The reaction yields of 58 radionuclides with the mass number A = 42-131 produced in the photofission of ¹⁹⁷Au by bremsstrahlung of end-point energies (E0) from 300 to 1100 MeV have been measured using the catcher foil technique. The reaction yields increase steeply with an increase of E0 up to 600 MeV and increase gradually above 600 MeV, indicating the resonance-type excitation function attributed to the (3, 3) resonance. The charge distribution was found to be well reproduced by a Gaussian function with the most probable charge (Zp) expressed by a linear function of A, i. e., Zp = RA + S, and with the A-independent full width at half maximum (FWHM_CD). The charge distribution parameters R, S, and FWHM_CD were found to be independent of Eο above 600 MeV, reflecting the resonance nature in photonuclear reactions at intermediate energies. The weighted mean values at E0≥600 MeV were R = 0.424 ± 0.001, S = 0.7 ± 0.1, and FWHM_CD = 2.2 ± 0.1 charge unit (c.u.). Based on these parameters, the symmetric mass yield distributions with the most probable mass Ap of 92 ± 1 mass unit (m.u.) and the width FWHM_MD of 39 ± 1 m.u. were obtained. The characteristics of the charge and mass yield distributions were discussed by referring to the literature data including those of proton-induced reactions.

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Positron annihilation processes in a liquid scintillator to be used for the detection of electron antineutrinos in the KamLAND project were studied. The liquid scintillator was a mixture of isoparaffin, pseudocumene and 2, 5-diphenyloxazole. We measured positron annihilation lifetime (PAL) spectra which contained important timing information on identification of the electron antineutrinos. The PAL spectra of a nitrogen-saturated liquid scintillator consisted of three lifetime components. Two fast components with the lifetime of 0.19 ± 0.05 ns and 0.48 ± 0.05 ns correspond to the mixture of para-positronium annihilation and free positron annihilation. The delayed component (3.41 ± 0.02 ns) with the intensity of 48.9 ± 0.3 % is ascribed to pick-off annihilation of ortho-positronium (ο-Ps) annihilation. To understand the 2γ annihilation processes of ο-Ps in the liquid, we examined the effects of dissolved gases (nitrogen, oxygen, helium and argon) and mixing ratio of isoparaffin and pseudocumene which were the main components of the liquid scintillator. The lifetime of ο-Ps was drastically shortened in the presence of oxygen, but not for other gases. The reaction rate constant for quenching of ο-Ps by oxygen was obtained from the O₂ concentration dependence of ο-Ps lifetime component. The pick-off annihilation of ο-Ps with the liquid scintillator is explained based on the bubble model. We discuss a possible microscopic picture of the surroundings of Ps in the liquid scintillator.

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The recoil properties of 37 radionuclides with the mass number A = 24-131 produced in the photonuclear reaction on ¹⁹⁷Au by bremsstrahlung of end-point energies (E0) from 300 to 1100 MeV have been investigated using the thick-target thick-catcher method with an aid of intensive chemical separations. The measured mean ranges, FW and BW, in the forward and backward directions, respectively, with respect to the beam axis show E0-independence above 600 MeV, reflecting the resonance character in photonuclear reactions at intermediate energies. The forward-to-backward yield ratios (F/B) are independent of the mass difference (ΔA) between a product (A) and the target (At =197) above ΔA = 66, and the weighted mean value of F/B =1.1 ± 0.1 is clearly distinguished from that of photospallation products with ΔA ≤ 45. The F/B ratios of proton-induced reactions at incident proton energies (Ep) of Ep ? 3 GeV are higher than those of photon-reactions and decrease with an increase of Ep, but become the same values as the photon-reactions at higher Ep. The mean kinetic energies T of the residual nuclei were deduced based on the two-step vector velocity model. The T values are independent of E0 above 600 MeV and increase with an increase of ΔA, reflecting the changes in the reaction mechanism from spallation to fission . The T values of photon-reactions at ΔA > 45 are quite different from those of the proton-reactions, in which the T values decrease with an increase of Ep up to about 3 GeV and become Ep-independent at higher Ep, though the T values of photon-reactions at ΔA ≤ 45 agree well with those of the proton-reactions, irrespectively of Ep. Recoils of ²⁴Na produced from ²⁷Al, ^natV ^natCu, ^natAg, and ¹⁹⁷Au are of special interest from a viewpoint of a change in the production mechanism with respect to At.

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The reaction products of laser-evaporated iron atoms and methyl iodide CH₃I are isolated in low-temperature Ar matrices and their Mössbauer spectra are obtained. The yields of the products vary depending on the concentration of CH₃I in the Ar matrix-gas. The Mössbauer parameters suggest that these species that are isolated in matrices are monomeric FeI₂ and dimeric Fe₂I₄, which is confirmed by molecular orbital calculations.

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Electron paramagnetic resonance (EPR) and photoacoustic spectral (PAS) studies were conducted on pure polyvinyl alcohol (PVA) film, ²³⁸U (VI) doped PVA film after gamma irradiation and ²³⁹Pu (IV) doped PVA film after in-situ alpha irradiation. Studies on gamma irradiated U (VI) doped PVA film revealed the formation of U (V) complex and free radical -CH₂-C (OH) CH₂-. The later was also formed in pure PVA film. The in-situ alpha-irradiation resulted in a broad line at g =1.998 ± 0.001 in addition to weak lines due to -C (O) =CH₂- and -CH₂-C (OH) CH₂-. The broad line appears to be due to high local concentration of these radicals which increased with longer in-situ irradiation. With long storage, a doublet, presumably due to interaction of -C (O) =CH₂- with I = 1/2 of ²³⁹Pu got evolved.

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Radiochemical procedures for preparing a multitracer using thermal neutron fission of ²³⁵U were established. The target material of ^natUO₂ mixed with the catcher material for the fission products was irradiated at the research reactor (KUR) of the Research Reactor Institute of Kyoto University. After irradiation, the multitracer solution mainly consisting of fission products was prepared by quick removing of the target material. The multitracer enables simultaneous tracing of various elements, and comparison of their chemical behavior under strictly identical experimental conditions.

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A simple and intuitive derivation of the formula describing the Doppler broadened (± 7.6 keV wide) γ-ray shape at 478 keV of ⁷*Li was developed. In application to analysis of PGA spectra of multi-elemental targets, a computer code based on the formula efficiently deconvoluted the 472 keV γ-ray from ²⁴Na superposed on the ⁷*Li line. The code also provided peak areas and the degradation constants of ⁷*Li in the spectra of samples.

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