  CMOS Monolithic Active Pixel Sensors (MAPS) are considered as an emerging
technology in the field of charged particle tracking. They will be used in the
vertex detectors of experiments like STAR, CBM and ALICE and are considered for
the ILC and the tracker of ATLAS. In those applications, the sensors are
exposed to sizeable radiation doses.
  While the tolerance of MAPS to ionizing radiation and fast hadrons is well
known, the damage caused by low energy neutrons was not studied so far. Those
slow neutrons may initiate nuclear fission of $^{10}$B dopants found in the
B-doped silicon active medium of MAPS. This effect was expected to create an
unknown amount of radiation damage beyond the predictions of the NIEL (Non
Ionizing Energy Loss) model for pure silicon.
  We estimate the impact of this effect by calculating the additional NIEL
created by this fission. Moreover, we show first measured data for CMOS sensors
which were irradiated with cold neutrons. The empirical results contradict the
prediction of the updated NIEL model both, qualitatively and quantitatively:
The sensors irradiated with slow neutrons show an unexpected and strong
acceptor removal, which is not observed in sensors irradiated with MeV
neutrons.
