High pressure xenon proportional tube detector for Tc-99m imaging

A novel nuclear medicine imaging camera is proposed based on a close packed array of thin walled aluminum proportional tubes having an entrance window at one end and operating in sealed mode at 40-60 atm. It can be shown that effects of the thin aluminum structure are minimal for 140 keV photons, and that stopping power similar to NaI can be readily achieved. Detectors having a diameter of 4 mm, length of 50 mm, and anode wire diameters of .01 and .02 mm were developed and tested. Tubes were operated in low gain (2-3) avalanche mode for optimal energy response. Energy resolution for /sup 99m/Tc was evaluated over a wide range of pressures. The 0.02 mm anode tubes produced energy resolution FWHM of 4.2% at 28 atm and increasing to 7.8% at 52 atm. The energy resolution of 0.01 mm anode tubes degraded less with pressure, producing 4.6% at 28 atm and increasing to 6.1% at 52 atm. Lifetime tests of a sealed 42 atm detector over 473 days and a 48 atm detector over 64 days revealed no detectable degradation of energy resolution. Operation of 0.02 mm anode detectors in collection mode (no avalanche) was also explored and produced highly stable performance and nearly pressure independent resolution of 7.5% in the range 28 to 52 atm. In summary, performance superior to NaI by a factor of two is feasible operating in the avalanche mode up to 48 atm and even higher pressures can be achieved in collection mode while still providing better resolution than NaI. Thus the xenon proportional tube employed in a close packed array can provide a photomultiplier free detector with acceptable sensitivity and considerably improved energy response while also providing many potential operational advantages over NaI.