In the next of our series on the inspirations behind journal covers, Hans-Peter Loock and Mark Chen, of Queen’s University in Canada, talk to us about neutrinos, and photographing the giant detector set up to find them, underground and underwater.
Mark: Neutrinos are subatomic particles, which, despite their tiny size, have played an important role in shaping the evolution of the early universe. Studying them is one way of connecting the physics of very small things with that of very large things, but detecting them is extremely challenging.
Our experiment follows on from the Nobel-prize winning Sudbury Neutrino Observatory (SNO) experiment, which used heavy water to detect neutrinos. In a new project, known as SNO+, a liquid scintillator – a material that luminesces when struck by an incoming particle – will be used. This should be able to detect neutrinos at a wider range of energies than the SNO experiment.
The SNO+ experiment involves a 12-metre diameter acrylic vessel filled with 780,000 kg of an organic liquid scintillator. When a neutrino hits the detector it will cause the scintillator to emit light. The light will be detected by the 9,500 photomultiplier tubes surrounding the vessel – each one is capable of detecting a single photon of light.
The whole thing is submerged in ultra-pure water and located in a 100-foot high cavity excavated in an active nickel mine near Sudbury, Canada, 2 km below the surface of the earth.
Peter: Before the experiment can begin, there are several important questions that need answering, and our paper attempts to address one of those.
The question is whether the scintillator liquid will actually work for the entire length of the experiment – for several years – or will it degrade?
In our study we have found that the biggest problem will be oxidation, and therefore the team will need to carefully avoid exposure of the scintillator to air.
To show this, we exposed the liquid scintillator to oxygen, and followed its degradation by 2D fluorescence spectroscopy. In the past my research group has used this technique to study the degradation of engine lubricants – but the opportunity to apply this technique to a neutrino detector was too fun to pass up!
From the art desk
Mark: In the photo, taken by a remotely-operated underwater camera, you can see the 12-metre spherical SNO+ detector, filled to 85% with water, and held in the centre of the water-filled cavity.
You can also see the ropes holding it in place, and criss-crossing over the top, and the hole for the “chimney” of the vessel.
Each of the shiny specks all around is an 8-inch diameter photomultiplier tube that will detect the scintillation light produced when neutrinos interact in the detector.
The neutrino detector is normally kept in the dark, and remotely operated lights installed on the structure provided the illumination for this photograph. The article appears today in our journal PCCP. The corresponding author is Hans-Peter Loock, and co-author and SNO+ Project Director Mark Chen provided the photograph.