Thomas Kittelmann

Neutron Position Sensitive Detectors for the ESS

The European Spallation Source (ESS) in Lund, Sweden will become the worlds leading neutron source for the study of materials. The instruments are being selected from conceptual proposals submitted by groups from around Europe. These instruments present numerous challenges for detector technology in the absence of the availability of Helium-3, which is the default choice for detectors for instruments built until today and due to the extreme rates expected across the ESS instrument suite. Additionally a new generation of source requires a new generation of detector technologies to fully exploit the opportunities that this source provides. The detectors will be sourced from partners across Europe through numerous in-kind arrangements; a process that is somewhat novel for the neutron scattering community. This contribution presents briefly the current status of detectors for the ESS, and outlines the timeline to completion. For a conjectured instrument suite based upon instruments recommended for construction, a recently updated snapshot of the current expected detector requirements is presented. A strategy outline as to how these requirements might be tackled by novel detector developments is shown. In terms of future developments for the neutron community, synergies should be sought with other disciples, as recognized by various recent initiatives in Europe, in the context of the fundamentally multi-disciplinary nature of detectors. This strategy has at its basis the in-kind and collaborative partnerships necessary to be able to produce optimally performant detectors that allow the ESS instruments to be world-leading. This foresees and encourages a high level of collaboration and interdependence at its core, and rather than each group being all-rounders in every technology, the further development of centres of excellence across Europe for particular technologies and niches.

Geant4 based simulations for novel neutron detector development

A Geant4-based Python/C++ simulation and coding framework, which has been developed and used in order to aid the R&D efforts for thermal neutron detectors at neutron scattering facilities, is described. Built upon configurable geometry and generator modules, it integrates a general purpose object oriented output file format with meta-data, developed to facilitate a faster turn-around time when setting up and analysing simulations. Also discussed are the extensions to Geant4 which have been implemented in order to include the effects of low-energy phenomena such as Bragg diffraction in the polycrystalline support materials of the neutron detectors. Finally, an example application of the framework is briefly shown.

Polycrystalline neutron scattering for Geant4: NXSG4

Abstract An extension to Geant4 based on the nxs library is presented. It has been implemented in order to include effects of low-energy neutron scattering in polycrystalline materials, and is made available to the scientific community. Program summary Program title: NXSG4 Catalogue identifier: AEUZ_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEUZ_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Free for non-commercial use (as per terms in LICENSE file) No. of lines in distributed program, including test data, etc.: 12281 No. of bytes in distributed program, including test data, etc.: 350468 Distribution format: tar.bz2 Programming language: C and C++. Computer: Any with a C or C++ compiler. Operating system: Linux, OSX. Classification: 7.6, 7.7, 8, 11.1, 11.7. External routines: Geant4 ( http://geant4.cern.ch/ ) Nature of problem: Simulation of neutron scattering in polycrystalline materials Solution method: Monte Carlo methods based on analytical formulas Running time: The example provided takes only a few seconds to run.

Detectors for the European Spallation Source

The European Spallation Source (ESS) in Lund, Sweden will become the worlds leading neutron source for the study of materials by 2025. First neutrons will be produced in 2019. It will be a long pulse source, with an average beam power of 5 MW delivered to the target station. The pulse length will be 2.86 ms and the repetition rate 14 Hz. The ESS is presently in a design update phase, which ends in February 2013 with a Technical Design Report (TDR). Construction will subsequently start with the goal of bringing the first seven instruments into operation in 2019 at the same time as the source. The full baseline suite of 22 instruments will be brought online by 2025. These instruments present numerous challenges for detector technology in the absence of the availability of Helium-3, which is the default choice for detectors for instruments built until today. Additionally a new generation of source requires a new generation of detector technologies to fully exploit the opportunities that this source provides. This contribution presents briefly the current status of the ESS, and outlines the timeline to completion. The number of instruments and the framework for the decisions on which instruments should be built are shown. For a conjectured full instrument suite, which has been chosen for demonstration purposes for the TDR, a snapshot of the current expected detector requirements is presented. An outline as to how some of these requirements might be tackled is shown. Given that the delivery of the ESS TDR is only a few months away, this contribution reflects strongly the content of the TDR.

High energy particle background at neutron spallation sources and possible solutions

Modern spallation neutron sources are driven by proton beams similar to GeV energies. Whereas low energy particle background shielding is well understood for reactors sources of neutrons (similar to 20 MeV), for high energies (100s MeV to multiple GeV) there is potential to improve shielding solutions and reduce instrument backgrounds significantly. We present initial measured data on high energy particle backgrounds, which illustrate the results of particle showers caused by high energy particles from spallation neutron sources. We use detailed physics models of different materials to identify new shielding solutions for such neutron sources, including laminated layers of multiple materials. In addition to the steel and concrete, which are used traditionally, we introduce some other options that are new to the neutron scattering community, among which there are copper alloys as used in hadronic calorimeters in high energy physics laboratories. These concepts have very attractive energy absorption characteristics, and simulations predict that the background suppression could be improved by one or two orders of magnitude. These solutions are expected to be great benefit to the European Spallation Source, where the majority of instruments are potentially affected by high energy backgrounds, as well as to existing spallation sources. (Less)

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.

Using Backscattering to Enhance Efficiency in Neutron Detectors

The principle of using strongly scattering materials to recover efficiency in detectors for neutron instruments, via backscattering of unconverted thermal neutrons, is discussed in general. The feasibility of the method is illustrated through Geant4-based simulations involving thermal neutrons impinging on a specific setup with a layer of polyethylene placed behind a single-layered boron-10 thin-film gaseous detector. The results show that detection efficiencies can be as much as doubled in the most ideal scenario, but with associated adverse contributions to spatial and timing resolutions of, respectively, centimeters and tens of microseconds. Potential mitigation techniques to contain the impact on resolution are investigated and are found to alleviate the issues to some degree, at a cost of reduced gain in efficiency.

Shielding optimization study for 10 B-Based large area neutron detectors with detailed Geant4 model

The European Spallation Source (ESS) sets the scope on replacing 3He tube detectors where it is reasonably achievable, consequently advanced neutron detectors require a signal-to-noise (S/N) ratio high enough to be competitive with 3He tubes and satisfy scientific requirements. Advanced local shielding could provide the improved S/N. The objective of the current study is to create a tool that can be used during the shielding optimization process. The study is performed with Monte-Carlo simulations using a Geant4 version extended with NXSG4, that is capable to handle the crystal structure of specific materials, therefore the effects of neutron absorption, coherent and incoherent scattering were simulated. Validation of the extended Geant4 code, developed at ESS is also part of the current study by comparing the simulated results with analytical calculations. A detailed and realistic model of the state-of-the-art Multi-Grid detector has been implemented, as it is almost the only prototype with published data on scattering effects. Simulations were performed for appropriate shielding materials with various monoenergetic neutron beams. A robust tool has been developed that could be effectively used to arise the S/N via optimizing the detector shielding for specific setups and requirements for all inelastic instruments.

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.

Scattered neutron background in thermal neutron detectors

Inelastic neutron scattering instruments require very low background; therefore the proper shielding for suppressing the scattered neutron background, both from elastic and inelastic scattering is ...