Applied Radiation and Isotopes 66 (2008) 886–889 Standardization of 241 Am, 124 Sb and 131 I by live-timed anti-coincidence counting with extending dead time Carlos J. da Silva a,b,Ã , A. Iwahara a , R. Poledna a , E.M. de Oliveira a , M.A.R.R. de Prinzio a , Jose´ U. Delgado a , Ricardo T. Lopes b a Laborato ´rio Nacional de Metrologia das Radiac - o˜es Ionizantes (LNMRI), Instituto de Radioprotec - a˜o e Dosimetria (IRD), Comissa˜o Nacional de Energia Nuclear (CNEN), Av. Salvador Allende, s/n-Recreio, CEP 22780-160 Rio de Janeiro, Brazil b Laborato ´rio de Instrumentac - a˜o Nuclear (LIN/PEN/COPPE/UFRJ), Caixa Postal 68590, CEP 21945-970 Rio de Janeiro, Brazil Abstract The National Metrology Laboratory for Ionizing Radiation (LNMRI)/Brazil has implemented a live-timed anti-coincidence system with extending dead time to complement the existing systems in its Radionuclide Laboratory for activity measurements of radioactive sources. In this new system, the proportional counter has been replaced by a liquid-scintillation-counter for alpha and beta detection. In order to test the performance of the new system, radioactive solutions of 131 I, 124 Sb and 241 Am have been standardized. In this work the measurement method, the results and the associated uncertainties are described and discussed. r 2008 Elsevier Ltd. All rights reserved. Keywords: Standardization; Anti-coincidence counting; Extending dead time 1. Introduction This work has been realized in order to test the performance of a new measurement system set up at the Radionuclide Group of the National Metrology Labora- tory for Ionizing Radiation (LNMRI). The anti-coinci- dence system has been used since 1973 for activity measurements of nuclides which present meta-stable levels (Bryant, 1967; Baerg et al., 1976; Santry et al., 1987) in the decay scheme, for complex decay schemes (Lu¨ etal., 2003) and also for tracing technique (Lucas, 1998). There is no basic difference between coincidence and anti-coincidence counting but in some cases the anti-coincidence counting presents advantages. Such a technique allows eliminating a series of corrections that must be taken into account when the technique of conventional coincidence counting is used such as dead time, resolution time and accidental coincidences and therefore, reducing significantly the uncertainty in the standardization. In the new system we used electronic modules developed especially by Labor- atoire National Henry Becquerel (LNHB)/France to work with the extending dead time and in the live-timed anti- coincidence counting method. 2. The 4p(a, b)-LS(c) anti-coincidence measurement system A schematic representation of the LNMRI 4p(a, b)- LS(g) anti-coincidence counting system is shown in Fig. 1. The counting cell is made of polyethylene painted inside with TiO 3 in order to reflect light. Two-photo multipliers based on a 3 00 Â 3 00 NaI(Tl) are incorporated in the gamma channel. Live-time measurements with an extending dead time device in the anti-coincidence system were used to eliminate dead-time corrections. Due to the difficulty in finding commercial electronic modules that work with extending dead time, Bouchard of the LNHB, developed an electronic module (MTR2) for the imposition of extending dead times (Bouchard, 2000), following concepts taken from the work of Gandy (1963). In the present experimental set-up of LNMRI two MTR2 modules are ARTICLE IN PRESS www.elsevier.com/locate/apradiso 0969-8043/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2008.02.029 Ã Corresponding author at: Laborato´rio Nacional de Metrologia das Radiac - o˜ es Ionizantes(LNMRI), Instituto de Radioprotec - a˜o e Dosimetria (IRD), Comissa˜o Nacional de Energia Nuclear (CNEN), Av. Salvador Allende, s/n-Recreio, CEP 22780-160 Rio de Janeiro, Brazil. Tel.: +55 21 2173 2875; fax: +55 21 442 1605. E-mail address: Carlos@ird.gov.br (C.J. da Silva).