WE PROPOSE TO DEVELOP AN ACCELERATOR BEAM-TEST INSTRUMENT TO DEMONSTRATE THE PERFORMANCE OF A NEW TYPE OF IMAGING CALORIMETER AIMED AT A FUTURE GAMMA-RAY/COSMIC-RAY MISSION. THE PROPOSAL WOULD FUND THE DEVELOPMENT CONSTRUCTION AND EVALUATION OF A PROTOTYPE INSTRUMENT WITH 150MM 150MM ACTIVE AREA AND CONSISTING OF 4 LAYERS OF SCINTILLATING FIBER X-Y TRACKERS AND 4 LAYERS OF CSI:NA CRYSTAL READ OUT BY WAVELENGTH SHIFTING (WLS) OPTICAL FIBERS. THE PROTOTYPE WOULD BE INSTRUMENTED WITH SILICON PHOTOMULTIPLIER (SIPM) PHOTODETECTORS AND CUSTOM ELECTRONICS UTILIZING LOW POWER ANALOG PIPELINE DIGITIZERS DESIGNED BY THE U. HAWAII GROUP. CALIBRATIONS WOULD BE PERFORMED USING ACCELERATOR RUNS AT BOTH TAGGED PHOTON BEAM FACILITIES AND A HEAVY ION ACCELERATOR. THIS PROJECT IS AIMED AT ADVANCING THE TECHNICAL READINESS LEVEL OF THE KEY DETECTOR TECHNOLOGIES FOR A FUTURE MIDEX (OR PROBE-CLASS) MISSION CONCEPT KNOWN AS THE ADVANCED PARTICLE-ASTROPHYSICS TELESCOPE (APT). THE INSTRUMENT DESIGN WAS DRIVEN BY THE REQUIREMENTS OF GAMMA-RAY SEARCHES FOR DARK MATTER (REQUIRING A PAIR TELESCOPE WITH AN ORDER OF MAGNITUDE IMPROVEMENT IN GEOMETRY FACTOR COMPARED TO FERMI LAT) AND PROMPT LOCALIZATION OF GAMMA-RAY TRANSIENTS SUCH AS THE COUNTERPARTS OF GRAVITATIONALWAVE SOURCES/NEUTRON-STAR MERGERS (BEST ACCOMPLISHED BY A VERY LARGE-AREA COMPTON TELESCOPE). A SUN EARTH LAGRANGE ORBIT WOULD REMOVE EARTH OBSCURATION PROVIDING THE LARGEST INSTANTANEOUS FIELD OF VIEW BUT WOULD REQUIRE THE USE OF A RELATIVELY THIN IMAGING CALORIMETER WITH A DEPTH LIMITED TO<6 RADIATION LENGTHS. THE SAME INSTRUMENT DESIGN WOULD PROVIDE MULTIPLE DIFFERENTIAL IONIZATION ENERGY LOSS (DE/DX) COSMIC-RAY MEASUREMENTS OVER A VERY LARGE AREA WITH A VERY DEEP DETECTOR. SUCH AN INSTRUMENT WOULD BE A POWERFUL COSMIC-RAY DETECTOR CAPABLE OF MEASURING THE ELEMENTAL ABUNDANCES OF VERY RARE ULTRA-HEAVY R-PROCESS COSMIC RAY NUCLEI FOR MATERIAL ORIGINATING OUTSIDE OUR SOLAR SYSTEM CONNECTING TO THE N-STAR MERGER SCIENCE. WITH THE ADDITION OF FOAM RADIATORS THE CSI DETECTORS COULD DETECT THE TRANSITION RADIATION X-RAYS FROM VERY-HIGH-ENERGY LIGHT COSMIC RAYS SPECIFICALLY BORON AND CARBON NEEDED TO DIFFERENTIATE MODELS OF COSMIC-RAY PROPAGATION OF IMPORTANCE TO INDIRECT DARK MATTER DETECTION. THE APT DETECTOR DESIGN WOULD INCORPORATE 20 LAYERS OF 5 MM THICK CSI:NA WITH CROSSED WAVELENGTH SHIFTING FIBER (WLS FIBER) READOUT INTERSPERSED WITH 20 XY SCINTILLATING OPTICAL FIBER TRACKER (SOFT) LAYERS USING INTERLEAVED 1.5MM ROUND SCINTILLATING FIBERS. THE 3M 3M 2.5M DETECTOR VOLUME CONSISTING OF PASSIVE PLASTIC SCINTILLATING FIBERS AND CSI CRYSTALS WOULD BE READ OUT ON THE SIDES WITH SIPM PHOTODETECTORS AND ANALOG-PIPELINE WAVEFORM DIGITIZERS. THESE ELECTRONICS WOULD PROVIDE ADEQUATE LOOK-BACK TIME TO INCLUDE SIGNALS FROM THE RELATIVELY SLOW CSI DETECTORS IN THE TRIGGER; ACHIEVING THIS CAPABILITY WITHIN THE POWER BUDGET CONSTITUTES ANOTHER MAJOR ACTIVITY OF THE PROPOSED RESEARCH. ULTIMATELY THIS WORK WOULD VALIDATE SIMULATION STUDIES THAT INDICATE THAT THE APT INSTRUMENT COULD ACHIEVE 10 TIMES THE SENSITIVITY OF THE FERMI LAT FOR GEV TEV GAMMA-RAYS AND PROVIDE MORE THAN AN ORDER OF MAGNITUDE IMPROVEMENT IN SENSITIVITY OVER ANY OTHER PROPOSED GAMMA-RAY EXPERIMENT IN THE MEV ENERGY RANGE WITH GRAVITATIONAL WAVE SOURCE LOCALIZATION TO BETTER THAN 1 DEGREE UNCERTAINTY. LIKEWISE THE COSMIC-RAY DETECTOR WOULD IMPROVE STATISTICS ON RARE HEAVY ELEMENTS AND HIGH-ENERGY LIGHTER NUCLEI BY ORDERS OF MAGNITUDE COMPARED TO ANY EXTANT EXPERIMENT.