197 TOTAL CROSS SECTIONS FOR INELASTIC SCATTERING OF CHARGED PARTICLES BY ATOMS AND MOLECULES. III. ACCURATE BETHE CROSS SECTION FOR IONIZATION OF HELIUM* AMitio Fnokutt and Y.-K. Kim The Bethe cross section ¢; for ionization of He byfast charged particles is accurately evaluated by a subtractlon ¢; = Ctot — dex, Where oto, 1s the total inelastic- resulting ‘counting’? ionization cross section for a particle of charge ze and velocity v = §e is oo, scattering cross section and ¢@.. the sum of all cdiserete- excitation cross sections. Our earlier work has given a highly precise value of oto: , and recent results on dis- _ Sragz [ = mv) 8 0.489 in (=) 2 =| + — 3526) , crete excitations enable one to determine oe... The where m is the electron mass, a) the Bohr radius, and R the Rydberg energy. Among numerous measurements, the electron-impact data by Smith are most consistent * Abstract of a paper published in Phys. Rev. 186, 100 (1969). Bethe asymptotic behavior near 1 keV incident electron energy. with our result und suggest a gradual attainment of the ELECTRO-OPTICAL TECHNIQUES FOR ULTRASENSITIVE RADIOPHOTOLUMINESCENT DOSIMETRY* Jacob Kastner, R. K. Langs, B. A. Cameron, Alichael Paesler and George Anderson The factor which has limited the sensitivity of pho- toluminescent dosimetry has been the “pre-dose” or matrix fluoresence background which is stimulated during readout by the usual continuous ultraviolet ex- posure. The signal-to-noise ratio has only been partially optimized by selective choice of filters and optical geometry. A microdosimetric system was conceived and in- vestigated which is potentially capable of sensing extremely low radiation dosages (in the order of micro- * Abstract of a paper given at the Second Int. Conf. on Luminescence Dosimetry, Gatlenburg, Tenn., September 23-26, 1968, and published in U.S. Atomic Energy Commission Report CONF-680920. rads). This system depends on the little-known fact that the decay time for the visible luminescence, which is @ measure of the absorbed dose, is at least ten times longer than the decay of the indistinguishable visible fluorescence (to UV) which is an inherent characteristic of unexposed silver phosphate glasses. The principle was confirmed some time ago using an ultraviolet laser and was reported elsewhere. In the hope of simplifying the reader, experiments have been carried out using an electro-optical Pockels shut- ter and much cheaper and less complex UV sources. To date we have been sucessful in obtaining a 2-to-1 signal-to-noise ratio from a Toshiba fluorod exposed to 10 mR. SPECTRAL ANALYSIS OF THERMOLUMINESCENT GLOW CURVES* B. G. Oltman, Jacob Kastner and C. AL. Paden An effort has been made to correlate the spectrum of the light from thermoluminescent materials, in* Abstract of a paper presented at the Second Int. Conf. on Luminescence Dosimetry, Gatlenburg, Tenn., September 2326, 1968 and published in CONF-680920. cluding the far UV, with various kinds of ionizing radiations. An apparatus is described which has been designed and constructed to provide well-controlled heating ramps and monochromator scanning capability. Results are given for such analysis for various inorganic thermoluminescent materials.