Dose

Figure 3. Flux as a Function of Energy at Beamline i04

The "set target dose" mode is particularly useful when changing beam size or energy, as it ensures that the beamline operates at a fixed dose, regardless of the changes in beam parameters. This is important because the flux profile across the energy range is not linear or constant (Figure 3).

Figure 3 shows the variation of the flux (number of photons per second) as a function of energy (eV) at Diamond Light Source beamline I04. The data were collected for a fixed beam size of 20 microns vertical and normalised to a 300 mA synchrotron ring current. The dots represent the real measured flux, while the smooth line is to illustrate the difficulty of fitting the data with a simple algorithm. The non-linear and non-constant nature of the flux profile is evident, highlighting the importance of using dose-aware data collection strategies.

Figure 4. Variation of Diamond Light Source Ring Current

Even for a particular energy, the flux can vary on different days due to variations in beamline commissioning and machine ring current (Figure 4). Working at a fixed dose and using the live flux at the time of the experiment allows for more rigorous comparisons between experiments on the same sample conducted months apart. This ensures that the diffraction power of the crystals remains consistent, which is not the case when working at fixed exposure.

The figure 4 shows the variation in the Diamond Light Source ring current over time. The left panels illustrate examples from December (2023), April, July, and September 2024, where the ring current varied significantly between and within synchrotron runs, often unnoticed by users. The right panel shows an example of a "decay mode" event, where the ring current steadily decreased over an 11-hour period due to the lack of electron re-injection. In such situations, the "Set Target Dose" mode is crucial for maintaining the consistency of experiments, as the exposure time will be automatically adjusted to compensate for the drop in flux.