Esben Bryndt Klinkby

Low dimensional neutron moderators for enhanced source brightness

In a recent numerical optimization study we have found that liquid para-hydrogen coupled cold neutron moderators deliver 3-5 times higher cold neutron brightness at a spallation neutron source if they take the form of a flat, quasi 2-dimensional disc, in contrast to the conventional more voluminous shapes used by now. In the present paper we describe a simple theoretical explanation of this unexpected behavior, which is based on the large difference in para-hydrogen between the values of the scattering mean free path for thermal neutrons (in the range of 1 cm) and its much larger equivalent for cold neutrons. This model leads to the conclusions that the optimal shape for high brightness para-hydrogen neutron moderators is the quasi 1-dimensional tube and these low dimensional moderators can also deliver much enhanced cold neutron brightness in fission reactor neutron sources, compared to the much more voluminous liquid D2 or H2 moderators currently used. Neutronic simulation calculations confirm both of these theoretical conclusions.

New developments in the McStas neutron instrument simulation package

The McStas neutron ray-tracing software package is a versatile tool for building accurate simulators of neutron scattering instruments at reactors, short- and long-pulsed spallation sources such as the European Spallation Source. McStas is extensively used for design and optimization of instruments, virtual experiments, data analysis and user training. McStas was founded as a scientific, open-source collaborative code in 1997. This contribution presents the project at its current state and gives an overview of the main new developments in McStas 2.0 (December 2012) and McStas 2.1 (expected fall 2013), including many new components, component parameter uniformisation, partial loss of backward compatibility, updated source brilliance descriptions, developments toward new tools and user interfaces, web interfaces and a new method for estimating beam losses and background from neutron optics.

Application of the MCNPX-McStas interface for shielding calculations and guide design at ESS

Recently, an interface between the Monte Carlo code MCNPX and the neutron ray-tracing code MCNPX was developed [1, 2]. Based on the expected neutronic performance and guide geometries relevant for the ESS, the combined MCNPX-McStas code is used to calculate dose rates along neutron beam guides. The generation and moderation of neutrons is simulated using a full scale MCNPX model of the ESS target monolith. Upon entering the neutron beam extraction region, the individual neutron states are handed to McStas via the MCNPX-McStas interface. McStas transports the neutrons through the beam guide, and by using newly developed event logging capability, the neutron state parameters corresponding to un-reflected neutrons are recorded at each scattering. This information is handed back to MCNPX where it serves as neutron source input for a second MCNPX simulation. This simulation enables calculation of dose rates in the vicinity of the guide. In addition the logging mechanism is employed to record the scatterings along the guides which is exploited to simulate the supermirror quality requirements (i.e. m-values) needed at different positions along the beam guide to transport neutrons in the same guide/source setup.

Monte Carlo Particle Lists: MCPL☆

Abstract A binary format with lists of particle state information, for interchanging particles between various Monte Carlo simulation applications, is presented. Portable C code for file manipulation is made available to the scientific community, along with converters and plugins for several popular simulation packages. Program summary Program Title: MCPL Program Files doi: http://dx.doi.org/10.17632/cby92vsv5g.1 Licensing provisions: CC0 for core MCPL, see LICENSE file for details. Programming language: C and C++ External routines/libraries: Geant4 , MCNP , McStas , McXtrace Nature of problem: Saving particle states in Monte Carlo simulations, for interchange between simulation packages or for reuse within a single package. Solution method: Binary interchange format with associated code written in portable C along with tools and interfaces for relevant simulation packages.

Optimization of moderators and beam extraction at the ESS

All instruments at the European Spallation Source (ESS), Lund, Sweden, are served by a carefully optimized moderator assembly, providing world-leading performance and excellent flexibility and upgradeability.

Material assessment for ITER's collective Thomson Scattering first mirror

ITERs Collective Thomson Scattering (CTS) system is a diagnostic instrument that will measure the plasma density and velocity through Thomson scattering of microwave radiation. Some of the key components of the CTS are quasioptical mirrors used to produce astigmatic beam patterns, which have impact on the strength and spatial resolution of the diagnostic signal. The mirrors are exposed to neutron radiation, which may alter the mirror properties or deform its structure. These changes may affect the collection of the scattered radiation and consequently decrease the quality of the measurements. In this work, three different materials (molybdenum (Mo), stainless steel 316L (SS-316L) and tungsten (W)) are considered for the first mirror of the CTS. The objective is to assess the suitability of these materials for this mirror and to provide a first ranking, considering the neutron radiation loads requirements defined by ITER, based on the resultant maximum Von Misses stresses and temperatures. For it, the neutron irradiation, and subsequent heat-load on the mirrors were simulated using the Monte Carlo N-Particle (MCNP) code. Based on the MCNP heat-load results, a transient thermal-structural Finite Element Analysis (FEA) of the mirror over a 400s discharge (reasonable number for computational tests, since an ITER discharge will be between 200 s and 1000 s), with and without mirror cooling, is performed. The results obtained in this preliminary analysis show that of the tested materials Mo and W are the most suitable materials for this application, being able to reliably sustain the thermal and structural stresses imposed by the neutron loads.

m=1 coatings for neutron guides

A substantial fraction of the price for a supermirror neutron guide system is the shielding, which is needed because of the gamma radiation produced as a result of neutron absorption in the supermirror layers. Traditional coatings have been made of nickel-titanium heterostructures, but Ni and Ti also have a fairly high absorption cross section for cold and thermal neutrons. We examine a number of alternatives to Ni as part of a study to reduce the gamma radiation from neutron guides. Materials such as diamond and Be have higher neutron scattering density than Ni, smaller absorption cross section, and when a neutron is absorbed they emit gamma photons with lower energies. We present reflectivity data comparing Ni with Be and preliminary results from diamond coatings showing there use as neutron guide coatings. Calculations show that Be and diamond coatings emit two orders of magnitude fewer gamma photons compared to Ni, mainly because of the lower absorption cross section.

Simulation tools for detector and instrument design

Abstract The high performance requirements at the European Spallation Source have been driving the technological advances on the neutron detector front. Now more than ever is it important to optimize the design of detectors and instruments, to fully exploit the ESS source brilliance. Most of the simulation tools the neutron scattering community has at their disposal target the instrument optimization until the sample position, with little focus on detectors. The ESS Detector Group has extended the capabilities of existing detector simulation tools to bridge this gap. An extensive software framework has been developed, enabling efficient and collaborative developments of required simulations and analyses – based on the use of the Geant4 Monte Carlo toolkit, but with extended physics capabilities where relevant (like for Bragg diffraction of thermal neutrons in crystals). Furthermore, the MCPL (Monte Carlo Particle Lists) particle data exchange file format, currently supported for the primary Monte Carlo tools of the community ( McStas, Geant4 and MCNP ), facilitates the integration of detector simulations with existing simulations of instruments using these software packages. These means offer a powerful set of tools to tailor the detector and instrument design to the instrument application.

Preparation for activation measurements of concrete and PE-B4C-concrete to be applied for shielding at the European Spallation Source

To improve the effect of the concrete below 10 MeV where iron has resonances in the cross section, a new concrete have been developed. The PE-B4C-concrete utilizes hydrogen containing PE to thermal ...

General use of low-dimensional moderators in neutron sources

The European Spallation Source (ESS) will use low-dimensional moderators for cold and thermal neutron production. Low-dimensional moderators deliver higher slow neutron brightness compared to conventional volume moderators. The concepts developed at ESS could be used in reactors and compact neutron sources to reach large increases in neutron flux to the experiments, compared to common practice. For reactors, the smaller volume of the moderators allows positioning them at the optimum flux region. For compact neutron sources, the reduced heat deposition and lower radiation environment allows for particularly efficient implementation of the ESS concepts. Using tube moderators in a compact source would allow reaching a higher brightness per unit yield at least a factor of 4, with respect to a spallation source.