## 2 Time Resolution  
From a technical point of view, the raw neutron flux that a beamline can produce is an important parameter in defining the time-resolution that is achievable, but it is certainly not the only relevant instrumental parameter. Intrinsic background in the scattering pattern arising from the instrument, scattering contrast and sample chemical composition, sample size, detector efficiency, and associated electronics all play a role. Although the technical differences between experiments on a pulsed neutron source vs. a continuous source must be considered, in some ways they are less relevant than the type of sample environments used to carry out experiments, which can have a profound effect on the quality of the resultant data. From a practical point of view, the time-scale over which the sample transforms from one physical state to another is the most important. However, equally important is how uniformly the sample can be perturbed. For instance, in a temperature jump experiment, temperature gradients are bound to develop. With the relatively large beam sizes produced by neutron scattering beamlines, one is detecting structural information over a thermal range. The faster the temperature jump, the larger the gradients, and the more complicated the data interpretation becomes. The data quality per frame in a time-resolved experiment depends on the data collection time, which in turn depends on the progress in time of the process being studied. For different applications, different degrees of statistical accuracy are required. For example, if the goal is to perform a detailed structural analysis, the statistical quality of the data has to be very high. However, if one is satisfied with understanding how the radius of gyration of a particle in solution or the growth of a lamellar peak in a polymer crystallization experiment changes with time, the counting statistics requirements can be lowered and the experiments can be performed in a much more expeditious manner. With the above points in mind, one can conclude that there is no definitive answer as to how fast a process one can resolve in a neutron scattering experiment. However, one should determine how fast a reasonable quality dataset can be acquired in order to address the problem and then assess whether or not the pertinent experiment is feasible to perform. New 'event mode' data acquisition will to some degree allow experimenters to have more leeway to carry out the experiment and afterwards decide on which time-resolution is feasible. 'Event mode' data acquisition involves retaining the position and time of each neutron detected, in contrast to traditional histogramming of detector data. Doing so makes it possible to parse a single data set collected into arbitrary time bins after data collection, which is extremely valuable for studies of time-dependent processes. It improves upon fast-frame data collection (i.e., a series of snapshots taken in quick succession) because it is not limited by the performance of the detector read-out, such as might be encountered with detectors used at synchrotrons. For time-resolved experiments where a trigger is used to initiate a non-reversible process and the data are collected in individual time frames, this capability was available in the early 1990s with time-resolutions of between 2 and 3 min/frame to study late stage spinodal decomposition in polybutadiene-polyisoprene blends, where one of the blocks was deuterated [4]. Similar time resolutions have been reported for polystyrene-polyisoprene diblock-copolymers [5]. Nearly a decade later, a researcher studying micelle-to-vesicle transformations in D\({}_{2}\)O was surprised about what was feasible and wrote: 'A measurement time of 60 s already results in astonishingly good statistics, although the total surfactant concentration is less than 1 mg/mL to avoid interparticle interaction effects' [6]. A decade later, in an experiment on the growth of mesoporous silica nanoparticles, using deuterium contrast variation, a time-resolution of 10 s/frame was reported.