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We define a virtual neutron experiment as a complete simulation of an experiment, from source over sample to detector. The virtual experiment (VE) will ideally interface with the instrument control software for the input and with standard data analysis packages for the virtual data output. Virtual experiments are beginning to make their way into neutron scattering science with applications as diverse as instrument design/upgrade, experiment planning, data analysis, test of analysis software, teaching, and outreach. In this paper, we summarize the recent developments in this field and make suggestions for future developments and use of VEs.

We here describe the result of simulations of 15 generic neutron instruments for the long-pulsed European Spallation Source (ESS). All instruments have been simulated for 20 different settings of the source time structure, corresponding to pulse lengths between 1 ms and 2 ms; and repetition frequencies between 10 Hz and 25 Hz. The relative change in performance with time structure is given for each instrument, and an unweighted average is calculated. The performance of the instrument suite is proportional to a) the peak flux and b) the duty cycle to a power of approx. 0.3. This information is an important input to determining the best accelerator parameters. In addition, we find that in our simple guide systems, most neutrons reaching the sample originate from the central 3-5 cm of the moderator. This result can be used as an input in later optimization of the moderator design. We discuss the relevance and validity of defining a single Figure-ofMerit for a full facility and compare with evaluations of the individual instrument classes.

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