  Electron-positron pairs, produced in intense laser-solid interactions, are
diagnosed using magnetic spectrometers with image plates, such as the National
Ignition Facility (NIF) Electron Positron Proton Spectrometers (EPPS). Although
modeling can help infer the quantitative value, the accuracy of the models
needs to be verified to ensure measurement quality. The dispersion of
low-energy electrons and positrons may be affected by fringe magnetic fields
near the entrance of the EPPS. We have calibrated the EPPS with six electron
beams from a Siemens Oncor linear accelerator (linac) ranging in energy from
$2.7$--$15.2$ $\mathrm{MeV}$ as they enter the spectrometer. A Geant4 TOPAS
Monte-Carlo simulation was set up to match depth dose curves and lateral
profiles measured in water at $100$ $\mathrm{cm}$ source-surface distance. An
accurate relationship was established between the bending magnet current
setting and the energy of the electron beam at the exit window. The simulations
and measurements were used to determine the energy distributions of the six
electron beams at the EPPS slit. Analysis of the scanned image plates together
with the determined energy distribution arriving in the spectrometer provide
improved dispersion curves for the EPPS.
