Nuclear Instruments and Methods in Physics Research A 568 (2006) 752–759 Absolute activity measurement of radon gas at IRA-METAS Philippe Spring a,Ã , Youcef Nedjadi a , Claude Bailat a , Gilles Triscone b , Franc - ois Bochud a a Institut Universiataire de Radiophysique Applique´e, Grand Pre´1, 1007 Lausanne, Switzerland b E ´ cole d’inge´nieurs de Gene`ve, rue de la Prairie 4, 1202 Geneva, Switzerland Received 12 June 2006; received in revised form 5 July 2006; accepted 25 July 2006 Available online 23 August 2006 Abstract This paper describes the system of the Swiss national metrological institute (IRA-METAS) for the absolute standardisation of radon gas. This method relies on condensing radon under vacuum conditions within a specified cold area using a cryogenerator, and detecting its alpha particles with an ion-implanted silicon detector, through a very accurately defined solid angle. The accuracy of this defined solid angle standardisation technique was corroborated by another primary measurement method involving 4pg NaI(Tl) integral counting and Monte Carlo efficiency calculations. The 222 Rn standard submitted by IRA-METAS to the Syste`me International de Re´fe´rence (SIR) at the Bureau International des Poids et Mesures (BIPM) has also been found to be consistent with an analogous standard submitted by the German national metrological institute (PTB). IRA-METAS is able to deliver radon standards, with activities ranging from a few kBq to 350 kBq, in NIST-Type ampoules, and glass or steel containers usable for calibrating radon-measuring instruments. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Wk; 29.30.Ep Keywords: Radon; Solid-angle particle counting; Primary standard; Traceability; 4pg counting 1. Introduction Radon is known to be the major component of natural irradiation of the population. Over 20 occupational epidemiological studies of radon-exposed underground miners unequivocally demonstrated that prolonged expo- sure to radon increases the risk of lung cancer [1]. Pooled studies of 65 000 miners found a linear relationship between radon exposure and lung cancer deaths [2,3]. This relationship was maintained even among a subgroup of miners that had lower exposures extending into the range for some homeowners [4]. These findings therefore suggest that residential radon also carries a risk of cancer. Improving radon metrology is then necessary for the traceability of secondary measurements of radon in air concentration. In spite of its important contribution to irradiation, only few intercomparisons of either radon standards or radon in air concentration have been carried out, and furthermore these have revealed a spread in the results of about 5% [5] to 40% [6]. Traceability chains for radon measurement reference systems in the national metrology institutes (NMI) are realised in two ways. In some cases, laboratories measure the gamma emitting daughters of 222 Rn gas traceable to 226 Ra Standard Reference Materials (SRMs) [6–9]. Alter- natively, measurements performed in some laboratories are traceable to radon primary standards in which radon itself is measured with an absolute method. To our knowledge, only two absolute measurement methods have been implemented. The first one, introduced by Piccolo [10], is based on the detection of alpha particles under an accurately measured solid angle. This system has also been implemented at the PTB [11] and at IRA-METAS. A second absolute method described in Ref. [12] involves a proportional counter of calculable 222 Rn efficiency and quantifiable active volume. This paper describes the system developed at IRA- METAS, the procedures to validate it and the first primary standardisation of 222 Rn submitted [13,14] to the Syste`me ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.07.055 Ã Corresponding author. E-mail address: philippe.spring@chuv.ch (P. Spring).