The calibration of Bonner sphere spectrometer

Bonner sphere spectrometer (BSS) is used in radiation protection measurement because of its wide energy range (thermal to MeV) and easy operation. Mitsubishi Heavy Industries (MHI) has used BSS to obtain neutron spectrum and has used the neutron spectrum to estimate neutron dose or induced activity. Calibration of BSS is important to estimate precise neutron dose or induced activity. MHI BSS was calibrated at National Institute of Advanced Industrial Science and Technology (AIST). The calibration results at AIST are in good agreement with calculation results.     

Response matrix of an extended Bonner sphere system

We have developed a system of Bonner spheres designed for use around high-energy accelerators. The upper energy limit of the system was extended using a lead radiator, which acts as an energy converter via the (n,xn) reaction. In addition, we use ¹¹C activation as an additional component integrated into the system and the spectra unfolding process. In the first version of the system, the lead radiator was present in only one sphere with diameter of 30.48 cm. The object of the present work was to investigate the geometry of the lead radiator and its use in moderators of several different sizes. As a result, we have developed a modular design and calculated the response matrix of the new system.     

Measurement and unfolding of neutron spectra using Bonner spheres

Neutron spectra from Am-Be, 252Cf sources and 2H + 2H and 2H + 3H reactions have been measured using a Bonner sphere system in conjunction with a 4 x 4 mm2 LiI(Eu) detector, and unfolded using the two codes BUNKI and MAXED. The BON unfolding algorithm is used with BUNKI. It has been observed that end test percentage between 1 and 3 and smoothing factor between 0.05 and 0.1 are optimal choices for the BUNKI code. A temperature parameter 1.0 is used for MAXED. Comparison with standard spectra shows that the shape of the spectra is fairly well reproduced. A coupling between the two codes is made and the solution spectrum from BUNKI is used as the default spectrum in MAXED.     

Performance of the UAB and the INFN-LNF Bonner sphere spectrometers in quasi monoenergetic neutron fields

In the framework of collaboration between the Universidad Autónoma de Barcelona and the INFN Frascati National Laboratories, an experimental Bonner Sphere neutron spectrometry exercise has been performed in the 2.5 MeV and 14.2 MeV quasi monoenergetic neutron beams of the ENEA Fast Neutron Generator. The neutron spectra at given distances from the accelerator target have been determined, taking advantage of the new unfolding FRUIT code, recently developed by the LNF group. The results show a good coherence between the two spectrometers, and between the measured and simulated data.     

Additional experimental tests of the Bonner sphere neutron spectrometer

In a continued effort to experimentally test Bonner sphere spectrometer systems, the results reported in this paper include: (1) a new partial illumination correction factor which has been determined to correct an error in the previous work, (2) a new spectral unfolding code, BONABS, which has been included in these tests (in addition to the previously used SWIFT code) and (3) scandium and uranium filtered beams which have been measured. The results indicate a small improvement in energy prediction capabilities due to the corrected partial illumination factor and reasonable agreement between the SWIFT and BONABS unfolding codes for the spectra tested.     

Characterization of Bonner sphere systems at monoenergetic and thermal neutron fields

The Institute for Radiological Protection and Nuclear Safety (IRSN) and the GFR, Universitat Autónoma de Barcelona (UAB) use Bonner spheres (BS) for neutron spectrometry at workplaces. The two systems, equipped with similar cylindrical 3He proportional counters, were simulated with the MCNP Monte-Carlo code to determine the response to neutrons of different energies for each polyethylene sphere. The BS systems were characterized at monoenergetic and thermal neutron fields. Measurements were performed at the Physikalisch-Technische Bundesanstalt (PTB) and at the National Physical Laboratory (NPL) standard laboratories, and with the newly characterized IRSN 'SIGMA' thermal neutron facility. The energy distribution of the reference neutron fluence was folded with the response functions for comparison purposes with the experimental data. In almost all cases related to monoenergetic neutrons, a good agreement between the experimental and the calculated count rates was found, and some discrepancies of a few per cent were observed in the thermal region.     

Propagation of uncertainties in unfolding procedures

A review of methods currently used to unfold particle spectra from measured pulse height distributions or other detector readings is given. It is pointed out that most of the measurements in particle spectrometry reveal ill-conditioned or ill-posed problems. The presentation which is given here for examples of such inverse problems is focussed on the algorithms used in the HEPRO unfolding program package of PTB. The question of uncertainty propagation is discussed for least-squares algorithms as well as for those based on maximum entropy. A first attempt has been made to quantify generally the “ambiguity” in ill-posed unfolding problems. The maximum entropy algorithm realized in the MIEKE code allows a clear distinction to be made between two parts of uncertainty, one part coming from ambiguity and one part coming from the usual uncertainty propagation. The resulting uncertainty matrix of the MIEKE code provides these two parts.     

Monte Carlo calculations and validation of a gold foil-based Bonner sphere system

The Grup de Física de les Radiacions (GFR) of the Universitat Autònoma de Barcelona (UAB), in collaboration with the Institute for Radiological Protection and Nuclear Safety (IRSN), has developed a passive Bonner sphere system (UAB-BSS), with gold foils as thermal neutron detectors, for application in pulsed neutron fields or in mixed neutron-photon fields with high photon intensities. In such fields, active devices suffer from saturation and dead-time effects. The MCNPX Monte-Carlo code has been used to determine the response to neutrons of different energies of each polyethylene sphere belonging to the BSS. The passive UAB-BSS system was characterised with the ISO (252)Cf reference source at the IRSN facilities. The energy distribution of the reference source neutron fluence was folded with the response functions for comparison with the experimental data. A good agreement between the experimental and calculated count rates was found.     

Application of Neural Networks for unfolding neutron spectra measured by means of Bonner Spheres

A Neural Network structure has been used for unfolding neutron spectra measured by means of a Bonner Sphere Spectrometer set. The present work used the “Stuttgart Neural Network Simulator” as the interface for designing, training and validation of a MultiLayer Perceptron network. The back-propagation algorithm was applied. The Bonner Sphere set chosen has been calibrated at the National Physical Laboratory, United Kingdom, and uses gold activation foils as thermal neutron detectors. The neutron energy covered by the response functions goes from 0.0001 eV to 10 MeV. Two types of neutron spectra were numerically investigated: monoenergetic and continuous. Good results were obtained, indicating that the Neural Network can be considered an interesting alternative among the neutron spectrum unfolding methodologies.     

Characterisation of the IPNE Bonner sphere spectrometer by comparison with the PTB system

An existing set of Bonner spheres of the Institute for Physics and Nuclear Engineering in Bucharest has been modified following the PTB design for application of an ³He-filled SP9 counter. By simple interpolations of the fluence responses of the PTB spheres, a preliminary fluence response matrix could be established for the IPNE Bonner sphere set. For further adjustments of the preliminary responses and for validation of the final fluence response matrix of the Romanian Bonner sphere spectrometer, calibration measurements with a reference ²⁵²Cf neutron source and joint measurements, including the PTB Bonner sphere spectrometer, were performed in a few neutron fields differing substantially in their spectral distributions. It is estimated that the integral neutron fluences and dose-equivalent values can now be determined with the Romanian spectrometer with uncertainties of about ±4% and ±8%, respectively.     

Characterization and utilization of a Bonner sphere set based on gold activation foils

Bonner sphere (BS) sets which use activation foils as the central thermal neutron sensor have advantages over active BS systems in certain environments, for example, pulsed fields, or fields with high photon components. In such environments, they may be the only type of neutron spectrometer which can be used. This paper describes work, using both measurements and calculations, to validate the response functions for a BS set based on gold activation foils. As an illustration of the use of such a system, a measurement is described of the contaminant neutron spectrum in the treatment room of a 21 MV hospital linear accelerator providing photon beams for radiotherapy.     

Bonner sphere neutron spectrometry at nuclear workplaces in the framework of the EVIDOS project

The Institute for Radiological protection and Nuclear Safety was engaged in the EC funded EVIDOS project to provide reference spectrometry data using its Bonner sphere system. The data were processed by means of two unfolding codes, NUBAY and GRAVEL, both provided by the Physikalisch-Technische Bundesanstalt. The NUBAY program, based on Bayesian parameter estimation methods, assumes a parameterised spectrum and provides posterior probability distributions for the parameters. The code GRAVEL, an iterative algorithm based on SAND-II, was used with various default spectra, among them the NUBAY solution. The BS measurements were used to establish the neutron fluence energy distributions and reference values for the neutron ambient dose equivalent. As this quantity depends strongly on the high energy neutrons, a sensitivity analysis was done by unfolding the BS data with GRAVEL using the NUBAY solution spectrum as default with various changes in the parameters of the high energy peak. This new method of analysing Bonner sphere data allowed the determination of reliable neutron spectra, as well as a very good estimate of the corresponding integral quantities with small associated uncertainties.     

Sensitivity study of CR39 track detector in an extended range Bonner sphere spectrometer

Bonner sphere spectrometers (BSS) are being used widely in neutron spectrometry since 1960. The response to neutrons of these moderating detectors spans over a broad energy range. This work discusses the measurement of the sensitivity of an extended range BSS hosting a CR39 nuclear track detector coupled to a boron converter. Two kinds of boron converters were tested, the first made by natural boron and a second one enriched in (10)B. The BSS response functions were calculated with Monte Carlo simulation using the MCNPX 2.4.0. and the FLUKA codes.     

Characterisation of a gold foil-based Bonner sphere set and measurements of neutron spectra at a medical accelerator

The characteristics of a Bonner sphere set with gold foils as the thermal neutron sensor are described. To illustrate the application of this neutron spectrometer in a pulsed field with an intense photon component, the results of measurements at a hospital electron accelerator are presented.     

Performance of different vector potential formulations in solvingmultiply connected 3-D eddy current problems

The authors describe their numerical experiences in applying FEM (finite-element method) solution techniques to a 3-D (three-dimensional) eddy-current problem with a coil-driven multiply connected conductor, the benchmark problem No.7 of the International TEAM Workshops. Several formulations have been tried using a magnetic vector and electric scalar potential or an electric vector and a magnetic scalar in the conductor and a magnetic vector or scalar potential outside. The problem has been solved at two frequencies. The authors briefly describe the formulations used and compare the performance. Magnetic field and current density plots are also compared. The advantages and disadvantages of the various versions are pointed out. The use of a magnetic scalar potential H rather than a magnetic vector potential A outside the conductor and the hole substantially reduces the number of degrees of freedom and thus the computational effort. The versions using it in the conductor yield relatively ill-conditioned systems. Also, at the higher frequency, the conditioning deteriorates considerably     

Neutron field measurements of the CRNA OB26 irradiator using a Bonner sphere spectrometer for radiation protection purposes

The present work deals with the Bonner sphere spectrometer (BSS) measurements performed, to support the authors' Monte-Carlo calculations, to estimate accurately the main characteristics of the neutron field of the (241)Am-Be-based OB26 irradiator acquired for radiation protection purposes by the Nuclear Research Centre of Algiers. The measurements were performed at a reference irradiation position selected at 150 cm from the geometrical centre of the neutron source. The spectrometric system in use is based on a central spherical (3)He thermal neutron proportional counter. The response matrix of the present spectrometer has been taken to be similar to the original Physikalisch-Technische Bundesanstalt (PTB) (Braunschweig, Germany) BSS's response matrix, with a five bins per decade energy group structure, as there is no significant difference in the BSS's physical characteristics. Thereafter, the authors' BSS measurements were used together with MCNP5 results to unfold the neutron spectrum by means of MAXED and GRAVEL computer codes from the U.M.G. 3.3 package, developed at PTB. Besides, sensitivity analysis has been performed to test the consistency of the unfolding procedure. It reveals that no significant discrepancy was observed in the total neutron fluence and total ambient dose values following the perturbation of some pertinent unfolding parameters except for the case where a 10 bins energy structure was assumed for the guess spectrum. In this latter case, a 5 % difference was observed in the ambient dose equivalent compared with the reference case. Finally, a comparative study performed between different counting systems together with MCNP5 and predictive formulas results shows that they were globally satisfactory, highlighting thereby the relevance of the unfolding procedure and the reliability of the obtained results.     

Performance aspects of mains connected small-scale wind turbines

Small-scale wind turbines are seen as potentially playing an important part in the future UK electricity generation mix. As such it is important that the necessary turbine and generator systems are well matched in order to maximise power output and energy capture. A turbulent wind model is described that is suitable for use with small-scale wind turbines. The model is used along with models of the turbine, generator and power conversion system to study potential problems that can exist with incorrect turbine design. The wind model is verified by comparison with measured data while the generator and power converter model are verified by laboratory test. A small 2.5 kW, `H' bladed vertical axis wind turbine is then studied to examine the effect that different blade sections may have in turbulent winds. A symmetrical blade section is seen to cause potential stall problems while these are avoided by the use of a cambered blade section     

New aspects in the unfolding of the nilpotent singularity of codimension three

This paper is concerned with three-dimensional vector fields and more specifically with the study of dynamics in unfoldings of the nilpotent singularity of codimension three. The ultimate goal is to understand the dynamics and bifurcations in the unfolding of the singularity. However, it is clear from the literature that the bifurcation diagram is very complicated and a complete study is far beyond the current possibilities, not only from a theoretical point of view but also from a numerical point of view, despite recent developments of computational methods for dynamical systems. Since all complicated dynamical behaviour is known to be of small amplitude, shrinking to the singularity for parameter values tending to the bifurcation parameter, the aim in this paper is especially to focus on a different aspect that might be interesting in the study of global bifurcation problems in the presence of such a nilpotent singularity of codimension three. The notion is introduced of 'traffic regulator' and the specific sets called the 'inset' and 'outset', which give a new framework for studying a transition map in a cylindrical neighbourhood of the singularity that contains all the non-trivial dynamics that can bifurcate from the singularity, focusing on the domains on which the transition map is defined. A list is also given of open problems which are believed to be helpful for future investigation of the bifurcations from the nilpotent triple zero singularity in 3.