Positron utilization research and development for materials engineering
Positron annihilation method

The positron is an antiparticle of the electron discovered by Anderson in 1832. It annihilates with the electron and the mass energy is emitted as gamma rays. Most of the annihilation occurs in the two-photon emission process in condenced phase. Measuring this annihilation gamma rays is the positron annihilation method. By measuring gamma rays, various information can be obtained.Simply, it is energy and time information.The energy is affected by the momentum of positrons and electrons at the time of annihilation, and the Doppler effect occurs.In material research, this makes it possible to measure the momentum of electrons at the time of annihilation, and is mainly sensitive to defects that change the rate of annihilation with inner-shell electrons.

Study in Insulating Matter

Fig.1 Formation mechanism of positronium in substance. Conventional positronium formation is a reaction between excess electrons and thermalized positrons, which is believed to be occurring up to a few picoseconds. New positronium formation process occurs when metastable electrons are accumulated at low temperatures in dark, by such as the ionization of previously injected positrons, and positrons that escape the conventional positronium formation diffuse to find metastabilized electrons. Therefore, as shown in the figure, it gradually increases. Also, because it occurs after diffusion, formation can be seen even after hundreds of picoseconds.

Fig.2 Positron annihilation lifetime measurement time dependence of the formation yield of ortho-positronium formation (75% of total positronium formation) in cyclohexane at 77 K (●; in darkness, ○; under visible light). At low temperatures in darkness, metastabilized electrons are formed and accumulated by ionization by incident positrons, and as a result, new positronium formation as shown in the figure 1 increases. As shown in the figure 1, the metastable electrons are extinguished by visible light, so only the component (about 16%) due to the conventional positronium formation remains.

Metal material research

Method development

The positron annihilation method is a cutting edge method on gamma ray measurements, and the method is also being developed.For example, by dramatically advancing the time resolution of positron annihilation lifetime measurement, new phenomena are captured and new research becomes possible. The same is true for count rates.Recently, by performing waveform analysis, we developed a method to perform detector discrimination and applied it to the positron annihilation method[7]. Until now, the signal from the detector was sent as it is to the analysis system, where data was stored and analyzed. We have developed a technology that transports the signal from the detectors with a single cable and performs waveform analysis to discriminate the detectors and to accumulate and analyze data.Even if the number of detectors is increased, it is not necessary to increase the analysis system proportionately, and it is possible to operate particularly large-scale devices economically.

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