  The electron drift in germanium detectors is modeled making many assumptions.
Confronted with data, these assumptions have to be revisited. The temperature
dependence of the drift of electrons was studied in detail for an n-type
segmented point-contact germanium detector. The detector was mounted in a
temperature controlled, electrically cooled cryostat. Surface events were
induced with collimated 81 keV photons from a $^{133}$Ba source. A detailed
analysis of the rise time of pulses collected in surface scans, performed at
different temperatures, is presented. The longitudinal anisotropy of the
electron drift decreases with rising temperature. A new approach, making use of
designated rise-time windows determined by simulations using
SolidStateDetectors.jl, was used to isolate the longitudinal drift of electrons
along different axes to quantify this observation. The measured temperature
dependence of the longitudinal drift velocities combined with the standard
electron drift model as widely used in relevant simulation packages results in
unphysical predictions. A first suggestion to modify the electron-drift model
is motivated and described. The results of a first implementation of the
modified model in SolidStateDetectors.jl are shown. They describe the data
reasonably well. A general review of the model and the standard input values
for mobilities is suggested.
