Influence of secondary fragments on energy spectrum measurements using a CR-39 stack detector in laser-driven heavy ion acceleration

Recent advances in laser-driven ion acceleration have brought GeV-class carbon acceleration closer to realization. For high-energy ion measurements, we have used a stack detector, which consists of plastic nuclear track detectors, such as CR-39 and aluminum energy absorbers. In such experiments, etch pit analysis from a large number of microscope images is required. To handle these massive datasets efficiently, the etch pit analysis was automated with the aid of machine learning. However, as the ion energy increases, secondary fragments generated within the CR-39 stack detector make a non-negligible contribution, and their impact on the measured energy spectra remains unclear. In this study, secondary fragments produced by GeV-class carbon ions incident on a CR-39 stack detector were examined at a conventional accelerator. The results show that the ratio of secondary fragments to incident carbon increases with depth, reaching more than 20%. In addition, comparison of linear energy transfer distributions with a Monte Carlo nuclear particle transport simulation revealed the production of Li, Be, and B. These findings highlight the importance of accounting for secondary fragments in precise high-energy heavy-ion measurements using CR-39 stack detectors.