| Research Abstract |
A monoenergetic positron beam line was constructed in order to develop a postitron moderator. In this beam line, positrons emitted from a 22Na soruce were used. The monoenergetic positrons were guided by a magnetic field and were implanted into the Al specimen. The conversion rate from high energy positrons to monoenergetic positrons was measured with a HP-Ge detector located at the sample chamber. In the present study, the moderators consist of tungsten foils, tungsten ribbons and nickel cones were developed.
The positron pulsing system designed in the present study consists of three main devices ; a reflection type chopper, a sub-harmonic prebuncher (SHPB) and a double harmonic buncher. The chopper generates a pulsed beam with beam width of 3-8 ns. The chopped beam is accelerated up to 250 eV,then it is guided into the SHPB.The SHPB compresses the pulse width of the chopped beam to about 2 ns. Then, the buncher compresses the pulse width to about 100 ps at the specimen. The fundamental frequency of the buncher, f_b, is 150 MHz. The operating frequency of the SHPB is a quarter of the fundamental frequency (f_b/4). The chopper frequency is f_b/4n MHz (n=1,2 ...), where the time interval between subsequent pulses can be varied. The lifetime spectra are obtained by measuring the time interval between the timing signal derived electrically from the pulsing system and the annihilation gamma ray detected by a BaF_2 scintillation detector.
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