Characterization of monolithic CMOS pixel sensor chip with ion beams for application in particle computed tomography

Abstract

Particle computed tomography (pCT) is an emerging imaging modality that promises to reduce rangeuncertainty in particle therapy. The Bergen pCT collaboration aims to develop a novel pCT prototype basedon the ALPIDE monolithic CMOS sensor. The planned prototype consist of two tracking planes forming arear tracker and Digital Tracking Calorimeter (DTC). The DTC will be made of a 41 layer ALPIDE-aluminumsandwich structure. To enable data acquisition at clinical particle rates, a large multiplicity of particles will bemeasured using the highly-granular ALPIDE sensor. In this work, a first characterization of the ALPIDE sensorperformance in ion beams is conducted. Particle hits in the ALPIDE sensor result in charge clusters whose sizeis related to the chip response and the particle energy deposit. Firstly, measurements in a 10 MeV4He microbeam have been conducted at the SIRIUS microprobe facility of ANSTO to investigate the dependence of thecluster size on the beam position over the ALPIDE pixel. Here, a variation in cluster size depending on theimpinging point of the beam was observed. Additional beam tests were conducted at the Heidelberg Ion-BeamTherapy Center (HIT) investigating the cluster size as a function of the deposited energy by protons and4Heions in the sensitive volume of the ALPIDE. Results show the expected increase in cluster sizes with depositedenergy and a clear difference in cluster sizes for protons and4He ions. As a conclusion, the variation in clustersize with the impinging point of the beam has to be accounted for to enable accurate energy loss reconstructionwith the ALPIDE. This does, however, not affect the tracking of particles through the final prototype, as forthat only the center-of-mass of the cluster is relevant.