  The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope
is a pair-conversion telescope designed to survey the gamma-ray sky from 20 MeV
to several hundreds of GeV. In this energy band there are no astronomical
sources with sufficiently well known and sharp spectral features to allow an
absolute calibration of the LAT energy scale. However, the geomagnetic cutoff
in the cosmic ray electron-plus-positron (CRE) spectrum in low earth orbit does
provide such a spectral feature. The energy and spectral shape of this cutoff
can be calculated with the aid of a numerical code tracing charged particles in
the Earth's magnetic field. By comparing the cutoff value with that measured by
the LAT in different geomagnetic positions, we have obtained several
calibration points between ~6 and ~13 GeV with an estimated uncertainty of ~2%.
An energy calibration with such high accuracy reduces the systematic
uncertainty in LAT measurements of, for example, the spectral cutoff in the
emission from gamma ray pulsars.
