Hitoshi Koizumi

Predominant decay channel for superexcited organic molecules

Photoabsorption and photoionization cross sections of organic molecules are systematically compared for elucidating relation between superexcitation and ionization. The cross sections examined are of alkenes, alkanes, alcohols, and ethers in the energy range of about 2 eV above the first ionization potential. Although the photoabsorption cross sections are much different from one another, the photoionization cross sections are similar in each molecular group. This result indicates that ions are formed only through direct photoionization and most of superexcited molecules dissociate to neutral fragments. Ionization efficiency curves are calculated under the assumption of no autoionization, and they well reproduce the observed ionization curves, which mainly depend on energy difference between the first and second ionization potentials.

Possibility of Radiation-Induced Degradation of Concrete by Alkali-Silica Reaction of Aggregates

The effect of Ar ion irradiation on the reactivity of crystalline and amorphous quartz to alkali has been examined for clarifying whether radiation from nuclear reactors accelerates the degradation of concrete by inducing alkali-silica reaction of aggregates. Distorted amorphous quartz generated on the surface of quartz by irradiation of a 200 keV Ar ion beam is at least 700 times and 2.5 times more reactive to alkali than crystalline and regular amorphous quartz, respectively. The high reactivity of the distorted amorphous quartz indicates that the degradation of concrete by alkalsilica reaction is possible to be induced by nuclear radiation even the aggregates are inert to alkali before the irradiation. The critical radiation doses for the degradation of aggregates containing crystalline quartz are estimated to be 5 × 1019 n/cm2 for fast neutrons with energy >0.1 MeV, and 5×1011 Gy for beta and gamma rays. They are 1×1019 n/cm2 and 0.5tiems;1011 Gy, respectively for aggregates containing amorphous quartz.

Radical formation in the radiolysis of solid alanine by heavy ions

Abstract Radical formation in solid α-alanine irradiated with 175-MeV Ar8+ and 460-MeV Ar13+, 220-MeV C5+, and 350-MeV Ne8+ ions were studied by the ESR method. The radical yield (number of radicals per incident ion) is constant below the critical fluence of about 1010 ions cm−2 for the Ar ions, 1011 ions cm−2 for the C ion, and 5 × 1011ions cm−2 for the Ne ion. Above the critical fluence, the yield decreases with increasing ion fluence. G-value of the radical formation was obtained from the constant yield at the low fluences. The G-value is not a simple function of LET. This is ascribed to the difference in lateral dose distribution of ion tracks. Assuming a simple cylindrical shape of the ion tracks, the average dose in and the radius of the ion tracks were estimated from the G-values. The radius is 8–25 nm, which is larger than the radius of 2–5 nm for 0.5–3 MeV H+ and He+ ion-irradiations. The fluence-yield curves were simulated with the cylindrical tracks and with the dose-yield relationship for the radical formation in γ-irradiated alanine. The simulated curves agree well with the experimental ones. With the cylindrical model of ion tracks, the variation of the radical yield at the high fluences can be estimated for solid alanine irradiated with several hundreds of MeV heavy ions.

Lyman‐α, Lyman‐α coincidence detection in the photodissociation of doubly excited molecular hydrogen into two H(2p) atoms

Photodissociation processes of the doubly excited states of H2 into H(2p)+H(2p) have been studied using a coincidence detection of two Lyman‐α photons. Coincidence spectra have been measured in the energy region of 29.0–36.0 eV. The intensity of the observed coincidence peak corresponding to two Lyman‐α photons increases with increasing energy from its threshold which is about 29 eV. The main precursor of the two H(2p) atoms is assigned to the doubly excited Q2 1Πu state.

Relation between track structure and LET effect on free radical formation for ion beam-irradiated alanine dosimeter

The yield and local concentration of free radicals generated from alanine (α-aminopropionic acid) by irradiation with 3 MeV H+ and He+ ions were examined by means of electron spin resonance (ESR) and ESR power saturation methods at room temperature. The G-value of the radical formation showed a marked dependence on linear energy transfer (LET) of the ions. The G-value for the H+ ion (average LET: 28 eV/nm) was almost the same as that for γ-irradiation and it was smaller by a factor of 1/4.7 for the He+ ion (average LET: 225 eV/nm). Combining the local concentration of the free radicals along the ion tracks with the G-values and the reported ion range, the radius of a track filled with free radicals was estimated to be 4 ≈ 5 nm by assuming a simple rod-shaped track with a constant radius and homogeneous distribution of the free radicals in it. The track radius scarcely depends on the LET within the range examined. The radiation energy deposited in the core region of the ion track was concluded to spread over the rod to generate free radicals.

Radical formation in the radiolysis of solid alanine by protons and helium ions

Abstract Radical formation in solid α-alanine irradiated with 0.5-, 1-, 2-, and 3-MeV H + , 0.5-, 1-, and 3-MeV He + ions were studied by means of electron spin resonance. The radical yield (number of radicals per incident ion) is constant below the critical fluence of about 10 12 ions cm −2 . Above the critical fluence, the yield decreases with increasing ion fluence. G -value of the radical formation was obtained from the constant yield at the low fluences. Assuming a simple cylindrical shape of the ion tracks, the average radii of the ion tracks were estimated from the G -values. The estimated value of the radius is from 2 nm to 5 nm. The dependence of the radical yield on the ion fluence was simulated with the cylindrical model of ion tracks and dose-yield relationship for γ-irradiation. The observed yield decreases more than the simulated one above the critical fluence. This difference is ascribed to higher damage on the solid alanine by the ion irradiation than by γ-irradiation.

Effective Optical Faraday Rotations of Semiconductor EuS Nanocrystals with Paramagnetic Transition-Metal Ions

Novel EuS nanocrystals containing paramagnetic Mn(II), Co(II), or Fe(II) ions have been reported as advanced semiconductor materials with effective optical rotation under a magnetic field, Faraday rotation. EuS nanocrystals with transition-metal ions, EuS:M nanocrystals, were prepared by the reduction of the Eu(III) dithiocarbamate complex tetraphenylphosphonium tetrakis(diethyldithiocarbamate)europium(III) with transition-metal complexes at 300 °C. The EuS:M nanocrystals thus prepared were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectroanalysis (ICP-AES), and a superconducting quantum interference device (SQUID) magnetometer. Enhanced Faraday rotations of the EuS:M nanocrystals were observed around 550 nm, and their enhanced spin polarization was estimated using electron paramagnetic resonance (EPR) measurements. In this report, the magneto-optical relationship between the Faraday rotation efficiency and spin polarization is discussed.

VUV light-induced electron emission from organic liquids

Abstract Electron emission from low vapor pressure liquids induced by vacuum ultravoilet light was measured as a function of photon energy between 6 eV and 10.8 eV. Absolute quantum yields were determined for 17 organic compounds comprising hydrocarbons, phthalates, phosphates, dicarboxylic esters, amines and polysiloxane fluids. The quantum yields rise from 10 −6 to 10 −1 with increasing photon energy. Systematic changes of the yields with the molecular structure of the liquids were observed. The photoemission threshold energies of 11 liquids have been determined in this work. The electron energy levels and the photoemission process are discussed by taking into account the electronic properties of the isolated molecules, the influence of charge polarization, and the transport of hot electrons through the liquid/vapor interface.

Photoionization quantum yield for liquid squalane and squalene estimated from photoelectron emission yield

Abstract An equation relating photoelectron emission yield to initial photoionization quantum yield in the condensed phase is derived. The equation is applied to the photoelectron emission yields from liquid squalane (C30H62, 2,6,10,15,19,23-hexamethyltetracosane) and squalene (C30H50, 2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene) previously reported by the authors. On the assumption that the thermalization distance of photoelectrons is of a similar value to that of electrons produced with X-rays the photoionization quantum yields of these liquids in the energy region between 7 and 10.8 eV are estimated. The photoionization quantum yields are evidently less than unity and increase with increasing photon energy. The quantum yield for squalane increases monotonically whereas the one for squalene shows a change in its slope around the photoionization threshold of the σ electrons.

Enhancement of Optical Faraday Effect of Nonanuclear Tb(III) Complexes

The effective magneto-optical properties of novel nonanuclear Tb(III) complexes with Tb-O lattice (specifically, [Tb9(sal-R)16(μ-OH)10](+)NO3(-), where sal-R = alkyl salicylate (R = -CH3 (Me), -C2H5 (Et), -C3H7 (Pr), or -C4H9 (Bu)) are reported. The geometrical structures of these nonanuclear Tb(III) complexes were characterized using X-ray single-crystal analysis and shape-measure calculation. Optical Faraday rotation was observed in nonanuclear Tb(III) complexes in the visible region. The Verdet constant per Tb(III) ion of the Tb9(sal-Me) complex is 150 times larger than that of general Tb(III) oxide glass. To understand their large Faraday rotation, electron paramagnetic resonance measurements of Gd(III) complexes were carried out. In this Report, the magneto-optical relation to the coordination geometry of Tb ions is discussed.