In 1964, Cribier and his colleagues experimentally confirmed, using elastic neutron scattering on niobium, that a triangular Abrikosov vortex lattice is formed in type II superconductors in a magnetic field B > Bc1. In the experiments, a first-order maximum was observed in the reflection of neutrons with a wavelength l = 5 A at an angle q = 20 relative to the incident beam from planes separated by a distance h (the height of the equilateral triangle of the structure). In what magnetic field was the experiment conducted?

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