Y. Ohkubo

“Multitracer” a new tracer technique — Its principle, features, and application

We established “Multitracer”, a new versatile radiotracer technique, for simultaneous tracing of a number of elements in various chemical, environmental, and biological systems. Metal foil targets (typically Au, Ag, Ge, Cu and Fe) are irradiated with C, N, or O ions accelerated up to 135 MeV/nucleon by RIKEN Ring Cyclotron. Radiochemical procedures have been developed to remove the target material leaving the nuclides as Multitracer solutions containing various radionuclides of Be, Na, Mg, K, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Te, I, Ba, Ce, Pm, Eu, Gd, Tb, Tm, Yb, Lu, Hf, W, Re, Os, Ir, Pt, and Hg. Multitracers enable efficient tracing of a number of elements, and comparison of their behavior under strictly identical experimental conditions. Such features will be demonstrated by means of an example of application to a model experiment for the study of removal mechanism of various elements from the ocean.

Multitracer study on transport and distribution of metal ions in plants

Transport and distribution of metal ions in rice and soybean plants were studied using multitracers produced by irradiating an Au target by 135 MeV/nucleon12C,14N, or16O ions accelerated by RIKEN Ring Cyclotron. The multitracer consisted of radioisotopes of the following elements: Be, Na, Sc, Mn, Fe, Co, Zn, Se, Rb, Sr, Y, Zr, Nb, Ag, Te, Ba, Ce, Pm, Eu, Gd, Tb, Tm, Yb, Lu, Hf, Ir, and Pt. Rice and soybean plants were grown in a nutrient solution and also on soil containing a multitracer. γ-ray spectroscopy of different parts of the plants showed that all the elements were more or less taken up by roots, while appreciable transport to leaves and seeds was observed only for Mn, Zn, Se, Rb, Sr, and Ba.

Preparation of carrier-free111mCd and105,106mAg and their chemical behavior

It was found that not only111mCd but also105Ag and106mAg sublimate at 1050 °C under a reduced pressure from Pd foils irradiated with α-particles. The sublimation method has enabled rapid separation of the isotopes and repeated use of valuable Pd foils. Adsorption onte α-Fe2O3 and coprecipitation with ferric hydroxide were studied by using radioactive tracers.

Strong Affinity between In and Al Impurities Doped in ZnO

In recent years, zinc oxide (ZnO) doped with group 13 elements (Al, Ga, In) as impurity donors is strongly expected for application to functional devices as n-type II–VI compound semiconductors. It is known that the electric conductivity of the semiconductor drastically changes depending on the type, concentration, and introduction methods of impurities. For the precise control of conductivity, therefore, it is essential to investigate the physical and chemical states of the impurities. With respect to the study of the electromagnetic field in the vicinity of dilute impurities, nuclear techniques with radioactive probes are very suited because of their high sensitivity. Taking this advantage, we have applied the timedifferential perturbed angular correlation (TDPAC) method to the investigation of local fields at donor sites in ZnO, employing Cd disintegrated from In, symbolized as 111Cdð 111InÞ hereafter, as the nuclear probe expecting the behavior of In as a donor impurity. In our previous TDPAC studies, we found that local fields at the probe nucleus in ZnO doped with dilute In, Ga and Al ions are distinctly different from that for undoped ZnO. Especially for the Al-doped ZnO, in addition, the doping effect on the 111Cdð 111InÞ probe is prominent even at as low Al concentration as 0.05 at.% (500 ppm). This observation implies a high probability for the local association of the 111Cdð 111InÞ probe and Al ion(s) as a result of their thermal diffusion in ZnO. Directing our interest to the probability of their association, we investigated the dilute limit of Al concentration at which their association presumed above can be observed. The present paper proves the presence of a strong attractive force between In and Al ion(s) in ZnO matrix by showing an experimental evidence that the 111Cdð 111InÞ probe can be locally associated with extremely dilute Al ion(s). Zinc oxide samples doped with various Al concentrations were separately synthesized by a solid-state reaction following our previous work. For the synthesized samples, we found from powder X-ray diffraction patterns that the lattice constants were unchanged from pure ZnO even for the 5000 ppm at.% Al-doped ZnO and new phases were not detected. TDPAC measurements were performed for the 111Cdð 111InÞ probe doped in each sample at the concentration of 100 ppt on the cascade rays with the intermediate state of I 1⁄4 5=2 having a half-life of 85.0 ns. In the present work, observation of TDPAC was made for the directional anisotropy, A22G22ðtÞ, as a function of the time interval between the cascade -ray emissions, t, during which the probe is perturbed by the outer surrounding field. Here, A22 denotes the angular correlation coefficient depending only on the nuclear properties and G22ðtÞ is the time-differential perturbation factor. For easy understanding of the effect of the Al doping on the present perturbation pattern, the TDPAC spectra of 111Cdð 111InÞ in undoped and 500 ppm Al-doped ZnO are firstly cited in Fig. 1 from our previous studies. The spectrum for the undoped ZnO in Fig. 1(a) exhibits an oscillatory structure typical of the electric quadrupole interaction between the probe nucleus and the extranuclear field (hereafter denominated Component A). Although the oscillating pattern reproducible with a single component is not clearly discernible due to the damped structure reflecting a wide distribution of the field at the probe, the static perturbation pattern for the Al-doped ZnO in Fig. 1(b) (denominated Component B) is also ascribable to the electric quadrupole interaction because the sample consists of no magnetic materials; accordingly, the distinct perturbation should be attributed to the change in the charge distribution around the probe by the Al doping. We thus performed least squares fits to the spectra in Fig. 1 with G22ðtÞ expressed as G22ðtÞ 1⁄4 G 22ðtÞ 2;0 þ X3

Systematic study of lanthanoid endohedral metallofullerenes: Production yields, HPLC retention time and reactor irradiation effects

We made systematic studies of lanthanoid metallofullerenes on the following three properties using the radiochemical method: (1) the relative production yields of metallofullerene species, (2) variation of the HPLC retention time among M(III)@C82 species, and (3) the effects of the reactor irradiation on the survival yields of each metallofullerene species. The production yields of M@C82 relative to La@C82 were found to decrease as the atomic number of M became larger and as the number of atom ratio, M/C, in the carbon rod became larger. On the other hand, the production yields of M1M2@C82, relative to that of LuM@C82 were found to increase for the larger atomic number. The retention time for the M(III)@C82 species in the Buckyprep column was found to become slightly longer for the larger atomic number but it becomes abruptly larger for Gd, and Tb by about 5%. The overall effects of the reactor irradiation on the survival yields (or retention yields) of M@C82 species were found to be (19.7±2.1)%.

Yields and Recoil Properties of Products from the Interaction of Natural Copper with Intermediate-Energy 14N, 15N and 40Ar Ions

Reaction residues in the interaction of copper with intermediateenergy N, 1 N and 4 Ar ions (26 to 135 MeV/u) have been studied using non-destructive y-ray spectrometry. Formation cross-sections and average forward ranges were measured for 58 products. The data were used to obtain the mass yield distribution and the longitudinal momentum transfer. The results are compared with those from the studies reported on the interaction of copper with heavy projectiles, and discussed in terms of the transition of the reaction mechanism from low energy to relativistic energy.

Search for the decay of 229mTh by photon detection

Summary A search for the isomeric transition of 229mTh at 3.5 eV was made by photon detection with a low-noise photomultiplier for samples prepared by a rapid chemical separation from the parent nuclide 233U by means of an anion exchange method. The photon emission caused by the disintegration of 229mTh was not observed for 229Th nitric acid solution samples. From the experimental results, the limits of the half-life of 229mTh as dissolved in HNO3 solution are discussed.

Formation energy of oxygen vacancies in ZnO determined by investigating thermal behavior of Al and In impurities

Thermal behavior and interacting nature of 100-ppm Al and ∼100-ppt In impurities doped in zinc oxide (ZnO) were investigated by means of the time-differential perturbed angular correlation method with the 111In(→111Cd) probe. Contrasting interactions between Al and In impurities were observed depending on different atmospheric conditions: (1) in air, Al and In impurities irreversibly associate with each other in the process of their thermal diffusion, but (2) in vacuum, their bound state formed in air dissociates by heat treatment at temperatures higher than 873 K, and this process is enhanced with increasing temperature. Detailed investigation of the thermal behavior of the impurities has revealed that the dissociation reaction is triggered by the formation of oxygen vacancies in the vicinity of the locally associated In-Al structure. A unique method to determine the activation energy of the oxygen-vacancy formation is presented with the estimated experimental value of Ea = 0.72(6) eV.

Radiochemical study on the mechanism of target fragmentation of Cu, Nb, Pr and Au targets induced by 12C and 40Ar projectiles

A thick-target thick-catcher experiment was performed to measure the formation cross sections and recoil momenta of products from target fragmentation of Cu, Nb, Pr, and Au by using gamma-ray spectrometry. Bombardments of C ions (180, 290, and 400 MeV/u) and Ar ions (290 and 650 MeV/u) were performed at the HIMAC facility in Japan. The results were discussed in comparison with systematics of fragmentation and used to deduce the prefragments in fragmentation process of the measured systems.

LIMITING BEHAVIOR OF TARGET FRAGMENTATION IN THE REACTIONS OF LANTHANIDE NUCLEI INDUCED BY HEAVY PROJECTILES

Nuclear reactions of165Ho induced by 1.0 GeV15N or 1.8 GeV14N ions and those of141Pr induced by 2.3 GeV or 3.8 GeV40Ar ions have been studied by off-line γ-ray spectrometry. Mass distributions of the products were confirmed to reveal a limiting characteristics at projectile energies beyond 2 GeV. Longitudinal momentum transfer was measured with a thick-target-thick-catcher technique. The results demonstrated that a provisional limiting of the recoil velocity appears in the energy range of 2–10 GeV, besides the ultimate limiting condition already pointed out. The coincidence in the onset energy of the limiting behavior was interpreted as the result of the existence of the limit heating of nucleus.