# Simultaneous SAXS/SANS Method at D22 of ILL: Instrument Upgrade
## 3 Results and Discussion
### Overall SAXS Performance
Nevertheless, the overall background level even for the short neutron collimation lengths is still low and allows high quality simultaneous SAXS/SANS experiments. Based on the \(q\)-range of characteristic interference features, scattering power of investigated samples, and experimental conditions (such as collimation length, SDD\({}_{\text{X-ray}}\), X-ray slit size, and measuring time), one needs to carefully optimize the experimental conditions to achieve the best quality simultaneous SAXS/SANS experiments. Luckily, the D22 instrument is currently equipped with a second fixed detector at 1.4 m from the sample position in addition to a movable detector, enabling extended SANS \(q\) ranges (from 0.003 up to 0.7 A\({}^{-1}\) in a single configuration). Hence, typical SANS experiments are performed with collimation lengths > 5 m. Thus, the high signal-to  
Figure 4: 1D SAXS profiles at various neutron collimation lengths and SDD\({}_{\text{X-ray}}\) of (**a**,**b**) 100 nm and (**c**,**d**) 26 nm silica NPs collected with the neutron beam switched on (orange curves) or off (blue) (simultaneous and asynchronous modes). The asynchronous mode represents a standard laboratory SAXS experimental condition where neutron shutter was closed and the nuclear reactor was almost switched off (below < 1.5 MW). SAXS/SANS simultaneous mode represents an experimental condition where the neutron shutter of D22 was open (reactor power = 43 MW). At long SDD\({}_{\text{X-ray}}\), the SAXS profiles in simultaneous mode are well matching those of the asynchronous mode, which leads to a good signal-to-noise ratio for the SAXS data. For short collimations, noticeable background which was approximately 4 and 8 times larger than that for standard lab SAXS measurements was found for both collimations 8 m (**b**) and 5.6 m (**d**), respectively. background level of the SAXS data detected at different collimation lengths (17.6, 8 and 5.6 m) allows state-of-the-art time-resolved in situ simultaneous SAXS/SANS measurements. This has been demonstrated by studies on the growth of gold nanorods using surfactant-assisted seed-mediated synthesis using simultaneous SAXS/SANS at a neutron collimation length of 8 m [23]. For SAXS/SANS data evaluation of the studied NP dispersions (Figure 5), a simple sphere model with a log-normally distributed radius coupled with a distribution width parameter implemented in SASfit software [33] was used. In SASfit, simultaneous fits of the model to the SAXS and SANS experimental data sets were performed with a convergence \(\chi<<1\). The scattering length density contrast \(\Delta\)SLD = \(\mathrm{SD_{NPs}}\) - \(\mathrm{SD_{solvent}}\) was fixed to the known values for silica to be 13.3 and 2.2 A\({}^{-2}\) for SAXS and SANS, respectively. The data can be nicely fitted with the simple model and reveals a particle diameter (with a distribution width) of 110 \(\pm\) 7 nm (nominal 100 nm) and 26 \(\pm\) 3 nm (nominal 26 nm), respectively. This simple example demonstrates that a simultaneous fit of a structural model to the SAXS and SANS data set can be easily performed on an absolute intensity scale and reveal the structural parameters, circumventing ambiguities which might be left if only one contrast (X-ray or neutron) is used. This is a particular advantage for in situ or operando studies of complex nanoscale systems which slowly change their structure, e.g., upon a reaction, crystallization, aggregation, gelation, particle formation, growth or aging process.