Calculates the absorption cross section of a small (sub-macroscopic) nonrotating black hole for massive scalar and Dirac particles in the low-energy (long-wavelength-compared-to-Schwarzschild-radius) limit, finding the Dirac cross section is exactly 1/8 the scalar one. This underlies the earliest, subatomic-scale phase of any stable-micro-BH growth calculation: before a hypothetical stable BH is large enough for classical Bondi accretion to apply, its capture rate for ambient particles is set by a quantum absorption cross section of this kind, not a classical gravitational cross section. relevance_note: Foundational physics for the “how fast can a genuinely microscopic stable BH first start growing” leg of the accretion-timescale argument, prior to the macroscopic-Bondi regime.