  Citation: Luca, A.; R ˘ adulescu, I.; Ioan, M.-R.; Fugaru, V.; Teodorescu, C.; Barna, C.; Tut , ˘ a, C.S.; Tugulan, C.L.; Antohe, A.; Lal ˘ au, I.; et al. Recent Progress in Radon Metrology at IFIN-HH, Romania. Atmosphere 2022, 13, 363. https://doi.org/ 10.3390/atmos13030363 Academic Editor: Cuco¸ s (Dinu) Alexandra Received: 26 January 2022 Accepted: 18 February 2022 Published: 22 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). atmosphere Article Recent Progress in Radon Metrology at IFIN-HH, Romania Aurelian Luca 1, *, Ileana Rădulescu 1 , Mihail-Răzvan Ioan 1 , Viorel Fugaru 1 , Constantin Teodorescu 1 , Cătălina Barna 1 ,Cătălin Stelian Tut , ă 1 , Cornel Liviu Tugulan 1 , Andrei Antohe 1 , Ioana Lalău 1,2 , Cătălina Cîmpeanu 1 and Cristian Postolache 1 1 Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH), 30 Reactorului St., P.O. Box MG-6, 077125 Măgurele, Romania; rileana@nipne.ro (I.R.); razvan.ioan@nipne.ro (M.-R.I.); vfugaru@nipne.ro (V.F.); constantin.teodorescu@nipne.ro (C.T.); catalina.barna@nipne.ro (C.B.); catalin.tuta@nipne.ro (C.S.T.); liviu.tugulan@nipne.ro (C.L.T.); antohe@nipne.ro (A.A.); ioana.lalau@nipne.ro (I.L.); catalinac@nipne.ro (C.C.); cristip@nipne.ro (C.P.) 2 Faculty of Physics, University of Bucharest, P.O. Box MG-11, 030018 Măgurele, Romania * Correspondence: aluca@nipne.ro; Tel.: +40-(0)21-404-6163 Abstract: The practical implementation of the European Council Directive no. 2013/59/EURATOM in Romania requires reliable indoor measurements of the radon ( 222 Rn) activity concentration in air. In Romania, several Testing Laboratories were designated for radon activity and/or radon activity concentration in air measurements by the Romanian National Commission for Nuclear Activities Control (CNCAN). The calibration of the instruments used for indoor radon activity concentration measurements is very important. IFIN-HH, through its Ionizing Radiation Metrology Laboratory (LMRI), performed advanced research in the field of radon metrology, using radon standard sources prepared by LMRI, its radon chamber facility and a new reference radon monitor. The most recent results are described in this article. The radon chamber facility from IFIN-HH was technically improved, and new equipment and methods were set up and tested in order to provide new calibration services for customers. Additionally, calibration of the radon monitors was performed, as well as of the systems with solid-state nuclear track detectors, used for radon in air activity concentration measurements. IFIN-HH/LMRI obtained the CNCAN designation as Calibration Laboratory for installations measuring the radon activity concentration in air. Keywords: radon ( 222 Rn); Ionizing Radiation Metrology; calibration method; activity concentration; radon monitor; solid-state nuclear track detector; uncertainty 1. Introduction The European Council Directive no. 2013/59/EURATOM [1] was implemented in the Romanian national legislation by Law no. 63/9.03.2018, which modified and completed Law no. 111/1996 regarding the safe deployment, regulation, authorization and control of nuclear activities. During the same year, Government Decision no. 526/12.07.2018 for the approval of the National Radon Action Plan (PNAR) was issued. More recently, three Or- ders of the President of the Romanian National Commission for Nuclear Activities Control (CNCAN) were issued: Order no. 185/22.07.2019 for the approval of the Methodology to determine the radon activity concentration in air inside buildings and at working places, Order no. 180/17.11.2020 for the approval of the Norms for personal dosimetry and radon, and Order no. 237/12.09.2019 for the approval of the Norms regarding the designation procedure for laboratories in the nuclear field. This complex legislation adopted by the European and national authorities from Romania (and other EU countries as well) reflects the significance of the risk of radon ( 222 Rn isotope) to public health [2]. The practical implementation of PNAR requires important efforts of several Romanian Government ministries, but also the involvement of many other institutions: local and central public authorities, academia (universities and national institutes for research and development), Atmosphere 2022, 13, 363. https://doi.org/10.3390/atmos13030363 https://www.mdpi.com/journal/atmosphere