Electronics and Equipment
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:
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Bit change during the gate, timestamped at the start of the gate.
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Bit change during the gate, time stamped on occurance.
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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:
-
Recoil-gamma data.
-
Decay of implanted nuclei.
-
Correlation between 1 and 2.
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