On paper, thermal neutrons seem ideal for probing the structure of crystallized biomolecules. At a few tenths of a nanometer, the de Broglie wavelength of thermal neutrons matches molecular bond lengths. And thermal neutrons interact strongly enough with matter to pick up structural information but weakly enough to pass through crystalline samples.
Another desirable property of thermal neutrons has to do with hydrogen. X rays, the most commonly used structural probe, interact with the atomic electric field; the more electrons an atom has, the stronger the interaction. With just one electron per atom, hydrogen, the most abundant element in biomolecules, is all but invisible in x-ray diffraction patterns.
Neutrons interact with nuclear spins. Though predictable, the strength of the interaction oscillates with apparent randomness across the periodic table. Hydrogen, it turns out, scatters neutrons with about the same strength as potassium.
Scattering strength also varies from isotope to isotope. Conveniently...