# Simultaneous SAXS/SANS Method at D22 of ILL: Instrument Upgrade
## 3 Results and Discussion
### Overall SAXS Performance  
For simultaneous SAXS/SANS experiments, it is important to achieve sufficient data quality in a given measuring time for both methods. For that purpose, it was important to reduce the background signal originating from gamma rays on the 2D X-ray detector of the SAXS instrument for all experimental conditions represented by different SANS collimation lengths and primary neutron beam fluxes and sizes, as well as different SDD\({}_{\text{X-ray}}\). As demonstrated above, this could be achieved by a targeted enhancement of the neutron and magnetic field shielding, as well as using a double-energy threshold X-ray detector. To demonstrate the success of these upgrades for the performance of the SAXS instrument, two NP samples of nominal sizes of 100 nm and 26 nm were studied by simultaneous SAXS/SANS experiments. The scattering data was collected while the nuclear reactor was in operation (neutron shutter opened, reactor power = 43 MW). SAXS data of the same samples were also collected when the neutron reactor was in the shutdown time (neutron shutter closed, reactor power < 1.5 MW). The latter experiments allow a comparison of the SAXS data of the investigated samples under simultaneous SAXS/SANS data recording conditions with standard laboratory standalone SAXS experiments, where only natural cosmic background contributes to the gamma background of the SAXS data. SAXS data of silica NP samples were collected for 15 min for both short (0.55 m) and long (1.63 m) SDD\({}_{\text{X-ray}}\) (Figure 4). For long SDD\({}_{\text{X-ray}}\), SANS measurements of both silica NPs were simultaneously acquired at a long collimation length of 17.6 m. For short SDD\({}_{\text{X-ray}}\), SANS data were simultaneously measured at two different collimation lengths of 8 m and 5.6 m for 100 nm and 26 nm silica NPs, respectively. The SAXS 2D detector patterns were corrected and normalized, respectively, for measuring time, sample thickness, transmission, flat field, solid angle covered by each detector pixel, and incident intensity. The data were calibrated to absolute intensities, and the \(q\)-scale was azimuthally averaged. Background (dark current) correction was performed by subtracting the background intensity measured on the detector at relevant operating conditions. For each neutron collimation length used, the background intensity on the X-ray detector was collected for several minutes while the neutron shutter was opened but the X-ray shutter was closed. Besides this, each SAXS data set was corrected for a constant background determined from a measurement of the corresponding sample with a closed neutron shutter (and reactor power < 1.5 MW). Figure 3: X-ray beam intensity before (no magnetic shielding, green line) and after (magnetic field shielding, black symbols) installing a 1 mm thick plate of a soft ferromagnetic alloy at the X-ray source. A magnetic field strength of approximately 2–5 Gauss at the X-ray source causes an X-ray beam drift. At a long collimation length (17.6 m), the SAXS 1D profiles are well matching those collected in absence of the high-energy gamma radiation environment (Figure 4a,c). This reveals that the SAXS data quality of a simultaneous SAXS/SANS measurement at those conditions (collimation length = 17.6 m, SDD\({}_{\text{X-ray}}\) = 1.63 m) is not compromised with respect to the background level of a state-of-the-art lab-based SAXS measurement. At a short distance SDD\({}_{\text{X-ray}}\) (0.55 m), SANS images were collected in a synchronous mode at collimation lengths of 8 m (for 100 nm NPs) and 5.6 m (for 26 nm NPs). In the latter configurations, the SAXS profiles exhibited a background intensity approximately 4 and 8 times higher than the standard lab-based SAXS measurements for both 8 m and 5.6 m collimation lengths, respectively (Figure 4b,d). This gamma background originated predominantly from the gamma rays produced by neutron capture within the sample.