THALLIUM BROMIDE X-RAY PHOTCATHODES

X-RAY PHOTOCATHODES ARE USED IN A LARGE NUMBER OF X-RAY IMAGING SYSTEMS, MAINLY IN THE FIELDS OF ASTRONOMY AND MEDICAL IMAGING. IN MOST APPLICATIONS, X-RAY PHOTOCATHODES ARE USED IN CONJUNCTION WITH A MICROCHANNEL PLATE (MCP) AND A POSITION-SENSITIVE DETECTOR. DEVICES INCORPORATING THIS COMBINATION ARE CAPABLE OF IMAGING X-RAYSWITH VERY HIGH SPATIAL AND TEMPORAL RESOLUTION. MANY DIFFERENT X-RAY PHOTCATHODE MATERIALS HAVE BEEN DEVELOPED TO DATE. THESE MATERIALS HAVE USUALLY BEEN ALKALI-HALIDES (E.G., CSL OR CSBR) AND TEND TO BE HYGROSCOPIC IN NATURE. FOR THIS REASON SPECIAL HANDLING AND TREATMENT OF THESE MATERIALS IS NECESSARY TO PREVENT THEM FROM DEGRADATION IN THE LABORATORY. IN ADDITION, X-RAYIMAGING SYSTEMS EMPLOYING THESE MATERIALS AS X-RAY PHOTOCATHODES SUFFER FROM LOW SENSITIVITY AND VERY POOR ENERGY RESOLUTION. WE PROPOSE TO DEVELOP THE SEMI-INSULATING SEMICONDUCTOR MATERIAL THALLIUM BROMIDE (TIBR) FOR USE AS AN X-RAY PHOTOCATHODE. BECAUSE OF THE HIGH ATOMIC NUMBER OF ITS CONSTITUENT ELEMENTS, TIBR HAS A VERY HIGH X-RAY STOPPING POWER AND A HIGH YIELD OF SECONDARY ELECTRONS. MOST IMPORTANTLY, WE HAVE GROWN CRYSTALS OF TIBR IN OUR LABORATORIES AND FOUND THAT THIS MATERIAL IS MECHANICALLY AND CHEMICALLY VERY STABLE. THE DEVELOPMENT OF THIS MATERIAL WOULD LEAD TO A SIGNIFICANT IMPROVEMENT IN THE UTILITY OF DEVICES EMPLOYING X-RAY PHOTOCATHODES.