H. Nakahara

Prompt gamma-ray analysis (PGA) of meteorite samples, with emphasis on the determination of Si

Neutron-induced prompt gamma-ray analysis (PGA) was applied to seven meteorite samples (Allende, Zagami, Acfer 209, ALH77005, ALH84001, EET79001 and Neagari). Samples were irradiated in both the thermal neutron and the cold neutron guided beams of JRR-3M at JAERI. Multiple samples of an Allende standard powder were analyzed for Si using two different methods: (1) the comparison method, using a Si standard, and (2) the mono-standard method, using Fe as an internal reference element. The Si concentrations determined by these two methods are in good agreement with literature values. The analytical sensitivity for Si using the cold neutron guided beam is∼14.3× higher than that for the thermal neutron guided beam. Other elements determined (B, Ca, Ti and S) also showed higher sensitivities using the cold neutron beam. The other meteorites studied showed some anomalous B and S values likely due to the effects of terrestrial weathering/contamination.

Two deformation paths in proton-induced fission of 232Th☆

Abstract Velocities of complementary fission fragments have been accurately measured by a double velocity time-of-flight method in the proton-induced fission of 232 Th with incident energies from 12.0 to 14.7 MeV. The results experimentally demonstrate for the first time the correlation among the threshold energy, the total fragment kinetic energy, and the mass division mode in fission: one type of the fission process has a lower threshold energy and leads to a compact scission configuration with reflection asymmetry, and the other type has a higher threshold energy and leads to a symmetric elongated scission configuration.

Production and characterization of actinide metallofullerenes

We have, previously, reported on the HPLC elution behavior of the Th, Pa, U, Np, and Am metallofullerenes and the UV/vis/NIR absorption spectra of the Th@C84 and U@C82 species. In this paper, the followings are reported: (1) Pu metallofullerenes were produced and their HPLC elution behavior was investigated using a radiotracer technique. The HPLC chromatogram of this metallofullerene was found to be almost the same as that of the Np and Am metallofullerenes. (2) The oxidation states of Th@C84 and U@C82 produced in macroscopic quantities were examined by XANES (X-ray absorption near edge structure) measurements. The oxidation state of the U atom in the C82 fullerene cage was estimated to be 3+ with the formal charge of the ionic molecule being U3+@C823-.

Pre-equilibrium process in 3He-induced reactions on 59Co, 109Ag, 181Ta and 209Bi

Abstract Excitation functions and mean projected recoil-ion ranges of radioactive nuclei produced in the 3 He-induced reactions were measured for the targets of 59 Co, 109 Ag, 181 Ta and 209 Bi. Most of the excitation functions showed characteristic features of the pre-equilibrium process. We proposed a new method, based on the hybrid model, for evaluation of the initial exciton number n 0 from the slope of excitation function for the reaction in which only a single neutron is emitted. With the deduced n 0 for ( 3 He, n) reaction, the observed excitation functions were compared with the theoretical calculations of the equilibrium and the pre-equilibrium model. The shape of the observed excitation functions for ( 3 He, n) reactions with four kinds of targets was compared and discussed in connection with the n 0 values.

Startup of transactinide chemistry in JAERI

The transactinide nuclides 261Rf and 262Db have been successfully produced in the 248Cm(18O,5n) reaction at 99 MeV and in the 248Cm(19F,5n) reaction at 100, 103, and 106 MeV, respectively, at the JAERI tandem accelerator. The on-line ion exchange experiments with an automated fast and repetitive liquid chromatography separation system were performed in the HNO3/HF system using Rf homologues 89mZr and 167,165Hf produced in the 89Y(p,n) and 152Gd(18O,xn) reactions, respectively. The radiotracers 88Zr, 175Hf, and 234Th were also prepared and the distribution coefficients on ion exchange resins were measured systematically in 1-11 M HCl and 1-14 M HNO3 with the batch method. It was found that anion exchange experiments of Rf in 8 M HNO3 and 9 M HCl provided information useful to extract the ionic radius of Rf and to verify the influence of relativistic effects.

Angular momentum effects on mass division in actinide fission

Angular momentum (/) effects on mass yield curves were studied for the fission from the same compound nucleus (CN) Np of the same excitation energy but with different angular momenta, produced by the Li+Th and p+U reactions. The effects were examined only for the complete fusion-fission events after careful estimation of the contribution of incomplete fusion-fission. It is found that the width of the whole mass yield curve is larger while the peak-to-valley ratio is smaller for the Li+Th fission at the same excitation energy (Ex) of the CN, the former value increases while the latter ratio decreases with Ex. Statistical model calculations incorporating angular momentum dependent fission barriers were made to reproduce the observed cross section data of evaporation residues and fission. The observations can well be explained if the angular momentum dependence of fission barriers is taken into account in the competition process of fission and neutron evaporation in the decay of a compound nucleus except the observed trend of the width of the mass yield curve which could not be explained straightforwardly.

Analysis of archaeological samples by the internal monostandard method of PGAA

Nondestructive determination of major and trace elements within voluminous materials has been demonstrated by prompt γ-ray neutron activation analysis. Some fragmented pieces of pottery bowls considered to have been produced in the late 18th century were analyzed by a new internal monostandard method, and noticeable differences of the elemental contents and significant correlations of the elemental distributions were observed among the samples. The possibility of identification of producing areas of unearthened material is demonstrated by the analytical results and the applicability of this method to the investigation of precious samples in a completely nondestructive way is verified in practice.

Some Basic Studies on Non-Destructive Elemental Analysis of Bulky Samples by PGAA

An internal monostandard method for PGAA is proposed for completely non-destructive analysis of elements in bulky samples by the use of a neutron beam guided out from the reactor. The method is essentially the same as the k0 method except that it corrects for the change of relative γ-ray counting efficiencies caused by the absorption and scattering of neutrons and absorption of γ-rays within the sample. Some examples of applications to archaeological samples of earthen wares, bronze mirrors, and to voluminous meteorite samples are demonstrated, and its future application to the in-situ studies of distribution and metabolism of a certain elements in animals is also explained.

Bimodal Nature of Low Energy Fission of Light Actinides

Studies of mass division phenomena in low energy proton-induced fission of light actinides are discussed. From experimental data of excitation functions and angular distributions of fission products, it has been found that there are two kinds of fission threshold energies (fission barrier heights); symmetric and asymmetric mass divisions require different threshold energies. Two kinds of scission configurations for certain mass division have been verified by accurate measurements of fragment velocities : compact and elongated scission configurations. A correlation between the two types of threshold energies and the two kinds of scission configurations has been revealed by observing the incident energy dependence of the yields corresponding to each compact and elongated scission configuration. It indicates that there are two independent deformation paths in the protoninduced fission of light actinides; one type of the fission process has a lower threshold energy and leads to a compact scission configuration with reflection asymmetry, while the other type has a higher threshold energy and leads to a symmetric elongated scission configuration.

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)%.