Applied Radiation and Isotopes 66 (2008) 905–908 Optimization of a coincidence system using plastic scintillators in 4p geometry M.S. Dias à , H. Piuvezam-Filho, M.F. Koskinas Instituto de Pesquisas Energe´ticas e Nucleares (IPEN-CNEN/SP), Centro do Reator de Pesquisas—CRPq, C.P. 11049, Pinheiros, 05422-970 Sa˜o Paulo, SP, Brazil Abstract Improvements recently developed at the Nuclear Metrology Laboratory of IPEN-CNEN/SP in Sa˜o Paulo were performed in order to increase the detector efficiency of a 4pbÀg coincidence primary system using plastic scintillators in 4p geometry. Measurements were undertaken and compared to the original system and Monte Carlo simulations of the extrapolation curves were calculated for this new system and compared to experimental results. For this purpose, the code Penelope was applied for calculating response functions for each detector and the code Esquema, developed at LMN, was used for simulating the decay scheme processes. r 2008 Elsevier Ltd. All rights reserved. Keywords: Activity; Plastic scintillator; Cobalt 60; Monte Carlo; Coincidence system; Disintegration rate 1. Introduction A4p(PS)bÀg coincidence primary system using plastic scintillators in 4p geometry has been recently developed at the Nuclear Metrology Laboratory (LMN) of IPEN- CNEN/SP in Sa˜o Paulo (Baccarelli, 2003; Baccarelli et al., 2003) and the nuclides 241 Am, 60 Co and 133 Ba have been previously standardized with this system. Although good results have been obtained by comparison to conventional 4pbÀg coincidence using a proportional counter (PC) as the 4p detector, the beta efficiency was low (60% for 60 Co) and some remedial modifications were undertaken. Modeling by Monte Carlo was introduced in order to predict the extrapolation curve. For this purpose, the code Esquema (Takeda et al., 2005; Dias et al., 2006) was modified to include response tables suitable for this new detector geometry. One factor that becomes important in the simulation is the beta-gamma efficiency, because of the solid nature of the beta detector. For PCs this effect is quite small (o1%) at low gas pressures. This factor was incorporated in the present simulation. 2. Experimental The 4p detector is a cylinder having a hole inside for introducing the radioactive source which is placed on a Collodion film held by a stainless steel or plastic ring, as shown in (Baccarelli, 2003). In this original coincidence system, the plastic scintillator is covered with Teflon on all sides except the surface coupled to the photocathode, in order to improve the light collection efficiency by diffuse reflection. This system has only one photomultiplier tube and the light produced at the scintillator side opposite to the photocathode loses more than 50% of the original light yield before it reaches the photocathode. As a result, the pulse height spectrum consists of two parts: one stemming from the scintillator side close to the phototube and the other stemming from the opposite side of the scintillator, as shown in Fig. 1. This asymmetry was removed by including an additional photomultiplier tube coupled to the plastic scintillator in a sandwich geometry. In this new geometry, the NaI crystal was placed orthogonally to the phototube axis, as close as possible to the source, as shown in Fig. 2. Two sets of electronics were tried. In the first set, the signals from both phototubes were summed with a sum-invert amplifier and 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.2008.02.033 à Corresponding author. Tel.: +55 11 3816 9176/9188. E-mail address: msdias@ipen.br (M.S. Dias).