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A Scintillating Way to Hunt Cosmic Rays

If you have ever watched lights sparkling from a spray of plastic fibers sticking out of lamp, you've watched part of the technique that scientists are developing for observing cosmic rays. They've sandwiched 20,000 polystyrene fibers between lead plates in the Scintillating Optical Fiber Calorimeter (SOFCAL), and connected the fibers to two video cameras, all to capture signs of cosmic rays zipping along at extreme energies.

"We're interested in in looking for changes in the energy spectra that indicate tha the acceleration sources in the galaxy can't accelerate above a certain energy," said Dr. Thomas Parnell, a co-investigator on SOFCAL. For several years, Parnell and other scientists at SSL, University of Tokyo, and Washington University have been hunting cosmic rays through the Japanese-American Cosmic-ray Emulsion Experiments (JACEE).

The term cosmic ray often is misapplied to any radiation coming from the heavens. To astronomers, it means one thing in particular: charged particles zipping along at high speed. One thing Parnell and others want to know is what accelerates cosmic rays - mostly protons and naked helium nuclei at these energies - to such high speeds in the first place.

optical fibers
Like organized spagehtti, optical fibers snake into and out of the central part of SOFCAL (left) and connect to the image intensifiers on the CCD cameras (right) that captures the scintillations of cosmic rays crashing through the lead plates.

SOFCAL image intensifiers

"The usual suspect is shockwaves in supernova remnants," Parnell explained. An explosion always spews materials outward. When a star dies, it blasts its own atmosphere into space, and that, in turn, will accelerate anything nearby when the shock wave hits.

Cosmic rays can't be focused the way light is, nor manipulated in mass spectrometers or other instruments that can precisely weigh gases or plasmas. They have to be observed indirectly, by looking for how they interact with "condensed" matter - the cold, slow stuff that planets and people are made of.

And that's part of how we study them. Like a bullet plunging through a wall, cosmic rays create a shower of debris when they intercept matter. Even the debris generates more showers until the original energy of the cosmic ray is scattered through the matter it hit, or the remains rifle through the other side and back into the cosmos.

Forensic evidence

You can reconstruct the size, speed, and direction of the bullet if you reassemble the jigsaw puzzle by calculating how much energy it took to fragment this piece of wood or that chunk of brick.

In cosmic ray physics, the "wall" often is layers of metals dense enough to intercept the cosmic rays and generate secondary showers of particles which bore invisible holes through layers of plastic and expose spots on the film emulsions.

After the package returns to Earth, the emulsions are developed and the plastic etched to reveal the holes. Then the scientists map the spots as they start reconstructing the various events.

SOFCAL will demonstrate a better way of doing this by using fiber optics and a solid-state TV camera. The technique was pioneered by scientists using particle accelerators which approach only a fraction of the energies which nature provides with cosmic rays.

The heart of SOFCAL - the portion that one day may be used in a satellite - is a stack of 0.5 mm (1/50th of an inch) square fibers sandwiched between lead plates. Each filling in this sandwich has two layers of fibers, one at right angles to the other, like an X-Y graph you had to do in algebra.

When a cosmic ray slams into the upper lead plate, it creates a cascade of secondary particles that shower through the fibers. Special coatings on the fibers emit flashes of light (scintillation) that are carried by the fibers to the TV cameras (image intensifiers make sure that none of the flashes are missed).

Forecast for showers

Meanwhile, each particle hits the next lead plate and creates its own shower of debris which is detected by the next layer of fibers, and so one, for a total of 10 times. When scientists reconstruct the data on the ground, this provides a picture of a complex lightning strike.

The first particle hitting the first level probably will yield one flash on the X plane, and one of the Y. The debris hitting the second level will produce lots of X and Y flashes at the same time, but in the vicinity of the first flash. It spreads outward in a complex, 3-D connect-the-dots game.

Instruments like SOFCAL have been used in nuclear particle accelerators (atom smashers), but this will be the first flight on a balloon to study cosmic rays. So, the SSL team added some standard instruments to compare their performance with SOFCAL.

SOFCAL instrument
Mark Christl, SOFCAL's principal investigator (left) takes some measurements through ground instruments. The completed instrument is bundled in styrofoam and other insulation to protect it during its journey in the stratosphere. Solar panels (right) are an experiment to see how they work at altitude. They power amateur radio equipment sending down video from a camera looking through the floor of the gondola. A computer monitors voltages, currents, temperatures and pressure.

Solar Panel

SOFCAL itself is sandwiched by two emulsion stacks like those that have been flown by JACEE and other projects for several years, and is topped by a Cerenkov detector that measures when particles pass through going faster than the speed of light. That's not a violation of the laws of relativity. Particles can travel faster than light if they are passing through a medium, like water or a gas, because there the speed of light is lower than in a vacuum (and that is the maximum). When this happens, the relativistic particles emit light at right angles to their line of travel. If you have ever seen pictures of the eerie blue glow at the bottom of a water-filled nuclear fuel tank, that's Cerenokv radiation caused by neutrons traveling faster than light in water.

The Cerenkov counter on SOFCAL is filled with transparent Teflon. Special light detectors - called photomultiplier tubes - capture and intensify the light. This serves as an announcement that a particle arrived and the SOFCAL scientists know in which video frame to look for evidence.

And what will the evidence suggest?

Models predict that the energy levels of these accelerated particles will peak at energies around 100 trillion or 1 quadrillion electron volts (or, 1 to 1,000 tera-electron volts, TeV). By comparison, the electrons that paint the images on this computer screen have an energy of only 10,000 electron-volts (10 keV).

SOFCAL Team PhotoThe usual suspects: The SOFCAL crew includes (left to right) Tom Koshut (USRA), Mark Christl (MSFC), Fred Berry (MSFC), J.O Jolley (MEVATEC), and Georgia Richardson (University of Alabama in Huntsville). Not pictured are Carl Benson, Tom Parnell, and Walt Fountain (MSFC) and Yoshi Takahashi and John Gregory (UAH).

One theory suggests that when these ultra-energetic particles strike condensed matter they will briefly create a cloud of quarks (the most basic particles) and gluons (particles that carry the force that "glues" matter together) like that at the moment of creation.

If SOFCAL works as planned, it could lead to advanced cosmic ray instruments aboard satellites - for months instead of hours of observations - and freeing scientists of the need to retrieve and analzye plastic and emulsion packages.

To dig further into SOFCAL, check:

"The Scintillating Optical Fiber Calorimeter Instrument (SOFCAL)." by M.J. Christl, et al. in "Gamma-Ray and Cosmic-Ray Detectors, Techniques, and Missions." Brian D. Ramsey and Thomas A. Parnell, eds. SPIE Vol. 2806, Aug. 7, 1996, pp 155-163.

Return to ballooning story.

Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack