  The non-linear energy response of the plastic scintillator EJ-260 is measured
with the MicroCHANDLER detector, using neutron beams of energy 5 to 27 MeV at
the Triangle Universities Nuclear Laboratory. The first and second order Birks'
constants are extracted from the data, and found to be $k_B = (8.70 \pm
0.93)\times 10^{-3}\ {\rm g/cm^2/MeV}$ and $k_C = (1.42 \pm 1.00) \times
10^{-5}\ {\rm (g/cm^2/MeV)^2}$. This result covers a unique energy range that
is of direct relevance for fast neutron backgrounds in reactor inverse beta
decay detectors. These measurements will improve the energy non-linearity
modeling of plastic scintillator detectors. In particular, the updated energy
response model will lead to an improvement of fast neutron modeling for
detectors based on the CHANDLER reactor neutrino detector technology.
