  The upgraded Facility for Advanced Accelerator Experimental Tests (FACET-II)
at SLAC National Accelerator Laboratory has been designed to deliver
ultra-relativistic electron and positron beams with unprecedented parameters,
especially in terms of high peak current and low emittance. For most of the
foreseen experimental campaigns hosted at this facility, the high energy
radiation produced by these beams at the Interaction Point will be a valuable
diagnostic to assess the different physical processes under study. This article
describes the X-ray and {\gamma}-ray detectors installed for the initial phase
of FACET-II. Furthermore, experimental measurements obtained with these
detectors during the first commissioning and user runs are presented and
discussed, illustrating the working principles and potential applications of
these detectors.
