Study of saturation of CR39 nuclear track detectors at high ion fluence and of associated artifact patterns

The occurrence of saturation in CR39 solid state nuclear track detectors has been systematically studied as a function of the incident ion (α particles and laser-accelerated protons) fluence and the etching time. When overexposed (i.e., for fluences above ∼ 108 particles cm2) and/or overetched, the CR39 detectors enter a saturated regime where direct track counting is not possible anymore. In this regime, optical measurements of saturated CR39 detectors become unreliable as well, since the optical response of the saturated detectors with respect to the ion fluence is highly nonlinear. This nonlinear optical response is likely due to scattering from the surface of irregular clumping patterns which have a diameter ∼20 μm, i.e., ten times larger than the diameter of individual tracks. These patterns, which aggregate many individual tracks, are observed to develop in highly saturated regimes. For fluences typical of high energy short pulse laser experiments, saturation occurs, inducing the appearance of artifact ringlike structures. By careful microscopic analysis, these artifact ring patterns can be distinguished from the genuine rings occurring below saturation and characteristic of low energy laser accelerated proton beams.