Thoron exhalation rate measurement – findings from a large worldwide intercomparison study

A global intercomparison study was conducted to measure the thoron (220Rn) exhalation rate from two building materials, with participation from five European laboratories and three Asian laboratories. The test samples—phosphogypsum and unfired clay—were circulated among the laboratories using a sequential proficiency testing scheme. The assigned values and their uncertainties were determined through recommended robustness analysis. For comparison, the classical method, which uses the arithmetic mean of all participants' results, was also applied. Individual measurement results were evaluated for bias, precision, and proficiency in accordance with ISO 13528:2022.
The assigned exhalation rates were (0.39 ± 0.15) Bq m−2 s−1 for phosphogypsum and (0.53 ± 0.15) Bq m−2 s−1 for unfired clay. Z-scores were below 3 for seven of the nine methods used. Bias (Rb) and precision (P) parameters were within 50 %, except in one case. Laboratories provided details on Type A and Type B uncertainties, revealing that detector calibration uncertainty was the dominant factor in most cases.
These findings underscore the need for more robust calibration methods to improve the accuracy of thoron measurements. The development of a harmonised standard would greatly enhance the consistency of thoron exhalation rate measurements. Such a standard should provide guidance on detector calibration, as well as key factors such as climate conditions during sample preparation and testing, procedures for determining exhalation rates and their uncertainties, and considerations for material aging and spatial variations.