Experiment Proposal - opera - Infn

Table 9: Number of electronic channels in each main detector.
Electronic detector Nb. of planes Nb. of channels/plane Total
Target Trackers 3 × 48 512 73728
Spectrometer RPC’s 3 × 22 560 36960
Spectrometer drift tubes 3 × 6 768 13824
Spectrometer XPC’s 3 × 2 790 4740
4.7.2 Data flow
To evaluate the expected data flow from the electronic detectors, we have considered a suitable safety
factor, accounting for the largest possible number of electronic channels.
A maximum counting rate induced by natural radioactivity of about 10 Hz per electronic channel is
assumed. Detector and electronic noise have to be added. Using the Hamamatsu 64-channel PMT with
a threshold corresponding to 1/3 photoelectron, a noise frequency lower than 5 Hz has been measured,
mainly coming from the photocathode thermoemission. The use of HPD’s for the scintillator readout
could increase the electronic noise by at least one order of magnitude as compared to the PMT for the
same threshold.
Taking into account possible noisy elements a mean electronic noise of the order of 50 Hz is considered
for the scintillator strips of the target. For RPC’s the noise is assumed to be of the same order. The
drift tubes of the Precision Trackers require an external time reference. The TDC common STOP could
be provided (within a time shorter than 512 ns) by the RPC’s associated with the spectrometer. Thus,
since the Precision Trackers do not participate in the trigger, the electronic noise of this detector does not
significantly disturb the acquisition system. For the data flow evaluation an electronic noise of 50 Hz
has also been assumed.
Table 9 gives the number of electronic channels for the three detectors. For each detected hit, at most
12 byte are stored (channel address, signal and time). The total rate amounts to about 90 Mbyte/s.
If the final electronic noise rate would be significantly higher, fast time correlations between X and
Y channels could be performed before signal digitization. These correlations could be used as external
signal to trigger the frond end readout cards and thus, reduce the noise. In any case, the acquisition
system must be able to support at least three times the above referred to rate.
4.7.3 Front end electronics
This part of the electronics is the only one which has to be adapted to each specific detector. In this
Section, we describe in particular the scintillator readout.
The front end unit includes the following components: the front end chip, the ADC’s, the local trigger,
the serial link controller and the programmable chip. Its schematic diagram is shown in Fig. 67.
One can make use of boxes close to the photodetector from where the data flow is transmitted by a
serial link. The discriminator signal is also sent to the readout interface. Other signals (external trigger,
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