Femtolensing of Gamma-Ray Bursts as a Limit on Low-Mass Primordial Black Holes

Between the evaporation window and the microlensing window lies a mass range, roughly ten to the seventeenth to ten to the twentieth grams, where a primordial black hole is too heavy to have evaporated and too light to lens starlight measurably. Wave optics offers one handle on that range: a gamma-ray burst lensed by such an object does not split into resolvable images, but the two unresolved paths interfere, imprinting an oscillatory fringe pattern on the burst's energy spectrum. The headline computed values are: strongest stated f = 0.08; window g = 1e17-1e20. The work is computational throughout: assumptions are stated, the arithmetic is deposited, and no result is presented as a measurement.