Low-cost data acquisition card for school-network cosmic ray detectors

The Cosmic Ray Observatory Project (CROP) at University of Nebraska/Lincoln and the Washington Area Large-scale Time coincidence Array (WALTA) at University of Washington/Seattle are among several outreach projects siting cosmic-ray detectors at local high schools in cities around North America, to study the origins and interactions of high-energy cosmic rays. In a collaboration between QuarkNet, the outreach program based at Fermilab, CROP, and WALTA, a low-cost data acquisition electronics card has been developed to collect and synchronize the data from each detector site. The cost per card is under US$500 for parts (for 15 boards), functionally replacing much more expensive electronics crates and modules at each high school site. The card has four analog discriminator inputs for photomultiplier tube signals, a four-channel time-to-digital converter (TDC) for local coincidence and time-over-threshold measurements with ∼1 ns resolution, programmable trigger logic via a CPLD and microcontroller, and a built-in low-cost GPS receiver/antenna module (via external cable) to provide event trigger time stamps with 50 ns accuracy. Temperature sensors and a barometer are also integrated to record environmental data along with the counter data. The card connects to any PC or laptop via a standard RS-232 serial port for data output and control. The microcontroller and CPLD are field programmable and thus make the card functionality flexible and easy to upgrade.