutral particles of extremely low mass. They travel at the speed of light and pass unhindered through just about all matter – including human bodies – leaving no trace. To be able to detect their creation (and annihilation), the experimental equipment has to be optimally sealed off. For one thing, the sun also produces neutrinos in its interior during nuclear fusion, blasting an immense amount of them towards the earth at any given time. For another, the high-energy radiation from the cosmos and from the atmosphere has to be avoided as well. The LEGEND experiment will therefore take place in the Gran Sasso Laboratory deep under the Apennine Mountains near the city of L’Aquila, well shielded from the cosmic radiation beneath 1,400 meters of rock.
“We have already processed 35 kilos of 76Ge from the 76GeO2 powder at IKZ for LEGEND-200 experiment; 80 kilos from the previous experiments GERDA (Europe) and MAJORANA (USA) are also to be reused for this,” says Sumathi. When will the experiment start? “The detectors are just being put into place in the experimental facility in Gran Sasso. Everything is expected to be ready for the experiment to start at the beginning of 2022.” And when it does, they will have to wait... wait... and wait... for a tiny peak at 2,039 kiloelectron volts – as the proof of neutrinoless double-beta decay.
LEGEND-200 is itself an intermediate step. In order to increase the experimental sensitivity even further, the consortium is already planning a future ton-scale experiment using 1,000 kilos of 76Ge. In the cleanroom of IKZ, the equipment for germanium crystal growth and other processes will therefore continue operating at full speed.
