Estudio de la anisotropía de una fuente de Am-Be de 111 GBq

Se ha estudiado la anisotropía de una fuente de Am-Be de 111 GBq (3Ci) mediante el uso de un pequeño motor que permite girar paso a paso la fuente situada en su posición de irradiación habitual. Las medidas se han realizado con un contador proporcional de 3He alojado en el interior de una esfera moderadora de 8” correspondiente a un sistema de espectrometría de esferas Bonner. Se reportan los resultados obtenidos y el factor de anisotropía determinado para esta fuente

Characterization of an 241AmBe neutron irradiation facility by different spectrometric techniques

An automated panoramic irradiator with a 3 Ci 241Am-Be neutron source is installed in a bunker-type large room at the Universidad Politécnica de Madrid (UPM). It was recently modified and a neutron spectrometry campaign was organized to characterize the neutron fields in different measurement points along the irradiation bench. Four research groups working with different Bonner Sphere Spectrometers (BSS) and using different spectral unfolding codes took part to this exercise. INFN-LNF used a BSS formed by 9 spheres plus bare detector, with cylindrical, almost point like, 6LiI(Eu) scintillator (4 mm x 4 mm, from Ludlum); UAZ-UPM employed a similar system but with only 6 spheres plus bare detector; UAB worked with a 3He filled proportional counter at 8kPa filling pressure, cylindrical 9 mm x 10 mm (05NH1 from Eurisys) with 11 spheres configuration; and CIEMAT used 12 spheres with an spherical 3He SP9 counter (Centronic Ltd., UK) with very high sensitivity due to the large diameter (3.2 cm) and the filling pressure of the order of 228 kPa. Each group applied a different spectral unfolding method: INFN and UAB worked with FRUIT ver. 3.0 with their own response matrixes; UAZ-UPM used the BUNKIUT unfolding code with the response matrix UTA4 and CIEMAT employed the GRAVEL-MAXED-IQU package with their own response matrix. The paper shows the main results obtained in terms of neutron spectra at fixed distances from the source as well as total neutron fluence rate and ambient dose equivalent rate H*(10) determined from the spectra. The latter are compared with the readings of a common active survey-meter (LB 6411). The small differences in the results of the various groups are discussed

Caracterización y empleo de "conos de sombra" en un laboratorio para calibración neutrónica

En laboratorios de calibración neutrónica, si las diferencias entre los espectros neutrónicos “en el lugar de trabajo” y los utilizados en la instalación de calibración son muy acusadas, resulta muy complicado obtener factores de normalización adecuados, siendo conveniente tratar de producir campos neutrónicos “realistas”, es decir, cuyo espectro energético sea similar al existente en el lugar de trabajo, lo que permitiría la calibración directa de los instrumentos dosimétricos. Uno de los métodos utilizados, es el método de los “conos de sombra”. En este trabajo se presentan el diseño, caracterización y empleo de los conos de sombra del Laboratorio de medidas neutrónicas del Departamento de Ingeniería Nuclear de la ETSII-UPM LMN-UPM), empleándose una fuente de 241Am-Be

Performance of artificial neural networks and genetical evolved artificial neural networks unfolding techniques

With the Bonner spheres spectrometer neutron spectrum is obtained through an unfolding procedure. Monte Carlo methods, Regularization, Parametrization, Least-squares, and Maximum Entropy are some of the techniques utilized for unfolding. In the last decade methods based on Artificial Intelligence Technology have been used. Approaches based on Genetic Algorithms and Artificial Neural Networks have been developed in order to overcome the drawbacks of previous techniques. Nevertheless the advantages of Artificial Neural Networks still it has some drawbacks mainly in the design process of the network, vg the optimum selection of the architectural and learning ANN parameters. In recent years the use of hybrid technologies, combining Artificial Neural Networks and Genetic Algorithms, has been utilized to. In this work, several ANN topologies were trained and tested using Artificial Neural Networks and Genetically Evolved Artificial Neural Networks in the aim to unfold neutron spectra using the count rates of a Bonner sphere spectrometer. Here, a comparative study of both procedures has been carried out

Neutron spectrometry using artificial neural networks for a bonner sphere spectrometer with 3He detector

Neutron spectra unfolding and dose equivalent calculation are complicated tasks in radiation protection, are highly dependent of the neutron energy, and a precise knowledge on neutron spectrometry is essential for all dosimetry-related studies as well as many nuclear physics experiments. In previous works have been reported neutron spectrometry and dosimetry results, by using the ANN technology as alternative solution, starting from the count rates of a Bonner spheres system with a LiI(Eu) thermal neutrons detector, 7 polyethylene spheres and the UTA4 response matrix with 31 energy bins. In this work, an ANN was designed and optimized by using the RDANN methodology for the Bonner spheres system used at CIEMAT Spain, which is composed of a He neutron detector, 12 moderator spheres and a response matrix for 72 energy bins. For the ANN design process a neutrons spectra catalogue compiled by the IAEA was used. From this compilation, the neutrons spectra were converted from lethargy to energy spectra. Then, the resulting energy ?uence spectra were re-binned by using the MCNP code to the corresponding energy bins of the He response matrix before mentioned. With the response matrix and the re-binned spectra the counts rate of the Bonner spheres system were calculated and the resulting re-binned neutrons spectra and calculated counts rate were used as the ANN training data set

A new response matrix for a 6 LiI scintillator BSS system

A new response matrix was calculated for a Bonner Sphere Spectrometer (BSS) with a 6LiI(Eu) scintillator, using the Monte Carlo N-Particle radiation transport code MCNPX. Responses were calculated for 6 spheres and the bare detector, for energies varying from 1.059E(−9) MeV to 105.9 MeV, with 20 equal-log(E)-width bins per energy decade, totalizing 221 energy groups. A comparison was done among the responses obtained in this work and other published elsewhere, for the same detector model. The calculated response functions were inserted in the response input file of the MAXED code and used to unfold the total and direct neutron spectra generated by the 241Am-Be source of the Universidad Politécnica de Madrid (UPM). These spectra were compared with those obtained using the same unfolding code with the Mares and Schraube matrix response

Study of a 10B+ZnS(Ag) neutron detector as an alternative to 3He-based detectors in Homeland Security

The response of a scintillation neutron detector of ZnS(Ag) with 10B was calculated, using the MCNPX Monte Carlo Code. The detector consists of four panels of polymethyl methacrylate (PMMA) and five thin layers of ~0.017 cm thick 10B+ZnS(Ag) in contact with the PMMA. The response was calculated for the bare detector and with different thicknesses of High-Density Polyethylene, HDPE, moderator for 29 monoenergetic sources as well as 241AmBe and 252Cf neutrons sources. In these calculations, the reaction rate 10B(n, α)7Li and the neutron fluence in the sensitive area of the detector 10B+ZnS(Ag) was estimated. Measurements were made at the Neutron Measurements Laboratory, Universidad Politécnica de Madrid, LMN-UPM, to quantify the detections in counts per second in response to a 252Cf neutron source separated 200 cm. The MCNPX computations were compared with measurements to estimate the efficiency of ZnS(Ag) for detecting the ? that is created in the 10B(n, α)7Li reaction. After validating new models with different geometries it will be possible to improve the detector response trying to achieve a sensitivity of 2.5 cps-ng252Cf comparable with the response requirements for 3He detectors installed in the Radiation Portal Monitors, RPMs. This type of detector can be considered an alternative to the 3He detectors for detection of Special Nuclear Material, SNM

Performance of B-10 + ZnS(Ag) neutron detectors in RPM for the detection of special nuclear materials

In homeland security, neutron detection is used to prevent the smuggling of Special Nuclear Materials in the fight against nuclear terrorism. Thermal neutrons are normally detected with 3 He proportional counters surrounded by a polyethylene box. However, due to the 3 He shortage in the radiation portal monitors reported in 2009, new procedures are being studied. In this work, Monte Carlo methods (using the MCNP6 code) have been used to study the neutron detection features of a 10B + ZnS(Ag) under real conditions inside a radiation portal monitor. The performance of neutron detection was carried out for 252Cf, HEU 235U (enriched at 70% and 94%) and 239Pu (enriched at 93%) under two different conditions, bare and shielded with lead and polyethylene. To mimic an actual situation occurring at border areas, when a vehicle was passing through the detection zone, a sample of Special Nuclear Materials, SNM, sited inside a cargo vehicle mixed with scrap was simulated, and the radiation portal monitors contained 10B + ZnS(Ag) neutron detectors. The responses were calculated for four different neutron detectors, two real detectors (N-15 and N-48) and two prototype detectors (N-15 plus and N-48 plus), which are based on the N-15 and N-48 geometries with an improvement in the amount of 10B and a higher polymethyl methacrylate, PMMA, thickness. Measurements were reported for the actuals detectors, and the response was greater than 200 cm from a neutron source of 252Cf in c/s-ng of 252Cf. An array of three N-15 or one array of N-48 are close to detect 2.5 c/s-ng of 252Cf as required the Pacific Northwest National Laboratory, PNNL. The PNNL is the laboratory that tests almost every alternative detector. It requires that each detector must have an absolute efficiency greater than 2.5 c/s-ng of 252Cf for such a 252Cf source, located 200 cm perpendicular to the geometric midpoint of the neutron sensor, complementing the ANSI recommendations that the detector must be able to detect the pass of a neutron source of 252Cf of 20,000 n/s at a distance of 200 cm, to be considered an attractive alternative for the 3 He neutron detectors. The N-15 plus system reached more than 2.5 c/s-ng of 252Cf. Therefore, the 10B + ZnS(Ag) detectors are an innovative and viable replacement for the 3 He detectors in the radiation portal monitor