Special Topic on “Initial instruments at the Second Target Station of the Spallation Neutron Source”

The Spallation Neutron Source (SNS) is an accelerator-based neutron facility at Oak Ridge National Laboratory (ORNL) (Mason et al., 2005). The First Target Station (FTS) of the SNS was completed in 2006 and now houses 19 operational instruments with an imaging beamline (VENUS) currently under construction (Bilheux et al., 2015).

The Spallation Neutron Source Second Target Station (STS) will be a new, high-brightness source optimized for producing cold neutrons via compact coupled moderators (Gallmeier and Remec, 2022). It will provide new transformational capabilities that complement the strengths of the current ORNL neutron sources (Adams et al., 2019). The high brightness of cold neutrons will enable time-resolved measurements of kinetic processes and provide more intense neutron beams to study smaller samples of newly discovered materials or materials under extreme environmental conditions. The STS will operate at a power of 700 kW with a pulse rate of 15 Hz, enabling simultaneous measurement of hierarchical architectures across a broad range of length scales. The STS is currently in the preliminary design phase, which is expected to complete in 2026, paving the way for the facility’s construction. The construction of STS will allow researchers to meet future scientific challenges in the coming decades.

Recently, an instrument selection process has been conducted to determine the eight instruments that would be built as part of the STS project. Twelve instrument proposals were developed in partnership with the user community and submitted to the instrument selection process. An external committee of 22 national and international experts reviewed and ranked the proposals, commenting that all 12 proposals detailed instruments that offer new capabilities to the scientific user community and should be built at some stage. Instrument selection culminated in August of 2021 with the announcement of the eight instruments to be constructed as part of the STS project. These were BWAVES (Mamontov et al., 2022), CENTAUR (Qian et al., 2022), CHESS (Sala et al., 2022), CUPI²D (Brügger et al., 2023), EXPANSE (Do et al., 2022), VERDI (Garlea et al., 2022), PIONEER (Liu et al., 2022), and QIKR (Ankner et al., 2023).

Four additional STS concepts are also detailed in this special edition, EWALD (Borgstahl et al., 2022), MENUS (An et al., 2022), M-STAR (Lauter et al., 2022), and TITAN (Winn et al., 2022), which were proposed but not selected for construction in the initial instrumentation phase. However, the STS has the capability to house 22 beamlines and these four instruments may be constructed at a later stage. In this special edition, a series of papers detail the design and expected performance of all 12 proposed instruments.

The high brightness beams at the STS will enable new experiments to become feasible, in which samples can be examined under multi-extreme conditions. The article by Haberl and co-workers outlines three sample environment concepts that facilitate experiments using extremes of pressure, temperature, and magnetic field.

The eight instruments that will be constructed as part of the STS project cover a large scientific footprint and take full advantage of the unique STS source characteristics. BWAVES, detailed in Mamontov et al. (2022), is a neutron spectrometer that can select the velocity of detected neutrons after a sample scatters them. It is the first inverted geometry spectrometer where the energy of detected neutrons can be chosen mechanically by a wide-angle velocity selector, irrespective of the limitations imposed by crystal analyzers or filters. As detailed by Qian et al. (2022), CENTAUR is a small- and wide-angle neutron scattering (SANS/WANS) instrument with diffraction capabilities to simultaneously probe atomic- to mesoscale structures in hierarchical systems. Simultaneous SANS/WANS and diffraction capabilities will be unique among neutron scattering instruments in the United States. As detailed in Sala et al. (2022), CHESS is a direct-geometry neutron spectrometer designed to detect and analyze weak signals intrinsic to small cross sections, such as small mass, small magnetic moments, or neutron-absorbing materials. CUPI²D, described in (Brügger et al., 2023, is a cold neutron imaging instrument designed to combine direct and indirect imaging across a broad range of length and time scales. The instrument is designed for applications that involve length scales from angstroms to micrometers and time scales from minutes to hours.

EXPANSE (Do et al., 2022) is an expanded angle neutron spin echo instrument designed to conduct high-energy resolution studies of dynamic processes in various materials. The wide-angle detector banks will provide a coverage of nearly two orders of magnitude in scattering wave numbers and approximately four orders of magnitude in Fourier times. This instrument will offer unique capabilities, such as direct measurements of slow dynamics in a time domain over wide Q ranges for time-resolved spectroscopic studies. PIONEER, described by Liu et al. (2022), is a single-crystal diffractometer optimized for studying small-volume samples (<1 mm³) in various sample environments. PIONEER will reduce sample volume requirements for single-crystal neutron diffraction experiments by more than one order of magnitude, allowing a detailed atomic-scale structural characterization of materials at the earliest stages of discovery when large crystals are typically unavailable.

QIKR (Ankner et al., 2023) is a horizontal sample surface reflectometer consisting of two independently operable end stations, one with the incident beam directed up and the other directed down. It will often be possible to collect complete specular reflectivity curves using a single instrument setting to provide a “cinematic” operation, in which the user “videos” the sample undergoing time-dependent changes.

VERDI, described in Garlea et al. (2022), is a wide bandwidth diffractometer with full polarization capability for powder and single crystal samples. The instrument will offer a high resolution at low momentum transfers with a high signal-to-noise ratio for routine measurements of small magnetic moment compounds from milligram-size samples.

This special topic journal issue describes the eight instruments that will be constructed as part of the STS project. Four additional instrument concepts not selected for construction in the initial instrumentation round are also detailed within this special edition.

This research used resources of the Spallation Neutron Source Second Target Station Project at ORNL under Grant No. ERKC2TS. ORNL is managed by UT-Battelle LLC for DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States.