Determination of the mean GCR level. Panel A: measured (N84; Nishiizumi et al. 1984) and red (R75; Rancitelli et al. 1975) dots) and the best-fit GCR-induced activity of ²⁶Al in the Apollo 15 deep drill core. Datapoints used for the fit are in black circles. Since activity at depths shallower than 20 g cm⁻² can be affected by SEPs, shallow points were not used in the fit. The gray hatched area denotes the 68% confidence interval defined in panel B. Panel B: χ₂ statistics for the fit of the modeled ²⁶Al activity shown in Panel A as a function of the modulation potential ϕ. The best-fit value of ϕ = 496 MV is shown by the arrow, while the 68% (defined as ) and 95% () confidence intervals are denoted by the dotted lines.