Nuclear reactors in the service of fundamental research

Power producing nuclear reactors are technological and engineering facilities, whose primary purpose has nothing to do with basic research. However, they turned out to be indispensible in one area of probing Nature’s innermost secrets, namely to find out whether or not the neutrino has a non-zero rest mass.

Neutrinos are one of the most elusive particles in nature. They are neutral (lack electric charge) and interact with matter extremely weakly; they can even penetrate the whole Earth without a noticeable chance of being absorbed. For the very same reason, they are extremely difficult to detect. They exist in three different forms (in elementary physics jargon called ”flavours”): electron-, muon- and tauneutrinos.

For a long time, one believed that neutrinos, just as photons, have no rest mass. But there have also been speculations, as well as some indirect indications, that they might have a non-zero, even if exceedingly small, rest mass. According to theory, these three types of neutrinos are built up from three different mass states. If these three masses are different, then the three different flavours should oscillate among each other. If one can observe oscillations between two neutrino types, it means that they have different masses, hence at least one of them must be larger than zero.

The simplest way to verify oscillations experimentally is to prove the absence of a particular type of neutrinos at a certain distance from a source, which emits just that type of neutrinos. However, the experiment can only be successful if the distance between the source and the detector matches the frequency of the oscillations. For maximum success there is an optimal minimum distance.

The first proof of the neutrino oscillations, and hence that of the existence of the neutrino mass, came from the Super-Kamiokande experiment in central Honshu in Japan (see the map below, Fig. 1). One proved the oscillations between muon- and tau-neutrinos, by measuring atmospheric neutrinos, which are given rise by cosmic radiation. With the help of direction sensitive detection methods, one performed measurements partly on neutrinos generated above the detection site, i.e. close to the detectors, and partly on neutrinos which were generated on the diagonally opposite side of the Earth. The distance, i.e. the diameter of the Earth, was perfect for confirming the existence of oscillations through the difference in the detection intensity between the two detectors. This is one of the two  measurements for which the Nobel prize in physics was awarded in 2015.

Figure 1. The site of the Kamioka and Super-Kamiokande 

experiment i Honshu, Japan, with the site of several 

nuclear reactors indicated by a grey ring around the 

detector facility 

However, for completeness and certainty, the other two types of neutrino oscillations, both related to electron neutrinos, had also to be shown. These were much harder to achieve, since there was no suitable source with a suitable distance to a detector in such a natural way as for the muon-tau-neutrino oscillations. It is at this point that nuclear reactors came in to the picture. Nuclear reactors are very intensive sources of electron-neutrinos (actually, antineutrinos, but this is an unimportant detail in the context), hence they are very suitable for measuring both  electron-muonneutrino as well as electron-tauneutrino-oscillations. It was even suggested that the operation of nuclear reactors can be monitored from a distance by measuring the neutrino flux.

It turned out that the site of the Super-Kamiokande-experiment was very fortunate in this respect. Namely, there are a number of nuclear reactors in form of a ring around the detector facility, at a distance of between 140 to 180 km (see Fig. 1 above). (These measurements were performed about 8 years before the Fukushima accident, hence all reactors were in operation then). The detector was re-built for the purpose of this experiment, and was re-named KamLAND. It was in this experiment that the oscillations between electron- and muon-neutrinos were verified. A paper on these experiments, published in the Physical Review Letters by the research group led by Professor Atsuto Suzuki at Tohoku University, Sendai, became the most frequently cited paper during a period in 2003. The use of nuclear reactors is mentioned even in the title of the paper: ”First results from KamLAND: Evidence for reactor antineutrino disappearance” (Fig. 2).

Now there was only the third type of oscillations to be found, namely that between the  electron- and tau-neutrinos. And even here the experiment had to be based on reactor neutrinos, but the reactors around KamLAND were not at a suitable distance. It was clear that the source should lie much closer to the detectors. At that point, the neutrino physicists asked for help from reactor physicists, and this is how I came into the picture.

 

Figure 2. Photo of the author and data of the pioneering publication on the detection of reactor neutrino oscillations. From a slide, courtesy of Prof. A. Suzuki.

It was on one of my numerous visits to Japan, in 2005, when I was about to meet my colleagues at Tohoku University in Sendai. I got a message from my first host and long-standing friend, Prof. Kojiro Nishina of Nagoya University, that the Vice President of Tohoku University, Prof. A. Suzuki, who found the second type of oscillations, wanted to meet me. He was trying to find a strong, movable neutrino source. During our meeting he asked me if it was possible to construct a movable nuclear reactor for neutrino experiments, in the race for finding the third type of neutrino oscillations.

His idea was to move the reactor under the ground, presumably vertically, since it is not easy to move a nuclear reactor on the surface in densely populated areas. That was no easy question. The idea of  SMRs (compact small- and medium size reactors), which exist now at least on the drawing board, did not exist then yet. I recommended to use an accelerator driven subcritical system, which has good safety margins and can be shut down and start up again between the moves. Nuclear vessels, such as submarines, icebreakers or aircraft carriers, could not come into the question, since the distance to the closes coastline was too long.

The KamLAND project with a movable nuclear reactor has not come about, the technical, safety and financial problems were simply prohibitively large. The third types of oscillations were instead verified not by putting a source to a suitable distance to an existing detector, rather by building a detector facility close to a large nuclear reactor site with several reactors. These experiments were performed first in China in 2012, then even in France and later on in South Korea. In all three measurements, neutrinos from nuclear power plants were used. The first, decisive experiments were made in China, around the six reactors of the Daya Bay site, with six antineutrino detectors. The distance between the detectors and the reactors varied between 0.5 and 1.5 kms. The movable reactor which Prof. Suzuki was thinking of, would not need to travel a long distance.

Personally, I was glad to hear about the experiments in KamLAND in Japan and Daya Bay in China, for the double reason that partly now I am a reactor physicist, but partly because I have some past in theoretical physics. I studied particle physics at the Lorand Eötvös University in Budapest and had strong interaction as the special field. My master thesis had the title ”Dual resonance model with SU(6)-symmetry for meson scattering”. Neutrinos interact through the weak forces, so my area was at a completely different area. Despite of this and that I have left the area of theoretical physics long ago, I am naturally delighted that my present field of work could contribute to fundamental science, and also that the question of neutrino oscillations and neutrino mass is now finally settled :) .