File:U.S. Department of Energy - Science - 271 014 003 (17575346394).jpg

ONE OF THE TWO END CAPS OF THE TIME PROJECTION CHAMBER (TPC), SEEN FROM THE BACK.

THE TIME PROJECTION CHAMBER IS A MAJOR PORTION OF THE POSITRON-ELECTRON PROJECT. TPC USES THE DRIFT OF IONIZED PARTICLES IN AN ELECTRIC FIELD TO RECORD PARTICLE TRACKS. IT MEASURES VELOCITY AND MOMENTUM OF A PARTICLE PASSING THROUGH THE CHAMBER AND PERMITS PRECISE IDENTIFICATION OF THE PARTICLE. THE CAPS ARE AT EACH END OF A GAS-FILLED CHAMBER ABOUT ONE METER LONG. A BEAM FROM THE PEP ACCELERATOR PASSES THROUGH THE CHAMBER VIA THE SMALL HOLE IN THE CENTER OF THE PICTURE, PARTICLES THAT ARE CREATED WHEN ELECTRONS AND POSITRONS INTERACT IN THE BEAM FLY OUT AND ENTER THE CHAMBER. THERE, THEY IONIZE ATOMS OF GAS, CREATING A STRING OF ELECTRONS WHICH ARE DRAWN TOWARDS THE DETECTORS IN THE END-CAPS. THE PIE-SHAPED WEDGES, SHOWN ABOVE, CONTAIN WIRES AND ELECTRONIC CIRCUITRY WHICH PERMIT PHYSICISTS TO RECONSTRUCT THE TRACK IN THREE DIMENSIONS AND IDENTIFY THE PARTICLES THAT CREATED IT.