Neutron field characteristics of Ciemat's Neutron Standards Laboratory

Design and testing of an irradiation room with low room-scattering for neutron calibration

INTRODUCTION

This laboratory plans to establish neutron reference radiation fields with three neutron sources to calibrate neutron-measuring devices. To perform calibration at multiple dose rates, neutron ambient dose equivalent rate H˙*(10) needs to range 1 μSv/h to 10 mSv/h. The lower limit requires that the maximum available calibration distance should be at least 4.5 m.

METHODS

To reduce room-scattered neutrons and extend the available calibration distance, MC simulations were conducted to determine the material and size of the irradiation room. A 3″ Bonner sphere and a LB6411 environmental neutron dosimeter were used to characterize the irradiation room.

RESULTS

A 14.32 × 14.32 × 12.00 m3 irradiation room was built based on simulation results. Floor, roof, and walls are made of 75 cm concrete covered by a coating layer of 2 cm BPE and 3 cm PE. Experimental maximum available calibration distance reaches 4.65 m. The range of H˙*(10) for calibration covers 1 μSv/h to 10 mSv/h. Neutron and photon H˙*(10) outside the room are within 0.19 μSv/h and 0.22 μSv/h, respectively.

Estimation with Monte Carlo of thermal neutrons in the FANT, using 252Cf and 241Am/9Be neutron source

The thermal neutron irradiation device (FANT), developed at the Neutron Measurements Laboratory of the Energy Engineering Department at Universidad Politécnica de Madrid, is a high-density polyethylene regular parallelepiped, with a rather uniform neutron fluence inside its irradiation chamber. It uses a Am95241/Be49 neutron source aiming to provide thermal neutron fluence rates. Neutron spectra and neutron fluences were estimated with Monte Carlo methods in the FANT irradiation chamber when a Cf98252 neutron source is used and were compared with the results obtained with the Am95241/Be49 source. Regardless of the neutron source, the largest contribution is due to thermal neutrons, producing also epithermal and fast neutrons. Per neutron emitted by the source, the use of Cf98252 produces a larger amount of neutrons.

Neutron spectrum unfolding based on generalized regression neural networks for neutron fluence and neutron ambient dose equivalent estimations

Neutron fluence and neutron ambient dose equivalent, H^*(10), are important physical quantities for neutron radiation protection and monitoring. They can be deduced from neutron spectrum, which is usually measured by multisphere system with proper unfolding methods. Novel unfolding methods on the basis of artificial intelligence, mainly artificial neural networks (ANNs), have been researched and developed. However, without normalization on network inputs, ANNs can not be applied to accommodate demands of various neutron field measurements for neutron spectrum unfolding in practice, because the neutron spectra for training the ANNs are mostly extracted from IAEA (2001), the integrals of which over neutron energy are unit fluences. Moreover, derived from an unfolded normalized spectrum, the true values of neutron fluence and H^*(10) are never to know. In this work, three normalization methods-zero-mean normalization method, min-max normalization method, and maximum-divided normalization method were used to process with the inputs of generalized regression neural networks (GRNNs), and a new method was proposed for neutron fluence and H^*(10) estimations derived from unfolded neutron spectrum based on GRNNs for the first time. Sixty-three neutron spectra were unfolded based on GRNNs with use of three normalization methods, and the corresponding neutron fluences and H^*(10) were obtained and compared. From the testing results, the GRNNs with the maximum-divided method is most effective to unfold neutron spectrum and to evaluate neutron fluence and H^*(10). The feasibility of the method was further studied through experiments by using Bonner sphere spectrometer in well characterized ²⁴¹Am-Be neutron field.

EXPERIMENTAL EVALUATION OF NEUTRON SHIELDING MATERIALS

A new proposed design of neutron shielding material-based on the commercial material Borotron UH050 with an addition of Al(OH)3-is evaluated in order to determine if its neutron and gamma shielding properties match those of a reference material, NS4FR. Neutron and gamma dosimetry measurements are performed, as well as neutron spectrometry measurements and Monte Carlo simulations. Negligible differences are found between the materials for neutron shielding, while significant differences are found for gamma shielding. The effect of Al(OH)3 addition to Borotron UH050 is to reduce neutron shielding properties while increasing gamma shielding properties. The resulting material is as efficient as NS4FR for neutron shielding but less efficient for gamma shielding-thicknesses 20% higher are required to match gamma shielding properties of NS4FR. Monte Carlo models of the materials are validated based on the performed measurements of neutron spectra and neutron and gamma ambient dose equivalent.

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 ~0.017cm 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 200cm. The MCNPX computations were compared with measurements to estimate the efficiency of ZnS(Ag) for detecting the α 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 2.5cps-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.