OBJECTIVES: THE MEAN DISTRIBUTIONS OF ISOTOPES AS FUNCTIONS OF GALACTOCENTRIC RADIUS (RGC DISTANCE FROM THE GALACTIC CENTER) ARE PROXIES FOR THE EXTENT OF STELLAR PROCESSING AND SIGNPOSTS FOR CHEMICAL VARIATIONS WITH TIME IN THE GALAXY. STUDIES OF ISOTOPE RATIOS VS. RGC THEREFORE PROVIDE THE CONTEXT FOR INTERPRETING THE SIGNIFICANCE OF SOLAR-SYSTEM STABLE ISOTOPE RATIOS. OUR OBJECTIVE IS TO CHARACTERIZE THE DISTRIBUTION OF THE THREE STABLE ISOTOPES OF SILICON 28SI 29SI AND 30SI IN THE INTERSTELLAR MEDIUM ACROSS THE GALAXY TO PROVIDE CONTEXT FOR INTERPRETING THE ISOTOPE RATIOS IN PRE-SOLAR GRAINS AND THE SOLAR SYSTEM AS A WHOLE. OUR SURVEY WILL CONSTITUTE THE FIRST NEW SYSTEMATIC INVESTIGATION OF INTERSTELLAR MEDIUM SI ISOTOPE RATIOS SINCE THE EARLY 1980S (EXCLUDING COSMIC RAYS). THE RESULTS WILL TEST GALACTIC CHEMICAL EVOLUTION (GCE) MODELS USED TO ASSESS THE ENVIRONMENT IN WHICH THE SOLAR SYSTEM FORMED.THE RATIOS OF SECONDARY TO PRIMARY SILICON ISOTOPES IN THE SOLAR SYSTEM ARE SURPRISINGLY LOW COMPARED WITH OLDER PRE-SOLAR SIC GRAINS FOUND IN METEORITES. THIS ABERRATION HAS BEEN USED AS POSSIBLE EVIDENCE FOR EXTRAORDINARY ENRICHMENT OF THE PRIMARY ISOTOPE 28SI BY SUPERNOVAE IN THE REGION IN WHICH THE SUN FORMED. OXYGEN ISOTOPES ALSO HINT AT SUPERNOVA ENRICHMENT IN THE FORM OF EXCESS 18O RELATIVE TO 17O. IN ORDER TO VERIFY OR CONTRAVENE THE IDEA THAT THE BIRTH ENVIRONMENT OF THE SOLAR SYSTEM WAS ATYPICAL OF THE GALAXY 4.6 GYR BEFORE PRESENT WE NEED AN UNDERSTANDING OF HOW RELEVANT STABLE ISOTOPE RATIOS HAVE EVOLVED WITH TIME AND PLACE IN THE GALAXY (I.E. OVER THE LAST 4.6 GYRS). IF OUR SOLAR SYSTEM FITS WITH THE GENERAL PICTURE OF SECULAR VARIATIONS IN STABLE ISOTOPE RATIOS IN THE GALAXY THEN IT WOULD SUGGEST THAT THE ANSWER TO THE QUESTION OF WHETHER WE ARE 'NORMAL' OR NOT IS AT LEAST IN PART IN THE AFFIRMATIVE. CONVERSELY IF OUR SOLAR SYSTEM EXHIBITS SIGNIFICANT DEPARTURES FROM THE AVERAGES EXPECTED FROM AN ANALYSIS OF THE DISTRIBUTION AND EVOLUTION OF ISOTOPES IN THE GALAXY THEN WE WILL BE IMPELLED TO SEARCH FOR EXTRAORDINARY CIRCUMSTANCES TO EXPLAIN THESE DEPARTURES IN ISOTOPIC ABUNDANCES (ENRICHMENT BY NEARBY SUPERNOVAE IS THE MOST OBVIOUS EXAMPLE). ISOTOPES OF SILICON ARE THOUGHT TO BE AN EXAMPLE OF THE LATTER CASE BUT A FIRM GALACTIC REFERENCE FRAME FOR INTERPRETATION OF THE SOLAR DATA IS NOT IN PLACE.APPROACH: THIS PROPOSAL IS TO CARRY OUT AN IN-DEPTH STUDY OF SILICON ISOTOPE RATIOS IN MOLECULAR CLOUDS ACROSS THE GALAXY USING THE GREEN BANK TELESCOPE (GBT) FOR THE 28SIO 29SIO AND 30SIO J=1-0 TRANSITIONS IN THE Q BAND (43 GHZ). OUR GROUP HAS ALREADY COLLECTED A LARGE FRACTION OF THE REQUIRED GBT DATA. WE REQUIRE FUNDS TO SUPPORT A GRADUATE STUDENT (A STUDENT HAS ALREADY BEEN ASSIGNED TO THE PROJECT) TO PROCESS THESE DATA AS PART OF THE STUDENT'S PHD PROJECT. PROCESSING INCLUDES COMPUTING THE EFFECTS THE SYSTEMATIC ERRORS THAT CAN ARISE FROM DIFFERENT CONTINUUM RADIATION FIELDS ON THE MEASURED ISOTOPOLOUGE RATIOS FILLING IN GAPS IN THE DATA WITH MORE OBSERVING WHERE NECESSARY AND CREATING GALACTIC CHEMICAL EVOLUTION CONSTRAINTS FROM THESE DATA.RELEVANCE TO EW: OUR STUDY IS DIRECTED ENTIRELY AT IMPROVING OUR UNDERSTANDING OF THE ISOTOPIC COMPOSITION OF THE SOLAR SYSTEM AT THE TIME OF ITS BIRTH. ABSOLUTE ISOTOPE RATIOS OF PRE-SOLAR GRAINS IN PARTICULAR AND THOSE OF THE SOLAR SYSTEM AS A WHOLE MORE GENERALLY CANNOT BE INTERPRETED WITHOUT A CLEAR UNDERSTANDING OF THE SPATIAL VARIABILITY AND TEMPORAL EVOLUTION OF ISOTOPE RATIOS IN THE GALAXY. VARIATIONS IN ISOTOPE RATIOS WITH GALACTOCENTRIC DISTANCE PROVIDE A PROXY FOR VARIATIONS WITH TIME BY ANALOGY WITH METALLICITY. THE DISPERSION IN SI ISOTOPE RATIOS ACROSS THE GALAXY TODAY ALSO AFFORDS A MEASURE OF THE VARIATIONS EXPECTED AT A GIVEN TIME FOR COMPARISON WITH THE DISPERSION SEEN IN PRE-SOLAR GRAINS.