The construction of low mass vertex detectors is of high interest for next generation collider experiments like Belle II. Test beam experiments with multi GeV particle beams and high resolution tracking telescopes provide an opportunity to obtain precise 2D images of the radiation length X/X0 of thin planar targets like detector modules. The method devel- oped to measure the radiation length uses hits from the reference telescope and requires no readout of the detector module under study. At the heart of a spatially resolved X/X0 measurement is a precise reconstruction of the particle’s hit position and scattering angle at the target plane. The main challenges are the alignment of the reference telescope and the calibration of its angular resolution. System- atical uncertainties can be minimized by conducting a calibration measurement, where the module under study is replaced by an aluminium target with a well known thickness profile. In order to demonstrate the capabilities of X/X0 imaging, a test beam experiment with the AIDA telescope has been conducted at the DESY test beam facility. The devices under test were a mechanical prototype of a DEPFET pixel module and a SVD Origami silicon strip sensor for the Belle II vertex detector. An overall data sample of more than 200 million tracks at 4 GeV has been collected. The data is sufficient to resolve bump bonds below the readout ASICs and to measure the thickness profile of the all-silicon DEPFET module.
Ariane Frey (Georg-August-Universitaet Goettingen (DE)) Benjamin Schwenker (University of Goettingen) Carlos Marinas (University of Bonn) Florian Jochen Lutticke (Universitaet Bonn (DE)) Phillipp Wieduwilt (University of Goettingen)