GREAT : Studies of Nuclei far from Stability by Tagging Techniques

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TDR allows each channel to be run independently, and then be associated in software for event reconstruction. Such a solution is possible as each data word is associated with a time-stamp generated by a global 100MHz clock. This allows all of the data from the target position and the focal plane to be collected with virtually no system dead time loses.

The correlation of events is defined in software by the physicist using the spatial and temporal constraints determined by the experiment being performed. This correlation can be as simple or as complex as the user decides, a simple example would be to start with the identification of recoils then to look back in time using a window centered on the time of flight of the recoils, for the gamma rays detected during that window. A more complex example would add in correlations between the decays of the implanted recoils before the data were stored.

The front end electronics (CFD's and shaping amplifiers) comprises commercial NIM/CAMAC units. The timestamping ADC card is a new design in VXI-D format, offering 32 independent channels (14 bits with sliding scale correction) and a pattern register. The ADC conversion and readout time is less than the shaping amplifiers busy period, so the only counting losses will be those due to pileup in the detector.

The timestamping requires the distrubution and synchronisation of a 100MHz clock. The clock distrubution takes advantage of the facilities built in to D-size VXI viz. a 100MHz clock, ClockSynch signal (to select one edge of the clock) and two star lines which arrive at each slot within a crate with less than 2ns skew. A module known as the Metronome controls the clock distrubution and maintains synchronisation of all the timestamps.

The pattern register unit (VME) implements 64 input channels as four groups. Each group has a gate input associated with it. The pattern unit is connected to the global timing metronome module and, via a SHARC link to the event builder. The pattern unit groups operate in one of three modes:

  1. Bit change during the gate, timestamped at the start of the gate.
  2. Bit change during the gate, time stamped on occurance.
  3. Pattern at leading edge of gate, time stamped on the leading edge of the gate.
The pattern unit may also be used to record information from which elements of the BGO shields were hit during an event.

Data collected in the ADC cards are timestamped using the start of their corresponding ADC gate input. The resulting data words are tagged with an address to identify which detector they come from and which signal they represent within that detector. The data items are then sent via point to point serial links (SHARC DSP links) into VME cards where SHARC DSP's recieve and buffer the information ready for processing. Processed data are sent to tape for storage using one or more data streams, where each data stream corresponds to one class of reconstructed event. In a typical RDT experiment there could be three streams:

  1. Recoil-gamma data.
  2. Decay of implanted nuclei.
  3. Correlation between 1 and 2.

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Last modified: Tue Jun 29 11:58:27 GMT Daylight Time 2004