Catching Rays
Home Topic 1:
The Cosmic Ray Puzzle
Topic 2:
Detectors on Balloons
Topic 3:
Topic 4:
Dark Matter?

Topic 3:

Assembling detector components is a delicate and exacting business.

Seo has pioneered the design, construction, testing and deployment of many instrument suites. But perhaps the two best known are the Advanced Thin Ionization Calorimeter (ATIC) and the next-generation equipment for the Cosmic Ray Energetics and Mass mission (CREAM). Both have flown repeatedly and have compiled numerous datasets that are still being analyzed. (For example, in November of 2008, the ATIC collaboration published an analysis of data taken in flights from 2000 to 2003. The result—see Topic 4: Dark Matter?—made news around the world and was reported at length in The New York Times.)

Scientists have known for decades how to determine the identity and energy content of subatomic particles. In fact, four CMNS physicists are currently doing just that for the Large Hadron Collider, the world's most powerful particle accelerator, at the CERN facility outside Geneva, Switzerland. But their detector weighs more than the Eiffel Tower. Seo's designs had to be light enough to carry on a balloon, but substantial enough to provide information on particles as heavy as iron nuclei traveling at enormous speeds.

Dr. Seo explains the CREAM electronics.

The configurations have evolved from ATIC into various versions of the CREAM suite. The latest CREAM design integrates five kinds of instruments, each with a different and difficult primary job. Starting at the top, the first component, called the Timing Charge Detector (labeled TCD in the diagram, right), detects the charge of the particle and its velocity by the light that is produced when particles pass through a plastic "scintillator."

A second component, the Transition Radiation Detector TRD), confirms the charge, measures a variable that indicates the ratio between the CR's mass and its velocity, and shows the particle's track.

The TRD is divided into two sections, with a "Cherenkov detector" in between. Cherenkov light is a blue flash that results when a particle is traveling faster than the speed of light in a particular material—in this case, a special plastic. (It's the light equivalent of the sonic boom that occurs when an airplane is traveling faster than the speed of sound in air.) The Cherenkov unit allows the CREAM suite to disregard lower-velocity particles, which do not produce the blue "boom."

Next in line is the Silicon Charge Detector (SCD), which contains about 200 semiconductor sensors, each less than half a millimeter thick, that can discern extremely small differences in charge. Multiple charge measurements with the TCD, CD, and SCD identify the incident particle charges by minimizing the effect of backscattered particles from the calorimeter. An imager next to the Cherenkov dector, called the "CherCam," provides an additional method of disregarding backscatter effects.


The bottom component is the calorimeter—that is, a device that measures the energy of incoming particles. The CRs smash into lightweight carbon "targets" interleaved with sheets of scintillator that further track the particles' paths. Finally, the particle debris that results from the target collision penetrates a set of tungsten plates. The more energetic the particle, the larger the torrent of debris recorded by the calorimeter.

"The easiest way to conceptualize it," Seo says, "is as a very compact air shower device. Of course, I need to use material that is a lot more dense than air to make these things happen quickly in small depths. That's the idea for the calorimeter."

Watch the current status webcam at and monitor and launch.

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