European Spallation Source ESS Research Articles

The future of ESS is bright

The European Spallation Source ESS has still a huge upgrade potential by using an accelerator ring structure for proton pulse compression that can change the long pulse to a medium pulse structure. Therefore, we consider the performance of a medium pulse structure on the existing ESS target, moderator and neutron instrumentation. A medium proton pulse will enhance the neutron peak brightness of thermal and cold neutrons by about one order of magnitude and even up to two orders of magnitude at shortest wavelengths used at ESS, largely increasing the performance of the ESS instruments for neutron scattering. The arguments for a medium pulse length are that it is best adapted to the resolution requirements of the ESS instruments, the coupled moderator, the typical long instruments; furthermore, that it seems feasible to realise a medium pulse length by advanced, slow extraction from an accelerator ring, and feasible for the rotating tungsten target to take the high-power load. We discuss the implications of a medium proton pulse length and its specific choice for the instrument resolution, and for moderator and target. The proposed upgrade is stimulated by the new project ESSnuSB for a neutrino super beam at ESS and motivated by the synergy effect of using a common pulse compressor ring. The upgrade will be a most economical and efficient path for the ESS to a next higher level from an already world leading long-pulse source, transforming with a medium pulse structure into an even much more powerful, future next generation neutron source.

Cryostat for the provision of liquid hydrogen with a variable ortho-para ratio for a low-dimensional cold neutron moderator

A significant contribution to the enhancement of the neutron brilliance achievable with Compact Accelerator-driven Neutron Sources (CANS) can be made by an optimized cold moderator design. When using liquid para-H2 as the moderating medium, the concept of low-dimensional cold moderators can be employed to increase the neutron brightness (as currently foreseen at the European Spallation Source ESS). Para-H2 shows a drop in the scattering cross section by about one order of magnitude around 15 meV, resulting in a large deviation between the mean free paths of thermal and cold neutrons. Taking advantage of this effect, the cold moderator geometry can be optimized to allow the intake of thermal neutrons through a relatively large envelope surface and then extracting them in an efficient way towards the neutron guides. One drawback of this solution is the lack of thermalization of the cold neutrons. In the context of the HBS (High Brilliance Neutron Source) project, efforts are made to overcome this problem by increasing the scattering cross section of the H2 in a defined way. The idea is to admix small amounts of ortho-H2, which maintains its large scattering cross section in the region below 15 meV. Like this, the neutron spectrum can be shifted towards lower energies and adjusted for the needs of the respective instruments. In a cooperation between TU Dresden and FZ Jülich, an experimental setup has been created to prove the feasibility of this concept. The main component of the experimental setup is a LHe-cooled flow cryostat that enables the separate condensation of a para-H2 and a normal-H2 flow and a subsequent mixing of the two in precise proportions. The resulting LH2 mixture at 17 - 20 K is fed into a small cold moderator vessel (approx. 200 ml). In this work, the current status of the setup is presented. The construction and commissioning of the mixing cryostat have been completed and first test runs show that different ortho-para-H2 mixtures can be produced. In the near future, the system will be ready for measurements at a neutron source.

ANNI – A pulsed cold neutron beam facility for particle physics at the ESS

Pulsed beams have tremendous advantages for precision experiments with cold neutrons. In order to minimise and measure systematic effects, they are used at continuous sources in spite of the related substantial decrease in intensity. At the European Spallation Source ESS these experiments will profit from the pulse structure of the source and its 50 times higher peak brightness compared to the most intense reactor facilities, making novel concepts feasible. Therefore, the cold neutron beam facility for particle physics ANNI was proposed as part of the ESS instrument suite. The proposed design has been re-optimised to take into account the present ESS cold moderator layout. We present design considerations, the optimised instrument parameters and performance, and expected gain factors for several reference experiments.

Assessment of environmental consequences of the normal operations of the ESS facility

As other accelerator based facilities, the European Spallation Source ESS facility will interact with the environment. The Swedish legislation requires a demonstration that the sum of the doses resulting from the exposure of any member of the public to ionizing radiation dose does not exceed the specified limit of 50 μSv/year. A radiological assessment has been produced to provide that demonstration. This evaluation was based upon the actual status of the ESS design. A graded approach was adopted through over the assessment allowing estimating dose for all radionuclides and exposure pathways, but the degree of detail in the assessment depend upon their relative radiological importance. The total dose was obtained making the sum of the contribution of all-important radionuclides treated realistically with that of all screened out radionuclides, derived by means a conservative method.

Fundamental physics possibilities at the European Spallation Source

The construction of the European Spallation Source ESS is ongoing in Lund, Sweden. This new high power spallation source with its long-pulse structure opens up new possibilities for fundamental physics experiments. This paper focusses on two proposals for fundamental physics at the ESS: The ANNI instrument and the neutron-anti-neutron oscillation experiment.

The Jülich high-brilliance neutron source project

With the construction of the European Spallation Source ESS, the European neutron user community is looking forward to the brightest source worldwide. At the same time there is an ongoing concentration of research with neutrons to only a few but very powerful neutron facilities. Responding to this situation the Julich Centre for Neutron Science has initiated a project for a compact accelerator driven high-brilliance neutron source, optimized for neutron scattering on small samples and to be realized at reasonable costs. The project deals with the optimization of potential projectiles, target and moderator concepts, versatile accelerator systems, cold sources, beam extraction systems and optimized instrumentation. A brief outline of the project, the achievements already reached, will be presented, as well as a vision for the future neutron landscape in Europe.

VSI@ESS: Case study for a vibrational spectroscopy instrument at the european spallation source

Neutron Vibrational Spectroscopy is a well-established experimental technique where elementary excitations at relatively high frequency are detected via inelastic neutron scattering. This technique attracts a high interest in a large fraction of the scientific community in the fields of chemistry, materials science, physics, and biology, since one of its main applications exploits the large incoherent scattering cross section of the proton with respect to all the other elements, whose dynamics can be spectroscopically detected, even if dissolved in very low concentration in materials composed of much heavier atoms. We have proposed a feasibility study for a Vibrational Spectroscopy Instrument (VSI) at the European Spallation Source ESS. Here, we will summarize the preliminary design calculations and the corresponding McStas simulation results for a possible ToF, Inverted Geometry, VSI beamline.

Experiments with Gravitationally-bound Ultracold Neutrons at the European Spallation Source ESS

Abstract Experiments with gravitationally-bound ultracold neutrons have made substantial progress in the last decade. They have been contributing to answer scientific questions ranging from gravity tests at micron distances, the direct search for dark matter particles as axions, and dark energy realizations. Comparing the present accuracy of around 10 -14 eV - achieved with a gravity resonance spectroscopy technique - with the by many orders of magnitude expected smaller size of inevitable systematic errors, one may conclude that the present experiments are heavily restricted by the limited strength of today's ultracold neutron (UCN) sources. We propose to build a dedicated UCN source at the European Spallation Source in order to perform experiments with gravitationally-bound UCN.

The Present Status of Particle Physics with Slow Neutrons

Abstract Slow neutrons are a privileged tool to explore the low-energy high-precision frontier of particle physics. This overview on the present status of neutron-particle physics is intended to serve as a basis for the ensuing discussion of the potential and future development of neutron-particle physics at the European Spallation Source ESS.

Status of the European Spallation Source ESS AB, the Instrument Selection Process, and a Fundamental Physics Beamline at the ESS

A general introduction to the status of the European Spallation Source ESS AB is given. As well as a general overview, the status of instruments and instrument design is presented. Particular attention is given to the instrument selection process, and how a proposal for a fundamental neutron physics beamline should be submitted. The contents of this presentation closely reflect the recently completed Technical Design Report for the ESS.

The wavelength frame multiplication chopper system for the ESS test beamline at the BER II reactor—A concept study of a fundamental ESS instrument principle

Contributing to the design update phase of the European Spallation Source ESS–scheduled to start operation in 2019–a test beamline is under construction at the BER II research reactor at Helmholtz Zentrum Berlin (HZB). This beamline offers experimental test capabilities of instrument concepts viable for the ESS. The experiments envisaged at this dedicated beamline comprise testing of components as well as of novel experimental approaches and methods taking advantage of the long pulse characteristic of the ESS source. Therefore the test beamline will be equipped with a sophisticated chopper system that provides the specific time structure of the ESS and enables variable wavelength resolutions via wavelength frame multiplication (WFM), a fundamental instrument concept beneficial for a number of instruments at ESS. We describe the unique chopper system developed for these purposes, which allows constant wavelength resolution for a wide wavelength band. Furthermore we discuss the implications for the conceptual design for related instrumentation at the ESS.

A 0.9-?eV direct backscattering spectrometer for the 50 Hz target station of the European Spallation Source ESS

The instrument presented is a pulsed-source backscattering crystal-analyzer spectrometer for highest energy resolution at an extended range of momentum transfers. The secondary spectrometer, based on Si(111) crystal analyzers, provides an energy resolution of 0.9 μeV. A second analyzer system using Si(311) crystals provides a more modest energy resolution of ∼6 μeV with an extended energy and Q range. The combination of high count rate, extended dynamic range in energy transfer along with the energy-resolution characteristics of classical reactor spectrometers results in a spectrometer that will be comparable with classical reactor-based backscattering spectrometers.

KARMEN upgrade and prospects at ESS

The KARMEN experiment at the ISIS spallation source has enhanced its sensitivity for ν μ → ν e oscillations in 1996 by the installation of an additional large area veto counter system. Consisting of 136 highly transparent plastic scintillator modules, the new veto is completely embedded inside the experiment's 7000 tonne steel blockhouse. It allows to eliminate the main background component in the search for ν μ → ν e appearance originating from high energy neutrons induced by deep inelastic scattering of cosmic ray muons in the massive shielding. First measurements after the detector upgrade show a substantial reduction of this background meeting the expected factor of 40. If no oscillation signal is seen after 3 years of measuring (1997–1999) KARMEN will be able to exclude mixing amplitudes of sin 22 Θ≃1 x10 -3. The ongoing KARMEN2 measurements will thus allow a decisive and reliable test of the entire parameter space favoured by the positive result of LSND. An even higher oscillation sensitivity can be achieved by experiments at future high intensity short spill proton accelerators such as the planned European Spallation Source ESS.