Measuring the ATLAS ITk pixel detector material via multiple scattering of positrons at the CERN PS

The ITk is a new silicon tracker for the ATLAS experiment designed to increase detector resolution, readout capacity, and radiation hardness, in preparation for the larger number of simultaneous proton–proton interactions at the High Luminosity LHC. This paper presents the first direct measurement of the material budget of an ATLAS ITk pixel module, performed at a testbeam at the CERN Proton Synchrotron via the multiple scattering of low energy positrons within the module volume. Using a four plane telescope of thin monolithic pixel detectors from the MALTA Collaboration, scattering datasets were recorded at a beam energy of 1.2GeV. Kink angle distributions were extracted from tracks derived with and without information from the ITk pixel module, and were fit to extract the RMS scattering angle, which was converted to a fractional radiation length x/X0. The average x/X0 across the module was measured as [0.89±0.01(resolution)±0.01(subtraction)±0.08(beam momentum band)]%, which agrees within uncertainties with an estimate of 0.88% derived from material component expectations.