How do we capture a ghost-like particle

If I tell you that there are billions of particles passing through your body every second, what kind of particle do you think they would be? The answer is neutrinos. Same as a ghost, these nearly massless and uncharged particles can pass through almost anything without provoke any interactions. Matters are virtually transparent to them because they can only interact through weak nuclear force. Neutrinos have three “flavors” – μ,τand e neutrinos. Geoneutrino is e anti-neutrino, who is produced when a neutron inside a radioactive element become a proton.

Figure 1. neutrino flavors

Figure 2. Beta-minus decay process

You may want to ask, why do we care about neutrinos? Let’s imagine our Earth is a moving vehicle, don’t you wonder what drives the engine of the Earth or how much energy we have left? Since geoneutrinos can travel through the Earth unimpededly from their origins, they carry integrated information about the abundances of the radioactive sources with them, which will provide insights of the power drives mantle convection, plate tectonics and geo-dynamo. So, how do we catch these ghost-like messengers? For decades, geologists and physicists are working side by side to solve this problem. In 2005, the KamLAND team first reported the detection of geoneutrino.

Currently, there are only four countries have detectors of neutrinos, which are Japan, Italy, Canada and China. Geoneutrinos detectors are mainly consistent three parts—reaction chamber, photo-multiplier and water volume. Reaction chamber are these giant underground tanks full of organic oil, which is the provider of protons and electrons. When an incident geoneutrino enter our detector’s reaction chamber, it will interact with a proton and generate positron and neutron. Two flashes of lights will be generated through this reaction: the prompt flash is from the positron and electron annihilation (cancel out each other); and the second flash comes from the neutron captured by a proton. These two flashes are highly synchronized (only 200 microseconds away) and only accessible to geoneutrinos, which effectively eliminates most backgrounds. In other words, these two flashes of lights indicate the capture of geoneutrinos.

Figure 3. Inverse beta decay process

To make the signals more accessible, an entire layer of photo-multiplier is attached to the outside of the reaction chamber. The water volume is located outside the photo-multiplier to protect the detector.  

Figure 4. Structure inside detector 

Geoneutrino study is expecting a new era of detection ability since a new deterctor JUNO, which has the ability to contain 20 kiloton organic oil, will come online. With more data available, maybe in the coming decades we can better constrain the abundance of the heat producing elements like U and Th in the Earth.