Applied Radiation and Isotopes 60 (2004) 415–418 The absolute standardization of 32 P and 204 Tl at LNMRI Paulo A.L. da Cruz, Akira Iwahara, Estela M.O. Bernardes*, Carlos J. da Silva Laborat ! orio Nacional de Metrologia das Radia@ * oes Ionizantes, Instituto de Radioprote@ * ao e Dosimetria, Comiss * ao Nacional de Energia Nuclear, Av. Salvador Allende, s/n-Recreio, CEP Rio de Janeiro 22780-160, Brazil Abstract 32 Pand 204 Tl solutions were standardized within the frame of the international key comparisons organized by bureau international des poids et mesures, in 2002. The activity concentration of 32 P was measured by counting solid sources in a4pb proportional gas flow counter and by liquid scintillation counting. The self-absorption in solid sources for 4pb counting and the presence of 33 P as an impurity were evaluated. The combined standard uncertainty for 32 P was 0.59% inthe4pb counting and 0.38% in the liquid scintillation counting. Liquid scintillation counting was used to measure the activity concentration of 204 Tl with combined standard uncertainty of 0.35%. r 2004 Elsevier Ltd. All rights reserved. Keywords: Standardization; Intercomparison; 4p proportional counting; Liquid scintillation; CIEMAT/NIST method 1. Introduction The standardization of the solutions of 204 Tl and 32 P were carried out at the National Laboratory for Ionizing Radiation Metrology (LNMRI), Brazil within the frame of the international key comparisons organized by bureau international des poids et mesures (BIPM), in 2002. The radionuclides were measured by liquid scintillation counting. The activity of 204 Tl was deter- mined by the CIEMAT/NIST method (Garcia-Toran˜o and Grau-Malonda, 1981) while to determine the activities of 32 P and the impurities the liquid scintillation counts were used with an algorithm developed by LNHB, France. The 32 P also was measured by 4pb counting method using a gas flow 4p proportional counter. The 204 Tl is a pure beta emitting radionuclide, which decays to the ground state of 204 Pb (97.1%) via b  decay and to the ground state of 204 Hg (2.9%) via electron capture. The maximum b  energy is 763.7 (2)keV and the K X-ray energies from the electron capture decay range from 68.895 to 83.028keV (BNM-CEA/LNHB, 2002). The recommended half-life by BIPM was 138177d. In a recent international key comparison, organized in 1997 by BIPM, a large spread in the results of the activity concentration was verified (Ratel and Cassette, 1999). Discussion on the some possible factors, whichmighthavecausedthisspread,canbefoundinthe article of Hult et al. (2000). The 32 P is a pure b  emitting radionuclide, which decays to the ground state of 32 S (100%) via b  decay. The maximum b  energy is 1710.4(6)keV (BNM-CEA/ LNHB, 1984) with recommended half-life of 14.26270.014d(Endt, 1990). The presence of impurities of the pure b  emitters 33 P and 35 S with half-lives of 25.3470.12d (Endt, 1990) and 87.3270.16d (BNM– DTA/LNHB, 1999), creates difficulties with the stan- dardization of this radionuclide. 2. The samples The 204 Tl solution was distributed by Laboratoire National Henri Becquerel (LNHB), France in a glass ampoule numbered 10 whose chemical composition was 31 mgg 1 of TlCl in HCl 0.1M with 3.585g mass. Impurities were not stated. 32 P was distributed by Physikalisch-Technische Bundesanstalt (PTB), Germany in a glass ampoule numbered 2002-1663 in a solution ARTICLE IN PRESS *Corresponding author. Fax: 55-21-2442-1605. E-mail address: estela@ird.gov.br (E.M.O. Bernardes). 0969-8043/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2003.11.051