Newly discovered optical effect allows IceCube Neutrino Observatory to deduce properties of ice crystals

Every second, 100 trillion neutrinos pass through the human body. These tiny, nearly massless particles travel enormous distances through space while carrying information about their sources and are created by some of the most energetic phenomena in the universe. But neutrinos are incredibly difficult to detect, requiring a first-of-its-kind detector that can “see” these nearly invisible particles.

On December 18, 2010, the IceCube Neutrino Observatory, located at the South Pole, was completed. Designed to search for high-energy cosmic neutrinos, the detector consists of an array of 5,160 optical sensors, called digital optical modules (DOMs), buried in a cubic kilometer of Antarctic ice. When a neutrino interacts with a molecule in ice, the resulting secondary charged particles emit blue light through a process called Cherenkov radiation. The light then travels through the ice and can reach some of the DOMs, where it is detected. The researchers can then reconstruct the particle’s energy and direction, a process that relies on knowledge of the optical properties of ice.

In 2013, the IceCube Collaboration reported a unique observation in which the observed brightness of a light source depends on the direction of the light, an effect called “ice optical anisotropy”. Until now, researchers have attempted to describe anisotropy with impurity-induced variations in absorption and scattering with limited results.

In a new study presented to cryosphere, IceCube reports an optical effect that has not been previously described. The effect is the result of the elongated ice crystals’ birefringent properties that bend light in two directions. The new insights gained were incorporated into a new birefringence-based optical model of ice used in detector simulation, SpiceBFR, which has substantially improved the interpretation of light patterns resulting from particle interactions in ice.

“The optical model of ice in use by the IceCube Collaboration has been in development since the early days of the predecessor AMANDA experiment,” said Dmitry Chirkin, an associate scientist at the University of Wisconsin-Madison. “For more than 20 years, we’ve been adding bits of discovery to our understanding of ice, including the disappearance of trapped air bubbles at depths well above the detector and that, at deeper depths, the South Pole ice sheet contains the cleanest ice on the planet Another discovery is the optical anisotropy of ice, which is the main topic of the study that was motivated by the new understanding in our paper.”

To improve on previous attempts to describe anisotropy, the collaborators looked closely at the effect of anisotropy and found a correlation between the depth development of ice crystal properties and the effect of anisotropy. This led the researchers to believe that the many small crystals that make up the ice were at play in the observed anisotropy.

“Things really got going when we realized that curved photon paths with small deviations of subdegrees per meter could accurately describe the data,” said Dr. Martin Rongen, a researcher at Johannes Gutenberg University Mainz ( JGU) and leader in analysis. “In fact, by calculating and simulating light diffusion through the polycrystalline ice present in the IceCube, where the crystals elongate on average along the direction of ice flow, a mean deviation emerges.”

For the study, the researchers ran simulations that modeled different paths that light could travel within the detector. They then compared the simulated data with a large set of calibration data taken from IceCube. The IceCube calibration dataset comprises data from 60,000 LEDs, equipped with all DOMs, emitting constant pulses of light into the ice, which are then used to calibrate the optical properties of the ice. From the comparison, the researchers were able to infer the average shape and size of the ice crystals inside the IceCube. This exciting new discovery drives the generation of new simulations and the adaptation of current reconstruction methods to account for the SpiceBFR model.

Not only will this new understanding help IceCube improve reconstructed neutrino interactions, it also has implications for the field of glaciology as a whole. “Ice crystal properties are studied in particular to understand ice flow mechanics, which can then be used to predict Antarctic mass balance and resulting sea level rise in a changing climate,” Rongen said.