  A performance evaluation of superconducting transition-edge sensors (TESs) in
the environment of a pion beam line at a particle accelerator is presented.
Averaged across the 209 functioning sensors in the array, the achieved energy
resolution is 5.2 eV FWHM at Co $K_{\alpha}$ (6.9 keV) when the pion beam is
off and 7.3 eV at a beam rate of 1.45 MHz. Absolute energy uncertainty of
$\pm$0.04 eV is demonstrated for Fe $K_{\alpha}$ (6.4 keV) with in-situ energy
calibration obtained from other nearby known x-ray lines. To achieve this small
uncertainty, it is essential to consider the non-Gaussian energy response of
the TESs and thermal cross-talk pile-up effects due to charged-particle hits in
the silicon substrate of the TES array.
