15 The set Au - Zr - Mo - Cr - Lu as multiple reactor flux monitor. The performance of 96 Zr and the analysis of inconsistencies Eduardo Montoya 1,* , Pablo Mendoza 1 , Patricia Bedregal 1 , Marco Ubillús 1 , Blanca Torres 1 , Isaac Cohen 2,3 1 Departamento de Química, Instituto Peruano de Energía Nuclear, Av. Canadá 1470, Lima, Perú 2 Universidad Tecnológica Nacional, Facultad Regional Avellaneda, Secretaría de Ciencia, Tecnología e Innovación Productiva. Av. Mitre 750 (1870) Avellaneda, Argentina 3 Universidad Tecnológica Nacional, Facultad Regional Buenos Aires, Departamento de Ingeniería Química. Avenida Medrano 951 (C1179AAQ) Buenos Aires, Argentina Resumen El uso simultáneo de Au, Zr, Mo, Cr y Lu, para la caracterización de facilidades de irradiación, con propósitos de análisis por activación neutrónica paramétrico, revela algunas inconsistencias sistemáticas en la determinación de los parámetros relevantes: flujos térmico y epitérmico, así como el exponente alfa (α), que describe el comportamiento no ideal del flujo neutrónico epitérmico. Se presenta un caso experimental que muestra que cuando los valores determinados son óptimos para la reacción 96 Zr(n,γ) 97 Zr, pueden ser inaceptablemente discrepantes para las reacciones 98 Mo(n,γ) 99 Mo y 50 Cr(n,γ) 51 Cr y viceversa. En las condiciones empleadas en este trabajo, se ha encontrado que el par de reacciones 96 Zr(n,γ) 97 Zr y 98 Mo(n,γ) 99 Mo no produce una solución única para los valores de los flujos neutrónicos térmico y epitérmico, independientemente de los valores que se asignen al parámetro α. Abstract The simultaneous use of Au, Zr, Mo, Cr and Lu, for the characterization of irradiation facilities with the purpose of parametric neutron activation analysis, reveals some systematic inconsistencies in the determination of the values of the relevant parameters: thermal and epithermal fluxes, as well as the exponent alpha (α), which describes the non ideal behaviour of the epithermal neutron flux. An experimental case is presented, showing that when the determined values are optimal for the reaction 96 Zr(n,γ) 97 Zr, can be unacceptably discrepant for the reactions 98 Mo(n,γ) 99 Mo and 50 Cr(n,γ) 51 Cr, and vice versa. Under the conditions of this work, it has been found that the pair of reactions 96 Zr(n,γ) 97 Zr and 98 Mo(n,γ) 99 Mo does not lead to a single solution for the values of the thermal and epithermal neutron fluxes, independently of the values given for the parameter α. 1. Introduction The use of the sets Zr - Au and Au – Cr – Mo as bare multi-isotopic neutron flux monitors, for the characterization of irradiation facilities, with purposes of parametric neutron activation analysis, is frequently mentioned in literature (see for example [1- 5]). It has been recently reported [5] that the sets Zr - Au and Au – Cr – Mo lead to considerably different values for the parameters f (thermal to epithermal flux ratio, φ t /φ e ) and α, an exponent which describes the departure of the epithermal neutron flux from the ideal behaviour. In order to study this reported inconsistency, a careful determination of the parameters f and α was performed in one of the irradiation facilities of the RP-10 nuclear research reactor at IPEN. 2. Experimental All the literature data were extracted from the following sources: atomic weights, reference [6]; isotopic abundances, thermal cross sections (σ 0 ), Q 0 = (resonance integral/cross- section ratios, I/σ 0 ), effective resonance energies, half lives, gamma intensities and k 0 values, reference [7]. The efficiency curve of the detector was initially determined at 210.7 mm, using standard sources of 133 Ba, 109 Cd, 57 Co, 137 Cs, 54 Mn, 65 Zn and 60 Co, from CANBERRA. Then, the activities of 165 Dy, 203 Hg, 51 Cr and 198 Au sources, prepared at the laboratory, were determined and used as secondary standards, together with the standard sources ____________ * Corresponding author: emontoya@ipen.gob.pe