  This report presents results from a background measurement campaign for the
CODEX-b proposal undertaken in August, 2018. The data were recorded in the
DELPHI side of the LHCb cavern behind a 3.2~m concrete shield wall, during
Run~2 proton-proton collisions with the goal of calibrating the simulation for
the full CODEX-b detector. The maximum flux rate in the DELPHI side of the
cavern was found to be around 0.6~mHz/cm$^2$ across a vertical plane just
behind the shield wall, parallel to the beam line. A detailed simulation under
development within the LHCb {\tt Gauss} framework is described. This includes
shielding elements pertinent for CODEX-b's acceptance -- the LHCb detector, the
shield wall and cavern infrastructure. Additional flux from tracks not in the
line of sight from the interaction point, but bent by the magnetic fields, are
incorporated. Overall, the simulation overestimates the background flux
compared to the measurement. Several cross-checks and avenues for further
investigations are described.
