  The focal plane of Athena's WFI consists of spectroscopic single photon X-ray
detectors that contain arrays of DEPFETs (DEpleted P-channel Field-Effect
Transistor) as well as ASICs that are used for steering, readout and analog
signal shaping. These components have to be examined regarding the effect of
ionizing radiation. A Total Ionizing Dose (TID) test was done with prototype
detector modules with 64x64 DEPFETs and one SWITCHER and VERITAS ASIC each. The
current design of the WFI detector head features a proton shield equivalent to
4 cm of aluminum in order to prevent a strong increase of leakage current in
the fully depleted 450 $\mu$m thick bulk of the sensor. This keeps the expected
doses and dose rates during the nominal mission relatively low ($\sim$5 Gy). It
is nevertheless important to study the current system in a dedicated TID test
in order to exclude unforeseen effects and to study any radiation related
changes that can have an effect on the very sensitive readout chain and the
detector performance. The combination of low doses, low dose rates, low
operating temperature (<-60{\deg}C) but high sensitivity on small changes of
the threshold voltages represent somehow unusual boundary conditions in
comparison to TID tests for standard radiation hard electronic components.
Under these circumstances it was found beneficial to do the test in our own
laboratory with an X-ray source in order to realize irradiation during nominal
operation conditions. Furthermore, it facilitated to take annealing effects
into account. Reasonably accurate dosimetry is achieved by measuring the X-ray
spectrum and intensity with the device under test. After irradiation to a total
dose of 14 Gy and subsequent annealing the threshold voltage of the DEPFETs
were shifted by a mean value of 80 mV, the performance remained unchanged apart
from a slight increase in readout noise by 10%.
