  Understanding the mechanism and symmetry of electron pairing in iron-based
superconductors represents an important challenge in condensed matter physics
[1-3]. The observation of magnetic flux lines - "vortices" - in a
superconductor can contribute to this issue, because the spatial variation of
magnetic field reflects the pairing. Unlike many other iron pnictides, our
KFe2As2 crystals have very weak vortex pinning, allowing
small-angle-neutron-scattering (SANS) observations of the intrinsic vortex
lattice (VL). We observe nearly isotropic hexagonal packing of vortices,
without VL-symmetry transitions up to high fields along the fourfold c-axis of
the crystals, indicating rather small anisotropy of the superconducting
properties around this axis. This rules out gap nodes parallel to the c-axis,
and thus d-wave and also anisotropic s-wave pairing [2, 3]. The strong
temperature-dependence of the intensity down to T<<Tc indicates either widely
different full gaps on different Fermi surface sheets, or nodal lines
perpendicular to the axis.
