In accordance with ISO/IC 17025:2017 testing laboratory shall have a procedure for monitoring the validity of results that includes regular participation in interlaboratory comparisons. Interlaboratory comparison RADON2021 was organized in the frame of the bilateral project between Serbia and Montenegro: “Measurement of the radon equilibrium factor in typical residential buildings in Serbia and Montenegro and harmonization of radon measurement techniques” in order to assess congruence of different measurement techniques, and then harmonize methods in case of inconsistencies. Harmonization is of particular importance to neighboring countries, in order to avoid inconsistencies in the classification of radon risk zones along border areas. Thus, six regional laboratories took part in the intercomparison – three from Belgrade and one from Novi Sad, Serbia; one from Podgorica, Montenegro; one from Banja Luka, Bosnia and Herzegovina. All laboratories participated in the indoor radon measurements, while 4 laboratories measured radon in the soil also. The measurement methods and detectors used by the laboratories were: • for air - EPA 402-R-92-004 – Indoor Radon and Radon Decay Product Measurement Devices Protocols, ch. 3.1. with RAD 7, RTM1688-2 and RadonEye; EPA 520/5-87-005 – 1987 EERF Standard Operating Procedures for Rn-222 Measurement Using Charcoal Canisters, ch. 3.4 using adsorption on charcoal canisters; ISO 11665- 4: Measurement of radioactivity in the environment – Air Radon 222: Part 4: Integrated measurement method for determining average activity concentration using passive sampling and delayed analysis with Electretes, and • for soil gas - ISO 11665-11:2016 Measurement of radioactivity in the environment — Air: radon-222 — Part 11: Test method for soil gas with sampling at depth by using active devices RTM1688-2 and RAD 7. The measurements were performed in real conditions where the radon concentration varied following a typical daily variation. Indoor radon concentrations were measured at four locations, while radon measurements in soil were performed at one location, all at the Vinča Institute site in the period from October 1 to October 8, 2021. The exposure period was consistent with a typical soil and indoor radon measurement protocols. The radon concentration in the soil was measured at depths of 20 cm and 80 cm. The robust mean and robust standard deviation were calculated as the mean of all measured radon concentrations (excluding outliers) and their standard deviation. Reference values of indoor radon covered the wide range of concentrations: (65 ± 9) Bq m-3 , (519 ± 80) Bq m-3 , (900 ± 160) Bq m-3 , (1980 ± 160) Bq m-3 , while in the soil reference values were (13350 ± 780) Bq m-3 and (26100 ± 2400) Bq m-3 at 20 cm and 80 cm depth, respectively. In order to assess whether there is a statistically significant difference between individual measurement results and the reference value, the Z - test was used as a criterion for success. If, for the calculated z value, |z| 3 result is not acceptable. All results of indoor radon measurements were satisfactory, except in the case of one measurement (for the lowest reference value) with electret, that was at the detection limit. Three laboratories had satisfactory results of radon concentration measurements in the soil, while the results of one laboratory were systematically lower, with z <-3, which indicated the need for additional analysis of the causes of systematic error and the possible need to change measurement protocols and/or re-calibration of used device.

X JUBILEE International Conference on Radiation in Various Fields of Research : RAD 2022 (Spring Edition) : book of abstracts; June 13-17, 2022; Herceg Novi, Montenegro