Central Laser Facility
Pierre Auger Observatory, Mendoza Province, Argentina
The Pierre Auger Observatory is the world's largest detector for measuring high energy cosmic rays. The CLF a remotely controlled laser facility at the center of this observatory. The facility produces a calibrated "test beam". Four fluorescence detector (FD) "eyes" of the observatory will view the CLF from the edges of the 55x55 km ground array. These eyes which record tracks produced by the passage of extensive air showers will also record tracks produced by the passage of laser light from the CLF.
As a laser pulse travels upward into the sky, the atmosphere scatters light out of the beam. At maximum laser energy and under clear viewing conditions, the amount of scattered laser light is equivalent to the amount of scintillation light produced along the path of a 200 EeV air shower. This energy is equivalent to that of a major league fastball, but contained in a single sub-atomic particle. By varying the laser energy, its air shower equivalent energy can be scanned across the dynamic range of the FD eyes including the region of the predicted GZK cutoff near 50 EeV.
Inside the CLF
The photo on the left shows Mike Roberts (University of New Mexico) and Lawrence Wiencke (University of Utah/ University of Chicago) inside the central laser facility. The laser beam goes up the vertical pipe behide them. The layout of the optical table behind them is shown on the right.
The CLF Design
The design builds on the experience from another interesting cosmic ray experiment called HiRes that operates lasers and other calibration systems in the desert environment. The CLF features a 355nm pulsed frequency tripled YAG laser. This wavelength falls the middle of the fluorescence spectrum produced by air showers. Downstream optics randomize the polarization so that a vertical beam will scatter equally toward all FD eyes. For redundancy, the beam optics will be configured in two paths. A computer controlled flipper mirror will select between them. The default path produces a vertical beam. A secondary path with beam steering has be added.
The brain of the system is a small embedded Linux computer with no moving parts. The computer controls the laser, the power controller, a radiometer to records the relative energy of each laser pulse, and GPS laser timing module. The 100 ns accuracy of this module allows physics blind separation of laser tracks from air shower candidates using event times only. (Neutrinos, for example, can also generate upward going tracks.) The CLF is housed in a modified connex type shipping container and powered by batteries charged by solar panels. Voltages and temperatures are automatically logged. A propane heater maintains indoor temperature above 10C. The entire system is operated remotely over a microwave internet link.
Major system components
|LASER||Frequency tripled YAG
8mJ @ 355nm
|CFR Ultra||Big Sky Laser|
-Si Probe for relative calibration
-Pyroelectric Probe for
|Low Power PC||6 watts, 11 serial ports,
no moving parts, Linux