The sensitivity of the IceCube Neutrino Observatory at the South Pole could be boosted by adding optical materials to the icy boreholes that contain its detectors – according to physicists in the US.
Encompassing 1 km3 of ice, IceCube comprises 86 cables, each up to 2.5 km long, suspended inside vertical boreholes in the ice. Attached to each cable are dozens of photomultiplier tubes (see figure), which record the Cherenkov radiation given off by the secondary particles created when incoming neutrinos collide with nuclei inside the ice.
In 2013, IceCube made the first every detection of cosmic neutrinos from throughout the universe and physicists are now thinking about how the detector could be upgraded. In a preprint on arXiv, Imre Bartos, Zsuzsa Marka and Szabolcs Marka of Columbia University in New York describe how filling sections of the boreholes with materials with desirable optical properties could boost IceCubes’s detection efficiency.
They first looked at the effect of surrounding each photomultiplier tube detector with a material with a higher index of refraction than ice – the idea being that the change in refractive index will focus light towards the photomultiplier tubes. The found that for every 0.1% increase in the material’s index of refraction, a 10% increase in light flux to the photomultipliers could be achieved.
The trio also looked at the effect of filling parts of the boreholes with a material that would shift the wavelength of the Cherenkov light from ultraviolet to visible wavelengths – the latter being easier to detect using photomultiplier tubes. Their calculations suggest that filling most of a borehole with a wavelength shifter, but not the region immediately surrounding each detector, could lead to a very large increase in sensitivity.
The team does, however, point out several challenges that would have to be overcome to implementing its scheme. These include dealing with the natural radioactivity of the optical materials and the effect of freezing on the optical properties.