Abstract The European Spallation Source (ESS) in Lund, Sweden, is designed to become the most powerful accelerator driven spallation neutron source in the world. ESS is currently under construction, and the first beam on target is planned for the beginning of 2026, with first user operation expected to start in 2026. As a key component of the neutron production, which was developed, built and tested at the Institute of Technology and Engineering, (ITE) of Forschungszentrum Juelich GmbH, the cryogenic moderator slows down high-energy neutrons released from the spallation target. To gain maximum neutron brightness for condensed and soft matter research, an optimized low dimension liquid para-hydrogen moderator has been developed. Hydrogen with a pressure around 10 bar, a temperature around 20 K and a para-hydrogen fraction of at least 0.995 will be utilized to interact with neutrons in a unique moderator vessel arrangement. This paper describes the engineering design, manufacturing and installation of the low dimension liquid para-hydrogen moderator for the ESS.

Background Research with neutrons is usually related to the use of fission-based research reactors or neutron spallation sources to offer neutron beams for science and industry. In recent years a novel way for the production of brilliant cold, thermal, and epithermal neutron beams has emerged with the availability of high current proton accelerator systems. These “High-Current Accelerator-driven Neutron Sources” (HiCANS) offer pulsed neutron beams with high peak brilliance close to present day neutron sources. Methods A project was launched at the Jülich Centre for Neutron Science for the development, design and demonstration of such an innovative high-current accelerator driven neutron source termed “High-Brilliance neutron Source” (HBS). The aim of the project is to construct a scalable neutron source as a user facility. The basic technical components consist of i) a high current proton accelerator with a proton energy below 100 MeV, ii) a compact neutron target, moderator and reflector unit and iii) a neutron extracting, and transport system optimized for neutron beams with high brilliance to serve a suite of high performing instruments with epithermal, thermal and cold neutrons for various applications. Results and Conclusions The HBS project will offer open access and services to the various and changing demands the scientific and also the industrial community asks for. The project offers flexible solutions to a broadest scale of applications in science and industry. The conceptual design of HBS as well as the technical design was published recently in a series of reports as blueprint of a HiCANS facility. HBS will complement and develop further the landscape of high-end neutron facilities in Europe. In addition, HBS will allow intense training and preparation for experiments at the highest level at flagship European neutron sources such as the ILL or, in the future, at the ESS.

Background Research with neutrons is usually related to the use of fission-based research reactors or neutron spallation sources to offer neutron beams for science and industry. In recent years a novel way for the production of brilliant cold, thermal, and epithermal neutron beams has emerged with the availability of high current proton accelerator systems. These “High-Current Accelerator-driven Neutron Sources” (HiCANS) offer pulsed neutron beams with high peak brilliance close to present day neutron sources. Methods A project was launched at the Jülich Centre for Neutron Science for the development, design and demonstration of such an innovative high-current accelerator driven neutron source termed “High-Brilliance neutron Source” (HBS). The aim of the project is to construct a scalable neutron source as a user facility. The basic technical components consist of i) a high current proton accelerator with a proton energy below 100 MeV, ii) a compact neutron target, moderator and reflector unit and iii) a neutron extracting, and transport system optimized for neutron beams with high brilliance to serve a suite of high performing instruments with epithermal, thermal and cold neutrons for various applications. Results and Conclusions The HBS project will offer open access and services to the various and changing demands the scientific and also the industrial community asks for. The project offers flexible solutions to a broadest scale of applications in science and industry. The conceptual design of HBS as well as the technical design was published recently in a series of reports as blueprint of a HiCANS facility. HBS will complement and develop further the landscape of high-end neutron facilities in Europe. In addition, HBS will allow intense training and preparation for experiments at the highest level at flagship European neutron sources such as the ILL or, in the future, at the ESS.

Background Research with neutrons usually relies on fission-based research reactors or neutron spallation sources to provide neutron beams for scientific and industrial applications. In recent years a novel way for the production of brilliant cold, thermal, and epithermal neutron beams has emerged with the availability of high current proton accelerator systems. These “High-Current Accelerator-driven Neutron Sources” (HiCANS) offer pulsed neutron beams with high peak brilliance close to present day neutron sources. Methods A project was launched at the Jülich Centre for Neutron Science for the development, design and demonstration of such an innovative high-current accelerator driven neutron source termed “High-Brilliance neutron Source” (HBS). The aim of the project is to construct a scalable neutron source as a user facility. The basic technical components consist of i) a high current proton accelerator with a proton energy below 100 MeV, ii) a compact neutron target, moderator and reflector unit and iii) a neutron extracting, and transport system optimized for neutron beams with high brilliance to serve a suite of high performing instruments with epithermal, thermal and cold neutrons for various applications. Results and Conclusions The HBS project will provide open access and cater to the various and evolving demands of the scientific and industrial communities. The project offers flexible solutions to a broadest scale of applications in science and industry. The conceptual design of HBS as well as the technical design was published recently in a series of reports as blueprint of a HiCANS facility. HBS will complement and develop further the landscape of high-end neutron facilities in Europe. In addition, HBS will allow intense training and preparation for experiments at the highest level at flagship European neutron sources such as the ILL or, in the future, at the ESS.

Molecular hydrogen occurs in two forms, the so-called ortho-hydrogen and para-hydrogen. At a temperature of 293 K and above, hydrogen has 75% of ortho- and 25% para-content. The content of para-hydrogen increases with falling temperature. At a temperature of 20 K almost 100% para-hydrogen is present. While hydrogen is cooled down and liquefied the exothermic natural conversion starts, but much slower than the liquefaction itself. However, the conversion can be accelerated by using a catalyst. To avoid an unintentionally evaporation due to the exothermic ortho to para conversion it is important to verify that the conversion is completed before the liquid hydrogen is stored, transported or used. To accelerate and to ensure a complete conversion an oversized ortho/para-catalyst is normally used. However, the activity of the catalyst decreases over time. Therefore, a continuous in-situ measurement of the ortho/para-ratio can contribute to effective use of the catalyst and can help to reduce hydrogen losses due to unintentional evaporation. A measurement system based on Raman-spectroscopy is currently being developed at the ZEA-1. First tests with the measuring system show promising results on a laboratory scale. The aim of these investigations is to develop a compact measuring system that measures the ortho/para concentration of liquid hydrogen in-situ.

The Cryogenic Moderator System (CMS) is equipped with a catalyst to convert hydrogen from the ortho state to the para state in order to keep desirably high parahydrogen fractions of 99.5%. An in-situ measurement system for the ortho and para fractions of liquid hydrogen (OPMS) by means of a Raman spectroscopy with an accuracy of 0.1% is being developed to detect any undesirable shift towards a high orthohydrogen fraction caused by neutron scattering driven para-to-ortho back conversion. We have demonstrated that our developed sapphire window, through which the laser and backscattered photon travel, has endured the required pressure and thermal cycles in a liquid hydrogen environment. Meanwhile, a Raman optics system has been developed using normal hydrogen. Eventually, we have succeeded in detecting the parahydrogen peak at J = 4, which is our primary goal, because the peak ratio of parahydrogen at 1245 cm(-1) to ortho hydrogen at 587 cm(-1) corresponds to 0.5%. It can be expected that it would achieve our requirement for liquid hydrogen with higher density.

The main, high-brightness neutron source for ESS is based on the low-dimensional moderator concept, and will serve the initial suite of neutron scattering instruments. In the HighNESS project several design options have been identified and investigated for a second source for ESS, intended to be complementary to the primary one. The emphasis of this project, completed in September 2023, was on the design of high-intensity sources, delivering Cold, Very Cold (VCN), and Ultra-Cold Neutrons (UCN). Remarkable results include: a cold moderator based on liquid deuterium capable of delivering an intensity close to a factor 10 greater than the ESS upper moderator; a VCN moderator based on solid deuterium at 5 K, surrounded by nanodiamond layers, delivering brightness above 40 Å an order of magnitude higher than a conventional cold moderator placed in the same location; and several design options for UCN sources based on the use of superfluid helium and solid deuterium. The use of these new sources would have a major impact on fundamental physics experiments and neutron scattering techniques. We investigate the possible impact that these concepts can have for compact sources, with particular emphasis on VCN.

At the Central Institute for Engineering, electronics, and Analytics (ZEA-1 by its acronym in German) of the Julich Research Center GmbH, a cryostat was developed for the European Spallation Neutron Source (ESS) and manufactured from a titanium-stabilized austenitic stainless steel alloy (EN 1.4571). After tungsten inert gas welding, the welds exhibited a black, patchy discoloration. After materialographic preparation, the phenomena were examined using microscopic methods to assess their impact on the strength of the weld and thus on the usability of the cryostat. Not only could the impact on the cryostat operation be assessed, but the formation mechanism could be elucidated and suitable measures could be outlined to avoid the black, patchy discoloration. Am ZEA-1 der Forschungszentrum J & uuml;lich GmbH wurde ein Kryostat f & uuml;r die europ & auml;ische Spallationsneutronenquelle ESS entwickelt und aus einer titanstabilisierten austenitische Edelstahllegierung (EN 1.4571) gefertigt. Die Schwei ss n & auml;hte zeigten nach dem Wolfram-Inertgas-Schwei ss en eine schwarze, fleckige Verf & auml;rbung. Zur Beurteilung der Auswirkungen auf die Festigkeit der Schwei ss naht und damit der Verwendbarkeit des Kryostaten wurden die Ver & auml;nderungen mittels materialographischer Pr & auml;paration und mikroskopischer Verfahren untersucht. Neben der Absch & auml;tzung der Auswirkungen auf den Betrieb des Kryostaten konnten der Bildungsmechanismus aufgekl & auml;rt und geeignete Ma ss nahmen zur Vermeidung der schwarzen, fleckigen Verf & auml;rbung aufgezeigt werden.

The European Spallation Source (ESS), presently under construction in Lund, Sweden, is a multidisciplinary international laboratory that, once completed at full specifications, will operate the world's most powerful pulsed neutron source. Supported by a 3 M Euro Research and Innovation Action within the European Union Horizon 2020 program, a design study (HighNESS) is now underway to develop a second neutron source located below the spallation target. Compared to the first source, which is located above the spallation target and designed for high cold and thermal brightness, the new source is being optimized to deliver higher intensity and a shift to longer wavelengths in the spectral regions of cold neutrons (CNs) (2 to 20 & Aring;), very cold neutrons (VCNs) (10 to 120 & Aring;), and ultracold neutrons (UCNs) (> 500 & Aring;). The second source consists of a large liquid deuterium moderator to deliver CNs and serve secondary VCN and UCN sources, for which different options are under study. These new sources will boost several areas of condensed matter research and will provide unique opportunities in fundamental physics. The HighNESS project is now entering its last year, and we are working toward the Conceptual Design Report of the ESS upgrade. In this paper, results obtained in the first 2 years, ongoing developments, and future perspectives are described.

The European Spallation Source (ESS), currently under construction, is based on a high-brightness, bi-spectral, low-dimensional moderator placed above a spallation target, intended to initially serve fifteen neutron scattering instruments. Within the upgrade path of ESS, the HighNESS project aims at designing a source complementary to this upper moderator, focusing on delivering a higher intensity and a colder spectrum of neutrons. We have investigated the use of solid ortho-deuterium at 5K as a source of very cold neutrons (VCNs). This source performs competitively as a high-intensity cold-neutron moderator, while also showing an order-of-magnitude flux increase in the very cold range above 40̊ A compared to a liquid deuterium moderator of similar volume and shape, also designed within HighNESS. The long-wavelength performance of the source can be improved further by encasing it in a thin layer of nanodiamonds.The cooling of a solid deuterium moderator placed so close to the spallation target of a high-power neutron source like ESS is very challenging, but may be feasible by augmenting the heat conductivity with the addition of low-density metallic foam structures within the moderator vessel. Such a source could provide unprecedented opportunities in fundamental physics research and neutron scattering using VCNs.

The High Brilliance Neutron Source (HBS) is a High-Current Accelerator-driven Neutron Source (HiCANS) under development at J & uuml;lich Centre for Neutron Science - Forschungszentrum J & uuml;lich. The unique specifications of the HBS require a neutron target with a high neutron emission able to operate in vacuum with a 70 MeV proton beam with 90 mA peak current and 1.6% duty cycle leading to an average thermal load of 100 kW. A tantalum target of 100 cm(2) irradiated area with an internal cooling structure which removes the heat quite homogeneously, provides a good compromise between low pressure and high heat transfer and ensures an homogeneous protons energy loss was designed and optimized by means of MCNP, CFD and FEM simulations. Such a target was successfully manufactured and its operational capability was demonstrated by operating it under a heat load of 1 kW cm(-2) from an electron gun.

A key aim of the HighNESS project for the European Spallation Source is to enable cutting-edge particle physics experiments. This volume presents a conceptual design report for the NNBAR experiment. NNBAR would exploit a new cold lower moderator to make the first search in over thirty years for free neutrons converting to anti-neutrons. The observation of such a baryon-number-violating signature would be of fundamental significance and tackle open questions in modern physics, including the origin of the matter-antimatter asymmetry. This report shows the design of the beamline, supermirror focusing system, magnetic and radiation shielding, and anti-neutron detector necessary for the experiment. A range of simulation programs are employed to quantify the performance of the experiment and show how background can be suppressed. For a search with full background suppression, a sensitivity improvement of three orders of magnitude is expected, as compared with the previous search. Civil engineering studies for the NNBAR beamline are also shown, as is a costing model for the experiment.

An optimized neutron producing tantalum target with an optimized internal microchannel cooling was developed for a 70 MeV proton beam with a peak current of 100 mA, a duty cycle of 1.43% and an average power of 100 kW on a target surface area of 100 cm2. In this work a target with microchannel cooling structure is described which matches with the proton's energy to minimize hydrogen implantation and to produce energy deposition with optimum homogeneity inside the target to minimize the thermal stresses. For the purpose of getting an optimal target design, the investigations of energy deposition, proton fluence, the spatial distribution of (p, n) reactions and the spatial distribution of stopping protons of the target with different microchannel geometries were performed with the particle transport code FLUKA. The resulting design produces a homogeneous proton fluence and energy deposition without hot spots. Furthermore, only 4.4% of the impinging protons accumulate in the metal target, which significantly decreases the risk of hydrogen embrittlement and blistering.

Presently under construction in Lund, Sweden, the European Spallation Source (ESS) will be the world's brightest neutron source. As such, it has the potential for a particle physics program with a unique reach and which is complementary to that available at other facilities. This paper describes proposed particle physics activities for the ESS. These encompass the exploitation of both the neutrons and neutrinos produced at the ESS for high precision (sensitivity) measurements (searches).

The ESS cryogenic moderator system (CMS) will be operated at a pressure of 1.1 MPa and a temperature of 17 K to cool two hydrogen moderators where a nuclear heating is estimated to be 6.7 kW for a 5-MW proton beam operation. A pressure control buffer (PCB) tank with a heater and a control release valve has been designed to mitigate a pressure fluctuation caused by the sudden nuclear heating when the proton beams turn on or off. Excessive gas produced by the heater will be released by the valve and be re-condensed via a small heat exchanger. The pressure fluctuations have been analyzed using a mass and energy conservation. The effects of liquid level, liquid and vapor temperatures in the PCB on the pressure fluctuation have been clarified. The optimal operational conditions have been determined based on the analysis results.

The High Brilliance neutron Source (HBS) is a project for a next generation neutron research facility, based on new concepts and recent technological advancements. As elementary processes it uses neither fission nor spallation, but instead low energy nuclear reactions in a very compact Target-ModeratorReflector (TMR) assembly. Our facility design results in very efficient production of neutron beams with high brightness. Key features of HBS are: (i) very competitive instrument performance, (ii) comparatively low construction and operation costs, (iii) resilience, (iv) sustainability, (v) flexibility, (vi) accessibility and (vii) scalability. Here we present the basic layout of the facility, elaborate on the mentioned key features and report on the commissioning of a small test setup.

The European Spallation Source, currently under construction in Lund, Sweden, is a multidisciplinary international laboratory. Once completed to full specifications, it will operate the world’s most powerful pulsed neutron source. Supported by a 3 million Euro Research and Innovation Action within the EU Horizon 2020 program, a design study (HighNESS) has been completed to develop a second neutron source located below the spallation target. Compared to the first source, designed for high cold and thermal brightness, the new source has been optimized to deliver higher intensity, and a shift to longer wavelengths in the spectral regions of cold (CN, 2–20 Å), very cold (VCN, 10–120 Å), and ultracold (UCN, >500 Å) neutrons. The second source comprises a large liquid deuterium moderator designed to produce CN and support secondary VCN and UCN sources. Various options have been explored in the proposed designs, aiming for world-leading performance in neutronics. These designs will enable the development of several new instrument concepts and facilitate the implementation of a high-sensitivity neutron-antineutron oscillation experiment (NNBAR). This document serves as the Conceptual Design Report for the HighNESS project, representing its final deliverable.

The ESS cryogenic moderator system (CMS) has been designed to circulate subcooled liquid hydrogen with a temperature of 17 K and a parahydrogen concentration of more than 99.5

The European Spallation Source (ESS) in Lund, Sweden, is designed to become the most powerful spallation neutron source in the world. As one subsystem of the Target Station, which was developed and built at Central Institute of Engineering, Electronics and Analytics – Engineering and Technology (ZEA-1) of Forschungszentrum Juelich, the cold Moderator slows down high-energy neutrons from the spallation process. To gain maximum neutron flux intensities along with high system availability for condensed and soft matter research, an optimized liquid hydrogen Moderator circuit has been developed. Hydrogen with a pressure around 1 MPa, a temperature around 20 K, and a para-hydrogen fraction of at least 0.995 will be utilized to interact with neutrons in a unique cold Moderator vessel arrangement. Hydrogen conversion from ortho- to para-hydrogen will be controlled using a catalyst. Two turbo pumps are arranged in series and circulate the cryogen. A helium refrigerator, the Target Moderator Cryoplant (TMCP), continuously recools the hydrogen mass flow. Pressure stabilization is achieved by a pressure control buffer. The individual ESS Cryogenic Moderator System (CMS) components, the first and second generation of hydrogen Moderators (BF1 and BF2) and a first draft of a deuterium Moderator upgrade are presented.

The NNBAR experiment for the European Spallation Source will search for free neutrons converting to antineutrons with a sensitivity improvement of three orders of magnitude compared to the last such search. This paper describes progress towards a conceptual design report for NNBAR. The design of a moderator, neutron reflector, beamline, shielding and annihilation detector is reported. The simulations used form part of a model which will be used for optimisation of the experiment design and quantification of its sensitivity.