SNO Requirements

The timing requirements of SNO cover a very wide dynamic range. On the one hand event positions are determined by relative PMT timings and, as light travels 30cm in a nsec, imply relative times need to be better than a nsec. On the other hand every event has an absolute time (Julian date) and this has to be accurate for the lifetime of the experiment. Relative time between events are important for studying a wide range processes from the time structure of a supernova burst or something as simple as just time-associating calibration events (like $^8 Li$ etc). Absolute times are need to set an event in an astronomical context.

A single clock cannot suffice for 2 distinct reasons:-

1. A single clock could not maintain an accuracy of an nsec over a period of years.

2. Julian date is ultimately defined by the relative orientation and position of the Earth relative to the Sun. For a variety of reasons this is not a perfect timepiece and relies on astronomical observations to maintain accuracy.

Instead SNO uses three clocks. Two are global: a 10MHz one and 50MHz one and one is local to each PMT.