Neutron Metrology Research Articles

Characterization of neutron fields from bare and heavy water moderated 252Cf spontaneous fission source using Bonner Sphere Spectrometer

In this work a calibrated Bonner Sphere Spectrometer (BSS), together with ISO shadow cones, was used to quantify the total and scattered components of bare and heavy water moderated 252Cf neutron fields. All measurements were performed with a BSS that was calibrated at the National Physical Laboratory (NPL), Teddington, UK, which is a global primary standard laboratory and world-leading facility for neutron metrology and neutron instruments calibration. The fields were characterized for source-spectrometer distances of 80, 100, 150 and 200cm; and at heights of 103 and 200cm from the facility floor. As expected, the scattered contribution was greatest at the farthest distance from the source and closer to the floor. Hence, at a distance of 200cm and a height of 103cm, the scatter added to the direct field up to 162% of the total neutron fluence and up to 61% of the ambient dose equivalent, while at the same distance and height of 200cm above the floor, these values were up to 146% and 52%, respectively. In the case of heavy water moderated 252Cf neutron fields, a shadow cone subtraction technique could not be implemented, however Monte Carlo simulations were utilized in order to differentiate between the direct and scatter components of the neutron fields. In this case, at a source–detector distance of 200cm and a height of 103cm, the scatter added to the direct field up to 148% of the total neutron fluence and up to 45% of the ambient dose equivalent, while at the same distance and a height of 200cm above the floor, these values were up to 134% and 42%, respectively.

Italian neutron sources

Many research activities, instrumental analysis, studies of radiation damage, etc., require neutron sources. The main neutron sources present in Italy are described in three different sections: nuclear research reactors, accelerator driven, and metrology stations. The nuclear research reactors of LENA (University of Pavia) and ENEA Casaccia are described in terms of irradiation facilities available, neutron flux for each of them and the main activities carried out by each research centre. In the second section, the Frascati Neutron Generator (FNG), the Frascati Beam-Test Facility (BTF) and their main features are reported. In the last section there is a detailed description of the institutional role and the main activities carried out in the field of neutron metrology by the National Institute for Metrology of Ionizing Radiation (INMRI) with a brief description of neutron sources of which the institute is endowed.

Neutron metrology laboratory facility simulation.

The Neutron Low Scattering Laboratory in Brazil has been completely rebuilt. Evaluation of air attenuation parameters and neutron component scattering in the room was done using Monte Carlo simulation code. Neutron fields produced by referenced neutron source were used to calculate neutron scattering and air attenuation.

Performance characteristics of a polyethylene collimator with an EJ-426 detector in neutron source localisation

In this paper, characteristics of neutron source localisation using an EJ-426 detector with a polyethylene collimator are presented. The detector is placed inside of the polyethylene collimator, where the collimator has a constant internal diameter and external diameter gradually increases towards the back end. The combined detector-collimator unit was mounted onto an equatorial mount and, data were collected from an ultra-fast mixed-field analyser. Neutron images have been produced to characterise the area investigated in real-time. Radiation images are fundamentally related to the detector efficiency and to the collimator geometry. In order to characterise the neutron source localisation capability of the polyethylene collimator, with an EJ-426 detector, experiments have been carried out using three different neutron sources. Results indicate that the existing design of the polyethylene collimator is suitable for localisation of low energy neutron sources. The measurements were performed in the low-scatter facility of the Neutron Metrology, National Physical Laboratory, Teddington, UK.

Energy measurement of fast neutron fields with a Recoil Proton Telescope using active pixel sensors

a The spectrometer ATHENA (Accurate Telescope for High Energy Neutron metrology Applications), in development at the LNE-IRSN, characterizes energy and fluence of fast neutron fields. The detector is a Recoil Proton Telescope and measures neutron energies in the range of 5 to 20 MeV. The system is intended to become a primary standard for both energy and fluence measurements. The most innovative part of ATHENA is made of three CMOS pixel sensors, thinned down to 50 microns thickness, allowing an accurate tracking of the recoil. The use of CMOS sensors and a thick silicon diode increase the intrinsic efficiency of the detector by a factor of ten compared with conventional designs. In this paper, we demonstrate the ability of the detector to measure neutron energies in the range of 5 to 20 MeV. Experimental investigations, using mono-energetic neutron fields produced by the AMANDE facility, indicate a good reconstruction of neutron energies. The present design is still under development. The accuracy of measuring 5 MeV and 14 MeV neutron energies was 15% and 5%, respectively.

International key comparison of neutron fluence measurements in monoenergetic neutron fields: CCRI(III)-K11

To ensure the validity of their national standards, National Metrology Institutes (NMIs) participate regularly in international comparisons. In the area of neutron metrology, Section III of the Consultative Committee for Ionizing Radiation is in charge of the organization of these comparisons. From September 2011 to October 2012, the eleventh key comparison, named CCRI(III)-K11, took place at the AMANDE facility of the LNE-IRSN, in France. Participants from nine NMIs came with their own primary reference instruments, or instruments traceable to primary standards, with the aim of determining the neutron fluence, at 1 m distance from the target in vacuum, per monitor count at four monoenergetic neutron fields: 27 keV, 565 keV, 2.5 MeV and 17 MeV.The key comparison reference values (KCRV) were evaluated as the weighted mean values of the results provided by seven participants. The uncertainties of each KCRV are between 0.9% and 1.7%. The degree of equivalence (DoE), defined as the deviation of the result reported by the laboratories for each energy from the corresponding KCRV, and the associated expanded uncertainty are also reported and discussed.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Review of neutron calibration facilities and monitoring techniques: new needs for emerging fields

Neutron calibration facilities and monitoring techniques have been developed since the middle of the 20th century to support research and nuclear power energy development. The technical areas needing reference neutron fields and related instruments were mainly cross section measurements, radiation protection, dosimetry and fission reactors, with energy ranging from a few millielectronvolts to about 20 MeV. The reference neutron fields and calibration techniques developed for these purposes will be presented in this paper. However, in recent years, emerging fields have brought new needs for calibration facilities and monitoring techniques. These new challenges for neutron metrology will be exposed with their technical difficulties.

Fluence measurement of fast neutron fields with a highly efficient recoil proton telescope using active pixel sensors

The spectrometer ATHENA (Accurate Telescope for High-Energy Neutron metrology Applications) is being developed at the LNE-IRSN and aims at characterising energy and fluence of fast neutron fields. The detector is a recoil proton telescope and measures neutron fields in the range of 5-20 MeV. This telescope is intended to become a primary standard for both energy and fluence measurements. The neutron detection is achieved by a polyethylene radiator for n-p conversion, three 50-µm-thick silicon sensors that use CMOS technology for proton tracking and a 3-mm-thick silicon diode to measure the residual proton energy. The use of CMOS sensors and silicon diode, owing to a large detection solid angle, increases the intrinsic efficiency of the detector by a factor of 10 compared with conventional designs. The ability of the spectrometer to determine the neutron energy was demonstrated and reported elsewhere. This paper focuses on the fluence measurement of monoenergetic neutron fields in the range of 5-20 MeV. Experimental investigations, performed at the AMANDE facility, indicate a good estimation of neutron fluence at various energies. In addition, a complete description of uncertainties budget is presented in this paper and a Monte Carlo propagation of uncertainty sources leads to a fluence measurement with a precision ∼3-5 % depending on the neutron energy.

Monte-Carlo calculations of the activation of the accelerator target holder and shadow cone during the calibration of the ITER diagnostic devices with monoenergetic neutrons

Abstract The ITER International Fusion Energy Organization has solicited IRSN Laboratory for Neutron Metrology and Dosimetry to study the possibility to calibrate, in monoenergetic neutron fields at 14 and 2.45 MeV, the neutron detectors to be placed inside the future fusion reactor. In addition to the estimate of the necessary irradiation times, the dose equivalent rates from some of the neutron activated beam line elements had been calculated to consider the cooling time mandatory before access. Neutron activation calculations have been performed with the Fluka Monte-Carlo code. The resulting dose equivalent rates depend strongly of the neutron beam intensity as well as the neutron energy. In the worst case, for 14 MeV neutrons at an emission rate of 1012 s−1, a cooling time of 24 h would be needed for a close access to the shadow cone. Several days would be mandatory in the case of the target holder.

A new recoil proton telescope for characterisation of energy and fluence of fast neutron fields

A new neutron spectrometer, called ATHENA (Accurate Telescope for High Energy Neutron metrology Applications), is under development at the LNE-IRSN. This detector, based on the Recoil Proton Telescope (RPT) principle, is dedicated to the characterization of mono energetic neutron fields produced at the AMANDE facility. In order to be considered as a primary measurement standard, ATHENA will measure, at the level of metrological quality, energy and fluence of the neutrons fields in the range [5 MeV;20 MeV]. Thanks to an innovative tracking of the proton recoils, using CMOS pixel sensors, and to a large acceptance, the RPT will be able to reconstruct both trajectories and energies of the recoil protons with high accuracy. The first prototype of the RPT has allowed validation of the detector principle.The expected performances, in terms of energy resolution and fluence are under investigation. Monte Carlo simulations, based on a MCNPX calculation, have been made. These simulations have allowed to compare theoretical predictions to experimental data obtained at the AMANDE facility.An excellent reconstruction of the 14 MeV neutron is already demonstrated. The results suggest that the RPT will measure energy and fluence of the AMANDE neutron's fields with a good precision and is aimed to become a measurement standard.

What is neutron metrology and why is it needed?

In some branches of metrology the quantity, or quantities, to be measured and the requirements for standards of this quantity, or these quantities, are clear-cut, although the accuracies required may not always be obvious. Good examples are time, length and mass. The quantities measured in neutron metrology and the requirements and uses of these quantities are less well known. This paper lays the foundation for subsequent more detailed discussions of the problems and issues faced by national metrology institutes when developing and providing standards.

Radionuclide neutron source characterization techniques

Radionuclide neutron sources are of fundamental importance to neutron metrology. They are the reference standards which allow quantities such as neutron fluence and neutron dose equivalent to be realized in laboratories around the world. In order to do this it is essential that the sources themselves are well characterized in terms of neutron emission rate, emission anisotropy and spectral fluence ΦE of emitted neutrons. A summary of the types of radionuclide neutron sources is given followed by detailed descriptions of the primary standardization techniques in use today, focusing on the manganese bath technique. Methods for determining the anisotropy of emission are discussed, followed by a brief description of the spectral fluence of the different source types. Finally, an overview of international comparison exercises in this area is included.

Thin tristearin deposits as solid hydrogen-rich layers for neutron metrology

Layers of solid material with a relatively high, constant content of hydrogen that is stable over a long period of time are needed for recoil proton telescopes and for precise determination of the cross-section ratio of 235U(n,f)/H(n,n). Thin layers of tristearin, a material that is relatively rich in hydrogen, have been prepared with an areal density of 0.05 up to 0.5mgcm−2 by evaporation in high vacuum. The surface quality was studied by AFM (atomic force microscopy). The thickness of the layers was estimated by several methods and its homogeneity was determined using an optical profilometer. The hydrogen amount in the deposited layer was estimated by combustion analyses.

Experimental characterization of the IRSN long counter for the determination of the neutron fluence reference values at the AMANDE facility

The neutron metrology and dosimetry laboratory has developed a long counter, on the basis of optimization work using Monte-Carlo simulations. The calibration of this new device was performed at the IRSN standard radionuclide source facility. Measurements were also realized at the monoenergetic neutron fields facility, AMANDE, to determine the dead time correction, the effective center and efficiency as a function of neutron energy. A good agreement was observed between the long counter results and the monoenergetic neutron fluence values determined with proton recoil counters. The results of the experimental characterization of the IRSN long counter are presented in this paper.

Design of a new long counter for the determination of the neutron fluence reference values at the IRSN AMANDE facility

Abstract The Laboratory for Neutron Metrology and Dosimetry of the Institute for Radiation Protection and Nuclear Safety has undertaken a study related to the optimization of the geometry of a long counter to determine the neutron fluence reference values at the AMANDE facility. This study was performed on the basis of a De Pangher long counter design. The materials, the dimensions and the geometry were varied and the consequences on the response function were studied. Different parameters were optimized to flatten the neutron response energy distribution, over a wide energy range, from a few eV’s to a few MeV’s. The aim of this paper is to present the results of the study performed by using intensive Monte-Carlo simulations which helped in the design of the new IRSN long counter.

Determination of semi-empirical relationship between the manganese and hydrogen atoms ratio, physical density and concentration in an aqueous solution of manganese sulphate

Abstract The Manganese sulphate solution has been used for neutron metrology through the method of Manganese Bath. This method uses physical parameters of manganese sulphate solution to obtain its corrections. This work established a functional relationship, using the gravimetric method, between those physical parameters: density, concentration and hydrogen to manganese ratio. Comparisons were done between manganese sulphate solution concentration from the Manganese Bath system of Laboratory of Metrology of Ionising Radiation and estimated values from the functional relationship obtained, showing percentage difference of less than 0.1%. This result demonstrates the usefulness in the correlation of the physical values of the solution to the MB.

Unfolding of Neutron Energy Spectra With Fisher Regularisation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A Monte Carlo unfolding method has been developed for neutron spectrometry with foil activation measurements. It is based on minimising least squares differences between measured and calculated activities under a Fisher regularisation constraint. The performance of the unfolding procedure is first illustrated by a set of 20 foil detectors to simulate reaction rates calculated for a typical neutron reactor energy spectrum. A demonstration of the method is next made using measured reaction rates of the Arkansas Nuclear One power plant benchmark spectrum of the Neutron Metrology File (NMF-90). Results of the method are compared with those obtained by the MAXimum Entropy Deconvolution (MAXED) code using the same input data and are found to be in good agreement with the measured activities. Our procedure has an advantage in unfolding the neutron energy distribution when an a priori solution for an underdetermined unfolding problem is lacking. This is illustrated by unfolding the fast neutron beam spectrum of the <emphasis emphasistype="smcaps">MedApp</emphasis> facility at the nuclear reactor FRM-II in the SAND-II expanded 640-energy group structure, using a set of 22 experimental activation rates without a guess solution. </para>

Novel recoil nuclei detectors to qualify the AMANDE facility as a standard for mono-energetic neutron fields

The AMANDE facility at IRSN-Cadarache produces mono-energetic neutron fields from 2 keV to 20 MeV with metrological quality. To be considered as a standard facility, characteristics of neutron field i.e. fluence distribution must be well known by a device using absolute measurements. The development of new detector systems allowing a direct measurement of neutron energy and fluence has started in 2006. Using the proton recoil telescope principle with the goal of increase the efficiency, two systems with full localization are studied. A proton recoil telescope using CMOS sensor (CMOS-RPT) is studied for measurements at high energies and the helium 4 gaseous μ-time projection chamber (μ-TPC 4He) will be dedicated to the lowest energies. Simulations of the two systems were performed with the transport Monte Carlo code MCNPX, to choose the components and the geometry, to optimize the efficiency and detection limits of both devices or to estimate performances expected. First preliminary measurements realised in 2008 demonstrated the proof of principle of these novel detectors for neutron metrology.

Energy and fluence calibration of the neutron spectrometer ROSPEC at the IRSN AMANDE facility between 70 keV and 4.5 MeV

The ROSPEC device is a multi-detector system, which has been designed by Bubble Technologies Industries (BTI at Chalk River, ON, Canada) to assess neutron spectra, and hence neutron dose quantities, at workplace fields. It is made up of six gaseous proportional counters that detect neutrons via the elastic (n,p) scattering (four hydrogenous counters) and with the (3)He(n,p)T reaction (two (3)He-filled counters). Results of the calibration of a similar rotating spectrometer (ROSPEC) have been described by Rosenstock et al.((1)). For energy and fluence calibration purposes, measurements were performed with the accelerator for metrology and neutron applications in external dosimetry (AMANDE) facility at the Laboratory of Neutron Metrology and Dosimetry (Institute of Radiation Protection and Nuclear Safety, IRSN, France). This facility provides monoenergetic neutron radiation fields from 2 keV to 20 MeV. Two kinds of experiments were carried out. First, the ROSPEC was used in its rotational mode for the ISO energies. Then, each detector was irradiated with all the available neutron energies, in a well defined position with the rotation of the device stopped. The energy values of the neutron beam were calculated using the TARGET code. A BC501-A liquid scintillation spectrometer provided the fluence values for energies beyond 1.2 MeV, a methane-filled SP2 counter from 800 keV to 1.4 MeV and an H(2)-filled SP2 counter from 144 to 800 keV. Reference data for 70 keV monoenergetic neutrons were obtained using the IRSN Long Counter. Results showed that the ROSPEC device was in agreement with the absolute neutron fluences within 10%. Moreover, the new energy calibration factors are in good agreement with those derived by BTI.

Comparison of experimental and computational neutron spectroscopy at a 14 MeV neutron generator facility

At any neutron production facility, the energy spectrum at any meaningful distance from the target will be modified. For the case of a facility used to provide reference irradiations of electronics and other devices at various target-to-device distances it is important to have knowledge of these spectral modifications. In addition, it is desirable to have the ability to generate near real-time measurement capability. Advances in neutron metrology have made it possible to determine neutron energy spectra in real time to high levels of accuracy. This paper outlines a series of experimental measurements and theoretical calculations designed to quantify the scattering effects for a 14 MeV neutron generator facility, and makes appropriate recommendations for near real-time measurements of these fields.