[en] We report a measurement of the neutron detection efficiency for an array of neutron detectors behind a lead-steel wall consisting of 10.16 cm thick lead bricks sandwiched between 3.18 cm steel plates. The neutron detection efficiency was obtained from the ratio of coincidence to single-arm proton cross sections using the gamma+²₁H -->p+n reaction. Knowledge of the neutron detection efficiency was necessary to extract the cross section for the exclusive ²H(e,e'n)¹H reaction, which we used to obtain the elastic magnetic form factor Gⁿ_M of the neutron. The measurements for three different neutron energies were carried out at the MIT-Bates Linear Accelerator Center with an incident electron beam energy of 254 MeV