High-resolution, back-side illuminated monolithic active pixel sensor for low-energy electron imaging

A new domain of application of monolithic active pixel sensors, with respect to particle tracking, has been triggered by the innovative idea of a nondestructive beam monitoring system in the extraction lines of a hadron-therapy center. The beam monitoring exploits secondary electrons emitted from a submicrometer thick Al foil, intersecting the beam. Electrons are accelerated in an electrostatic field. The detection of low-energy electrons, up to 30 keV, is required. The sensitivity to these energies is obtained by thinning the detector, originally fabricated in a standard VLSI technology, down to the thickness of the radiation sensitive layer. A thin entrance window, in the order of 100 nm, is provided. Monolithic active pixel sensors for low-energy electron imaging can be prospectively used in several domains: in bioscience for cell process study using radiotracers, e.g., ³H(18 keVβ^-) or fluorescence imaging exploiting the Hybrid Photodiodes principle, in safety or environmental studies for neutron imaging with converters directly deposited or in micro-beam facilities for position resolving in studies of living cell irradiations. The low-energy electron imaging capabilities for installation inside an HPD test facility and the results obtained with a ³H marked source are shown. The detector used is the 1×10⁶ pixel MIMOSA V chip. The back-thinning up to the epitaxial layer was applied, resulting in a high resolution, back-side illuminated imager.