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The development of a fast time-sorter and its use in the measurement of the lifetime of positrons in aluminium and mica Jones, Garth


A new type of fast "time-sorter" has been developed, employing a germanium diode as the detecting element. This instrument converts time delays between coincident events into a pulse amplitude distribution, which is then analyzed by a "kick-sorter", enabling a complete coincidence resolution curve to be recorded simultaneously. The resolution curves obtained with this apparatus were found to have half-widths of about 1 milli-microsecond, comparable with those obtained by delayed coincidence circuits. Using this time-sorter, the lifetimes of positrons in Aluminium and Mica were then studied. The positron life-time in Aluminium was obtained by measuring the centroid shifts between the coincidence resolution curves resulting from the observation of a Na²² source embedded in Al, and an assumed "prompt" source of cascade gamma rays, As⁷⁶. This latter source was employed for the comparison because the energies of its coincident gamma rays are similar to the 1.28 Mev and 511 Kev. gamma rays of Na²². This eliminates the problem of centroid shifts due to inequalities in pulse-height and rise-time between the two sources. The positron life-time in mica was obtained by comparing the centroid shifts of the resolution curves resulting from the annihilation of positrons in mica and in Aluminium. The measured positron lifetime in Aluminium is (1.6 ± 0.4) x 10[power -10] sec, and the lifetime in Mica is longer than this value by (0.7 ± 0.4) x 10[power -10] sec.

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