Material Measurement of an ATLAS Pixel Module via Multiple Scattering

In order to cope with the high intensity of proton collisions at the future High-Luminosity LHC, the ATLAS experiment is currently building a new all silicon inner tracker, the ITk. The five innermost layers of the ITk will consist of hybrid pixel detector modules, whose high granularity paired with a low material budget ensures excellent tracking and vertexing performance. Conventionally, the material budget of the modules is estimated from material content expectations for the components, and is only experimentally verified after commisioning in dedicated runs of the detector. In our talk we will present the first measurement of the radiation length of an ATLAS ITk pixel module via the multiple scattering of low energy electrons within the module. The measurement is performed with a tertiary electron beam of single-GeV-scale energy at the CERN Proton Synchrotron. A four-plane telescope with thin monolithic pixel detectors from the MALTA collaboration has been used to record datasets for several beam energies and telescope configurations. Kink angle distributions within the ITk pixel module are derived from broken-line tracks within the telescope, and a fit is used to extract the RMS scattering angle, which is then converted to a fractional radiation length via the inverse of the Highland formula for multiple scattering. The measurement results in a 2D map of the pixel module’s radiation length with O(10%) localized uncertainty and sub-mm resolution and is furthermore compared to the expectation from empirical estimates based on the module composition.