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Description
The MIRACLES instrument is the time-of-flight backscattering instrument of the ESS. This spectrometer is designed to provide a high energy resolution and flexible resolution for the study of the dynamics of molecules and atoms in biological systems, energy materials and other functional materials.[1]
The experimental station is a key system of MIRACLES, consisting of a concrete building (cave) and a control room attached. The cave has a threefold purpose: (i) it hosts the scattering system (vessel, analyzer, detectors) and the sample environment, shielding the radiation stemming from the sample and the scattering systems receiving the neutron beam, limiting the dose rate outside to operation levels; (ii) it shields the radiation from outside (skyshine, cosmic particles) from reaching the detectors; (iii) it is the main building for scientific activities, giving access to the sample position and the sample preparation areas.
Two main considerations define its design. On one hand, MIRACLES is the ESS instrument with the highest integrated flux on sample, and thus the radiation released by the sample and scattering characterization system will be higher than other ESS instruments. On the other hand, the MIRACLES cave is constrained by a restricted space envelope assigned. This envelope has determined the ground plan (corner-snipped rectangular), the shielding solutions in this wall (heavy concrete instead of normal concrete) and the layout of e.g. the sample preparation areas, utilities, etc,… Additionally, the use of heavy concrete to reduce the thickness of the walls, drives the weight of the cave beyond this floor load limit of 20 T/m2. To solve this issue, the first line of blocks (foundations) consists of extended blocks that increase the contact area with the floor. Therefore, the design of the experimental station has been a challenge with all these boundary conditions.
Neutronics calculations, carried out using MCNP6.2, using the nuclear data library ENDF-B/VIII for neutrons, except for cadmium isotopes, which used JENDL-5 neutron data libraries to account for prompt gamma emissions from cadmium. The JENDL-5 library was proposed because ENDF/B-VIII misses the formation of high-energy gamma photons in the neutron capture process by cadmium. In typical reactor problems, for which ENDF cross sections were developed, this limitation is not significant. However, it becomes critical in this calculation, as gamma generation varies significantly depending on the library. To validate the use of JENDL-5 instead of ENDF/B-VIII, we compared both libraries for cadmium isotopes. Several reaction cross sections were evaluated using MCNP6.2 and JANIS to ensure that both libraries align in shared cross-section reactions.
The detailed neutronics calculations were carried out on a detailed 3D design of the experimental cave. Additionally, the neutronics calculations had to consider the number of feedthroughs and their location for integration of electrical and utility infrastructure. The aim is to provide a maximum radiation dose of 1.5 µSv/h (including both neutrons and gamma photons) outside the cave, except for the sample access, considered a controlled area (maximum dose 12.5 µSv/h), conveniently fenced and linked to personnel safety and radioprotection systems to restrict access.
Three extreme scenarios were considered: (i) no sample (the incident beam directly hits the beam-stop, releasing gamma photons of high intensity through neutron capture); (ii) vanadium sample (the incident beam hits the sample which scatters neutrons in all directions and they prompt gamma photons all around the vessel) ; (iii) cadmium foil in the neutron beam path (the incident beam passes through a 1 mm Cd sheet before reaching the sample, producing high-energy gamma rays). These scenarios defined the boundary conditions for the final design of the MIRACLES cave.