32 research outputs found
Impact of crystallite size on the performance of a beryllium reflector
Beryllium reflectors are used at spallation neutron sources in order to
enhance the low-energy flux of neutrons emanating from the surface of a cold
and thermal moderator. The design of such a moderator/reflector system is
typically carried out using detailed Monte-Carlo simulations, where the
beryllium reflector is assumed to behave as a poly-crystalline material. In
reality, however, inhomogeneities in the beryllium could lead to discrepancies
between the performance of the actual system when compared to the modeled
system. The dependence of the total cross section in particular on crystallite
size, in the Bragg scattering region, could influence the reflector
performance, and if such effect is significant, it should be taken into account
in the design of the moderator/reflector system. In this paper, we report on
the preliminary results of using cross-section libraries, which include
corrections for the crystallite size effect, in spallation source neutronics
calculations.Comment: ICANS-XXII
Simulating neutron transport in long beamlines at a spallation neutron source using Geant4
The transport of neutrons in long beamlines at spallation neutron sources
presents a unique challenge for Monte-Carlo transport calculations. This is due
to the need to accurately model the deep-penetration of high-energy neutrons
through meters of thick dense shields close to the source and at the same time
to model the transport of low-energy neutrons across distances up to around 150
m in length. Typically, such types of calculations may be carried out with
MCNP-based codes or alternatively PHITS. However, in recent years there has
been an increased interest in the suitability of Geant4 for such types of
calculations. Therefore, we have implemented supermirror physics, a neutron
chopper module and the duct-source variance reduction technique for low-energy
neutron transport from the PHITS Monte-Carlo code into Geant4. In the current
work, we present a series of benchmarks of these extensions with the PHITS
software, which demonstrates the suitability of Geant4 for simulating long
neutron beamlines at a spallation neutron source, such as the European
Spallation Source, currently under construction in Lund, Sweden.Comment: ICANS-XXII
Overcoming High Energy Backgrounds at Pulsed Spallation Sources
Instrument backgrounds at neutron scattering facilities directly affect the
quality and the efficiency of the scientific measurements that users perform.
Part of the background at pulsed spallation neutron sources is caused by, and
time-correlated with, the emission of high energy particles when the proton
beam strikes the spallation target. This prompt pulse ultimately produces a
signal, which can be highly problematic for a subset of instruments and
measurements due to the time-correlated properties, and different to that from
reactor sources. Measurements of this background have been made at both SNS
(ORNL, Oak Ridge, TN, USA) and SINQ (PSI, Villigen, Switzerland). The
background levels were generally found to be low compared to natural
background. However, very low intensities of high-energy particles have been
found to be detrimental to instrument performance in some conditions. Given
that instrument performance is typically characterised by S/N, improvements in
backgrounds can both improve instrument performance whilst at the same time
delivering significant cost savings. A systematic holistic approach is
suggested in this contribution to increase the effectiveness of this.
Instrument performance should subsequently benefit.Comment: 12 pages, 8 figures. Proceedings of ICANS XXI (International
Collaboration on Advanced Neutron Sources), Mito, Japan. 201
Probing single-particle and collective states in atomic nuclei with Coulomb excitation
A series of experiments and developments, related to stable and radioactive isotopes, have been carried out. These studies have focused on measuring the low-lying excitations of spherical and deformed nuclei using electromagnetic (Coulomb) excitation and also on developments in detector technology for upcoming radioactive ion beams facilities. The low-lying excitations in the nuclei 107,109Sn and 107In have been investigated using low-energy Coulomb excitation at the REX-ISOLDE facility at CERN. The measured reduced transition probabilities were compared to predictions of nuclear structure models. In addition, a relativistic Coulomb excitation experiment was carried out using the FRS at GSI with the nucleus 104Sn. These radioactive ion beam experiments provide important constraints for large-scale-shell-model calculations in the region of the doubly magic nucleus 100Sn. A stable Coulomb excitation experiment was also carried out in order to explore the properties of low-lying structures in the nucleus 170Er. These measurements resulted in new data for the reduced transition matrix elements in this nucleus. The results were compared to predictions of models of deformed nuclei. The last study contained in this work is related to the design of a new detector system, to be deployed at the upcoming radioactive ion beam facility FAIR. A prototype of the detector was tested with a 180 MeV proton beam and the results were compared to Geant4 simulations. The results highlight important constraints for the design of the full detector system
Probing single-particle and collective states in atomic nuclei with Coulomb excitation
A series of experiments and developments, related to stable and radioactive isotopes, have been carried out. These studies have focused on measuring the low-lying excitations of spherical and deformed nuclei using electromagnetic (Coulomb) excitation and also on developments in detector technology for upcoming radioactive ion beams facilities. The low-lying excitations in the nuclei 107,109Sn and 107In have been investigated using low-energy Coulomb excitation at the REX-ISOLDE facility at CERN. The measured reduced transition probabilities were compared to predictions of nuclear structure models. In addition, a relativistic Coulomb excitation experiment was carried out using the FRS at GSI with the nucleus 104Sn. These radioactive ion beam experiments provide important constraints for large-scale-shell-model calculations in the region of the doubly magic nucleus 100Sn. A stable Coulomb excitation experiment was also carried out in order to explore the properties of low-lying structures in the nucleus 170Er. These measurements resulted in new data for the reduced transition matrix elements in this nucleus. The results were compared to predictions of models of deformed nuclei. The last study contained in this work is related to the design of a new detector system, to be deployed at the upcoming radioactive ion beam facility FAIR. A prototype of the detector was tested with a 180 MeV proton beam and the results were compared to Geant4 simulations. The results highlight important constraints for the design of the full detector system
Application of automated weight windows to spallation neutron source shielding calculations using Geant4
We present an implementation of a general weight window generator for global variance reduction in Geant4 based applications. The implementation is flexible and can be easily adjusted to a user-defined model. In this work, the weight-window generator was applied to calculations based on an instrument shielding model of the European Spallation Source, which is currently under construction in Lund, Sweden. The results and performance of the implemented methods were evaluated through the definition of two figures of merit. It was found that the biased simulations showed an overall improvement in performance compared to the unbiased simulations. The present work demonstrates both the suitability of the generator method and Geant4 for these types of calculations. (C) 2015 Elsevier By. All rights reserved
Two level scheme solvers for nuclear spectroscopy
A program for building level schemes from gamma-spectroscopy coincidence data has been developed. The scheme builder was equipped with two different algorithms: a statistical one based on the Metropolis method and a more logical one, called REMP (REcurse, Merge and Permute), developed from scratch. These two methods are compared both on ideal cases and on experimental gamma-ray data sets. The REMP algorithm is based on coincidences and transition energies. Using correct and complete coincidence data, it has solved approximately half a million schemes without failures. Also, for incomplete data and data with minor errors, the algorithm produces consistent sub-schemes when it is not possible to obtain a complete scheme from the provided data. (C) 2011 Elsevier B.V. All rights reserved
Systematic study on the performance of elliptic focusing neutron guides
In neutron scattering experiments there is an increasing trend towards the study of smaller volume samples, which make the use of focusing optics more important. Focusing guide geometries based on conic-sections, such as those with parabolic and elliptic shapes, have been extensively used in both recently built neutron instruments and upgrades of existing hardware. A large fraction of proposed instruments at the European Spallation Source feature the requirement of good performance when measuring on small samples. The optimised design of a focusing system comes after time consuming Monte-Carlo (MC) simulations. Therefore, in order to help reduce the time needed to design such focusing systems, it is necessary to study systematically the performance of focusing guides. In the present work, we perform a theoretical analysis of the focusing properties of neutron beams, and validate them using a combination of Monte-Carlo simulations and Particle Swarm Optimisations (PS0s), where there is a close correspondence between the maximum divergence of the beam and the shape of the guide. The analytical results show that two limits can be considered, which bound a range of conic section shapes that provide optimum performance. Finally, we analyse a more realistic guide example and we give an assessment of the importance of the contribution from multiple reflections in different systems. (C) 2015 Elsevier B.V. All rights reserved
Impact of crystallite size on the performance of a beryllium reflector
Beryllium reflectors are used at spallation neutron sources in order to enhance the low-energy flux of neutrons emanating from the surface of a cold and thermal moderator. The design of such a moderator/reflector system is typically carried out using detailed Monte-Carlo simulations, where the beryllium reflector is assumed to behave as a poly-crystalline material. In reality, however, inhomogeneities in the beryllium could lead to discrepancies between the performance of the actual system when compared to the modeled system. The dependence of the total cross-section in particular on crystallite size, in the Bragg scattering region, could influence the reflector performance, and if such an effect is significant, it should be taken into account in the design of the moderator/reflector system. In this paper, we report on the preliminary results of using cross-section libraries, which include corrections for the crystallite size effect, in spallation source neutronics calculations
Generation of thermal neutron scattering libraries for liquid para-hydrogen and ortho-deuterium using ring-polymer molecular dynamics
In this paper we present results of combining ring-polymer molecular dynamics with the LEAPR and THEMR modules of NJOY to generate thermal neutron scattering libraries for liquid para-hydrogen and ortho-deuterium. We present the methodology and show that it produces results that are in good agreement with data from both recent available measurements and previous theoretical studies. We also present some simple benchmark Monte-Carlo simulations compared with other available libraries