LIDAR - Steerable laser calibrates UV monitoring system

Somebody is standing on the rim of the Earth's atmosphere and dropping cosmic bowling balls from about waist high, which raise one heck of a cosmic racket when they smash into the atmosphere. Lawrence Wiencke and a team of researchers from all over the United States and as far away as South Australia are so determined to locate the cosmic bowler that they've set up a steerable laser system to monitor the lower 10 km of the atmosphere from the western Utah desert.

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Somebody is standing on the rim of the Earth's atmosphere and dropping cosmic bowling balls from about waist high, which raise one heck of a cosmic racket when they smash into the atmosphere. Lawrence Wiencke and a team of researchers from all over the United States and as far away as South Australia are so determined to locate the cosmic bowler that they've set up a steerable laser system to monitor the lower 10 km of the atmosphere from the western Utah desert.

Well, there aren't really any bowlers or bowling balls up there, but that's the analogy that Wiencke used during an interview to describe the massive 1020-eV subatomic particles that are much higher energy than any man-made accelerator can produce and that comprise the highest-energy cosmic rays observed to date. According to theory, the microwave background caused by the Big Bang should significantly attenuate high-energy cosmic rays before they reach the Earth, unless the sources are very close.1 "But if you measure these particles, reconstruct their directions, and compare where they come from in the sky to objects that astronomers know of, you don't see any obvious overlap," Wiencke said. "So where these [particles] come from is really a mystery."

To try to solve the mystery a multicenter research team has set up the High Resolution Fly's Eye (HiRes) astrophysics experiment, which includes a recently upgraded steerable laser system, at Dugway Proving Ground, UT (see Fig. 1). Wiencke (University of Utah; Salt Lake City, UT) described the upgraded experimental system in July at the 44th annual meeting and exhibition of the International Society for Optical Engineering (SPIE) in Denver, CO.

The HiRes currently consists of two mirror systems separated by 12.6 km on the desert floor and a steerable laser adjacent to one of the mirror systems. One mirror system, HiRes1, contains 21 mirrors, each with an area of 3.5 m2 and a 256-photomultiplier-tube (PMT) camera at the focal plane (see Fig. 1). The other mirror system, HiRes2, contains 42 similar mirrors. Even though positions and pointing directions of the mirrors in both systems are fixed, both systems cover 360° degrees in azimuth. HiRes1 observes elevations from 3.5° to 16°, and HiRes2 observes elevations from 3° to 30°, Wiencke said.


FIGURE 1. HiRes astrophysics experiment is conducted from two detector stations 12.6 km apart in the Utah desert (only one is shown in photo). The mirror detection system, designed to measure light from cosmic rays, also offers unique capabilities as part of a lidar system.
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Reading the traces

When cosmic bowling balls enter the atmosphere, they basically break up into more particles and produce what Wiencke described as a cosmic shower. "At the brightest spot in the shower-where there are the most particles-you might have upwards of 1010 particles going across the sky," he said. These particles excite the nitrogen in the atmosphere, which emits UV photons isotropically upon returning to its original energy level. Consequently, the detector system is optimized for 355-nm light because of its proximity to the 351-nm line of nitrogen fluorescence in air.

The laser system, located adjacent to HiRes2, can be steered in any direction above the horizon and is based on a 355-nm frequency-tripled YAG light source (see Fig. 2). It is used as a calibration source and also to probe the atmosphere.

"Because the occurrence of high-resolution particles is very rare [maybe a few a year], we have to look over a lot of atmosphere," Wiencke noted. "So we look out to distances that are maybe 30 and 40 km away. At those distances aerosols will change the transparency of the atmosphere. The amount of light that you see can be dimmed by different amounts depending on how much aerosol there is between the light produced by the cosmic rays and the detector."


FIGURE 2. A 355-nm pulsed YAG laser system is at the heart of the LINUX-controlled steerable laser system that scans the atmosphere and calibrates a UV detection system.
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By observing scatter of the pulsed 355-nm laser beam as it passes through the atmosphere, the detection system can calculate how much light the detector would receive from a cosmic shower in that direction. The new laser system, which was constructed in December 1998 and has been operating since April 1999, replaces a previous laboratory model and provides the researchers for the first time with a ruggedized instrument that will operate reliably over time in desert conditions and that also is powerful enough to scan distances 30 to 40 km away. During the coming year, the researchers hope to add an additional laser next to HiRes1 that will increase system accuracy through the ability to perform cross checks. The steerable lasers are computer controlled using a LINUX operating system.

Eventually the team hopes to deploy 10 detectors and lasers to probe the mystery of the cosmic bowlers. "We're looking into ways to reconstruct where the air shower is online," Wiencke said. "So that within a minute or so we can slew the laser beams around, point them in that direction in the sky and fire a pattern of shots and check that part of the atmosphere to verify that things are working and that we can really see that far."

Hassaun Jones-Bey

REFERENCE

  1. T. O'Halloran, P. Sokolsky, and S. Yoshida, Physics Today, 31 (Jan. 1998).

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