Journal of Environmental Quality 31:2001-2007 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Plant and Environment Interactions
Tree-Ring Strontium-90 and Cesium-137 as Potential Indicators of Radioactive Pollution
Akira Kagawa*,a,
Toru Aokib,
Naoki Okadac and
Yukio Katayamad
a Wood Anatomy and Quality Lab., Forestry and Forest Products Res. Inst., Tsukuba Norin P.O. Box 16, Ibaraki 305-8687, Japan
b Radioisotope Res. Center, Kyoto Univ., Sakyo, Kyoto 606-8502, Japan
c Grad. School of Agriculture, Kyoto Univ., Sakyo, Kyoto 606-8502, Japan
d Div. of Human Environment, The Univ. of Human Environments, Okazaki, Aichi 444-3505, Japan
* Corresponding author (akagawa{at}ffpri.affrc.go.jp)
Received for publication November 3, 2001.
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ABSTRACT
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To examine whether tree rings can be used to detect or assess local historical 90Sr or 137Cs fallout, such as that resulting from the Hiroshima atomic bomb, radial distribution of 90Sr and 137Cs in trees was examined. We studied a gymnosperm [Japanese cedar, Cryptomeria japonica (L. f.) D. Don] and an angiosperm (Japanese persimmon, Diospyros kaki Thunb.) tree species from the vicinity of the atomic bomb hypocenter, and from other locations in Japan. A significant amount of 137Cs was detected in tree rings formed before 1945, indicating lateral migration of Cs. In contrast, the specific activity of 90Sr in the Hiroshima Japanese cedar showed the highest level in 1945, due to relatively immobile characteristics of Sr compared with Cs. Strontium-90 and Sr analyses in tree rings helped identify and distinguish between residual 90Sr activity from the Hiroshima atomic bomb and the atmospheric nuclear testing. This indicates the possibility of detecting or assessing previous local 90Sr pollution through with tree-ring analysis.
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INTRODUCTION
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TREE RINGS have been used as biomonitors of various environmental parameters such as temperature, precipitation, and heavy metal pollution (Schweingruber, 1996; Epstein and Krishnamurthy, 1990; Hagemeyer, 1993; Katayama et al., 1993). Some elements in tree rings have been related to soil acidification (Dewalle et al., 1991, 1999; McClenahen et al., 1989; Guyette et al., 1992). Cutter and Guyette (1993) suggested species suitable for dendrochemical study. The advantage of using tree rings as a biomonitor is the prospect of exact dating and the tree's potentially great age, which makes historical reconstruction of environmental information feasible.
The possibility of using tree rings for historical assessment of radioactive pollution has also been studied by several researchers. One of the earlier attempts was the analyses of 137Cs in tree rings that confirmed lateral movement of 137Cs and pointed out the difficulty of using 137Cs for reconstruction of radioactive pollution history (Brownridge, 1984; Katayama et al., 1986). Another attempt was made by analyzing 14C in tree-ring cellulose of pinyon (Pinus edulis Engelm.) growing near the first atomic bomb test site. These results showed that duration and/or concentration of 14CO2 exposure were not sufficient to increase tree-ring 14C activity (Leavitt and Long, 1989). Kozák et al. (1993) studied the possibility of using 3H in tree rings, which is immobile just like hydrogen atoms in C–H bonds in cellulose, and therefore nonexchangeable like 14C in cellulose. They analyzed the tritium concentration of several spruce trees having grown in a tritium-contaminated area, showing the feasibility of retrospective evaluation of local tritium fallout. Kudo et al. (1993) studied trees grown in a Pu-contaminated area in Nagasaki and found that Pu is immobile, and that the Pu in the tree rings of 1944–1946 was a minor contribution to the concentration profile compared with that from global fallout.
On the other hand, 90Sr in gymnosperm trees shows abundance patterns similar to the cumulative deposition of 90Sr, indicating the relatively immobile character of Sr in nonfunctioning cells and therefore demonstrating the possibility of monitoring local 90Sr fallout by tree-ring analysis (Momoshima and Bondietti, 1990; Bondietti et al., 1989, 1990; Okada et al., 1990). Gymnosperm trees such as Japanese cedar are known to be more useful for this kind of study than angiosperm trees. They have less Sr relocation than angiosperms, possibly because they have less developed ray parenchyma cells, which act as radial transport channels in sapwood. On the other hand, angiosperm trees form vessel networks, and these networks contribute to radial translocation. Furthermore, mineral uptake of Japanese cedar takes place in a relatively shallower part of soil (Momoshima and Bondietti, 1994) that enables rapid responses to radiochemical changes of soil surface (Chigira et al., 1988). Aoki et al. (1998) have studied Sr movement in the Japanese cedar stems by injecting Sr solution into the middle part of sapwood, and demonstrated that upward movement of Sr takes place by sap flow; however, there was also some radial movement accompanying it. Thus, some minor incorporation of Sr into sapwood cells is possible, even after cells are formed. While there can be radial movement of Sr caused by sap flow, the range of the movement should be limited within the sapwood region at the time of incorporation. Buzinny et al. (2000) have studied 90Sr and 137Cs in pine tree biomass (needle, root, and tree rings) from the vicinity of the Chernobyl nuclear power plant, where the pine trees were planted after the pollution incident. The 90Sr activity of the tree rings often showed the highest activity at the innermost ring (formed in 1990), although the tree ring was formed after the pollution incident. This also indicates strong potential of monitoring 90Sr pollution with tree rings formed during a pollution incident.
Our main objective in this study was to test the suitability of tree rings for detecting or assessing local 90Sr and 137Cs fallout. For this purpose, we used trees that had grown in Hiroshima when the atomic bomb had been dropped. The atomic bomb, equivalent to 15 kilotons of TNT, was detonated at 580 m above ground level in the Hiroshima city area at 0815 h, 6 Aug. 1945 (Shigematsu, 1993). Immediately after the event, measurement of the residual radiation intensity in Hiroshima was conducted (Pace and Smith, 1959) with Geiger counters; however, no measurements of the 90Sr and 137Cs fallout distribution had been conducted until the 1970s. To assess the resulting 90Sr and 137Cs pollution from the atomic bomb blast, Hashizume et al. (1978) measured the activity of these nuclides in soil samples collected in the 1970s from within 30 km of the hypocenter. However, the subsequent fallout from atmospheric nuclear tests had already made it impossible to distinguish between the fallout from the Hiroshima atomic bomb and that from nuclear tests. The use of tree rings may resolve this problem, because under favorable circumstances, they may record surrounding environmental information and make historical reconstruction possible. Reconstructing the spatial distribution of 90Sr fallout is especially important, because many individuals from Hiroshima suffered from leukemia, which was caused by 90Sr incorporation into bone.
We compared samples of two tree species from the area of "black rain" that contained fission products of 235U (Roesch, 1987), with samples of the same species from other parts of Japan. Here we report specific activity of 90Sr (90Sr/Sr) and 137Cs activity of these sample trees together with radial distributions of alkali and alkali earth metals, then discuss radial migration of these nuclides in tree stems and the suitability of tree rings for monitoring local 90Sr and 137Cs fallout.
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MATERIALS AND METHODS
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Tree Sampling
A gymnosperm tree and an angiosperm tree, Japanese cedar and Japanese persimmon, were collected from the area of "black rain" (Fig. 1
; Roesch, 1987). Both tree species grow over a wide region in Japan and Japanese cedar is the longest-living tree species in Japan (up to 1000 yr old). Long-lived tree species with a large geographic distribution are considered desirable for dendrochemical study (Cutter and Guyette, 1993). However, due to the unavailability of tree species living from before 1945 in this region and time-consuming analysis of wood 90Sr (1 man-month for 10 data points), we sampled (cut down) one tree of each species. The Japanese cedar and the Japanese persimmon trees were situated 16 km and 11 km away from the hypocenter, respectively. Both trees were exposed to the "black rain" in 1945. The Hiroshima Japanese cedar grew in a temple precinct in a rural area, and we also sampled four orthogonal cores from four Japanese cedar trees at the same site for crossdating the Hiroshima Japanese cedar tree. A Japanese cedar and a Japanese persimmon were also collected from Asahi in Yamagata Prefecture and Tanba in Kyoto Prefecture, respectively (Fig. 1). The Yamagata Japanese cedar grew in a Japanese cedar plantation in Yamagata University experimental forest, and Kyoto Japanese persimmon was collected at a private house. Data for the sampled trees are listed in Table 1. Annual precipitation average in Hiroshima is about 1550 mm and that of Kyoto is 1600 mm, with both regions having more rain in summer than in winter. Annual precipitation average in Yamagata is 1850 mm, with significant snowfall in the winter.
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Fig. 1. A map showing study sites. A Japanese cedar was sampled at A, 16 km away from the hypocenter; a Japanese persimmon was sampled at B, 11 km away from the hypocenter; and C is the hypocenter of the atomic bomb detonation. Dotted area shows where the "black rain" fell.
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Sample Preparation
Stem sections at breast height were cut and then sliced into disks. Because of the obscurity of Japanese persimmon tree rings, we observed stained cross-sections under a microscope. All the tree rings were cut into 1- to 5-yr intervals with a wood chisel and were subjected to a series of analyses. Approximately 100 g of separated wood from each interval was used for the analyses of 90Sr and 137Cs. In the analyses of Japanese cedar samples, we used 15 tree-ring intervals of Hiroshima Japanese cedar and 5 from Yamagata Japanese cedar. Japanese persimmon samples were separated into intervals of 4 to 5 yr and all of the tree rings were used for the analyses. Each wood sample was put into a glass beaker covered with aluminum foil and carbonized in a muffle furnace at 320°C. The carbonized wood was pulverized in a mortar and then ashed at 380°C, the lowest possible temperature, since Cs can be vaporized at higher temperature (Koh et al., 1999). The weight of sample ash was about 1% of the original wood weight.
Chemical Analysis
Concentrations of Cs and other alkali metals were measured by instrumental neutron activation analysis (INAA), and samples for INAA were prepared every one to four tree rings. Neutron irradiations were conducted at the Research Reactor Institute of Kyoto University and the Institute for Atomic Energy at Rikkyo University. After each irradiation, 
rays were counted with a high-purity Ge detector. Concentrations of Mg, Ca, and Ba were also measured by the same method. The 137Cs activity of the ash samples was measured directly by ray spectrometry with a low-background, high-purity Ge detector.
Analysis of 90Sr was performed according to the procedure standardized by the Science and Technology Agency of Japan (The Science and Technology Agency, 1979) with some details modified (Aoki, 1999) as follows. Each ash sample was dissolved in 2 M HCl and concentrations of Sr and Ca in the solution were measured by atomic absorption spectrometry and inductively coupled plasma mass spectroscopy (ICP–MS). We calculated Sr and Ca concentrations in each tree ring from these measurements. Then Sr carrier was added to the solution to make 90Sr easily precipitable with it, and oxalate was precipitated at pH 4.0 to 4.2. The oxalate was separated on a membrane filter (pore size 0.45 µm, diameter 47 mm) and then dissolved into dilute nitric acid. Strontium in this solution was separated from Ca by repeating the Sr(NO3)2 precipitation process with fuming HNO3 until the Ca concentration became less than 10% of the Sr concentration. After the separation of Sr from Ca, radiochemical trace impurities were coprecipitated with iron hydroxide (10 mg Fe) at pH 8. Then, BaCl2 solution (10 mg Ba) was added to the solution and was precipitated with sodium chromate at pH 5 to remove radioactive Ba isotopes such as 140Ba. After measuring final Sr and Ca concentrations, recovery rate of the Sr carrier was calculated. Then, SrCO3 was precipitated by the addition of Na2CO3 on a membrane filter (pore size 0.45 µm, diameter 25 mm) to form of a thin disk layer. The 90Sr activity (count/min) of the disk layer was measured with a low background 2
gas flow GM counting system (Aloka [Tokyo, Japan] LBC 451, FC-103C). The SrCO3 disk layers had been left for more than two weeks before the measurements to achieve 90Y ingrowth. Counting efficiency was calculated by measuring seven 90Sr standards with different thickness (10–50 mg Sr) to correct for the ß ray self-absorption effect. The 90Sr and 90Y counting efficiency of samples ranged from 30 to 32%. The background counting rate was 0.8 to 1.0 counts/min for blank SrCO3 precipitate. The data were corrected for background, Sr recovery rate, counting efficiency, and radioactive decay.
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RESULTS AND DISCUSSION
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Radial Distributions of Cesium-137 and Alkali Metals
Hiroshima Japanese cedar rings formed long before 1945 contained significant amounts of 137Cs (Fig. 2)
. Since no 137Cs existed in the environment before 1945 in Hiroshima, this result indicates inward transport of Cs, as has been previously reported (Brownridge, 1984; Katayama et al., 1986; Chigira et al., 1988). The Cs distribution pattern of Hiroshima Japanese cedar was similar to that of other alkali metals (Na, K, Rb), and the concentration of every alkali metal showed an inward increase around the sapwood–heartwood boundary, except for Na in Yamagata Japanese cedar (Fig. 3)
. This pattern is common to Japanese cedar trees (Okada et al., 1987), and the greater concentration level in the heartwood than in the sapwood also supports inward Cs movement from sapwood to heartwood of the tree species. If we assume constant uptake of Cs in each year, then Cs has to be moved inward to achieve greater Cs concentration in heartwood. In contrast to Japanese cedar, radial distributions of alkali metals in Kyoto Japanese persimmon were almost constant. Elemental analyses of various tree species have shown that each tree species has intrinsic distribution patterns of elements (Okada et al., 1993).
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Fig. 2. Radial distributions of 137Cs in Japanese cedar trees. Results of Hiroshima Japanese cedar are shown with error bar of ±1 standard deviation and a data point shown as b.d.l. means that the measurement was below the detection limit. Data are not corrected for radioactive decay.
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Fig. 3. Radial distribution of alkali and alkali earth metals, where concentrations are shown on a dry wood basis and vertical lines in the figure show sapwood–heartwood boundaries. The Cs of Kyoto Japanese persimmon is shown with error bars of ±1 standard deviation. Concentration of Ca was analyzed by two methods. ICP represents the data by inductively coupled plasma mass spectrometry analysis and INAA by instrumental neutron activation analysis. Although Mg is not an alkali earth metal, it is included in the figure.
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The 137Cs activity was about six times greater in the Yamagata Japanese cedar than in the Hiroshima Japanese cedar. In Japan, 137Cs and 90Sr depositions are highest along the northwestern side of the main island of Japan, indicating geographical differences, since these areas usually have a heavier snowfall in winter brought by the Siberian air mass (Katsuragi, 1983).
Radial Distributions of Strontium-90 and Alkali Earth Metals
Strontium and Ca were almost evenly distributed throughout the stem and trends were parallel to each other (Fig. 3), as has been observed in Japanese cedar trees in other studies (Okada, et al., 1990). The 90Sr/Sr trends of Japanese cedar trees in Japan showed trends similar to the cumulative deposition of 90Sr, due to the immobility of Sr (Momoshima et al., 1995; Aoki et al., 1995). In contrast, a peak was observed in 1945 in the 90Sr/Sr distribution of Hiroshima Japanese cedar (Fig. 4)
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Fig. 4. Radial distribution of 90Sr/Sr in Hiroshima and Yamagata Japanese cedar. Because of 90Sr radioactive decay (half life 28.8 yr), 90Sr/Sr data were corrected for decay based on tree-ring age.
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A significant amount of 90Sr existed at rings 1941–1942. However, 90Sr/Sr of rings 1941–1942 was 58% of that of ring 1945, and the value became almost zero in rings 1927–1928, indicating less lateral movement of 90Sr than 137Cs. A possible explanation for the existence of 90Sr in tree rings formed before 1945 is the radial 90Sr movement through ray parenchyma cells or 90Sr incorporation into sapwood tracheids by sapflow. When the 90Sr incorporation into the 1945 ring of Hiroshima Japanese cedar occurred in the year 1945, there might also have been some 90Sr incorporated into the tree rings formed before 1945 (sapwood tracheids), carried by sap flow. However, sap flow is limited within the sapwood region and therefore this mechanism of 90Sr incorporation should be limited within the sapwood at that time. The 90Sr/Sr profile of tree rings before 1945 would therefore indicate the degree of radial 90Sr mobility in the stem by both mechanisms and thus infer the efficacy of Sr in dendrochemical studies. Once Sr is incorporated into heartwood region, there would be little or no relocation, since Japanese cedar trees have a well-defined heartwood region, lower heartwood radial permeability, and lower moisture content compared with other tree species (Kuroda, 1991; Cutter and Guyette, 1993).
To decay-correct the 90Sr/Sr value of tree rings, we had to take the radial migration effect into account. The activity of 90Sr halves every 28.8 yr due to radioactive decay, while the 90Sr/Sr values became half every 4 to 5 yr in tree rings before 1945 due to the radial migration effect. This means the magnitude of this migration effect is much less than that of radioactive decay; therefore, we assumed the radial migration effect to be negligible when decay-correcting 90Sr/Sr data. Immobility of Sr in Japanese cedar is also shown by the cation exchange characteristic of Sr between sap and cell walls. In fact, most Sr in Japanese cedar exists within cell walls (Okada et al., 1990; Momoshima et al., 1995).
The decay-corrected 90Sr/Sr profile of Hiroshima Japanese cedar differs from several reports for Japanese cedar (Chigira et al., 1988; Momoshima et al., 1995). Japanese cedar trees that were not affected by the atomic bomb show a 90Sr/Sr pattern like that of Yamagata Japanese cedar. The pattern is similar to the cumulative deposition of 90Sr, with a maximum in the early 1960s or thereafter (Fig. 5
; Katsuragi, 1983), and minimal 90Sr existing before 1945. This effect can also be seen in the 90Sr/Sr profile of Hiroshima Japanese cedar, with a small rise in 1963, coinciding with the maximum of 90Sr atmospheric fallout.
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Fig. 5. Variation of 90Sr/Sr profiles among the species and the sites in Japan; the data are not corrected for radioactive decay.
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Geographical Variation of Strontium-90/Strontium Level
Although 90Sr/Sr fluctuations were similar to the deposition pattern, tree-ring 90Sr/Sr levels varied considerably among the sampling sites. For example, Yamagata Japanese cedar showed two to five times higher 90Sr/Sr values than Hiroshima Japanese cedar. Observational records of 90Sr deposition show similar geographical distribution as that of 137Cs, with greater deposition on the northwestern side and reduced deposition on the southwestern side of the main island of Japan (Katsuragi, 1983, National Institute of Radiological Sciences, 2000). The same tendency was also observed in soil 90Sr activity data throughout Japan. The greatest activity was observed in Yamagata (6.3 GBq/km2 ) and this value is three to four times greater than the Japan Inland Sea area (1.5–2.1 GBq/km2 ) where Hiroshima is situated. Kyoto falls into the region of lower soil 90Sr activities where cities nearby are between 1.5 and 1.6 GBq/km2. Soil 90Sr levels in 1967–1970 at the closest observation stations to the sampling sites were significantly correlated with 90Sr specific activity levels in tree rings formed at that time (Fig. 6 ; Kamata, 1979), irrespective of the distance between the trees and stations (up to 90 km).
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Fig. 6. The relationship between tree-ring 90Sr specific activity and soil 90Sr activity. Correlation coefficient was significant at the 1% level.
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The 90Sr/Sr level in the 1960s and 1970s of the Hiroshima Japanese cedar and Hiroshima Japanese persimmon were almost identical, regardless of species or element concentration, although the 90Sr/Sr variations were different (Fig. 5). Kyoto Japanese persimmon showed the highest 90Sr/Sr level at the outermost rings. A detectable amount of 90Sr was measured in Kyoto Japanese persimmon in tree rings formed in 1920, indicating inward movement of Sr, and the Sr movement of Japanese persimmon was far more than that of Japanese cedar. These differences were probably due to physiological and wood structural difference between gymnosperm and angiosperm trees. Angiosperm trees, like Japanese persimmon, have more developed ray-parenchyma cells in tree stems (which control the radial movement of substances) than gymnosperm trees. Furthermore, Japanese persimmon is a diffuse porous species and forms a vessel network. This network can also contribute to the radial transport of minerals. The 90Sr/Sr pattern difference between Hiroshima and Kyoto Japanese persimmon, the former showing maximum values at the innermost rings and the latter at the outermost rings, may be due to the atomic bomb, but 90Sr/Sr was clearly not effective as a pollution indicator. In fact, 90Sr in tree rings of gymnosperms, like Japanese cedar, is known to have greater potential for monitoring local 90Sr fallout than angiosperm trees (Momoshima and Bondietti, 1994).
Possibility of Assessing Strontium-90 Pollution from the Hiroshima Atomic Bomb
There are three major preconditions for retrospective monitoring of radioactive pollution with tree rings. First, the radial migration of the radionuclide across tree rings has to be minor. Second, most of the radionuclide has to be incorporated into the outermost tree ring (for example, the 1945 ring in the case of Hiroshima Japanese cedar). Third, the radionuclide has to be incorporated into tree rings above the detection limit. While isotopes such as 3H of C–H bonds and 14C in tree ring cellulose are completely immobile, 90Sr in tree rings can be relocated, a shortcoming that makes exact dating of a pollution event difficult. However, for the study of local radioactive pollution, 90Sr has favorable characteristics because it can be deposited in the humus layer or soil for years (Kamata, 1979), while 14C and 3H can mainly exist in the forms of CO2 and H2O and easily disappear from the surrounding environment after their introduction. This increased residence time of 90Sr in soils makes incorporation of 90Sr into tree rings more likely; otherwise, such low-level pollution like that of the Hiroshima atomic bomb could not be detected.
It has been reported that fallout residual radioactivity originated from the Hiroshima atomic bomb is so weak that one cannot distinguish between 90Sr from the Hiroshima atomic bomb and 90Sr from atmospheric nuclear tests at present. However, the results showed that the detection of the 90Sr pollution from the Hiroshima atomic bomb was possible with tree rings. Radial distributions of 90Sr/Sr in Japanese cedar trees from various parts of Japan, which are unaffected by the Hiroshima atomic bomb, are known to show a maximum in the 1960s or thereafter, and only a little 90Sr/Sr is present in the 1945 ring (Aoki et al., 1995; Momoshima et al., 1995), reflecting a 90Sr deposition pattern of atmospheric nuclear tests as shown by Momoshima and Bondietti (1994). Our results for Yamagata Japanese cedar show the same trend.
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CONCLUSIONS
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A significant amount of 137Cs was detected in tree rings formed long before the introduction of the anthropogenic radionuclides into the surrounding environment, indicating inward movement of Cs in tree stems. Radial distribution of Cs along with other alkali metals also supports the idea of inward movement of these elements from sapwood to heartwood. The 137Cs level was higher in Yamagata Japanese cedar than in Hiroshima Japanese cedar, probably reflecting the geographical differences in 137Cs deposition. A clear indication of the atomic bomb fallout could not be observed from the 137Cs data.
Decay-corrected 90Sr/Sr distribution of Hiroshima Japanese cedar showed a peak at the 1945 ring with a steep increase from 1941 to 1945, and a gradual decrease from 1945 to 1992, completely different from the trend of Yamagata Japanese cedar. Considering the fact that all the Japanese cedar trees from other parts of Japan consistently show the trend similar to the cumulative atmospheric 90Sr fallout pattern, a peak at 1945 is due to the fallout from the atomic bomb. A small peak was also observed for 1963 in Hiroshima Japanese cedar, coinciding with the maximum atmospheric 90Sr fallout pattern (Katsuragi, 1983). The 90Sr/Sr distribution of Hiroshima Japanese cedar thus reflected both the local and global fallout pattern. Although a small amount of 90Sr migrated to tree rings formed before 1945, the mobility of Sr was far less than that of Cs, making Sr a desirable element for dendrochemical study. Considering the difference in 90Sr/Sr trends between Japanese cedar of Hiroshima and other parts of Japan, there is the possibility of reconstructing spatial distribution of 90Sr fallout in Hiroshima from the atomic bomb by the 90Sr/Sr peak in 1945, because 90Sr contribution from atmospheric nuclear testing was minimal in those rings.
Both 90Sr and 137Cs levels of trees from various parts of Japan showed wide geographical variation and 90Sr specific activity in trees was significantly correlated with soil 90Sr activity data.
Further studies of the 90Sr/Sr distribution with sampling several Hiroshima Japanese cedar trees from the polluted area are necessary to evaluate the feasibility of reconstructing spatial distribution of 90Sr fallout in Hiroshima.
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ACKNOWLEDGMENTS
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We especially thank Professor T. Nagatomo for providing the Hiroshima Japanese cedar, without which this research could not be accomplished. We thank Professor Y. Nobori and Professor K. Minato for providing the other tree samples, and we also thank Professor J. Azuma for his advice and the staff of the Radioisotope Research Center at Kyoto University for their help in radiochemical analysis. In conducting instrumental neutron activation analysis (INAA) elemental analysis, we thank the staff of the Research Reactor Institute at Kyoto University and the Institute for Atomic Energy at Rikkyo University for their support.
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