  Precise knowledge of the location of the material in the LHCb vertex locator
(VELO) is essential to reducing background in searches for long-lived exotic
particles, and in identifying jets that originate from beauty and charm quarks.
Secondary interactions of hadrons produced in beam-gas collisions are used to
map the location of material in the VELO. Using this material map, along with
properties of a reconstructed secondary vertex and its constituent tracks, a
$p$-value can be assigned to the hypothesis that the secondary vertex
originates from a material interaction. A validation of this procedure is
presented using photon conversions to dimuons.
