Measurement of the distribution of ^207Bi depositions on calibration sources for SuperNEMO

The SuperNEMO experiment will search for neutrinoless double-beta decay ($0νββ$), and study the Standard-Model double-beta decay process ($2νββ$). The SuperNEMO technology can measure the energy of each of the electrons produced in a double-beta ($ββ$) decay, and can reconstruct the topology of their individual tracks. The study of the double-beta decay spectrum requires very accurate energy calibration to be carried out periodically. The SuperNEMO Demonstrator Module will be calibrated using 42 calibration sources, each consisting of a droplet of ²⁰⁷Bi within a frame assembly. The quality of these sources, which depends upon the entire ²⁰⁷Bi droplet being contained within the frame, is key for correctly calibrating SuperNEMO's energy response. In this paper, we present a novel method for precisely measuring the exact geometry of the deposition of ²⁰⁷Bi droplets within the frames, using Timepix pixel detectors. We studied 49 different sources and selected 42 high-quality sources with the most central source positioning.