The flashing pathway at your feet is detecting the presence of cosmic ray particles from outer space. Stand on one of the glass panels.  Did it flash?  If so, that means a particle carrying a cosmic ray's energy from outer space has passed through the earth's atmosphere, through the roof of this building, through your body, and finally through detection equipment below the panel to trigger the flash.

Don't worry.  Cosmic ray particles aren't dangerous to people on earth.  However, astronauts and pilots who travel in outer space and high in the earth’s atmosphere do have to worry about their increased exposure.

Cosmic ray particles are actual pieces of the far-away places astronomers study.  They are the only direct material link we have to the far reaches of our galaxy and the rest of the Universe.  Scientists study cosmic rays to learn more about supernovae, quasars, neutron stars and black holes.

Where do cosmic rays come from?
Some cosmic ray particles come from the Sun.  Others come from within our own galaxy or other galaxies far, far way... One of the great mysteries in modern astrophysics is the source of the highest energy cosmic ray particles and just how they are accelerated to these energies.

1. A secondary cosmic ray particle travels through the atmosphere, the roof of the building, the glass floor panel, and the detection equipment below the panel. It may continue to penetrate many meters into the Earth depending on its energy.

2. When the particle hits the detection equipment, it passes through two special slabs of plastic called scintillators.  This plastic contains special chemicals that result in a tiny flash of light being emitted when a particle passes through. The scintillators are wrapped with black material to prevent light from entering.  As a result, any light observed is due only to penetrating particles.

3. The flash of light is carried from the scintillators through long strands of fiber optics beneath the floor to light detection equipment in the wall.

4. The light is detected by a photomultiplier tube.  The photomultiplier converts the tiny light flash into an electrical pulse that in turn is converted to a digital signal.

5. The photomultiplier tube is very sensitive, and as a result 
sometimes emits random noise pulses that are not due to light flashes arriving at the tube. We therefore look for a light flash from each scintillator slab arriving at the photomultiplier at the same time (i.e., in coincidence).  This tells us that a particle passed downward through both scintillators. To do this, the digital signals from each scintillator are sent to a logical "AND" gate that gives a digital output only when there are two inputs at the same time.

6. The digital output controls a switch that briefly turns on light bulbs beneath the glass floor panel so that you can "see" that a cosmic ray particle has arrived!