Advanced Neutron Moderators for the ESS

Thermal and cold neutrons are used in a wide array of different experiments investigating the sub-micrometer properties of matter. Neutrons are typically produced at reactor or spallation sources and subsequently cooled to the wanted thermal or cold energy levels by employing neutron moderators. The main increases in thermal and cold intensity are achieved by upscaling the power density of reactors or proton beam power of spallation sources. Reactor development saturated in the 1960s with the construction of the continuous, compact, high-power-density reactors HFIR, Oak Ridge, and ILL, Grenoble. Today these sources are still the most intense neutron sources. The short-pulsed sources SNS and J-PARC are the most powerful spallation sources in the world; although less intense than ILL and HFIR, these sources provide more useful neutrons because of their pulsed beam structure.
This thesis focuses on the European Spallation Source (ESS), which is currently being constructed in Lund, Sweden. The ESS will be a long-pulsed spallation source (pulse length 2.86 ms) driven by a 5-MW proton beam impinging on a rotating tungsten target. The ESS will be the world's most intense neutron source in terms of brightness, but it will also be the first spallation source to outperform reactors in terms of the integrated intensity of thermal and cold neutrons.
Experiments at modern facilities use less than one millionth of the neutrons created in neutron source. Much of this inefficiency can be attributed to the moderator system. The imperfections of moderator systems originate from the highly isotropic slowing-down and thermalizing processes, premature leakage (fast neutron escape), neutron absorption and suboptimal geometrical congurations. The inefficiency of moderator systems implies a potential gain in efficiency for neutron sources, which generates an interest in moderator development. Many facilities have proposed and applied advanced moderator concepts to better utilize the produced neutrons. The topic of this thesis is the study of these advanced moderator concepts.
Chapters 1 to 6 briefly summarize the historical development of neutron sources. The ESS is briefly introduced. Then the governing physics is outlined as well as the main principles behind neutron sources and advanced moderators. Chapters 7 to 10 (further detailed below) present novel work in the form of three papers (two published articles, one submitted) and two conference proceedings.
Chapter 7 comprises two conference proceedings and describes the development from the moderator system at the ESS suggested in the Technical Design Report (TDR) to the new moderator baseline (accepted in March 2015), known as the butterfly moderator. The chapter outlines the development process from TDR through the pancake moderator and to the butterfly moderator, and presents various key results. Ultimately, it is shown how this redesign and optimization results in a significant increase in cold and thermal brightness relative to the TDR proposal.
Chapter 8 is a study in which MCNPX simulations are transferred to ROOT and
analyzed. A method for reconstructing the full emission distribution of the moderator brightness is developed. The ESS pancake moderator (and butterfly moderator in the sub-appendix) is studied. The brightness distributions are fitted to analytical functions that have been implemented in McStas. This enables more precise predictions of the expectations from ESS, which is not only a key requirement for experiments at ESS but also enables neutron instruments to be significantly better optimized before their construction. This, in turn, is expected to contribute signicantly to the overall quality of the ESS.
Chapter 9 suggests a novel type of broad-spectrum moderator. This moderator
concept is based on the idea that heavy metals, such as lead and bismuth, are inefficient moderator materials. The article investigates this idea through enriched ²⁰⁸Pb. The article shows that the inability of these materials to moderate can be exploited to design a moderator that reflects neutrons from surrounding moderators of different spectral temperatures, with little change in energy. This results in the emission of a broad neutron spectrum (or multiple spectra) from the lead element. Since lead can also serve as a reflector filter, the geometry can be configured such that the broadspectrum lead moderator acts as a reflector filter for a cold moderator positioned behind it, thus increasing the neutron yield below the lead Bragg edge whilst still producing a broad spectrum of neutrons.
Chapter 10 is an experimental paper carried out in the framework of the LENS collaboration. The experiment investigates the concept of a single-crystal reflector filter - a reflector filter that also transmit neutrons in the thermal energy range because of the delta-function-like Bragg edge in a single crystal. The experiment compares single-crystal sapphire, sapphire powder and void. Sapphire was used, since no other single-crystal candidates (diamond, pyrolytic graphite and lithium uoride) could be obtained within the cost and time constraints of the experiment. Unfortunately, sapphire does not notably increase neutron yield, but the experiment proves the viability of a single-crystal reflector filter and indicates a potential regain of the thermal neutrons lost to a conventional reflector filter, with little or no loss of the cold neutrons below the Bragg edge.