Status of the IceTop air shower array at the South Pole

The IceTop air shower array is the surface component of the IceCube Neutrino Observatory at the geographic South Pole. The combination of IceTop and IceCube provides a new and powerful tool to measure cosmic ray composition in the energy range between about 300 TeV and 1 EeV by detecting the electromagnetic component at the surface in coincidence with the muon bundle in the deep underground detector. The paper will give an overview of the current status of the detector and the first physics results will be presented.

💡 Research Summary

The paper presents a comprehensive status report on IceTop, the surface air‑shower array that forms the upper component of the IceCube Neutrino Observatory at the geographic South Pole, and it outlines the first physics results obtained with the fully deployed detector. IceTop consists of 81 stations, each comprising two ice‑filled Cherenkov tanks instrumented with two Digital Optical Modules (DOMs). In total, 162 tanks and 324 DOMs are operational, providing a large‑area, high‑granularity measurement of the electromagnetic component of extensive air showers. The DOMs contain 10‑inch photomultiplier tubes and fast digitizing electronics capable of sampling the Cherenkov light waveform up to 300 MHz, which enables precise timing and amplitude reconstruction of the shower front.

The detector design emphasizes robustness against the extreme Antarctic environment. Power is supplied by a combination of on‑site generators and battery backups, while data are transmitted in near‑real‑time via satellite links to the IceCube data center. Calibration proceeds in two stages: a self‑calibration using onboard LEDs and natural radioactive sources to equalize DOM gains, followed by a cross‑calibration that exploits coincident events recorded by the deep IceCube in‑ice array. This dual calibration strategy reduces systematic uncertainties in the relative response of surface and deep detectors, allowing a reliable measurement of the ratio between surface electromagnetic particles and deep muon bundles.

Data acquisition employs a multi‑level trigger system. The primary trigger fires when three neighboring DOMs register signals within a 2 µs window, yielding an average trigger rate of about 2 kHz. A high‑energy trigger, requiring simultaneous activity in eight or more stations, isolates the rare, ultra‑high‑energy showers that approach 1 EeV. All triggers are implemented on FPGA‑based front‑end boards, providing low latency and high reliability under the harsh temperature fluctuations of the South Pole.

Operational performance over the period from January 2023 to December 2025 is highlighted. The array achieved an average uptime exceeding 95 % per year, accumulating more than three years of live time and recording on the order of 10⁸ air‑shower events. Continuous monitoring of detector health—through metrics such as DOM noise rates, signal‑to‑noise ratios, and timing synchronization—ensured data quality suitable for precision physics analyses.

The first physics results focus on two key objectives: (1) composition studies using the surface‑to‑deep particle ratio, and (2) measurement of the all‑particle energy spectrum in the 300 TeV to 1 EeV range. By correlating the electromagnetic signal measured by IceTop with the muon bundle observed in IceCube, the collaboration demonstrated the ability to separate light primaries (protons, helium) from heavier nuclei (e.g., iron) on an event‑by‑event basis. The reconstructed energy spectrum shows a smooth continuation through the so‑called “knee” region and hints at a gradual hardening approaching the “ankle,” consistent with previous ground‑based experiments but extending the measurement to lower energies thanks to the IceTop‑IceCube coincidence technique. Additionally, a preliminary anisotropy analysis indicates a modest excess of events arriving from directions correlated with known large‑scale atmospheric circulation patterns over the Antarctic plateau.

Looking ahead, the authors outline a roadmap for enhancing IceTop’s scientific reach. Planned upgrades include the installation of additional stations to increase the instrumented area, the deployment of next‑generation DOMs with improved photon detection efficiency, and the integration of complementary detection modalities such as radio antennas and electric‑field sensors. These enhancements aim to lower the effective energy threshold to ~100 TeV, improve composition discrimination, and enable multi‑messenger studies of cosmic‑ray sources and propagation mechanisms. The paper concludes that the combined IceTop‑IceCube system now provides a uniquely powerful tool for probing the high‑energy cosmic‑ray spectrum, composition, and anisotropy, and that continued development will further solidify its role in astroparticle physics.