Cylindrical Moderator Research Articles

Optimization of the Second Target Station cold source moderators using an automated workflow

The Second Target Station (STS) at the US Department of Energy’s Oak Ridge National Laboratory is designed to become the world’s highest peak-brightness spallation source of cold neutrons. Successful completion of the STS, which is currently in the preliminary design phase, will provide transformative new capabilities to examine novel materials for future technologies. At STS, neutrons will be generated by spallation reactions in a solid tungsten target. They will be moderated and thermalized in two cold (20 K) para-hydrogen moderators. Careful optimization of these moderators is essential to the project’s success. To find optimal moderator designs, an advanced optimization workflow integrates high-fidelity neutronics calculations using the Monte Carlo N-Particle (MCNP) transport code MCNP6.2 with state-of-the-art optimization algorithms in the Dakota optimization toolkit. For each design iteration, a parametrized solid CAD geometry is generated in Creo and automatically converted into an unstructured mesh geometry by Attila 4MC for the neutronics calculation with MCNP. Iterations repeat until optimal designs are found. This paper presents the results of a sensitivity and optimization study for the cylindrical and tube moderators. Both moderators can be optimized for maximum peak brightness, maximum time-integrated brightness, or any combination between these extremes. Maximum peak brightness is achieved by using smaller optimal dimensions of the moderators, whereas maximum time-integrated brightness is achieved by using larger dimensions. A Pareto front details the designs that optimally balance both brightness metrics. The Pareto front can be found in only 40–110 iterations with 4–5 design parameters when using the efficient global and Pareto-set optimization algorithms in Dakota. Additionally, important engineering constraints can be taken into account, such as the coupling between the cylindrical moderator radius and aluminum vessel wall thicknesses required to ensure structural integrity of the vessels. This interaction has a significant impact on the resulting optimal designs. Our new, highly efficient, fully automated optimization workflow will be used to optimize additional STS components in the future and can be adopted for design and optimization studies at other experimental neutron and accelerator facilities.

Development of single moderator-based neutron spectrometer for neutron spectrometry in reactor core

Measuring the neutron spectrum in the reactor core is quite difficult to do. The commonly used technique is using a multi-foil activation-based neutron spectrometer. This technique provides precise positional measurement but low accuracy and is difficult and expensive to manufacture. A technique for measuring the neutron spectrum in the reactor core has been developed using a single moderator-based neutron spectrometer. This technique is simple, easy to make, and inexpensive. The measurement time is relatively fast, and the results are quite accurate. This technique uses a single cylindrical moderator and places seven passive Au detectors with varying depths. The response of each detector is calculated using the Monte Carlo program MCNPX. The neutron spectrum measurement in the reactor core was carried out by placing the spectrometer above the core for a few minutes. The foil activity was measured using a gamma counter, and the neutron spectrum was calculated using the UMG unfolding program. After measuring the neutron spectrum with a single moderator-based neutron spectrometer in the Triga Mark II reactor core, Kartini, compared with the simulation results, the comparison results show values that are close to the same, where the difference is around 2.2%.

The NCT-WES directional neutron spectrometer: validation of the response with monoenergetic neutron fields

A directional neutron spectrometer named NCT-WES (Neutron Capture Therapy Wide Energy Spectrometer) was developed for quality assurance of the therapeutic neutron beam in Neutron Capture Therapy (NCT). NCT-WES operates as a “parallelized” Bonner spheres spectrometer, embedding six semiconductor-based thermal neutron detectors in a collimated cylindrical moderator. With the objective of validating the simulation model used to derive its response matrix, irradiations in reference monoenergetic fields were performed at National Physical Laboratory (UK). As the energy distributions of neutron beams in NCT extend from keV to a few MeV, monoenergetic fields in this domain were chosen, namely 71.5 keV, 144.2 keV, 565.1 keV, 841.9 keV and 1200.4 keV. The results of the experiment confirm the correctness of the NCT-WES simulation model, within an overall uncertainty lower than ± 2%.

A liquid hydrogen tube moderator arrangement for SNS second target station.

Liquid hydrogen filled tubes arranged in a triangular shape surrounded by light-water premoderators were investigated as cold moderators coupled to a neutron production zone of a tungsten target fed by a 1.3 GeV proton beam in a short-pulse mode. A moderator concept optimized for the tube length, premoderator thickness, and target position was found to deliver the highest pulse-integrated neutron brightness suitable to send cold neutron beams for neutron scattering instrumentation geared toward medium resolution and small samples. Reducing the tube diameter from 50 to 10mm boosts the cold neutron brightness (E < 10meV) by a factor of 2.25, also with enhancements seen by downsizing the height of cylindrical moderators in earlier studies. Attempts were made to characterize this novel moderator system with regard to pulse shapes, local brightness distribution, and angular distribution of emission at the neutron emission port.

Measuring the near-target neutron field of a D–D fusion facility with the novel NCT-WES spectrometer

A new directional neutron spectrometer called NCT-WES (Neutron Capture Therapy Wide Energy Spectrometer) was recently developed within the INFN ENTER_BNCT project. This device constitutes a more compact and portable alternative to state-of-the-art directional neutron spectrometers. The device was used for the first time to characterise the near-target field produced by an accelerator-driven D–D fusion neutron source. NCT-WES operates as a “parallelised” Bonner spheres spectrometer, embedding six semiconductor-based thermal neutron detectors in a cylindrical moderator. Owing on a cylindrical collimating aperture, the device exhibits sharply directional response. To account for the non-uniform irradiation condition experienced in the near-target field, a dedicated NCT-WES response matrix was developed. The neutron spectrum at 0^{circ } from the D–D neutron target, determined by means of the FRUIT unfolding code, is coherent with that previously derived with Bonner spheres. NCT-WES proved to be a promising device for angular spectrometric characterisation of neutron-emitting targets.

A thermal neutron source for the CERN radiation Calibration Laboratory

This work presents the design, construction and experimental characterisation of a lightweight and low-cost thermal neutron assembly, to be used with the existing Am–Be source irradiator of CERN radiation Calibration Laboratory (Cal Lab). The assembly consists of a cylindrical moderator (18 cm diameter, 25.5 cm height and 5.5 kg weight) and an optional reflector box (5 cm thick walls, 20 kg weight). The moderator is tailored to fit on the Am–Be source in its irradiation position, while the box encloses the detector under test during the irradiation. The exposure volume delimited by the box is 30 × 30 × 30 cm3. The thermal neutron fluence at the exposure location, i.e., 30 cm from the source, was optimized by FLUKA Monte Carlo (MC) simulations. The simulations were validated with measurements performed with a bare 3He proportional counter. The thermal neutron fluence at the nominal irradiation position is 7.43 × 102 cm−2s−1 with the cylindrical moderator only, and 5.75 × 103 cm−2s−1 with the cylinder and the reflector box, with the detector placed at the centre of the box. The thermal neutron fluence inside the box is rather uniform (variation <5%).

Analysis by Monte Carlo of thermal neutron flux from a 241Am/9Be source for a system of trace analysis in materials

Neutron techniques to characterize materials have a wide range of applications, one of the major developments being the identification of terrorist threats with chemical, biological, radiological, nuclear and explosives (CBRNE) materials. In this work, a thermal neutron irradiation system, using a241Am/9Be source of 111 GBq inside polyethylene cylindrical moderators, has been designed, built and tested. The geometry of moderator and the neutron source position were fixed trying to maximize the thermal neutrons flux emitted from the system. Therefore, the system is in fact a thermalized neutron source taking advantage of the backscattered neutrons, achieving thermal fluence rates of up to 5.3x102 cm−2 s−1, with dominantly thermal spectra. Samples can be placed there for several hours and thereafter be measured to identify their component elements by NAA (Neutron Activation Analysis).Through Monte Carlo techniques employing the MCNP6 code (Pelowitz et al., 2014), four different configurations with polyethylene cylinders were simulated to choose the most adequate geometry. The theoretical model was then replicated in the neutronics hall of the Neutron Measurements Laboratory of the Energy Engineering Department of Universidad Politécnica de Madrid (LMN-UPM), carrying out experimental measurements using a BF3 neutron detector. A high agreement between MCNP6 results and the experimental values measured was observed.Consequently, the system developed could be employed in future laboratory experiments, both for the identification of trace substances by NAA and for the calibration of neutron detection equipment.

Dosimeter for Measuring the Ambient Dose of Neutrons with Energy from 10–4 MeV to 1 GeV Based on a Heterogeneous Moderator

The construction of a dosimeter for measuring the ambient dose of neutrons with energy from 10–4 MeV to 1 GeV based on a cylindrical polyethylene moderator with lead and cadmium inserts and a 3He counter of thermal neutrons is proposed. Numerical Monte Carlo simulations based on the GEANT4 and SHIELD programs were applied to choose the optimal design of the dosimeter and calculate its energy sensitivity function. The device can be used for operational and stationary radiation monitoring in neutron fields on high-energy accelerators. A comparison of the dosimeter characteristics with foreign analogs is given.

Neutron spectrometry of a liquid Lithium based (p, n) beam at SARAF facility using the broad-energy range directional spectrometer CYSP

An intense neutron beam produced via 7Li(p, n)7Be reaction on a newly developed high-power liquid Lithium target (LiLiT) is available at SARAF (Soreq Applied Research Accelerator Facility) facility, Israel. Its spectrum was determined using the CYSP (CYlindrical SPectrometer), a new directional neutron spectrometer which incorporates the functionality of the Bonner spheres in a single cylindrical moderator. The advantages of this spectrometer are that the neutron spectrum is derived in only one exposure, and that its directional response is not affected by room- and air-scattered neutrons. The data from the CYSP spectrometer were analysed using the FRUIT unfolding code.

Measurements of neutronic characteristics of rectangular and cylindrical coupled hydrogen moderators

ABSTRACT Extensive simulation calculations were performed in the design studies of the coupled hydrogen moderator for the pulsed spallation neutron source of the Japan Proton Accelerator Research Facility (J-PARC). It was indicated that a para-hydrogen moderator had an intensity-enhanced region at the fringe part, and that pulse shapes emitted from a cylindrical para-hydrogen moderator gave higher pulse-peak intensities with narrower pulse widths than those from a rectangular one without penalizing the time-integrated intensities. To validate the peculiar distribution and advantages in pulse shapes experimentally, some measurements were performed at the neutron source of the Hokkaido University electron linear accelerator facility. It was observed that the neutron intensity was enhanced at edges of the para-hydrogen moderators, whereas it decreased at the same part of the ortho-rich-hydrogen moderator, where the dimension of those moderators was 50 mm in thickness and 120 mm in width and height. The spatial distribution and pulse shapes were also measured for a cylindrical coupled para-hydrogen moderator that has the same dimensions as for the coupled moderator employed for J-PARC. The measured results from the cylindrical moderator were consistent with the results obtained in the design studies for the moderator for J-PARC.

CHARACTERIZATION OF AN IN-HOUSE DEVELOPED MULTI-CYLINDRICAL MODERATOR NEUTRON SPECTROMETER.

This article describes the characterization of an in-house developed multi-cylindrical moderator neutron spectrometer, which consists of a cylindrical 3He proportional counter and cylindrical moderator shells of different sizes. The response matrix of the spectrometer was calculated by Monte Carlo simulations for neutron energies from 1 × 10-8 to 10 MeV and verified with measurements in 0.144 MeV, 1.2 MeV and 241AmBe neutron standard fields. Influence of scattered neutrons was properly eliminated from the measured response using the shadow cone technique. The calculated and measured responses were in good agreement in most cases. Differences were <10% for all moderated counter configurations, while larger deviations were observed for the bare counter configuration. The performance of the neutron spectrometer in terms of spectrum unfolding was verified in the 241AmBe neutron standard field, showing reliable neutron spectrum and fluence rate in the energy range up to 10 MeV as investigated in this work.

The spatial dose rate and energy characterization of a P 385 D-D Neutron Generator using a nested neutron spectrometer and a tissue equivalent proportional counter

A nested neutron spectrometer (NNS) and a tissue equivalent proportional counter (TEPC) were used to measure the spatial dose equivalent and energy neutron fluence distribution of a P 385 D-D Thermo Fisher Scientific Neutron Generator located in the mixed field dosimetry facility at the University of Ontario Institute of Technology. The P 385 Neutron Generator uses the D-D nuclear reaction to generate neutrons with an expected average energy of 2.5 MeV. The NNS uses a He-3 proportional counter to measure the neutron capture count rate as a function of nested cylindrical moderator thickness. Using the NNS and associated unfolding software, the energy neutron fluence distribution at various locations from the P 385 neutron generator were determined along with other radiation protection quantities of interest. Measurements of the dose equivalent rate were also conducted at the same locations using a 5-inch spherical TEPC and compared to the NNS results. The TEPC was further used to measure the dose equivalent as a function of radial distance from the neutron generator target and also for different P 385 operating conditions.

DEVELOPMENT OF SIMULATED WORKPLACE FIELDS AT KRISS FOR PERFORMANCE TEST OF NEUTRON PERSONAL DOSEMETERS.

Simulated workplace neutron fields have been developed at the Korea Research Institute of Standards and Science (KRISS). An (241)Am-Be neutron source and a cylindrical moderator composed of stainless steel and heavy water were installed in a 10-cm-thick concrete block with dimensions of 150 × 120 × 120 cm(3) The neutron energy spectrum at a distance of 66.5 cm was measured using a Bonner sphere spectrometer and was found to agree with the spectrum obtained from the Monte Carlo N-Particle Extended simulation to within 5 %. The neutron fluence-to-personal dose equivalent conversion coefficients were (20.8-43.6) pSv·cm(2) and were thus in good agreement with those of reactor fields. The results showed that the KRISS-simulated workplace neutron fields can be used for performance tests and the calibration service of neutron personal dosemeters.

Experimental test of a newly developed single-moderator, multi-detector, directional neutron spectrometer in reference monochromatic fields from 144 keV to 16.5 MeV

A new directional neutron spectrometer called CYSP (CYlindrical SPectrometer) was developed within the NESCOFI@BTF (2011–2013) collaboration. The device, composed by seven active thermal neutron detectors located along the axis of a cylindrical moderator, was designed to simultaneously respond from the thermal domain up to hundreds of MeV neutrons. The new spectrometer condenses the performance of the Bonner Sphere Spectrometer in a single moderator; thus requiring only one exposure to determine the whole spectrum. The CYSP response matrix, determined with MCNP, has been experimentally evaluated with monochromatic reference neutron fields from 144keV to 16.5MeV, plus a 252Cf source, available at NPL (Teddington, UK). The results of the experiment confirmed the correctness of the response matrix within an overall uncertainty of ±2.5%. The new active spectrometer CYSP offers an innovative option for real-time monitoring of directional neutron fields as those produced in neutron beam-lines.

Investigation of the Characteristics of Neutron Fields in Channels of a Graphite Prism and Cylindrical Moderators Near the Bars-6 Reactor Cores

We present results investigating the characteristics of neutron fields in three channels of a graphite prism with a radionuclide neutron source 238Pu(α, n)9Be, located at its geometric center, and in the central channels of cylindrical moderators installed near the cores of the reactor BARS-6.

Development of portable long counter with two different moderator materials

A portable, light-weight long counter (LC) with small dimensions was developed. This LC consists of a (3)He thermal neutron counter, a cylindrical moderator and outer shields. It was designed to have an almost flat response in a neutron energy range of 0.4 eV to 5 MeV. The portable LC has a radius of 11 cm and a length of 39 cm. Its weight was successfully reduced to 15 kg. Polystyrene was employed instead of polyethylene for the front part of the moderator in order to increase the sensitivity to low-energy neutrons. The response function calculated using the MCNP code was consistent with the results of experiments using monoenergetic neutron calibration fields.

Thermoluminescence measurements of neutron streaming through JET Torus Hall ducts

Thermoluminescence detectors (TLD) were used for dose measurements at JET. Several hundreds of LiF detectors of various types, standard LiF:Mg,Ti and highly sensitive LiF:Mg,Cu,P were produced. LiF detectors consisting of natural lithium are sensitive to slow neutrons, their response to neutrons being enhanced by 6Li-enriched lithium or suppressed by using lithium consisting entirely of 7Li. Pairs of 6LiF/7LiF detectors allow distinguishing between neutron/non-neutron components of a radiation field. For detection of neutrons of higher energy, polyethylene (PE-300) moderators were used. TLDs, located in the centre of cylindrical moderators, were installed at eleven positions in the JET hall and the hall labyrinth in July 2012, and exposure took place during the last two weeks of the experimental campaign. Measurements of the gamma dose were obtained for all positions over a range of about five orders of magnitude variation. As the TLDs were also calibrated in a thermal neutron field, the neutron fluence at the experimental position could be derived. The experimental results are compared with calculations using the MCNP code. The results confirm that the TLD technology can be usefully applied to measurements of neutron streaming through JET Torus Hall ducts.

Moderator for neutron activation with the photoneutrons produced by a LINAC

Using Monte Carlo methods a polyethylene moderator has been designed to induce activation using the photoneutrons field of a 15 MV linear accelerator for radiotherapy. In the calculations the photoneutron spectrum at 1 m from the isocenter was used as a source term and the neutron spectra were calculated in the center of different size cylindrical moderators. The best size was selected defining the thermal-to-fast-neutron ratios as a figure of merit. The moderator was built and its performance was evaluated by inducing the activation of Mn dissolved in water, silver coins and souvenir coin. The thermal neutron fluence rate was determined with the Mn samples being 9.96 × 105 cm−2 Gyx−1.

Coupled calculation of the influence of the para to ortho conversion in a cryogenic hydrogen moderator

Abstract We will analyze the dynamics of ortho and para-hydrogen in a moderator. The energy transferred by neutrons during moderation tends to turn para into ortho, and the opposite reaction, ortho decaying to para at low temperatures, is very slow if not accelerated. Thus, there is a chance that ortho-hydrogen represents actually a significant fraction of the total hydrogen. Besides the moderator needs to be recirculating, and the Fluid Dynamics must be modeled, as radiation heat could accumulate in some spots, and raise the hydrogen temperature. This, in turn, would affect the moderating ability of the hydrogen, since ortho has a much greater cross section than para.A Computer Fluid Dynamics (CFD) model has been used to perform the thermohydraulic calculations for a cylindrical moderator, and obtain an estimation of the temperatures in it. Using those temperatures, the ortho-para balance is estimated, taking into account the different properties of the two forms, and the neutronic calculations have been redone, as a first iteration of the coupled calculations. A comparison of the results between pure para and a more realistic H 2 moderator is given and its consequences are discusse.

Flat-response neutron detector using spatial distribution of thermal neutrons in a moderator

We propose a novel neutron detector that realized flat-response using information of a spatial distribution of thermal neutrons in a moderator. The proposed detector consists of a 3He position sensitive proportional counter (PSPC) and a cylindrical moderator surrounding the 3He PSPC. The cylindrical detector is irradiated by neutrons along the cylinder axis. The spatial response of the 3He PSPC is used to correct the detector response into flat-response. We adopt a weighting method to achieve flat-response, in which detected neutrons weighted depending on their detected positions are accumulated as the detector response. Through Monte Carlo simulation studies, we confirm that the flat-response neutron detector can be realized by correcting the response of the proposed detector using the weights determined by a multiple least square method (MLSM). Additionally, fundamental property of the 3He PSPC is experimentally investigated to check applicability to the proposed flat-response neutron detector. We conclude that we should take account of the end effect when determining the weights and correcting the detector response.