external exposure facility Laboratory for retrospective Radon and Thoron dosimetry Paul Schofield D. Pressyanov et al., Journal of Environmental Radioactivity 166 (2017) 181-187 The infrastructure for retrospective radon and thoron dosimetry at Sofia University in Bulgaria, completed in 2015 within the DoReMi project framework, provides a Radon and Thoron Exposure Facility (RTEF) capable of exposures to ²²²Rn (1–2000 kBq/m³) and ²²⁰Rn concentrations under static or dynamic temperature regimes using a programmable thermostat. Its main purpose is to provide low-dose retrospective dosimetry for epidemiological studies where radon or thoron is the primary risk agent or a confounder, and it can also support detector response studies and measurement method development. The facility is available to external users following agreement. Located at Faculty of Physics, Sofia University.