EXPERIMENTAL AND MONTE CARLO EVALUATION OF THE NEUTRON FLUX OF AN ASSEMBLY WITH TWO AmBe SOURCES Tufic Madi Filho 1,2, , Ruy Barros de Lima 1 , He ´lio Yoriyaz 1 and Antonio Carlos Hernandes 2 1 Instituto de Pesquisas Energe ´ticas e Nucleares, IPEN/CNEN-SP, Cidade Universita ´ria, Travessa R, 400 CEP 05508-900, S~ ao Paulo, SP, Brasil 2 Universidade Cruzeiro do Sul (UNICSUL), Av. Dr. Ussiel Cirilo, 225 CEP 08060-070, S~ ao Paolo (SP), Brasil This work aimed to determine the irradiator thermal (under-cadmium) and fast (over-cadmium) neutron fluxes, of the Nuclear Experimental Laboratory of the Nuclear Engineering Center (CNEN–IPEN, Sa ˜o Paulo, Brazil), and the possibility of this irradiator use for Neutron Activation Analysis (NAA), by the absolute method. To establish the facility specifications, the neutron flux values along the irradiator axis were determined experimentally and calculated by Monte Carlo method. The irradiator presents the advantage of supplying a stable neutron flux for a long period, eliminating the need to use standard material (comparative method), so that the process becomes agile, practical and economical. INTRODUCTION The Neutrons Activation Analysis (NAA) method is a highly sensitive non-destructive technique for the elemental composition determination in samples. It has been particularly useful in the simultaneous determination of elements in complex samples of sev- eral natures. It consists basically of submitting a sam- ple to a neutron flux, generated by reactors or of radioactive sources, then to accomplish gamma spec- trometry of the irradiated material identifying and quantifying the sample components. Some methods of NAA are used in many laboratories (1–4) . Among the several methods used are: (a) comparative method, (b) absolute method, (c) instrumental method (INAA) and (d) chemical method (RNAA), a descrip- tion of each of these methods is given in the IAEA- TECDOC-1215 (5) . In this work, the NAA absolute method was used for analysis of irradiated samples. In order to use the absolute method, an essential parameter is the neutron flux. For obtaining experi- mentally the neutron flux, the activation gold foil technique was used. The flux obtained using Monte Carlo method was simulated by MCNP-4B code. MATERIALS AND METHODS Neutron irradiator design Basically, this facility consists of a 0.5 cm thick, 120.0 cm long and 98.5 cm diameter aluminum cylinder (filled with paraffin), with two AmBe sources 592 GBq each. The cylinder has a central tube of 8.0 cm diameter in all its length and two perpendic- ular cylindrical cavities which cross the geometric centre, where the neutron sources are placed. The two neutron sources are positioned symmetrically and face to face, at the same distance from the geo- metric centre. In Figure 1 one photo of the irradiator is shown. Details about the axes configuration are shown in the Monte Carlo simulation, Figure 2. Figure 2. Modelled irradiator sources using MCNP.  Corresponding author: tmfilho@ipen.br Figure 1. Irradiator photo with details of neutron source position. Radiation Protection Dosimetry (2005), Vol. 115, No. 1–4, pp. 412–414 doi:10.1093/rpd/nci150 ª The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org