Detection of Extensive Air Showers with HiRes
The two HiRes sites (called HiRes-1 and HiRes-2) are operated as completely separate detectors. HiRes-1, operating since 1996, was designed and built by the University of Utah; HiRes-2, completed in 1999, was designed at Nevis Laboratories.
HiRes-2 Electronics
HiRes-2 differs from HiRes-1 in that the sample-and-hold data acquisition system of the older site is replaced by an FADC system which continuously digitizes the photomultiplier signals at 10 MHz (8-bit dynamic range). This system uses small commercial 24-bit digital signal processors (DSPs) which, among other tasks, carry out trigger formation and basic monitoring.
In addition to the signals from each of the 256 photomultipliers of each mirror, a 16 tube analog signal for each of the 16 horizontal and 16 vertical rows of tubes is formed with 16 times lower gain than the individual channels. A first level trigger decision is based on these sums. The figure demonstrates the basic principle: a shower signal appears as light sweeping across the cluster, appearing in one analog sum while disappearing from a neighboring sum.
On top of normal data-taking, all 13,440 channels are read out approximately every 1.6 s, regardless of whether the trigger condition is fulfilled. These "snapshots" allow simple monitoring of the photomultipliers and can be used to detect stars crossing the field of view of a photomultiplier. Tracking stars provides a useful tool to verify the pointing accuracy of the telescope, understand aberration effects, and detect clouds in the field of view. However, the amount of data is challenging: a 2.5 μs snapshot yields about 3.4 Mbyte of data.
