  Back-n is a white neutron experimental facility at China Spallation Neutron
Source (CSNS). The time structure of the primary proton beam make it fully
applicable to use TOF (time-of-flight) method for neutron energy measuring. We
implement the electronics of TOF measurement on the general-purpose readout
electronics designed for all of the seven detectors in Back-n. The electronics
is based on PXIe (Peripheral Component Interconnect Express eXtensions for
Instrumentation) platform, which is composed of FDM (Field Digitizer Modules),
TCM (Trigger and Clock Module), and SCM (Signal Conditioning Module). T0 signal
synchronous to the CSNS accelerator represents the neutron emission from the
target. It is the start of time stamp. The trigger and clock module (TCM)
receives, synchronizes and distributes the T0 signal to each FDM based on the
PXIe backplane bus. Meantime, detector signals after being conditioned are fed
into FDMs for waveform digitizing. First sample point of the signal is the stop
of time stamp. According to the start, stop time stamp and the time of signal
over threshold, the total TOF can be obtained. FPGA-based (Field Programmable
Gate Array) TDC is implemented on TCM to accurately acquire the time interval
between the asynchronous T0 signal and the global synchronous clock phase.
There is also an FPGA-based TDC on FDM to accurately acquire the time interval
between T0 arriving at FDM and the first sample point of the detector signal,
the over threshold time of signal is obtained offline. This method for TOF
measurement is efficient and not needed for additional modules. Test result
shows the accuracy of TOF is sub-nanosecond and can meet the requirement for
Back-n at CSNS.
