Abstract
The European Spallation Source (ESS) will provide long neutron pulses for experiments on a suite of different instruments. Most of these will perform neutron data acquisition in event mode, i.e. each detected neutron will be characterised by one absolute timestamp and pixel identifier pair. Slow controls metadata from EPICS, such as sample environment measurements and motor positions, will also be timestamped at their source, so that all data and metadata are streamed as a list of events instead of histograms. A flexible data aggregation and streaming system is being developed combining both open source third-party software and in-house development. This is to be used at ESS and other neutron scattering facilities like ISIS and SINQ, replacing legacy solutions by a shared software collection maintained by a cross-facility effort. The architecture of the Apache Kafka-based system, its metadata forwarding and NeXus file writing components are presented, along with test results demonstrating their integration and the scalability in terms of performance.
Highlights
Architecture of the data aggregation and streaming system for the European Spallation Source neutron instrument suite
Neutron detector data acquisition in event mode relies on information that is provided by the European Spallation Source (ESS) timing system, such as accurate timestamps for neutron events coming from the detector readout electronics and accelerator pulse sequence numbers
The system architecture differs from the ESS architecture in that the detector data do not come from Event Formation Unit (EFU), but from data acquisition electronics through the Instrument Control Program (ICP), together with metadata; the ICP is responsible for writing NeXus files
Summary
Acquisition of neutron instrument data is frequently done in histogram mode, in which detector data over a region is acquired for an interval and presented as a set of accumulated counts. Each neutron count on the detector generates an individual event consisting of a pixel number, which uniquely identifies the location where the neutron hit the detector, and a timestamp. Postponing histogram generation provides additional flexibility to the data acquisition process, as the conversion of neutron detection events into a histogram results in loss of information (the individual timestamps are lost and the final histogram only gives information as detailed as the binning and integration time allow). Can the data in an event list later be binned in any desired resolution, individual (sets of) events can be filtered out depending on metadata parameters like, for example, sample environment information. The highest data rates are expected to arise from neutron event data; these will be discussed followed by the ESS approach to timing and timestamping
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