SANSARA - Small-Angle Neutron Scattering Instrument

Leader: M.V. Avdeev

 

The SANSARA (Small-Angle Neutron Scattering And RAdiography) instrument is a small-angle neutron scattering (SANS) diffractometer combined with a neutron radiography (NRG) station located at beamline 10a with cold neutrons. The construction of a new advanced small-angle neutron scattering instrument at IBR-2 with the most efficient realization of the capabilities of neutron scattering, is an important part in the development of structural methods for studying nanosystems at the reactor. Following the current trends in the development of neutron centers, the instrument is a general-purpose small-angle diffractometer aimed at providing a wide range of possibilities for conducting SANS experiments. The optimization of the setup follows the conventional configuration of a SANS instrument at a cold neutron source. To enhance the efficiency of using cold neutrons, it is proposed to combine a SANS diffractometer with a neutron radiography station.

 

1. Scientific program

The SANSARA (Small-Angle Neutron Scattering And RAdiography) setup is a small-angle neutron scattering (SANS) diffractometer combined with a neutron radiography (NRG) station located at beamline 10a with cold neutrons.

Small-angle neutron scattering is one of the widely used methods of structural studies of nano-objects – systems whose properties are determined by structural features at a level of 1-100 nm. The scientific program of small-angle instruments includes a variety of research areas:

• Complex fluids, including magnetic fluids, surfactant solutions, anisotropic fluids, liquid crystals, etc.

• Magnetic nanocomposites
• Polymers, including magnetic polymers
• Biological macromolecules, membranes, vesicles
• Liposomes, including magnetosomes
• Dispersions of carbon materials
• Inhomogeneities in structural materials

 

The use of neutron scattering in the study of nanosystems is determined by two factors: (1) wide possibilities of contrast variation based on the isotopic substitution of atoms in the systems under study; (2) magnetic neutron scattering, which allows obtaining information on magnetic correlations in magnetic systems. Thus, the construction of a new advanced small-angle neutron scattering instrument at IBR-2 with the most efficient realization of the capabilities of neutron scattering, is an important part in the development of structural methods for studying nanosystems at the reactor.

The construction of the instrument is fully consistent with the current trends in methodological development. Over the past five years, general-purpose small-angle diffractometers have been designed and are successfully operating at almost all neutron sources, both pulsed and steady-state. Their main task is to meet a huge demand for this technique.

 

2. Scientific and methodological groundwork

The YuMO time-of-flight small-angle neutron scattering instrument [1], which is oriented to work with the thermal moderator (T = 300 K) and uses collimation with the direct view of the moderator [2], is successfully operating at the IBR-2 pulsed reactor. To obtain a scattering curve in the range q = 0.05 - 5 nm^-1, a special procedure is used with a continuous calibration to the vanadium standard placed in front of the detector. The time spent on the calibration is compensated by a high peak intensity when using the thermal moderator, which reduces the characteristic measurement time per one curve to an interval of 10 - 90 min (depending on the cross section of the sample).

The main advantage of YuMO is a two-detector system with large-area ring detectors for detecting isotropic scattering and central openings in the detectors allowing the direct beam to pass. This feature makes it possible to realize a record (~100) dynamic q-range (scanning range in one measurement run). Thus, the setup can be effectively used to study changes in the nanoscale range (10-100 nm) in real time (time resolution down to 1 min).

This feature, however, imposes some limitations. Since the instrument is optimized for the thermal moderator, there is a limitation in resolution at the minimum q-value, which narrows the sensitivity in the size range (submicron region). Also, because of the direct view of the source, the background level is comparatively high. The detector openings complicate the use of conventional designs of position-sensitive detectors (PSD), as well as the use of direct measurement of the transmission and corresponding calibration.

The design of the new SANS instrument is aimed at eliminating the above limitations, which is possible due to the availability of a cold moderator. The natural compensation will be a decrease in the average beam intensity at the sample. The availability of a large-area 2D PSD with the conventional procedure for obtaining and calibrating scattering curves at two detector positions (short and long flight paths) will allow experiments for a large number of equilibrium systems, including systems with scattering anisotropy (oriented systems, magnetized magnetic systems). In turn, this will make it possible to focus the YuMO scientific program on the study of kinetic phenomena.

3. Conceptual design of the instrument

 

The concept, simulation and selection of the optimal configuration and moderator temperature, as well as estimates of the instrument parameters were published in [3].

The schematic layout of the proposed instrument is presented in Fig. 1. For the implementation of the project, it is proposed to use channel 10 of the IBR-2 reactor, which is currently split into two beamlines: 10a and 10b. At present, the GRAINS reflectometer (reflectometer with a horizontal sample plane) is operating at beamline 10b [4]. Beamline 10a is equipped with a head part with a neutron guide (supermirror m = 2) and a background chopper in the ring corridor. Also, the head part is followed by the previously installed multi-slit optical beam deflector (bender), a supermirror m = 2, bending angle of 8 ° at a length of 2 m.

The main factors determining the parameters of SANSARA are: (1) minimization of fast neutron background; (2) measurement in a wider (with respect to small q-values) range of momentum transfer.

The implementation of (1) is provided by the tangential nature of beamline 10 relative to the reactor core and the use of a neutron bender with the beam axis diverted from the direct view of the neutron moderator.

The implementation of (2) is provided by the availability of a cold moderator at beamline 10.

Fig. 1. Schematic layout of SANSARA.

A replaceable beam-forming system together with an additional shutter are installed behind the bender (Fig. 2). The system provides the following options: NRG (radiography) and SANS (small-angle neutron scattering). Switching between the options is provided by three separate beam-forming units in the vertical direction (one unit for NRG and two units for SANS) via remote control.

Fig. 2. Conceptual view of the beam-forming system of the SANSARA instrument in the experimental hall of IBR-2. Main units: (A) movable shutter with a vacuum tube; (B) mechanical part for changing the vertical position of neutron guides to form a beam; (C) bender; (D) NRG platform; (E) NRG vacuum tube; (G) two SANS beam-forming units (neutron guide and vacuum tube).

For the most efficient use of neutron scattering in terms of sensitivity to the size range, the SANS instrument with the conventional configuration requires a cold moderator with a temperature of 30 K. The availability of cold neutrons additionally allows the use of neutron-optical devices to separate fast and cold neutrons and significantly reduce the background level at the sample. Also, to enhance the efficiency of using cold neutrons, this research technique can be relatively easily combined with other methods, such as neutron radiography and tomography. As a result of the use of a bender (beam deflector) optimized for a temperature of 30 K, the instrument is intended only for operation in the cold mode of the moderator.

 

Expected technical parameters

Beam size	50´50 mm2
Neutron wavelength range:	0.5 - 15 Å
q-range	0.001 - 1 Å–1
Angular resolution	5 - 20 %
Sample dimensions	5´5´1 - 20´50´50 mm
Neutron flux at sample position	1.0´106 cm–2 s–1
Detector	2D PSD, efficiency > 50% (0.2 nm) 64´64 - 80´80 cm2, resolution 5´5 - 10´10 mm2 count rate 105 - 106 s–1

 

5. Expected scientific results

The SANS technique, which is used in solving a wide range of fundamental and applied problems within a broad research scope related to the nanoscale structure of matter, remains one of the most popular methods among neutron scattering techniques. Such kind of problems are very important and of great current interest in various sciences, including condensed matter physics, physics and chemistry of complex liquids and dispersed systems, including solutions of surface-active agents and polymers, biophysics and molecular biology, materials science. The most important area of application of small-angle scattering is the analysis of the structure of disordered systems using non-destructive testing with an emphasis on obtaining direct structural information about systems with chaotic and partially ordered arrangement of density inhomogeneities with sizes of the order of 1 - 100 nm. This includes dispersed structures of alloys, powders, and glasses (phase separation mechanisms, particle size and degree of polydispersity), structural features of polymers in various states of aggregation, weight and geometric characteristics of biological macromolecules and their complexes, biological supramolecular structures, such as biological membranes and viruses.

 

References

 

1. I.Kuklin, A.V.Rogachev, D.V.Soloviov, O.I.Ivankov, Y.S.Kovalev, P.K.Utrobin, S.A.Kutuzov, A.G.Soloviev, M.I.Rulev, V.I.Gordeliy. Neutronographic investigations of supramolecular structures on upgraded small-angle spectrometer YuMO, J. Phys.: Conf. Ser. 848 (2017) 012010.
2. YM.Ostanevich, Time-of-flight small-angle scattering spectrometers on pulsed neutron sources, J. Macromol. Chem., 15, 91-103 (1988).
3. Avdeev M.V., Eremin R.A., Bodnarchuk V.I., Gapon I.V., Petrenko V.I., Erkhan R.V., Churakov A.V., Kozlenko D.P. Concept of small-angle diffractometer in classical configuration at the cold moderator of the IBR-2 reactor. Surf. Investigation 12 (4) (2018) 638-644.
4. V.Avdeev, V.I.Bodnarchuk, V.I.Petrenko, I.V.Gapon, O.V.Tomchuk, A.V.Nagorny, V.A.Ulyanov, L.A.Bulavin, V.L.Aksenov. Neutron time-of-flight reflectometer GRAINS with horizontal sample plane at the IBR-2 reactor: possibilities and prospects. Crystallography Reports 62(6) (2017) 1002-1008).