THERMAL TILE BOND INSPECTION BY GAMMA RAY SCATTERING

EXTERNAL THERMAL PROTECTION FOR THE SPACE TRANSPORTATION SYSTEM ORBITER IS PROVIDED BY SPECIALLY MACHINED INSULATING TILES WHICH ARE INDIVIDUALLY MOUNTED BY ADHESIVE BONDING TO THE OUTER SKIN. DURING APPLICATION THE ADHESIVE MAY CRACK, FORM VOIDS, OR SIMPLY FAIL TO BOND TIGHTLY TO THE SURFACE. CURRENTLY THERE ARE NO RELIABLE NONDESTRUCTIVE TECHNIQUES TODETERMINE THE EXISTENCE OR SIZE OF BONDING DEFECTS. PHASE I RESEARCH WILL EXAMINE THE FEASIBILITY OF FINDING A POSSIBLE REMEDY IN GAMMA RAY SCATTER COUNTING, A FORM OF RADIOMETRY WIDELY USED TO MEASURE THICKNESS AND COMPOSITION OF THIN FILM COATINGS. UNLIKE CONVENTIONAL RADIOGRAPHY OR RADIOMETRY, GAMMA SCATTERING REQUIRES ACCESS TO JUST ONE SURFACE, AND THROUGH PROPER COLLIMATION CAN BE MADE TO IGNORE SURFACE FEATURES, SENSING ONLY DEFECTS IN A LAYER DEEP UNDER THE SURFACE. THE APPARATUS FOR SCANNING THESE DEEP LAYERS CAN BE MADE EXTREMELY RUGGED AND LIGHTWEIGHT; DETECTORS AND ELECTRONICS SIMILAR TO THOSE REQUIRED FOR A HAND-HELD INSPECTION UNIT HAVE BEEN SUCCESSFULLY BOOSTED INTO SPACE. IN PHASE I A BREADBOARD GAMMA SCATTER UNIT WILL BE TESTED FOR CAPABILITY OF CHARACTERIZING CONDITION OF A LAYER OF ADHESIVE MATERIAL. A PROTOTYPE INSPECTION DEVICE WILL BE DEVELOPED IN PHASE II.