Speaker
Description
In UVSOR synchrotron facility at Institute for Molecular Science, an ultra-short pulsed gamma-ray source was developed by the 90-degree interaction between a 750-MeV electron beam and a Ti:Sa laser pulse. The maximum energy of the gamma rays is 6.6 MeV and the pulse width is calculated to be 5 ps (FWHM). Using the ultra-short pulsed gamma rays, gamma-ray-induced positron annihilation spectroscopy (GiPAS) was developed. In GiPAS, a sample is irradiated with 6.6 MeV gamma rays and positrons are generated by pair production inside the sample. The positrons lose energy in the same sample and then emit annihilation gamma rays. When positrons are trapped in defects, the positron lifetime is increased due to the low surrounding electron density. This property can be used for nondestructive analysis of atomic-scale defects formed in materials. GiPAS has several advantages compared with conventional positron annihilation spectroscopy using radioisotopes. (i) It enables defect analysis of thick material in a few centimeters because positrons are created throughout a bulk material via pair production. (ii) Source contribution of positrons annihilated in the covering material is negligible. In conventional radioisotopes, about 10%~15% of positrons annihilate in the source shealing films and their positron lifetimes are included in a lifetime spectrum. In contrast, in GiPAS, positrons are generated directly from gamma rays. (iii) Gamma rays are injected externally into the sample and positrons are generated inside the sample, allowing measurement of samples in special environments such as in-situ measurements. (iv) As the generation of gamma rays can be fully controlled by the laser, the laser signal can be used as the start signal for positron lifetime measurements. This enables measurements with a high signal-to-noise ratio and high time resolution. Multiple measurement methods for GiPAS are currently being developed at UVSOR. In gamma-ray-induced positron annihilation lifetime spectroscopy, a positron lifetime measurement system with a time resolution of 140 ps in FWHM was developed using eight BaF2 detectors and two digital oscilloscopes (DSOs). Gamma-ray-induced age-momentum correlation using two DSOs and two BaF2 detectors was also developed. In this workshop, features of the gamma-ray source and the current status of GiPAS will be presented.
Presentation mode | Onsite |
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