  In systems with multiple radiation detectors, time synchronization of the
data collected from different detectors is essential to reconstruct
multi-detector events such as scattering and coincidences. In cases where the
number of detectors exceeds the readout channels in a single data acquisition
electronics module, multiple modules have to be synchronized, which is
traditionally accomplished by distributing clocks and triggers via dedicated
connections. To eliminate this added cabling complexity in the case of a new
radioactive gas detection system prototype under development at the French
Atomic Energy Commission, we implemented time synchronization between multiple
XIA Pixie-Net detector readout modules through the existing Ethernet network,
based on the IEEE 1588 precision time protocol. The detector system is
dedicated to the measurement of radioactive gases at low activity and consists
of eight large silicon pixels and two NaI(Tl) detectors, instrumented with a
total of three 4-channel Pixie-Net modules. Detecting NaI(Tl)/silicon
coincidences will make it possible to identify each radioisotope present in the
sample. To allow these identifications at low activities, the Pixie-Net modules
must be synchronized to a precision well below the targeted coincidence window
of 500-1000 ns. Being equipped with an Ethernet PHY compatible with IEEE 1588
and synchronous Ethernet that outputs a locally generated but system-wide
synchronized clock, the Pixie-Net can operate its analog to digital converters
and digital processing circuitry with that clock and match time stamps for
captured data across the three modules. Depending on the network configuration
and synchronization method, the implementation is capable to achieve timing
precisions between 300 ns and 200 ps.
