Electronics and Data acquisition
In this section, the essence of the data acquisition system is described. A detailed explanation can be seen in Appendix A. For the outer detector, see also the next section.
The signal from a PMT is sent to a front end module called an ``ATM'' (Analog Timing
Module). We use 934 ATM modules. The signals from 12 PMTs are handled in each ATM module.
If the signal exceeds the threshold value, equivalent to about 0.32 p.e., a rectangular
pulse (200nsec in width and 11mV in height) is generated. This is called the ``HITSUM
signal''. In the ATM module, both timing and charge of the signal are also digitized.
HITSUM signals are summed and used for the trigger. The summed signal goes through a
discriminator which determines the trigger threshold. The value of the trigger threshold
was set to 320 mV which is equivalent to about 29 hits, or approximately 5 
6 MeV. This
means that a global trigger is generated when 29 PMTs are hit in any 200nsec time window.
The trigger rate is about 11Hz and has been stable during the course of the experiment as
shown in Fig 1. The trigger efficiency is described elsewhere.
(Actually, the the present trigger threshold is getting lower in accordance with the improvements of the data acquisition system and the water purification system.)
The digital data from ATM modules are sent to 8 online computers for data acquisition. These data are transferred to an online host computer via FDDI. They are then sent to the offline computer located outside of the mine via an optical fiber cable. The data is transferred every ten minutes, and the size of the file corresponding to ten minutes is about 70MByte. The offline host computer saves all the data in a tape library, converts the data of ADC and TDC counts to the units of photo electrons and nano-seconds, respectively, and distributes the data to analysis computers for preliminary reduction.
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Figure 1: |
The trigger rate in Super-Kamiokande as a function of time. |
