The stopping-power of mica for α-particles

Mica is commonly used in the laboratory for windows in counters and ionization chambers, and for absorption screens to determine the ranges of high-energy particles. For this reason, it is a matter of importance to have reliable values for the stopping-power of mica and for the variation of stopping-power with velocity. For computing the ranges of α-particles and protons, it has been customary to use a mean value, 1·43 mg/sq cm of mica as equivalent to 1 cm of air at 15° C and 760 mm pressure. The present experiments were undertaken in order to supply more accurate data. It has been found, however, that reliable values cannot be given because of the variation in mica from one sample to the next. Variations as great as 3% have been found for mica of the same kind (Green Madras). The best data have been plotted in fig. 3. If an error of 2% is not serious, this curve can be used: but for more accurate work, the stopping-power of the particular mica must be determined.