Mineo Imamura

Cosmogenic nuclides in the Kirin chondrite

Abstract Radioactive and stable cosmogenic nuclides have been determined in the various fragments of the Kirin chondrite (H5). Experimental results obtained from our samples are described and compared with similar data obtained in other laboratories. Cosmogenic 54 Mn, 22 Na, 60 Co, 26 Al, 53 Mn, 40 K (in the metal phase), and light noble gases were measured. Based on these data, the irradiation history of this meteorite can be explained in terms of (1) a multi-stage exposure which involves a first-stage irradiation for 10 7 years and a second stage for about 4×10 5 years, and (2) depth effects in the productions of the nuclides in 4π(second stage) and 2π(first stage) geometries. These conclusions are consistent with those of our previous work which was based on a limited number of samples.

Aging of wood: Analysis of color changes during natural aging and heat treatment

Abstract The color properties of aging wood samples from historical buildings have been compared with those of recent wood samples that were heat treated at temperatures ranging from 90°C to 180°C. The results of kinetic analysis obtained by the time-temperature superposition method showed that the color change during natural aging was mainly due to a slow and mild oxidation process. In other words, heat treatment could accelerate the changes in wood color that occur during aging. In one sample, the color change (ΔE* ab ) after 921 years at ambient temperature was almost equivalent to that of heating (artificial aging) approximately for 6.7 h at 180°C. The results have been interpreted that the aging and the subsequent change in wood color begin at the time of tree harvesting.

COSMOGENIC 10BE AND 26AL IN METAL, CARBON, AND SILICATE OF METEORITES

Abstract Cosmogenic nuclides 10 Be and 26 Al were determined in various meteorites, including chondrites stony irons and irons. In an effort to obtain information concerning shielding conditions for cosmic ray irradiation in meteorites, these two nuclides were measured in separated metal phases, where they are high-energy, products and in separated stone phases, where they are lower-energy products. Graphite phases, representing the single smallest mass target, were also separated from the IA group irons and a high production rate of 10 Be was determined in carbon. As an example of a large chondrite, Tsarev (L5) was examined by measuring several fragments recovered from different depths. Using measurements of fragments of the Jilin chondrite, the shielding indexes were formulated as a function of depth during its first-stage 2π irradiation. Based on the data obtained in this work and the literature cited, shielding in meteorites, in general, is expressed quantitatively by two universal parameters, as follows: k ′ 2 , a measure of the irradiation hardness or the spectral shape of the flux; and k ′ 1 , a measure of the intensity of the irradiation. For chondrites these parameters have been related numerically to the size of the preatmospheric body and the sample depth in it.

A Method to Estimate the Fast-Neutron Fluence for the Hiroshima Atomic Bomb

A new method to estimate the fast-neutron fluence of the Hiroshima atomic bomb is proposed. 63 Ni produced by the 63 Cu(n, p) 63 Ni reaction provides a unique measure by which to estimate the fast-neutron fluence of the Hiroshima/Nagasaki atomic bombs, because the half-life of 63 Ni is 100 years and 70% of the 63 Ni produced in a copper piece presently exists after 50 years. Using the neutron spectrum given in DS86 and the estimated cross section, we found that a piece of copper of about 10 g which was exposed at a point around 100 m from the hypocenter gives a measurable amount of 63 Ni using a low-background liquid scintillation counter. For the measurement of 63 Ni, accelerator mass spectrometry also seems to be applicable.

Tandem accelerator mass spectrometry of 10Be/9Be with internal beam monitor method

Abstract The 3.5 MV tandem accelerator at the University of Tokyo has been adapted for the accelerator mass spectrometry for 10Be/9Be using a unique technique in which 9Be from 9Be17O− is simultaneously monitored while 10Be is counted. It has been proved that the technique is appropriate for precision measurements. A reproducibility of better than 5% is attainable. Data are presented on 10Be in the volcanic lava samples from the north-east Japanese arc volcanoes.

26AL UPTAKE AND ACCUMULATION IN THE RAT BRAIN

Abstract To investigate the cause of Alzheimers disease (senile dementia), 26Al incorporation in the rat brain was studied by accelerator mass spectrometry (AMS). When 26Al was injected into healthy rats, a considerable amount of 26Al entered the brain (cerebrum) through the blood-brain barrier 5 days after a single injection, and the brain 26Al level remained almost constant from 5 to 270 days. On the other hand, the level of 26Al in the blood decreased remarkably 75 days after injection. Approximately 89% of the 26Al taken in by the brain cell nuclei bound to chromatin. This study supports the theory that Alzheimers disease is caused by irreversible accumulation of aluminium (Al) in the brain, and brain cell nuclei.

The10Be method of dating marine sediments — Comparison with the paleomagnetic method

Abstract Simultaneous study of10Be activity, magnetic reversal, mineralogy and trace elements in a 7 m long core from∼20°S, 170°W in the Pacific Ocean has been made. The10Be and magnetic reversal chronology are found to be in agreement, indicating validity of the10Be method of dating. The possible sources for the variations of10Be are also discussed.

High-energy p-Li neutron field for activation experiment

Abstract A p- 7 Li quasi-monoenergetic neutron field for activation experiment has been developed at energies between 18 and 38 MeV. The absolute neutron intensity was accurately determined from the number of 7 Be nuclei produced in the 7 Li target which is equal to the number of the monoenergetic peak neutrons released in the 4π direction. The angular distribution function of the peak neutrons is necessary for the estimation of the monoenergetic neutrons emitted in the forward direction for the sample irradiation. The distribution function was measured for proton energies of 20, 30 and 40 MeV and was collected from papers for energies between 25 and 800 MeV to get the universal curve. Activation experiments of various natural and enriched samples were performed at this quasi-monoenergetic neutron field. Here we show the neutron activation cross sections of the 197 Au(n, 2n) 196 Au, 197 Au(n, 4n) 194 Au and 59 Co(n, 2nα) 54 Mn reactions as examples.

Measurements of Activation Cross Sections on Spallation Reactions for 59Co and natCu at Incident Neutron Energies of 40 to 120 MeV

In our previous work, we have measured the neutron spallation cross sections of 12C and 209Bi in the 20-to 150-MeV energy range. Here in this succeeding study, the neutron activation cross sections on spallation reactions for 59Co and natCu have been measured in the quasi-monoenergetic p-7Li neutron fields in the 40-to 120-MeV energy range which have been established at two cyclotron facilities of Takasaki Research Establishment of Japan Atomic Energy Research institute (TIARA) and the Institute of Physical and Chemical Research (RIKEN). The neutron spectrum has been measured with the TOF (Time of Flight) method using an organic liquid scintillator, and the absolute value of peak neutron fluence has been determined from the proton recoil counter telescope at TIARA and the activation method of lithium target at RIKEN. In this study, the following seventeen neutron activation cross section data are reported which produce 56Mn, 54Mn, 52Mn, 58Co, 57Co, 56Co, 55Co, 59Fe from 59Co reactions and 56Mn, 59Fe, 60Co...

A low-level needle counter

Abstract A small needle type gas counter has been developed for high sensitive β counting of solid sources as weak as 0.01 cpm.