A new time-of-flight small-angle scattering instrument at the Helmholtz-Zentrum Berlin: V16/VSANS
aInstitute for Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, Berlin 14109, Germany, bInstitut Laue-Langevin, 6 rue de Jules Horowitz, 38042 Grenoble, France, cESS Target Division, European Spallation Source, Box 176, 22100 Lund, Sweden, and dInstitute of Physics, Humboldt University Berlin, Newtonstrasse 15, Berlin 12489, Germany
*Correspondence e-mail: email@example.com, firstname.lastname@example.org
Small-angle scattering methods have become routine techniques for the structural characterization of macromolecules and macromolecular assemblies like polymers, (block) copolymers or micelles in the spatial range from a few to hundreds of nanometres. Neutrons are valuable scattering probes, because they offer freedom with respect to scattering length density contrast and isotopic labelling of samples. In order to gain maximum benefit from the allotted experiment time, the instrumental setup must be optimized in terms of statistics of scattered intensity, resolution and accessible range in momentum transfer Q. The new small-angle neutron scattering instrument V16/VSANS at the Helmholtz-Zentrum in Berlin, Germany, augments neutron guide collimation and pinhole optics with time-of-flight data recording and flexible chopper configuration. Thus, the available Q range and the respective instrumental resolution in the intermediate and high momentum transfer regions can be adjusted and balanced to the individual experimental requirements. This renders V16/VSANS a flexible and versatile instrument for soft-matter research.
The past decade has seen tremendous progress in the instrumentation and application of neutron and X-ray scattering on soft-matter systems (Teixeira et al., 2008; Zanchetta & Cerbino, 2010). In particular, small-angle neutron and X-ray scattering have become indispensable methods in structural soft-matter research. These techniques are very widely used, for example, in the field of polymeric colloids or biological macromolecules (Ballauff, 2001; Svergun & Koch, 2003; Jacques & Trewhella, 2010). Concomitantly, the demand and need for new or updated instruments offering enhanced performance and/or new opportunities increases.
Small-angle neutron scattering (SANS) instruments employing the time-of-flight (TOF) method work with a broad spectrum of neutrons, the wavelength of which is proportional to their respective TOF values. The TOF is defined as the time that a given neutron needs to move from a certain position, for example from the neutron source, to the detector. Compared with conventional instruments that use a monochromatic beam of neutrons, the TOF method offers extended options in designing and analysing an experiment. In particular, the utilization of a broad wavelength band measures the scattered intensity I(Q) over a broader range of momentum transfer Q in a single experiment [Q = (4π/λ)sin(θ/2), where θ is the scattering angle and λ is the wavelength of the incident radiation]. However, applying the TOF method is much more demanding with respect to data acquisition and treatment (Ostanevich, 1988).
The first demonstrations and descriptions of the TOF method for SANS at the pulsed neutron sources IBR-30 in Dubna (Russia) and IPNS-II in Argonne (USA) can be traced back to the late 1970s (Gladkikh et al., 1977; Mildner, 1978; Carpenter & Faber, 1978). General principles and the applicability of the method were demonstrated in this work: for example, the prevention of the so-called `frame overlap' problem. After the emission of a pulse, the neutrons will move with different speeds towards the detector. Care must be taken that the fastest neutrons of the following pulse do not overtake the slowest ones of the preceding pulse and cause frame overlap. Since the frequency is fixed at pulsed neutron sources, this sets limits on both the width of the wavelength band and the length of the flight path between the neutron source and the detector (Crawford & Carpenter, 1988).
Since then, further TOF-SANS instruments have been constructed at pulsed sources at the IPNS in Argonne (USA), the KENS pulsed neutron source in Tsukaba (Japan), ISIS in Chilton (UK), LANSCE in Los Alamos (USA) and the SNS in Oak Ridge (USA) (Ishikawa et al., 1986; Seeger et al., 1990; Niimura et al., 1992; Heenan et al., 1997; Thiyagarajan et al., 1997; Otomo et al., 1999; Zhao et al., 2010). To our knowledge, the current user-accessible TOF-SANS instruments are LQD at LANSCE, LOQ at ISIS, YuMO in Dubna and EQ-SANS in Oak Ridge (Seeger et al., 1990; Serdyuk, 1995; Heenan et al., 1997; Zhao et al., 2010). There are other instruments being commissioned or under construction at ISIS (SANS2D), ILL in Grenoble, France (D33), and J-PARC in Tokai, Japan (HI-SANS) (Dewhurst, 2008; Teixeira et al., 2008; Shinohara et al., 2009).
The LOQ instrument at ISIS has been in operation for over 25 years. It utilizes neutron wavelengths between 2.2 and 10 Å and accesses a Q range between 0.006 and 1.4 Å−1, yielding a dynamic Q range of maximum Q over minimum Q (Qmax/Qmin) of 230 when using all detectors simultaneously. The standard dynamic Q range employing only the main detector is 40. An average flux of 2 × 105 neutrons cm−2 s−1 at the sample is reported for a typical instrumental setup (http://www.isis.stfc.ac.uk/instruments/loq/technical/loq-technical-information7485.html ).
At LANSCE, LQD has likewise been operational for more than two decades (Seeger et al., 1990). The most recent information reports an accessible wavelength band between 1.5 and 15 Å at 20 Hz and a Q range of 0.003 ≤ Q ≤ 0.5 Å−1 (http://lansce.lanl.gov/lujan/instruments/docs/LQD.pdf ). This corresponds to a dynamic Q range of about 170.
The YuMO instrument in Dubna, Russia, bears its name in honour of the (TOF-)SANS pioneer Yuri M. Ostanevich (Serdyuk, 1995). Utilizing sample-to-detector distances (SDs) of between 1.5 and 12.5 m, it accesses Q values between 0.008 and 0.7 Å−1 and provides a flux of thermal neutrons between 6 × 106 and 3.7 × 107 cm−2 s−1 (Serdyuk, 1995). However, these are old specifications, and the recent refurbishment of the IBR-2 reactor and the new start of the instrument will bring upgrades and improvements (Erhan et al., 2011).
The newest instrument is EQ-SANS, operating at the SNS in Oak Ridge, USA. The neutron source operates at 60 Hz, which considerably reduces the applicable wavelength band. By operating the frame-definition choppers in the so-called `frame-skipping' mode, the widths of the effective waveband can be extended and the instrument can effectively be run equivalent to a 20 Hz SANS machine, yielding a Q range between 0.003 and 0.4 Å−1 for SD = 5 m (Zhao et al., 2010), corresponding to Qmax/Qmin ≃ 130. The technical specification given on the Oak Ridge National Laboratory web site reports a total coverage of momentum transfer 0.002 < Q < 1.4 Å−1 and an integrated neutron flux between 107 and 109 cm−2 s−1 at the sample (http://neutrons.ornl.gov/instruments/SNS/factsheets/Instrument_6.pdf ).
Originally designed for pulsed sources, the TOF method has become attractive for instruments at continuous reactor-based neutron sources too, because here the neutron pulse is generated with a system of choppers, and hence the pulse length, and thus the accessible Q range and wavelength resolution, can be adjusted to selected requirements (Dewhurst, 2008). The new instrument V16/VSANS at the BER II reactor in Berlin is the first SANS instrument to use the TOF option at a continuous reactor source and thus allows measurements in a flexible and broad Q range. The acronym VSANS stands for `very small-angle neutron scattering' and refers to a special collimation mode the instrument will offer: multi-pinhole grid collimation will allow the determination of scattered intensities down to a momentum transfer region of 10−4 Å−1.
The present report is restricted to the `classical' neutron-guide collimation that the V16/VSANS instrument employs as its standard mode, focusing on the utilization of the TOF technique to meet particular experimental requirements. In the first part, the theory and construction of the instrument will be described. In the second part, experimental data will be discussed and compared with the theoretical expected values. We conclude with a comparison of the instrument's performance with that of other instruments.
V16/VSANS is located on the NL4C guide at the BER II reactor behind a supermirror-coated multichannel bender. The cross section of the neutron guides expands from 40 × 40 mm before the bender to 80 × 80 mm at the instrument in order to reduce the divergence of the neutron beam and enlarge the usable cross section. Using Au-foil activation measurements, an average neutron flux of (3.6 ± 0.1) × 108 cm−2 s−1 was determined with deactivated choppers at the entrance of the collimation section. Depending on the configuration of choppers, collimation and apertures, the neutron flux at the sample site can vary over a large range (see §2.2). As an example, the neutron flux of the primary beam as a function of wavelength for a high-collimation configuration determined at the detector site is depicted in Fig. 1.
Note that an incident beam containing the whole wavelength range depicted in Fig. 1 could be emitted to the sample in the form of pulses. However, the respective chopper configuration would exhibit large inaccuracies in the wavelength λ. Sufficiently accurate chopper configurations (average Δλ/λ < 0.1) employ small wavebands exhibiting widths between 2 and 6 Å (see §2.2).
The overall length of V16/VSANS from first chopper to beam dump (see Fig. 2) is 32.45 m, of which approximately 2 × 12 m are allotted to the collimation and detector sections, respectively. The first two choppers are positioned very close to each other at distances of 20.35 and 20.32 m from the sample. They can be rotated in parallel or in opposite directions and thus can serve as pulse choppers, either separately or in combination. Chopper 1 has an opening aperture φ1 of 11.1°, corresponding to the width of the neutron guide (60 × 60 mm) at this radial height. The opening aperture of chopper 2, φ2, is 22.2°. Thus, employing the first chopper as a pulse chopper allows the generation of short pulses with a good time resolution Δt at a given chopper frequency ν, while chopper 2 will generate longer pulses but with a transmission twice as high. Utilization of chopper 1 as the pulse chopper is the setup of choice for small pulses and thus high accuracy Δλ/λ, which leads to a good resolution in momentum transfer ΔQ/Q. Combining choppers 1 and 2 allows the generation of even smaller pulses. Employing chopper 2 as the pulse chopper increases the neutron flux on the sample and is therefore advised for test measurements and/or weakly scattering samples.
Choppers 3 (φ3 = 16.72°) and 4 (φ4 = 60°) are utilized to tailor the neutron pulse according to the requirements of the experiment. Chopper 4 is the waveband chopper of the array and will define the widths of the waveband transmitted by the chopper array. Chopper 3 serves to avoid frame overlap, i.e. it cuts out slow neutrons with such a long wavelength that they might be overtaken by fast neutrons of the following pulse, leading to a false TOF assignment for the respective neutrons. The choice of opening aperture for chopper 3 of 16.72° takes the broadening of the pulse at the respective distance into account.
All four choppers can be run up to a maximum speed of 3000 r min−1, corresponding to pulse lengths between 0.62 and 1.24 ms when employing chopper 1 or 2 as pulse chopper, respectively. The requirement to operate the choppers from low up to relatively high speeds in order to create a broad range of pulse lengths places limits on the widths of the opening apertures. Therefore, the opening aperture of waveband chopper 4 was set to 60°, although a wider one might be desirable for many cases. However, wider apertures would be technically very challenging for the desired rotating speed range.
The chopper configuration can be adjusted to the individual needs of a given experiment. If the resolution in momentum transfer Q plays a minor role, or in the case of weakly scattering samples, a setup can be chosen having a broad wavelength band and high flux but poorer time resolution Δt/t. In cases where a good resolution ΔQ/Q is needed or for strongly scattering samples, the resolution Δt/t can be improved at the expense of the transmitted neutron flux. The adjustable parameters are the choice of pulse chopper, the phase shifts ϕi of the other choppers compared with the pulse chopper and the frequencies νi. The opening aperture φi values and the positions si along the x axis are fixed.
The rotating frequency ν of the pulse chopper determines the width of the transmitted waveband between λmax and λmin. Small frequencies will yield a broad band, while higher frequencies will narrow the range. The positions of λmax and λmin (but not the width of the band λmax − λmin) can be set by changing the phase shift ϕi, namely by adjusting the phase shift ϕ4 of the waveband chopper 4. The corresponding formulae and a detailed discussion are given in §2.2, Theory.
The standard collimation system at V16/VSANS using neutron guides can be set to a collimation distance between the source and sample apertures of 1, 2, 4, 6, 8, 10 or 12 m by positioning the guide elements accordingly (see Fig. 2). In each collimation configuration, smaller 40 × 40 mm source apertures can be placed behind the final neutron guide. These reduce the divergence of the beam and thus increase the accuracy of the determination of the scattering angle θ. Using the various collimation and sample-to-detector distances in combination with a 0.9 × 1 m tube detector allows a Q range from about 0.003 to 0.6 Å−1 to be scanned. The exact range depends on the waveband utilized, as outlined above. The upper limit can be further extended by translating the detector along the y axis, which is perpendicular to the beam direction.
The time-of-flight method employing choppers was established decades ago and different setups have since been developed, e.g. for neutron diffractometry and spectroscopy (Seidl et al., 1954; Sköld, 1968; Steichele & Arnold, 1973; Copley, 1988; van Well, 1992; Copley & Udovic, 1993). In the following, a simple formalism is described which allows a discussion of the impact of the chopper configuration of the V16/VSANS instrument on the wavelength resolution in quantitative terms.
The total transmission T(λ) of a given chopper system will be given by the ratio of the number N(λ) of neutrons after passing all the choppers to the number N0(λ) before entering the first chopper site. Knowledge of this parameter is of crucial importance, e.g. in order to avoid overlap between the time frames of two adjacent neutron pulses.
T(λ) can be expressed as a function of N0(λ) and of the transmissions Ti of each individual chopper i as a function of wavelength and distance Δsi,
where Nc is the total number of choppers and K is a normalization constant. The length Δsi is the distance from a given chopper to the neutron source, e.g. a spallation source.
For instruments with a continuous source of neutrons like V16/VSANS, the neutron pulse will be generated by the first chopper. Since the flux is not a function of time at a continuous source, the transmission of the first chopper is simply given by the ratio of the opening aperture to the full angle. After passing the first chopper, however, the transmissions become wavelength dependent. Moreover, the transmission behaviour of a given chopper will be a function of its frequency νi as well as of the relative phase shift compared with the first chopper.
Using ϕi as the symbol for the phase shift between chopper i and the pulse chopper, the transmission of chopper i as a function of time t can be defined as follows:
Setting the result of the function as νi for the above boundaries guarantees that the integration of Ti(t, ϕi, νi, φi) over time from t = 0 to t = νi−1 will yield the fraction of time the chopper is in the open state relative to the duration of the whole chopper cycle without further constraint as is required for the usage in equation (1).
To calculate the transmission according to equation (1), equation (2) must be converted to the wavelength domain. Moreover, the length of the neutron pulse τ has to be taken into account, as without doing so the whole wavelength spectrum would be compressed into an infinitely small time interval. In our case τ equals the time interval in which the pulse chopper is in the open state.
We introduce the `starting time' t0 as the time at which an ensemble of neutrons with different wavelengths leaves the exit of chopper 1 (or any other chopper acting as the pulse chopper) at position s1. t0 = 0 when chopper 1 opens and t0 = τ when chopper 1 closes. It is assumed that the wavelength distribution of this ensemble is independent of time t. Moreover, we express time t in equation (2) via the wavelength of the neutrons and the distance Δsi between the pulse chopper and chopper i using the well known relation of de Broglie and obtain
where m is the mass of one neutron, h is Planck's constant and t0 is in the range 0 ≤ t0 ≤ τ.
Equation (3) can be solved via numerical integration over t0 from 0 to τ. In the case of V16/VSANS, the first chopper creates the neutron pulse and thus τ equals the time interval in which this chopper is in the open state:
Here, φ0 depicts the angular value for a full rotation of the chopper, i.e. 2π or 360°.
These equations are valid under the condition that both time t = 0 and time t0 = 0 are defined as the time point when the pulse chopper opens. Since the phase shift ϕ between chopper j and chopper i is by definition the angular difference between the central points of their respective opening apertures φj and φi, the phase shift ϕi denoted in the above equations includes the shift due to the different opening apertures of these choppers as well as the angular difference between the aperture centres ϕj,i:
Note that, according to this definition, ϕj,i is negative for the case when the state in which chopper i is open is shifted to higher values of t than chopper j.
The individual chopper transmissions Ti calculated according to equation (5) are the fraction of neutrons passing chopper i with reference to the white beam as a function of wavelength. The loss of neutrons at the pulse chopper is therefore taken into account for each calculation of Ti. Calculating the transmission for more than one chopper according to equation (1) as a product of each Ti therefore requires a correction for this excess loss for each additional chopper. The normalization constant K is thus given by the expression
T(λ) is closely related to the resolution of the experiment in the time domain and, since the TOF is proportional to λ for a constant SD, to the resolution in the wavelength domain too. The maximum transmission equals the ratio of the opening aperture of the pulse chopper φ1 to φ0. In this case, the corresponding wavelength can pass the chopper setup for the full length of time it is in the open state, τ. Accordingly, the TOF for this wavelength can simply be determined with an accuracy of ±τ/2. Wavelengths with a lower transmission will exhibit a correspondingly better accuracy Δt in the determination of the TOF. Therefore
Fig. 3 illustrates the above relations in a qualitative sketch. Fig. 3(b) depicts a path versus time diagram for neutrons emitted by the pulse chopper at a given distance from the waveband chopper and the detector. The trajectories of the neutrons are visible here as straight lines. The slope of these lines is dependent on the velocity, i.e. the wavelength λ, of the respective neutrons. In the upper section of Fig. 3(b), the times when neutrons of a given wavelength λ reach the detector can be read. The absolute time t is of minor interest here. For the calculation of the wavelength of a detected neutron, the TOF measured as the difference between pulse initiation and time of detection is needed. Therefore, we introduce the symbol t* for the TOF:
where t*min is the TOF of the fastest neutrons and t*max = t*min + ν−1.
It is obvious that only a waveband spanning from a minimum wavelength λmin to a maximum wavelength λmax can pass the choppers (red and blue lines in Fig. 3). These limiting wavelengths can pass the chopper array along just a single trajectory, i.e. the transmission is nearly zero. Between a lower and an upper wavelength limit λ1 and λ2, neutrons can pass the array for the whole opening interval τ of the pulse chopper (yellow and green shaded areas). For these wavelengths, the transmission and likewise Δt reach a maximum value. Below and above these wavelengths, T(λ) decreases linearly to zero (see Fig. 3a, black line). Accordingly, the accuracy of the determined Δt/t*, and concomitantly Δλ/λ, are nearly zero for λmin and λmax. For λ1 and λ2, these values are given by τ/t*1 and τ/t*2, respectively.
Experiments on V16/VSANS are completely electronically controlled via the computer program CARESS, which has been developed at the Helmholtz-Zentrum Berlin and is used at many instruments there as control software. CARESS allows the user to control every important instrument parameter via input over a terminal interface or window menus. Pre-edited scripts can be uploaded to the program, and thus a whole set of measurements can be performed automatically without further manual input or attendance of instrument users.
Initial visualization and assessment of the raw data can be performed with the program EGRAPH, developed at the Department for Data Processing at the Helmholtz-Zentrum Berlin. This program allows the export of raw data into the NeXus format, as well as into a histogram of the data with self-defined binning schemes (Klosowski et al., 1997). The exported data can in turn be loaded into the program MANTID (http://www.mantidproject.org/Main_Page ). For this purpose, a modified version of the MANTID LoadRaw software module is employed, which was adapted by Dr Wolf-Dieter Stein from the Helmholtz-Zentrum Berlin. Within the MANTID environment, predefined functions or self-programmed Python scripts can be used for further data processing. Experimental data were normalized to the neutron flux and the transmission of the sample. Reference measurements were subtracted from the sample data and radially averaged to obtain the scattered intensity I(Q) as a function of the modulus of momentum transfer Q. The scattering from a water sample with a thickness d = 0.1 cm was employed to normalize the radially averaged data, approximating I(Q, H2O, 0.1 cm) ≃ 1 cm−1 for all wavelengths λ. Since the differential cross section per solid angle per unit volume for water depends on both wavelength and sample thickness and can differ slightly for different instruments, this simplification would result in estimated inaccuracies of about ±25% for the present experiments (Lindner, 2000). Therefore, the resulting differential cross sections were verified and corrected using the scattering from a standard glassy carbon material (batch No. M25) which was kindly provided by Jan Ilavsky from the Argonne National Laboratory, USA (Zhang et al., 2010).
V16/VSANS is designed to allow individual adjustment of instrument parameters to the experimental requirements. The primary question addresses the values of neutron flux and wavelength resolution. An increase in flux is at the expense of resolution and vice versa. Thus, the preference towards a better resolution or a higher neutron flux has to be chosen for a given sample. While a strong scatterer allows us to measure with lower flux, weaker scatterers need to be measured with higher flux to obtain the data within a reasonable time. In the absence of fine structure, or if the determination of it is not needed, the flux can be increased at the expense of the TOF resolution. In the following, these issues will be discussed and quantitatively exemplified.
The neutron flux can be controlled by means of the chopper configuration: the opening aperture of the pulse chopper determines the number of neutrons entering the instrument as well as the pulse length τ for a given frequency. This rotation frequency can be lowered, which broadens the wavelength interval λmax − λmin [see Fig. 3 and equation (4)]. This will likewise increase the pulse length and thus directly affect the parameter Δt/t* = Δλ/λ.
The choice of waveband from λmin to λmax is thus related to the flux as well as to the experimental resolution. One may choose values for λmin and λmax so that the selected waveband lies within the maximum flux of the incident spectrum (see Fig. 1). This will lead to a maximum flux at the sample site, but not necessarily to a maximum number of scattered neutrons. The scattering probability increases with increasing wavelength, so one might obtain a higher number of scattered neutrons with a λmin even larger than the wavelength with maximum incident flux. Longer wavelengths will, for a given scattering law, be preferentially scattered to higher angles θ with a lower Δθ/θ.
Besides their improved resolution in t*, long wavelengths thus additionally exhibit an improved angular resolution. However, one has to take care that the sample transmission for the given wavelengths remains sufficiently high to avoid multiple scattering, which places an upper wavelength limit for a given sample.
All in all, the optimal chopper configuration depends strongly on the transmission properties of the individual sample. Typical soft-matter particles dispersed in pure D2O usually exhibit a high transmission. Hence, if a high resolution is required, the utilization of long wavelengths is advised. In the presence of water, e.g. in the case of contrast matching, one has to determine the transmission profile of the sample and carefully adjust the chopper frequency and wavelength range.
Fig. 4 exemplifies the above issues. The scattered intensity I(Q) was determined for a sample of latex spheres (R ≃ 54 nm) dispersed in D2O (1 wt%) using different wavebands λmin to λmax. The latex spheres consisted of polystyrene covered with a small shell of poly(N-isopropylacrylamide). The sample was kindly provided by Dr Yan Lu from the Helmholtz-Zentrum Berlin.
While the chopper frequency was the same in all three examples (ν1 = ν4 = 770 r min−1), the phase differences between the pulse and waveband choppers were different (50, 95 and 150°), leading to wavebands of 2.8–8, 7.6–12.8 and 13.5–18.8 Å, respectively. While in the first configuration the broadest Q range is covered in a single measurement, the resolution in Q is lowest. Employing a smaller waveband in the second configuration leads to a reduced range in Q but to a lower minimum Q as well as to a greatly enhanced resolution. A further shift to even higher wavelengths in the third configuration does not lead to an accordingly large gain in resolution. Here, it is probably the polydispersity of the sample particles that places a limit on the resolution in Q. Moreover, multiple scattering occurring at large wavelengths may cause additional smearing of the data, which would thwart the improved Δt/t* in this case.
The first configuration in the above example represents a good setup to measure samples with reduced demands on resolution in Q. One may likewise probe a broad Q range with this configuration. The second and third chopper setups are suitable to meet demands for higher resolution or a lower minimum Q.
The inset of Fig. 4 illustrates that the possibilities offered by the TOF mode are not just restricted to the configuration of the choppers. For comparison, the scattered intensities of the latex particles as a function of Q are shown again in the inset for the chopper configuration using the waveband between 2.8 and 8 Å, while in the second data set only the high wavelengths between 7 and 8 Å were employed. The resolution of the latter data set is noticeably increased by neglecting the signals from neutrons with low wavelengths. Thus, the TOF mode offers the opportunity to adjust the resolution for a given Q region during data reduction and offers the possibility of resolution optimization even after the end of the experiment [for a detailed discussion of Q binning and resolution issues, see Seeger & Hjelm (1991)].
The scattered intensity I(Q) from the spherical latex particles (see Fig. 4) exhibits distinct maxima and minima and thus serves as a sensitive probe for Q resolution. In the first step, I(Q) was determined for this colloidal system over a broad range of Q and compared with data obtained from the second SANS instrument V4 at the Helmholtz-Zentrum Berlin (Keiderling & Wiedenmann, 1995). Fig. 5 depicts the two sets of I(Q) determined at the two instruments.
The V4 instrument achieves a better resolution in momentum transfer in the low Q region due to the smaller pixel size of its detector (0.5 × 0.5 cm). However, even at intermediate momentum transfer the resolution of the V16/VSANS instrument becomes superior through its employment of the TOF mode, which allows the determination of a broad Q range with increasingly enhanced resolution towards higher values of Q. Note that, for the above example, just two measurements at V16/VSANS would in fact have been sufficient to determine the most important regions of I(Q) versus Q. A first measurement using a sample-to-detector distance of 11.23 m with a waveband 2.8 ≤ λ ≤ 8 Å would have provided an overview of the scattering characteristics of the particles as well as data extending to sufficiently high momentum transfer Q (see Fig. 4). A second measurement at the same sample-to-detector distance using wavelengths between 7.6 and 12.8 Å would have resolved the main structural features of the particles.
Fig. 6 depicts I(Q) versus Q for a set of `round robin' reference data (Hellsing et al., 2012; Rennie et al., 2013) determined on the D11 instrument at the ILL in Grenoble, France, and on V16/VSANS. The latex samples were provided by Adrian R. Rennie (Department of Physics and Astronomy, Uppsala University, Sweden) and were intended to facilitate the comparison of data between different instruments, namely to check and unify the absolute calibration (http://www.cansas.org/wgwiki/index.php/Latex_Round_Robin ).
The data sets from the two different instruments agree well, showing that the calibration procedures worked out for the TOF mode of V16/VSANS allow reliable absolute scaling of the scattered intensity I(Q). The Q resolution achieved at V16/VSANS is lower than that at D11, again because of the smaller pixel size of the latter instrument. However, choosing a waveband comprising higher wavelengths would further improve the resolution of the data set obtained at V16/VSANS.
With V16/VSANS, a new small-angle neutron scattering instrument is now available to users at the Helmholtz-Zentrum in Berlin, Germany. The key feature of the instrument is the employment of the time-of-flight mode. This not only allows us to measure a wider Q range in a single measurement but also enables the experimenter to adjust the resolution in Q according to the requirements of a given sample through the configuration of the choppers and selection of the sample-to-detector distance. In this manner, the scattering statistics of a given sample on the one hand, and the Q range and concomitant resolution on the other hand, can be carefully balanced to achieve an optimal result, making V16/VSANS a valuable new tool for soft-matter research.
The authors thank Dr Yan Lu from the Helmholtz-Zentrum Berlin for the synthesis of the latex particles. We express our gratitude to Dr Sylvain Prévost from the Helmholtz-Zentrum Berlin for his assistance during the setup of the instrument, as well as for the measurements of the latex particles at V4. Many thanks are expressed to Adrian Rennie for providing the `round robin' reference samples. We further thank Dr Wolf-Dieter Stein from the Helmholtz-Zentrum Berlin for support dealing with the MANTID software and the import module.
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