We observed cosmic ray showers candidates since the beginning of the FD commissioning with a rate of a few per hour. These events have specific characteristics compatible with cosmic ray showers produced in the atmosphere. PMT signals are 300 ns up to 1 microsecond wide, and are small. The signals show a characteristic time sequence when passing from one pixel to the next. The tracks start from the bottom of the Camera (which is consistent with a cosmic ray shower arriving from the top) and their total duration is between 5-9 microseconds.

The first cosmic ray shower candidate as seen online on May 23, 2001. The data were taken with the WinNT test DAQ and display program developed by A. Mentschikov. The picture shows on the upper left plot the pixel matrix with the triggered pixels in green. On the upper right plot, the start of the trigger time for each row. On the lower right, FADC traces for selected pixels.

Another candidate event, displayed with the Online Event display developed by P. Facal.

More FADC traces of the same event.

Another candidate event.

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