In situ estimation of ice crystal properties at the South Pole using LED calibration data from the IceCube Neutrino Observatory

Abstract

Abstract. The IceCube Neutrino Observatory instruments about 1 km3 of deep, glacial ice at the geographic South Pole. It uses 5160 photomultipliers to detect Cherenkov light emitted by charged relativistic particles. An unexpected light propagation effect observed by the experiment is an anisotropic attenuation, which is aligned with the local flow direction of the ice. We examine birefringent light propagation through the polycrystalline ice microstructure as a possible explanation for this effect. The predictions of a first-principles model developed for this purpose, in particular curved light trajectories resulting from asymmetric diffusion, provide a qualitatively good match to the main features of the data. This in turn allows us to deduce ice crystal properties. Since the wavelength of the detected light is short compared to the crystal size, these crystal properties include not only the crystal orientation fabric, but also the average crystal size and shape, as a function of depth. By adding small empirical corrections to this first-principles model, a quantitatively accurate description of the optical properties of the IceCube glacial ice is obtained. In this paper, we present the experimental signature of ice optical anisotropy observed in IceCube light-emitting diode (LED) calibration data, the theory and parameterization of the birefringence effect, the fitting procedures of these parameterizations to experimental data, and the inferred crystal properties.

Low energy event reconstruction in IceCube DeepCore

“…The parallel nature of photon propagation makes such a task well suited for graphics processing units (GPUs) [16,17]. Such an approach has allowed for a refined description of the optical properties of the ice by simulating LED calibration devices and fitting to the observed in situ calibration data [12,14]. Physics simulations of particle showers also rely on MC-based photon propagation.…”

“…The first is to accurately model the physics and instrumentation to the best extent possible within the full MC simulation chain. To a large extent this includes the modeling of the optical properties of the ice, where significant progress has been made over the past few years that led to an improved description of the observed ice anisotropy based on birefrigence [14] and a more realistic mapping of layer undulations [15]. Including these effects has resulted in much better agreement with in situ calibration data.…”

“…After cuts, the events range in energies from 10 TeV to 10 PeV, a regime where the astrophysical neutrino flux is expected to transition above the atmospheric neutrino background. The simulation relies on GPUs for photon propagation [17] and incorporates our most up-to-date knowledge of the ice, namely birefringence [14] and recent updates to ice layer undulations [15]. The left panel of Fig.…”

“…In reality, we now know that the ice is more complex and exhibits additional anisotropies. These include an axial dependence now attributed to birefrigence of the polycrystals in the ice, which leads to a higher photon yield along the axis of glacial ice flow [14]. Elevation changes of ice isochrons, or ice layer undulations, have also been mapped out in detail throughout the detector volume [15].…”

“…The left panel shows the number of events per bin, 𝑁, distributed over cos Θ s for the benchmark MC sample consisting of electron neutrino and antineutrino interactions contained within a fiducial volume [3]. The effects of birefrigence [14] and ice layer undulations [15] are included in the simulation, but are not included in either of the event reconstructions shown. A ringing effect is visible when the Mie model is used (gray), which disappears when the updates discussed in Section 3 are included (red).…”