A system for positron beam-based Doppler broadening spectroscopy of the formation and evolution of monovacancy defects in silicon is described. The apparatus allows in situ ion implantation at low temperatures (similar to 50 K) followed by positron beam assay. First measurements for 6 keV He implantation, at post-implant temperatures between 60 and 300 K are presented. Benefits and drawbacks of this system are discussed. (c) 2005 Elsevier B.V. All rights reserved.