  The high-luminosity LHC (HL-LHC) upgrade is setting now a new challenge for
particle detector technologies. The increase in luminosity will produce a
particle background in the gas-based muon detectors that is ten times higher
than under conditions at the LHC. The detailed knowledge of the detector
performance in the presence of such a high background is crucial for an
optimized design and efficient operation after the HL-LHC upgrade. A precise
understanding of possible aging effects of detector materials and gases is of
extreme importance. To cope with these challenging requirements, a new Gamma
Irradiation Facility (GIF++) was designed and built at the CERN SPS North Area
as successor of the Gamma Irradiation Facility (GIF) during the Long Shutdown 1
(LS1) period. It features an intense source of 662 keV photons with adjustable
intensity, to simulate continuous background over large areas, and, combined
with a high energy muon beam, to measure detector performance in the presence
of the background. The new GIF++ facility has been operational since spring
2015. In addition to describing the facility and its infrastructure, the goal
of this work is to provide an extensive characterization of the GIF++ photon
field with different configurations of the absorption filters in both the
upstream and downstream irradiation areas. Moreover, the measured results are
benchmarked with Geant4 simulations to enhance the knowledge of the radiation
field. The absorbed dose in air in the facility may reach up to 2.2 Gy/h
directly in front of the irradiator. Of special interest is the low-energy
photon component that develops due to the multiple scattering of photons within
the irradiator and from the concrete walls of the bunker.

Instrumentation and Detectors;High Energy Physics - Experiment