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We developed a subtraction circuit to measure the saturation spectra of rubidium atoms without using a lock-in amplifier. An instrumentation amplifier (IA), which is a type of differential amplifier, was used. We designed our IA to have an adjustable gain of 4.3 - 21 to obtain saturation spectra with a good signal-to-noise ratio. With our IA, we tried to deduce the saturation spectrum by subtracting the Doppler signal from the transmission spectrum of rubidium atoms enclosed in a glass cell. As a result, the obtained saturation spectra have six peaks originating from the atomic and cross-over resonances, and their line widths are 11.2 MHz. This number of peaks is the same as that detected with a lock-in amplifier, and the line width is smaller than that detected with a lock-in amplifier. Therefore, we confirmed that our IA has equivalent performance to a lock-in amplifier for measuring the saturation spectrum of atoms.

Solute atoms greatly influence irradiation damage structures. For example, voids are formed in pure Ni at 573 K, but they are not formed in Ni-Sn alloys irradiated by fission neutrons at 573 K to 0.2 dpa. In this study, the interaction between solute atoms and vacancies and correlation between void growth and solute atoms in neutron-irradiated Ni-Sn alloys were investigated by positron annihilation spectroscopy. Positron annihilation lifetime measurements showed that void density was lower in Ni-0.05 at%Sn than pure Ni. In Ni-0.3 at%Sn, single vacancies, di-vacancies and small stacking fault tetrahedra (SFTs) were formed and the size did not change below 0.1 dpa. In Ni-2 at%Sn, single vacancies and SFTs were formed and the size also did not change below 0.1 dpa. In Ni-Sn alloys, the total intensities of lifetime related to vacancy type defects were more than 60%. Vacancy concentration was higher than in pure Ni and a large number of positrons annihilated at them. From positron annihilation coincidence Doppler broadening (CDB) measurements, the ratio of annihilation of positrons and low-momentum electrons increased in Ni-Sn alloys compared with pure Ni. The ratio increased with increasing the solute atom concentration. As positron affinity of Sn is lower than that of Ni, positrons are attracted to Sn atoms more strongly than Ni. Complexes of vacancy clusters and Sn atoms are formed and the number of Sn atoms adjacent to vacancy clusters increases with the solute atom concentration. It is expected that Sn atoms change the electron state of vacancy clusters, which leads to the increase of annihilation ratio between positrons and low-momentum electrons.