How do neutrino detectors work

"A beautiful detector"

Neutrinos more than live up to their nickname as “ghost particles”. Although these elementary particles are created in numerous different processes like a myriad and flood the entire universe, they mostly traverse matter unhindered. Detecting them usually requires huge detectors located deep in the ice of the Antarctic, for example, or consisting of huge water tanks. In contrast, the neutrino detector from the COHERENT collaboration is downright handy and, with a weight of less than 15 kilograms, even portable. With this new type of detector, the researchers have now been able to detect a type of neutrino interaction that has been theoretically predicted for decades, but has never been observed before. The team presented their results in the specialist magazine "Science". Welt der Physik spoke about this with the participating scientist Björn Scholz from the University of Chicago.

Björn Scholz from the University of Chicago (left in the picture)

World of physics: where do neutrinos come from?

Björn Scholz: Neutrinos are generated in large quantities, for example in nuclear fusion processes inside the sun. However, they are very volatile particles that have a very low probability of interacting with other matter. Therefore they can fly through a lot of matter - for example the earth - without making themselves noticeable. And so it took decades before scientists were able to prove it at all.

What is the difference between the new experiment and previous ones?

In addition to the size of our detector, this is primarily the energy range that we have probed. When a neutrino flies through a detector and hits a particle, for example an atomic nucleus or an electron, it can be compared to a game of billiards: the neutrino transmits momentum and energy during this collision - and the higher the energy of the neutrino, the more Of course, the energy that is deposited in the detector is also higher. The higher this energy, the easier it is to discover such a signal. Therefore, all previous experiments were designed for high-energy neutrinos. In our experiment, on the other hand, we move in a low to medium energy range. The energy that a neutrino transfers to an atomic nucleus is negligible here. That is why the signal we were looking for was very small.

Where did the neutrinos for your experiment come from?

We performed the experiment at the Neutron Source SNS spallation at Oak Ridge National Laboratory in the United States. This is where neutrons are generated, and a large number of neutrinos are also produced as a by-product, so to speak. The advantage here is that we know the energy spectrum of these neutrinos very precisely and can therefore make good predictions about what the signal should look like in the detector. The SNS has provided us with a corridor in the basement that is only twenty meters away from the neutrino source. That was of course perfect, because it gave us an extremely large number of neutrinos that flew through our detector.

What does the detector look like?

It is a cylindrical detector made of cesium iodide and doped with sodium. It is about a foot long, has a diameter of twelve centimeters and is in a copper tube. If I may put it that way: this is a wonderful detector.

What happens if a neutrino in the detector encounters an atomic nucleus in the detector material?

In contrast to high-energy neutrinos, the neutrino interacts with an entire atomic nucleus. It collides with him, the atomic nucleus flies away and triggers a cascade of further such “billiard ball events” in the detector material. So it triggers other atomic nuclei, which in turn fly on, and so on. The recoil of these atomic nuclei creates light that we can record. From this we can infer the energy of the original neutrino. We recorded data over several months and recorded the collision of a neutrino with an atomic nucleus about every two days. Ultimately, we got a beautiful spectrum of energy that looks exactly as the theory predicted.

Prototype of the neutrino detector

In contrast to all other existing neutrino detectors, the detector you use is very small. How is that possible?

For the energy range we are investigating, the coherent neutrino scattering at atomic nuclei has the largest cross-section that there is for neutrinos. This interaction is much more likely than, for example, the collision of a neutrino with the electron of an atom. It is therefore possible to make the detector much smaller. Our detector only weighs around 14.5 kilograms, which is easy to carry. So we are talking about a real miniaturization of neutrino technology.

What was the motivation for the experiment?

On the one hand, it is an interaction that was predicted decades ago in the Standard Model of particle physics, but has never been observed before. By demonstrating this process, we provide further confirmation of the standard model. On the other hand, this interaction via the electroweak force is also relevant for research into dark matter. Because if dark matter interacted with normal matter not only via gravity, but also via the electroweak force, a similar reaction could take place. Therefore, we first wanted to prove that this process even exists.

What is the next step with the research?

We have so far only been able to roughly determine the cross-section - i.e. how likely the reaction is in a certain energy range. Precision measurements are required here. We could then use these to search for deviations from the standard model of particle physics, for example. Another interesting question revolves around the neutrino's magnetic moment. Because according to a minimal extension of the standard model, this elementary particle should actually have a magnetic moment. So far, however, nobody has proven this - and so it is still in the stars whether it really exists. By measuring the recoil spectrum in these interactions between atomic nuclei and neutrinos more precisely, we may also get information about the magnetic moment.