Search for Cosmic-Ray Antiparticles with Balloon-borne and Space-borne Experiments

This thesis discusses two different approaches for the measurement of cosmic-ray antiparticles in the GeV to TeV energy range. The first part of this thesis discusses the prospects of antiparticle flux measurements with the proposed PEBS detector. The project allots long duration balloon flights at one of Earth’s poles at an altitude of 40 km. GEANT4 simulations were carried out which determine the atmospheric background and attenuation especially for antiparticles. The second part covers the AMS-02 experiment which will be installed in 2010 on the International Space Station at an altitude of about 400 km for about three years to measure cosmic rays without the influence of Earth’s atmosphere. The present work focuses on the anticoincidence counter system (ACC). The ACC is needed to reduce the trigger rate during periods of high fluxes and to reject external particles crossing the tracker from the side or particles resulting from interactions within the detector which would otherwise disturb the clean charge and momentum measurements. The last point is especially important for the measurement of antinuclei and antiparticles.

💡 Research Summary

This dissertation presents two complementary strategies for measuring cosmic‑ray antiparticles in the GeV–TeV range: a balloon‑borne experiment called PEBS and the space‑borne AMS‑02 spectrometer. The first part describes the design, mission profile, and performance expectations of PEBS, a detector suite that includes a transition‑radiation detector, time‑of‑flight system, silicon‑fiber tracker, and an electromagnetic calorimeter, all housed inside a 0.8 T superconducting magnet. PEBS is intended to fly from one of the Earth’s poles at an altitude of 40 km for a total of about 100 days, giving an acceptance of roughly 0.4 m² sr. Because the instrument operates inside the residual atmosphere, the author performed detailed GEANT4‑based PLANETOCOSMICS simulations to quantify how atmospheric nuclei, ionisation losses, and geomagnetic cutoff modify both primary and secondary particle fluxes. The simulations show that about 30 % of the traversed column density remains at 40 km, leading to significant attenuation of primary particles and, more importantly, to the production of secondary antiparticles (especially antiprotons) and to energy‑loss effects for positrons. By comparing simulated spectra with existing balloon data, the work derives altitude‑ and rigidity‑dependent correction factors that must be applied to recover the true top‑of‑atmosphere antiparticle fluxes. The analysis also evaluates the combined tracking‑calorimeter efficiency and energy resolution, demonstrating that PEBS can achieve a statistical precision sufficient to probe positron‑to‑electron ratios at the 10⁻⁴ level, provided the atmospheric corrections are applied.

The second part focuses on the Alpha Magnetic Spectrometer (AMS‑02), scheduled for a three‑year mission on the International Space Station starting in 2010. The detector architecture mirrors that of PEBS but adds a Ring‑Imaging Cherenkov counter and a high‑granularity electromagnetic calorimeter. The central element examined in detail is the Anticoincidence Counter (ACC), a cylindrical array of sixteen 8 mm‑thick plastic scintillator panels (diameter 1100 mm, height 830 mm) surrounding the silicon tracker. Scintillation light is wavelength‑shifted, transported via clear fibers, and read out by photomultiplier tubes (PMTs). The ACC must operate in a 0.8 T magnetic field, consume less than 800 mW, weigh under 54 kg, and survive launch vibrations and thermal cycling. The thesis documents the production, quality‑control, and space‑qualification of each component: uniformity tests of the scintillator panels, optimization of fiber‑to‑PMT coupling, magnetic‑field tests of the PMTs, and full‑system integration tests using test‑beam particles and atmospheric muons. Measured detection efficiencies exceed 99.99 % for charged particles, with inefficiencies confined to geometric gaps between panels and to occasional PMT gain fluctuations. The ACC reduces the trigger rate during high‑flux periods by more than an order of magnitude and, crucially, vetoes particles entering the tracker from the side or secondary particles generated inside the detector. This clean‑event selection is essential for the identification of rare antiparticles such as anti‑helium nuclei.

In the concluding comparison, the author emphasizes that PEBS and AMS‑02 are complementary: PEBS provides long‑duration exposure in the near‑atmosphere, requiring sophisticated atmospheric corrections, while AMS‑02 benefits from the vacuum of space, eliminating atmospheric uncertainties but demanding stringent background rejection via the ACC. Both experiments rely on a 0.8 T superconducting magnet to achieve precise momentum measurements and on high‑efficiency anticoincidence systems to suppress spurious signals. The work demonstrates that, with accurate atmospheric modeling and a well‑characterized ACC, the next generation of cosmic‑ray experiments can reach the sensitivity needed to test dark‑matter annihilation models, baryogenesis scenarios, and the possible existence of anti‑nuclei in the cosmos.