Extended Q-Range Small-Angle Neutron Scattering Diffractometer (EQ-SANS)

Fig 1. Extended Q-Range Small-Angle Neutron Scattering Diffractometer at SNS. Click image for a larger view.

The EQ-SANS Diffractometer (see Fig. 1) is designed to study non-crystalline, nano-sized materials in solid, liquid, or gas forms such as polymers, proteins and other large biological molecular complexes in solution, and micelles. It is designed to have high useable neutron flux, high wavelength resolution (or precision), and wide Q-coverage. The combined properties of high precision and wide Q-coverage will make this instrument suitable to be simultaneously used as a low- to medium-resolution diffractometer, thus opening many exciting opportunities for new sciences. An example is the study of protein-membrane interactions, where protein signals appear at low Q while lipid signals show up at high Q. Other applications include membrane fusion, and engineering materials, where simultaneous monitoring of domain small angle scattering and crystal diffraction is of great interest.

The smallest accessible Q-value is 0.004 Å^-1 while the largest Q-value accessible with the baseline instrument is 4 Å^-1, as displayed in Table 1.  The design allows for an upgrade to provide additional detectors at higher scattering angles to access even higher Q-values if desired. Because of this broad Q-range, the SNS SANS instrument is also referred to as the Extended-Q SANS. The useable flux at the sample position for comparable resolution will be as high as or higher than at existing SANS instruments. This, together with the uniquely wide Q range, makes this a best-in-class instrument.

The minimum accessible Q-value on the Extended-Q SANS is obtained when the low angle detector is at its maximum distance of 22 m (8 m detector-to-sample), giving a minimum Q value of ~ 0.004 Å^-1 when 11 Å neutrons are used (3rd frame). The maximum usable Q value of ~4 Å^-1 is given by the use of 1 Å neutrons at the highest detector angle of the moveable two-dimensional detector array. This Q-value can be extended by adding detectors at higher angles in a future upgrade. Since biological samples frequently have time-dependent properties, experimentally desired (e.g. kinetics) or undesired (e.g. aggregation), the ability to simultaneously collect data over all the required Q-range is often a prerequisite to study these systems.

Data collection time depends both on scattering properties of the sample and on the required Q range. For strong scatterers, such as polymers at high concentrations, collection of a single data set within one minute will be possible for all accessible Q ranges. For weak scatterers, such as proteins in dilute solutions, the per data set collection time less than 10 min will be possible when Q down to 0.004 Å^-1 is needed.

For SANS measurements, neutrons from the guide are scattered by the sample. The time of arrival of a neutron at a detector gives the total time of flight (TOF) from moderator to sample to detector, and this time in turn can be used to deduce the wavelength of the incident neutron. Knowledge of the scattering angle comes from the location of the neutron detection event within the two-dimensional detector area. These parameters provide complete information about the momentum transfer Q in the scattering event. Carrying out this process for a large number of incident neutrons builds up a pattern that shows the probability of scattering as a function of the momentum transfer in the scattering process.

The basic layout of the instrument is illustrated in Fig. 2. Neutrons from the coupled H₂ moderator are guided to the sample position at 14 m from the moderator via a combination of a short channel beam bender and a straight neutron guide. The bender is effectively nine narrow curved supermirror neutron guides in parallel, with very thin supermirror-coated septa between the adjacent guides. This arrangement allows the guides to have a relatively short radius of curvature (~95 m) while still transmitting neutrons down to short wavelengths. Each of the parallel guide channels has a cross-section of ~0.4 x 4 cm². The first ~2 m of the bender are in the primary shutter in the target monolith, while the remaining ~1 m of the bender is downstream from the shutter. The bender is followed by a 5 m long straight guide. The combination of bender plus straight guide results in a line-of-sight distance of ~8 m from the moderator for fast neutrons.

Fig 2. Sketch of the EQ-SANS diffractometer. Click image for a larger view.

Two single-disk neutron bandwidth limiting choppers are located at distances of 5.3 m and 7.8 m from the moderator, and a dual-disk neutron bandwidth limiting chopper is located at 9.8 m. The openings in these chopper disks are sized to provide a bandwidth of 4.4 – 3.0 Å when operated at the design speed of 60 Hz, varied to match the frame bandwidth between for detector positions between 15 m and 22 m from the moderator. This 4.4 – 3.0 Å band can be centered at any desired wavelength by changing the chopper phasing, and by adjusting this phase this instrument can use the wavelength range ~1 Å – 4 Å (first frame), ~4 Å – 8 Å (second frame), or even higher frames depending on the need of the particular experiment. Additionally, by reducing the speed of the choppers, pseudo 30 Hz (or lower) operation can be realized with correspondingly wider wavelength bandwidth.

This incident beam line also includes a 100 cm long secondary shutter starting at ~10 m from the moderator and linked to the PPS system (see below). The secondary shutter contains beam transport optics or apertures, which are rotated out of the beam when the shutter is closed. A low-efficiency beam monitor in the incident beam provides wavelength spectrum data for the incident neutrons. The incident beam line, including secondary shutter, beam monitor, and all three choppers, is surrounded by massive shielding made of steel, heavy concrete, and normal concrete in proportions optimized to provide the desired shielding characteristics. This shielding is designed to reduce radiation doses at its surface to below 0.25 mrem/hr when the facility is operating at 2 MW.

The neutron guide system transports the beam to a sample position. Slits upstream from the sample position control the size and the angular divergence of the neutron beam incident on the sample. Neutrons that are scattered by the sample will be counted by a two-dimensional detector array located in an evacuated scattering tank downstream from the sample. This detector is made up of an array of ³He proportional counters with a total active area of 100 x 100 cm² and a resolution of <=8 mm, as illustrated in Table 2. This detector array can be moved along the scattering tank between ~15 m and ~22 m from the moderator. The instrument design includes room for a future upgrade to include a high-angle detector array to extend the Q-range. A state-of-the-art data acquisition system, located in a cabin on the instrument platform, collects and organizes the data from the detector array(s) to present this information.

The primary user access will be through a gate into the controlled instrument area. A Personnel Protection System (PPS) linked to the primary and secondary shutter ensures that personnel can enter the controlled instrument area only when the one or both of these shutters is closed. The PPS can direct closure of the secondary shutter and can also force shutdown of the accelerator in order to prevent unsafe radiological conditions.

Documents

The following documents contain more detailed information about the expected performance and design of the EQ-SANS diffractometer:

Instrument Fact Sheet (PDF 853 Kb)

Conceptual Design and Performance Analysis of the Extended Q-range, High Intensity, High Precision Small Angle Diffractometer (PDF 998 Kb)

Chopper Design and Operation for the EQ-SANS (PDF 433 Kb)

Optical Components for the Extended Q-Range Small Angle Diffractometer at the SNS (PDF 369 Kb)