PERFORMANCE AND APPLICATIONS OF COOLED COMPOSITE BOLOMETERS IN THE FIELD OF IONIZING-RADIATION METROLOGY

Two major limitations met in the field of ionizing radiation metrology, i.e. energy resolution and detection energy threshold. may be improved by one or two orders of magnitude by using composite bolometers cooled below 300 mK and operating in the pulse mode. The major advantages are: possibility of choosing the absorber (from diamond to BGO: Al2O3,Ge, Si, BaF2, LiF, etc), the absence of statistical noise (phonon quanta are in the 10(-4) eV range) and ultimate resolution in the 10(-1) eV range for small bolometers. The theoretical and experimental limits are analyzed and solutions for extraneous noise limitations in a complete system are described. The most recent results in alpha-particle and gamma-ray spectrometry using a new design, the composite-composite bolometer, are given. In these, a resolution of 10.5 keV was obtained for 6 MeV alpha particles and < 2.5 keV for Am-241 18 keV X rays using a 24 g sapphire bolometer. The ideal bolometer concept is also discussed. In this, the simultaneous detection of quanta (due to ionization, fluorescence, ballistic phonons) and thermal energies will make it possible to have lower thresholds by amplification in the target. It should thus be possible to determine the characteristics of the particles (their precise arrival time and the positions of the impacts) and provide improvements in the field of integral counting for the absolute measurement of radioactivity.