Studying Soft-matter and Biological Systems over a Wide Length-scale from Nanometer and Micrometer Sizes at the Small-angle Neutron Diffractometer KWS-2

The overall goal of this experiment is to investigate the size and the arrangement of various polymeric morphologies in water solution over length scales between tens of angstroms and one micron. This method can help answer key questions in the soft matter and biophysics fields, such as polymer melds and solutions, amphiphilic block copolymer assemblies, gels, colloids, protein denaturation, liposomes drug carriers. The main advantage of this technique is that the structural and morphological characterization is carried out over a wide range of lengths with adjustable resolution on a single neutron scattering instrument.

This demonstration takes place at the KWS-2 diffractometer of the Julich Center for Neutron Science, Maier-Leibnitz center. The diffractometer has three working modes, including a conventional pinhole mode. Neutrons first pass through a velocity selector, optionally through a beam chopper, and collimator apertures at the entrance and the sample, before they are scattered by the sample.

A position sensitive detector registers the scattered particles. A beam-stop prevents the direct beam from saturating the detector and defines the minimum scattering angle that can be reached for a given instrument configuration. The second mode is a high intensity focusing mode.

In it, there are lenses before the sample and a larger collimator sample aperture. These allow measurement of larger samples with the same resolution as the pinhole mode. The third mode can achieve a lower minimum wave vector transfer than the pinhole mode.

This is done with a smaller collimator entrance aperture and lenses. It also uses a smaller high resolution position sensitive detector and a smaller beam-stop. Place the small high resolution detector tower at the end of the unit.

Begin at the sample stage of the diffractometer to position the prepared samples. Here, the samples are in place on the sample stage and in the beam line. The samples include polystyrene particles in water and heavy water, a diblock copolymer in heavy water, and references samples.

With the samples in position, leave the sample area and close the lead door to continue. In the control room, move to the control computer to start the measurement software. This is the main screen of the control software.

In the next steps, the focus will be on the functions at the left of the screen. First, select the Configuration function to get to the Configuration window. From there, select the User Data menu option.

This leads to identification and comment fields that should be completed. When done, leave by clicking on Save. Next, select the Sample function.

In the window that opens, there will be a list of samples and positions along the left. Select one of these and begin to enter the requested information. The entered information includes the sample title, the sample beam window, information on the sample thickness, and a comment.

Move to Close and click it to save the information. Enter the information for each sample in the experiment. When done with all the samples, leave the Sample window by clicking Close.

Back in the Configuration window, save all configurations using the File menu options. Then, close the Configuration window. In the main screen, select the Definition function.

The Definition window is used to define the experimental setup and measurement program. Select the Sample function which opens the Select Samples window. In the Select Samples window's known samples column, choose the 12 samples that must be measured.

Use the blue arrow to move these to the selected samples field. To reorder an entry in the selected samples, select it and use the vertical blue arrows. If necessary, change the information fields for the samples.

Leave the window by clicking Save/Close. Now, choose the Detector function from the Definition window. This opens the Definition of Measurements window.

Move to the Selector field and choose the appropriate values for the wavelength. Go to the Measurement area and select Standard to choose a static measurement. Go to the End Conditions area to choose the proper time unit for the measuring time.

Continue by going to the Select Detector and Collimator Distances area. Here, the fields for the experiment measurement's time, lens and polarizer settings, and the collimation distance are filled in. With this configuration entirely defined, click the New button.

This will fix the configuration and store it in the table below. After the entire set of configurations is defined, click Save/Close. The program will generate a list of measurements.

These can be sorted using the sorting conditions at the bottom of the menu. Remove measurements or adjust their measurement time as needed. Leave this screen by clicking Save/Close.

Then, in the Definition window, click Close to return to the main screen. Proceed by choosing the Control function. On the new screen, log in to lock the session for generating the control script.

Select Loop Definition to check the uploaded measurement program. Choose the Current Values tab to see the visualization of the instrument parameters during the measurements. When ready, press the Start button and answer the safety related questions to start the measurements.

Across the top of the screen there is information about the collimator position, the detector position, and active devices. The middle part of the screen has information about the current sample and the status of the beam shutter and lenses. Along the bottom of the screen is information related to timing of the measurements, reports on detector and monitor intensity, event counting braids, and details on the velocity selector and the chopper parameters.

After the measurements are completed, open the data processing software. From the opening screen, choose the Start New Session option on the right side of the window. Then, click the data processing tab.

Across the top of the right hand region, there is a horizontal slider. Use it to define the number of conditions used in the experiment. Next, identify the yellow pencil symbols which indicate rows that require input.

Run numbers should be supplied for the empty cell, EC, the blocked beam, BC, and calibration samples. The center fields are for the run numbers of measurements with strong forward scattering. The EB fields are for the run numbers of empty beams.

Check the boxes below to calculate the transmission of samples. Enter the appropriate values for these fields in each experiment. Once the table is filled in, identify all of the rows that have buttons with green arrows and click on each of them.

This loads information necessary for data processing. Next, name each of the yellow columns by clicking on the head of the column and providing a label. When done naming the columns, move to the new button and click it.

This generates the list of files to be processed. Name the list before continuing. Then click the Add button to load the data files.

Move to the Transmission button and click on it to find the transmission of each sample. The results will appear in a generated table. Choose Project to save results in the current project.

Next, click the I(x, y)button to perform correction and calibration of 2D data. Follow this with clicking the I(q)button for correction, calibration, and radial averaging of data. The data from these actions is accessible through the folders in the window at the bottom of the screen.

This is the scattering pattern for polystyrene particles with a radius of 500 angstroms, using a detector distance of eight meters and a wavelength of five angstroms. This pattern is for polystyrene particles with a 1000 angstrom radius, the detector at 20 meters, and a wavelength of 20 angstroms. This final pattern is for particles with a 4000 angstrom radius, a detector distance of 17 meters with lenses in the secondary high resolution detector, and a wavelength of seven angstroms.

In all cases the scattering pattern is isotropically distributed around the beam-stop, which blocks the transmitted beam. Here is the corrected and calibrated cross section for polystyrene particles in heavy water with a radius of 500 angstroms. The diffractometer can cover a wide Q range in conventional pinhole mode by varying the detection position and using one or more wavelengths.

The data showed the form vector features of spherical particles. At high Q the profile is dominated by the solvent and is flat. These cross sections for different radii polystyrene particles are corrected for solvent contribution.

At high Q the slope is minus four, which is typical for spherical objects. Measurements of micelles in heavy water lead to this two dimensional and radially average scattering pattern in pinhole mode. Use of the tunable resolution mode with higher resolution reveals the fine structure of peaks.

Once mastered, this technique can be done in 24 hours if it is performed properly. While attempting this procedure, it's important to remember to plan the experiment based on the scientific goals. Following this procedure, other methods like optical microscopy and cryo-TEM can be performed to help determine the overall and the local morphology of the investigative system as an aid for interpreting the complex scattering data.

After its development, this technique paved the way for researchers in the field of soft matter and biophysics to explore polymer and block copolymer morphologies, proteins and supramolecular particles, gels and colloidal system, in health and technology applications. Don't forget that working with neutrons can be extremely hazardous and precautions, such as tradition protection measures, should always be taken while performing this procedure.