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H*(10) Y Fluencias En Un Irradiador De Neutrones Con Una Fuente De Ra-Be

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H*(10) Y Fluencias En Un Irradiador De Neutrones Con Una Fuente De Ra-Be ISSSD 2020 ONLINE _________________________________________________________________________________ H*(10) y fluencias en un irradiador de neutrones con una fuente de 226Ra-Be Bedher O. Vega-Cabrera1,*, Héctor René Vega-Carrillo2, Víctor M. Viera Castillo1, César J. Guevara Pillaca1, Patrizia E., Pereyra Anaya1 María E. López Herrera1, Daniel F. Palacios Fernández1 1Pontificia Universidad Católica del Perú, Sección de Física. Av. Universitaria 1801, Apartado 1761, Lima – Perú. 2Universidad Autónoma de Zacatecas, Unidad Académica de Estudios Nucleares, C. Ciprés 10, Fracc. La Peñuela, 98068 Zacatecas, Zac. México. * E-mail: [email protected] Resumen Un irradiador de neutrones es un moderador con una fuente isotópica que es usado para enseñanza, entrenamiento y actividades de investigación. Normalmente, el moderador tiene puertos de irradiación radial y/o axial. Con el fin de utilizar el irradiador de neutrones de forma segura y óptima, deben conocerse los niveles de dosis y el espectro de la fluencia de neutrones. En este trabajo se utilizaron métodos Monte Carlo para estimar las fluencias de neutrones en tres rangos de energías: térmicos, epitérmicos y rápidos en siete puertos de un irradiador de neutrones con una fuente 226Ra-Be. El irradiador revestido de plomo contiene parafina wax como medio moderador de neutrones y sus puertos están asignados a diferentes distancias de la fuente de neutrones. El equivalente de dosis ambiental, debido a los neutrones, se estimó a 100 cm lateralmente y a 10 cm por encima del irradiador de neutrones. Palabras claves: Código MCNP 6.1.0; fluencia de neutrones; H*(10); 226Ra- Be. ______________________________________________________________________________ 93 Proccedings of the ISSSD 2020 Volume 2 ISSSD 2020 ONLINE _________________________________________________________________________________ 1.- INTRODUCCIÓN Desde el descubrimiento del neutrón en 1932 por James Chadwick, diversas fuentes de neutrones han sido desarrolladas y usadas hasta el día de hoy en investigación y en aplicaciones médicas e industriales [OIEA, 1997]. Un irradiador de neutrones (neutron howitzer en inglés) es considerado una fuente de neutrones, empleado en experimentos de laboratorios y universidades con el fin analizar la composición elemental de muestras (análisis por activación neutrónica), determinar el flujo de neutrones (espectrometría), estudiar la respuesta de detectores y de dosímetros personales (dosimetría de neutrones), entre otros [Cox et al., 1968, Thompson et al., 1961]. Un irradiador de neutrones está conformado mínimamente por una fuente isotópica emisoras de partículas alfas (241Am, 210Po, 239Pu, 226Ra) acopladas a elementos de bajo número atómico como el 9Be, garantizando la producción de reacciones (,n) [Barros et al., 2014, Ali et al., 2015, Holden et al., 2004]: La fuente de neutrones al tener alta actividad de radiación se encuentra dentro de un contenedor que contiene un medio moderador (H2O, C2H4 o parafina). Para caracterizar diferentes muestras y/o detectores, de acuerdo al objeto de la investigación, es necesaria la determinación de la distribución de energía y de flujo tanto radial como angular en cada puerto de irradiación [Didi et al., 2015, Asamoah et al., 2011, Zevallos-Chávez et al., 2005, AlTaani et al., 2017, Kotb et al., 2018]. La tabla 1, muestra diferentes tipos de irradiadores de neutrones empleados en diferentes instituciones en función a la geometría del contenedor, actividad de la fuente, número de puertos de irradiación y el medio moderador. ______________________________________________________________________________ 94 Proccedings of the ISSSD 2020 Volume 2 ISSSD 2020 ONLINE _________________________________________________________________________________ Tabla 1.- Tipos de irradiador de neutrones Investigador / Tipo de Fuente de N° Puertos Medio País geometría Neutrones de moderador irradiación Didi A., et al., (2015). Cilíndrica 226Ra-Be (1 fuente) 7 Parafina wax Marruecos (3 mCi = 111 MBq) Asamoah M., et al., Cilíndrica 241Am-Be (1 fuente) 2 Agua (2011). Ghana (20 Ci = 740 GBq) Marchese N., et al. Paralelepípedo 241Am-Be (4 fuentes) 1 Agua (2020). Italia (3 Ci = 111 GBq) Juan Yury Zevallos- Cilíndrica 241Am-Be (2 fuentes) 1 Polietileno Chávez, et al. (2005). (16,2 Ci 600 GBq) Brasil A. AlTaani, et al. Paralelepípedo 226Ra-Be (1 fuente) 5 Parafina (2003). Siria (3,5 mCi = 129,5 MBq) Kotb N. A., et al., Cubo 241Am-Be (6 fuentes) 6 Parafina wax (2018). Egipto (5 Ci =185 GBq) En especial, el isótopo 226Ra con vida media de 1600 años emite partículas alfas con energía de 4,77 MeV en su proceso de decaimiento radiactivo. De igual manera sucede con su progenie (222Rn, 218Po, 214Po y 210Po), emitiendo partículas alfas con energías de 5,5; 6,0; 7,69 y 5,3 MeV respectivamente. La característica principal de la fuente de neutrones 226Ra-Be se encuentra en su intensidad de emisión, debido a que se producen aproximadamente 502 ± 10% neutrones por segundo por cada 106 partículas alfas emitidas, a diferencia de las fuentes de 241Am-Be, 210Po-Be y 239Pu-Be cuyas tasas de emisión de neutrones representan aproximadamente la sexta, séptima y octava parte respectivamente, es decir (82,0 ± 9,8%; 73,0 ± 9,6%; y 65,0 ± 9,2%) [Vega-Carrillo et al., 2016]. El espectro de neutrones producidos por la fuente isotópica de 226Ra-Be proporciona un amplio rango energético con un valor promedio de 4,4 MeV, con una mayor contribución de neutrones aproximadamente desde 3,5 – 8 MeV. ______________________________________________________________________________ 95 Proccedings of the ISSSD 2020 Volume 2 ISSSD 2020 ONLINE _________________________________________________________________________________ El departamento de Física de la Universidad Nacional Mayor de San Marcos (UNMSM), Lima – Perú, cuenta desde hace muchos años con irradiador de neutrones con fuente isotópica de 226Ra-Be. Su uso ha sido limitado y no se ha evidenciado reporte alguno sobre los niveles de radiación tanto en su interior como alrededor del contenedor. Por tal razón, como una primera aproximación, a través del código de simulación MCNP6.1.0 se modeló un irradiador de neutrones con geometría cilíndrica lleno de parafina con siete puertos de irradiación. El objetivo fue estimar la fluencia de neutrones en cada puerto, tomando en cuenta tres rangos de energía: térmicos (< 0,5 eV), epitérmicos (0,5 eV - 10 keV) y rápidos (10 keV – 20 MeV) [Smilgys, 2013, Guan et al., 2016]. Debido a la distribución de los puertos de irradiación en el contenedor y por fines de protección radiológica, se estimó el equivalente de dosis ambiental H*(10) alrededor del irradiador. 2.- MATERIALES Y MÉTODOS 2.1.- Irradiador de neutrones 226Ra-Be Teniendo en cuenta las características geométricas, tipo de fuente, número de puertos de irradiación y material moderador, se verificó la existencia de un irradiador de neutrones con las mismas características en el laboratorio de Física Nuclear de la Universidad de Sidi Mohamed Ben Abdellah (USMBA), ciudad de Fez en Marruecos [Didi et al., 2015] y en el Instituto de Física de la Universidad de Miskolc, ciudad del mismo nombre en Hungría [Béla et al., 2020] (figura 1). El irradiador de neutrones ubicado en la Universidad Nacional de San Marcos consta de 7 puertos de irradiación de aluminio de 30 cm de longitud, 1,5 cm de radio interno y 1,6 cm de radio externo, alojados verticalmente dentro de un contenedor cilíndrico de 46,75 cm de longitud y 24,75 cm de radio, lleno de parafina wax de 0,93 g.cm-3 de densidad como medio moderador de neutrones. El contenedor está rodeado por plomo de 1 cm de espesor como material de blindaje frente a los rayos gammas generados en las interacciones de los ______________________________________________________________________________ 96 Proccedings of the ISSSD 2020 Volume 2 ISSSD 2020 ONLINE _________________________________________________________________________________ neutrones con los núcleos de los diferentes elementos existentes. La actividad de la fuente de 226Ra es de 20 mCi. (a) (b) (c) Figura 1.- Ubicación y comparación de los irradiadores de neutrones con fuente de 226Ra- Be, a). Universidad Nacional Mayor de San Marcos (Lima – Perú), b). Universidad de Sidi Mohamed Ben Abdellah (Fez – Marruecos) y c). Instituto de Física de la Universidad de Miskolc (Miskolc – Hungría). 2.2.- Distribución radial y angular de los puertos de irradiación Para conocer si los siete puertos de irradiación se distribuyen en posiciones angulares uniformes en un semicírculo dentro del contenedor de parafina wax, se empleó la herramienta semi-automatizada ―Web Plot Digitizer‖ de libre acceso, basada en la extracción de data desde imágenes (https://automeris.io/WebPlotDigitizer/index.html). Este procedimiento se llevó a cabo a través de la adquisición de una foto en posición axial (por encima del irradiador). Se ingresaron tres posiciones establecidas por el usuario en coordenadas polares. El origen, que representa la posición de la fuente de neutrones 226Ra- Be, la posición del puerto de irradiación N° 1 (P1) con coordenadas (7 cm; 0°) y la posición del puerto de irradiación N° 6 (P2) con coordenadas (14 cm; 90°). Posteriormente, el ______________________________________________________________________________ 97 Proccedings of the ISSSD 2020 Volume 2 ISSSD 2020 ONLINE _________________________________________________________________________________ programa facilitó la posición radial y angular (r, θ) de los otros puertos de irradiación. La figura 2 muestra el procedimiento indicado. (a)
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