The Borexino detector is located at the Laboratori Nazionali de Gran Sasso, Italy. Borexino is an actively used detector, and experiments are on-going at the site. The goal of the Borexino experiment is measuring low energy, typically below 1 MeV, solar neutrinos in real-time. The detector is a complex structure consisting of photomultipliers, electrons, and calibration systems making it equipped to take proper measurements of the low energy solar neutrinos. Photomultipliers are used as the detection device in this system as they are able to detect light for extremely weak signals.
Solar neutrinos are able to provide direct insight into the core of the Sun because that is where the solar neutrinos originate. Solar neutrinos leaving the Sun's core reach Earth before light does due to the fact solar neutrinos do not interact with any other particle or subatomic particle during their path, while light (photons) bounces around from particle to particle. The Borexino experiment used this phenomenon to discover that the Sun releases the same amount of energy currently as it did a 100,000 years ago.