NSCL Cosmic Ray Experiment

Originally a Word Document, PDF form: cosmic_directions.pdf

How the CRD Works | Operating the CRD | Experiment | Analysis

How the Cosmic Ray Detector (CRD) Works

A scintillator is a material that gives off light when struck with a charged particle, such as a nucleus.  Believe it or not, the scintillators on the cosmic ray detector are made of plastic.  They are wrapped in layers of aluminum foil, construction paper, and electrical tape to block out normal light.  That way, the only light in the scintillator is produced by cosmic rays.  When a cosmic ray strikes the plastic, it excites (or gives energy to) an electron in one of the atoms.  The excited electron wants to give up its energy, so it emits a photon (a small packet of light).  Since light given off by an electron isn’t very bright, the photon gets detected in a photomultiplier tube. 

Inside the photomultiplier tube, the photon strikes an electron and gives it enough energy to leave its atom.  Then, the negatively charged electron gets pulled toward a plate of positively charged metal.  Once it hits the metal, it frees other electrons.  These electrons hit the next piece of positively charged metal, which frees even more electrons.  This process continues until there are a million electrons.  They strike the last metal plate, which registers the electrical signal as a single count.

You’ll notice that the detector has two scintillators, not just one.  This allows you to control the direction of the cosmic rays that you are counting.  If you use only one scintillator, the cosmic rays may come from any direction.  If you set the detector to “coincidence,” the detector will only count the rays that pass through both scintillators.  The rays that it detects will come in at an angle roughly perpendicular to the scintillator paddles.  When a cosmic ray strikes the scintillator(s) and results in a count, we call it an event.