Rudolf Mößbauer received his Nobel Prize in 1961, only three years after he reported on what was later known as recoilless nuclear resonance fluorescence – or the Mößbauer effect in short. With an age of 32 at the time of the award, he belongs to the ten youngest Nobel Laureates ever. This early success probably facilitated his decision of eventually changing his research focus entirely after accepting a position at the Technical University of Munich in 1965. Around 1970 he became interested in the neutrino, an elusive elementary particle, which had been detected for the first time in 1957. In his 2001 Lindau lecture, Mößbauer would state: “I fooled around with the Mößbauer effect for 15 years and then I had it. […] When the neutrinos came up, I immediately caught fire.” And indeed, the neutrino and its peculiar properties should stay at the focus of his research interest until his retirement in 1997 and beyond. Of the 12 lectures he gave in Lindau during his life, 8 had the neutrino and its properties as their topic (1979, 1982, 1985, 1988, 1994, 1997, 2000, 2001).
In the present lecture, Mößbauer gives a very clear and concise overview of the history of the neutrino, its properties, sources and the scientific questions connected to it at the time. He differentiates three problems in particular: the solar neutrino problem, the question of neutrino mass (traditionally, neutrinos were assumed to have zero mass) and neutrino oscillations. Interestingly, from a today’s view, the three problems are actually but one: the solar neutrino problem, which boils down to the fact that for a long time only one third of the expected solar neutrinos were detectable on earth, could be largely resolved around the turn of the century. At that time experiments showed neutrino oscillations, which also implied a non-zero neutrino mass. However, Mößbauer was not actively involved in these discoveries. Major contributors were the teams running two large underground detectors, the Sudbury Neutrino Observatory in Canada and the Super-Kamiokande in Japan.
In any case, in 1982 the field was still open. In the second part of his talk, Mößbauer describes how his team is using a neutrino detector attached to the nuclear fission reactor in Gösgen, Switzerland in order to search for neutrino oscillations. Since the interaction of neutrinos with matter is so weak, neutrino detectors have to be huge: Mößbauer mentions how the Swiss Army helped the scientists to move their 1 000 ton apparatus into place. And still, despite the massive detector size, measurement periods of half a year were needed for a single curve, as Mößbauer explains. As a comparison, the Super-Kamiokande experiment mentioned above was even bigger, employing a 50 000 ton detector.
Despite the fact that Mößbauer’s work in Gösgen did not result in groundbreaking contributions to the field of neutrino physics, his Lindau lectures on neutrinos are particularly valuable. That is partly due to his well-structured and clear way of presenting. But first of all, his Lindau lectures cover the development of a highly topical research field over a significant period of time (1979-2001) and thus allow for a unique insight into the - sometimes apparently rather slow - workings of science.