Figure 3

CR-39 calibration curves for protons (gray), helium ions (red) and carbon (C⁶⁺, blue) from 0.1 MeV to 70 MeV ion energy, after 30 min of etching in 6.25 M NaOH at 70 ℃. Note the logarithmic abscissa. The symbols depict the most probable diameters and the error bars the ± σ-widths of the pit diameter distribution for each data point. Calibration data points obtained at the ion accelerator are plotted with squares, carbon results from the Thomson parabola ion spectrometer are plotted with circles, and He ion (alpha particle) data from a ²³⁹Pu source are plotted with diamonds. The dashed lines show the scaling of deposited energy within 2 µm of CR-39 using a stopping power model based on SRIM data. Since the stopping power model deviates towards both high and low energy limits of the data, we used an analytic fit (Eq. 2, continuous lines) for a better match. As a measure of the error of the fit, the shaded bands represent the variance (± 2.35σ) of the residuals between measured data and the fit. The fit parameters and variance values are given in Table 1.