Realization and characterization of a 220 Rn source for calibration purposes R. Buompane a,n , V. Roca b,c , C. Sabbarese a,c , F. De Cicco a , C. Mattone b , M. Pugliese b,c , M. Quarto b a Dipartimento di Matematica e Fisica, Seconda Università degli Studi di Napoli, Caserta, Italy b Dipartimento di Fisica, Università degli Studi di Napoli “Federico II”, Napoli, Italy c Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Italy HIGHLIGHTS  A thoron source realized by well characterized samples containing thorium.  Methodology for measuring exhalated thoron activity by gamma ray spectrometry.  Stable on time thoron specific activity concentration in air.  Radon isotopes spectroscopy by electrostatic collection. article info Available online 22 March 2013 Keywords: Thoron metrology Thoron chamber Thoron source Thoron in air abstract The recent interest for measuring 220 Rn activity in air and the following development of the corresponding measurement techniques require the improvement of standards for the calibration and characterization of the measurement devices. Due to the short half-life of the 220 Rn, the adopted techniques for the production of 222 Rn sources are not always reliable. In this paper a methodology for realizing a thoron known activity starting from a 232 Th source will be presented and discussed. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction The 220 Rn, usually called thoron, is a radon isotope with an half life of 55.8 s (Monographie BIPM-5, 2004). Its concentration in the domestic environment is normally much lower than that of the radon. Although the abundance of the natural 232 Th can be higher than that of 238 U, due to its short half life, the thoron air specific activity depends strongly on the distance from the origin point of the gas (Nikezic and Stevanovic, 2007) and often it becomes negligible. But, in some cases, the activity of thoron cannot be ignored (Tokonami, 2010): high levels of thoron activity can be found in weakly ventilated areas with rich in thorium sedimentary and granitic rocks, such as mines (Bigu, 1991), tunnels, cellars, etc.. In addition to the evaluation of the thoron activity for reasons connected to public health, an important role play the measure- ments of radon activity concentrations in soil gas, for the addi- tional information that they allow to obtain about the distribution of the long living isotope measured in a site of geologic interest (De Cicco et al., 2010, 2012). The measurement of thoron activity carried out using active or passive detectors requires the calibration of the instrument. While there exist well established methodologies for creating atmo- spheres of known activity of 222 Rn, for thoron these methodolo- gies are still in progress. The main problem in the realization and the management of standard atmosphere of thoron is the short half live of the isotope, which does not allow to employ the techniques which usually are adopted for radon. In this context it is important to develop and use calibration facilities (as thoron chamber) (Gargioni et al., 2003; Kobayashi et al., 2005; Yu et al., 2002; Röttger et al., 2009, 2010) and appropriate detection techniques suitable for the independent determination of radon and thoron (Roca et al., 2004a). The work here presented is focused on a methodology for the development and characterization of thoron sources for the realization of reference atmospheres. A similar approach, which uses the γ-spectrometry for the characterization of a 220 Rn source, was used by Röttger et al. (2010), but the present work proposes important differences. The first one is the evaluation of the source activity by difference between the 220 Rn activity of a sealed source and that one of the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes 0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.03.042 n Corresponding author. Tel.: +39 0823274814; fax: +39 0823274605. E-mail addresses: raffaele.buompane@unina2.it, rb.buompane@gmail.com (R. Buompane). Applied Radiation and Isotopes 81 (2013) 221–225