Journal of Nuclear and Radiochemical Sciences, Vol. 7, No. 2, pp. 33-35, 2006

Measurement of 26Al in Terrestrial Silicate Rock, Revisited
Seiichi Shibata,*,a Yasuhiro Okuda,a Koichi Takamiya,a Yuichi Oki,a Takayuki Sasaki,b Hiroyuki
Matsuzaki,c and Koh Sakamotod
aResearch Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan
Department of Nuclear Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japanc
Department of Nuclear Engineering and Management, School of Engineering, The University of Tokyo, Bunkyo-ku,Tokyo 113-0032, JapandDepartment of Chemistry, Faculty of Science, Kanazawa University, Kakuma, Kanazawa 920-1192, Japanb

Received: August 10, 2006; In Final Form: October 16, 2006
Cosmic-ray-induced 26Al in terrestrial silicate rock was remeasured by accelerator mass spectrometry (AMS) in order
to confirm the value reported more than 30 years ago. The result of (5.2 ± 1.0) × 106 26Al atoms/g SiO2 for the surface
rock sample was consistent with the previous result determined by a low background γ − γ coincidence spectrometrysystem within experimental uncertainties. The lower amount of 26Al was found in the deeper part of the rock samples. From the result, the surface exposure age and the erosion rate at the sampling location were estimated to be 1.4
× 105 y and 3.7 × 10-4 cm/y, respectively.

1. Introduction
Al (T1/2 = 7.2 × 105 y) observed in terrestrial and extraterrestrial matters provides one of the important clues to the deciphering fossil records stored in those materials and allows
investigations of their irradiation history by cosmic radiation.
In their pioneering work in 1968, Tanaka et al.1,2 searchedfor 26Al produced in terrestrial silicate rock (chert) by the interaction of muons with 28Si, which is the most abundant isotope
in the earth’s crust except for 16 O, in order to investigate thesecular variation of cosmic radiation intensity. They chemically extracted and purified aluminum from several tens of kgof silicate rock collected from an area with a low erosion rate.
The aluminum separated was ignited to Al2O3. Each 100 g of
the Al2O3 was pressed into a disk of 7 cm in diameter and 2 cmthick. The annihilation γ rays from the decay of 26Al in thesesamples were measured using a low-background γ−γ coincidence spectrometer consisting of two 7.6 × 7.6 cm NaI(Tl)crystals installed in the underground counting facility. 3
Unfortunately, they were not able to obtain the positive results,
and reported the 26Al activities of 0.02 ± 0.12 dpm/10 kg SiO2
for the surface rock and of 0.00 ± 0.08 dpm/10 kg SiO2 for thesample collected at a depth of 24 meters of water equivalent(mwe).
In 1975, Hampel et al.4 measured again the same sample
prepared by Tanaka et al.1 by means of a low-level γ−γ coincidence spectrometry system consisting of two 15 × 10 cmNaI(Tl) crystals with improved statistics. For the measurement, four original disks of the sample 1A prepared from thesurface rock by Tanaka et al.1 was repressed into a larger disk
(a 402 g disk of Al2O3) of 15 cm in diameter and 1.65 cm thick.
The result was reported to be 0.0068 ± 0.0028 dpm 26Al/kg
SiO2. Based on the result, they discussed the muon flux during
the last several million years and the erosion rate at the collecting location of the measured sample. In the conclusion of theirpaper, a more quantitative measurement in the future wasexpected for wide applications of cosmic-ray-produced longlived nuclides in geology, geophysics, and cosmic ray physics.
For trace analyses of long-lived nuclides, accelerators have
now extensively been applied as high-energy and extremely
26*Corresponding author. E-mail: shibata@rri.kyoto-u.ac.jp.
FAX: +81-72-451-2632.

high-sensitive mass spectrometers, accelerator mass spectrometry (AMS). By the advances in AMS, it became possible toapply cosmogenic radionuclides produced in situ in terrestrial
samples for studies of erosion and sedimentation rates and surface exposure ages. Recently, quartz, which is geologicallyabundant mineral, is used as one of the ideal samples for thesestudies.
We have applied the AMS method to measure the trace
amount of cosmogenic radionuclides produced in terrestrial
and extraterrestrial substances. In the course of this study, 26Al
induced by cosmic ray secondaries in terrestrial silicate rockwas measured in order to confirm the result reported byHampel et al.4 While the large amount of the rock sample isneeded for the radioactivity measurement of the cosmogenic
radionuclides, only a few grams of the rock is sufficient todetermine 26Al by AMS.
In this paper, we report the result for the AMS measurementof 26Al in the terrestrial silicate rock collected at the surface
and deeper parts of rock in the same sampling place as reported
in Tanaka et al.1
2. Experimental Procedure
A sample of silicate rock was collected at an altitude ofabout 500 m in Kutami, Yaotsu-cho, Kamo-gun, Gifu-prefecture, Japan. Details of the sampling place were alreadyreported by Tanaka et al.1,2 In the first sampling in December,
2000, samples were collected at the surface, 2.5 mwe and 12mwe of rock, and in the second sampling in June, 2003, samples at the surface, 2.5 mwe, 5 mwe, and 12 mwe.5
The chemical separation method of aluminum from large
quantities of the rock samples (> 10 kg) developed by Tanaka
et al.1 was improved for the AMS measurement in this work.5
The rock sample was crushed and pulverized after the intruded
clay was removed by brushing. 0.2 – 1.5 g of the powderedsample was dissolved in mixed acid solution of HNO3, HF, andHClO4 by heating under pressure. The solution was evaporated
in order to volatilize silicon as SiF4. The residue was dissolvedin concentrated H 2 SO 4 and evaporated to dryness again inorder to decompose Al-Si complex and to expel the excess HF.
Then the residue was leached with concentrated HCl, and thesolution obtained was evaporated. Finally the residue was dissolved in H2O.

© 2006 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/15/2006

Shibata

J. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006

TABLE 1: Al Content Determined by Neutron Activation
Analysis
SampleDepth (mwe*)

Aluminum content** (%)

1-001201

surface0.70 ± 0.01

2-0012012.5

6.49 ± 0.07

3-00120112

1.40 ± 0.02

1A-030620

surface0.87 ± 0.01

1B-030620

surface2.94 ± 0.02

2-030620

2.5

6.96 ± 0.04

3-030620

56.93 ± 0.05

1A-1968***

surface0.74*mwe: meter of water equivalent**uncertainty: 1 σ***1A-1968: Tanaka et al. (Ref.1)
TABLE 2: 26Al Measured in Silicate Rock Samples
Al atoms / g SiO2 ( ×106)

26Sample

Depth (mwe)1-001201

surface

4.6 ± 1.22-001201

2.5

1.7 ± 1.33-001201

120.8 ± 0.5

1A-030620

surface

6.4 ± 2.3

1B-030620

surface

9.4 ± 4.5

2-030620

2.5< 2.23-030620

5

< 1.9

3. Results and Discussion
The aluminum contents determined by neutron activation
analysis are summarized in Table 1. The experimental uncertainties attached to the aluminum contents refer only to thosestatistically estimated in γ-ray counting. The aluminum contents of the surface rock samples of 1-001201 and 1A-030620
are consistent with that determined by Tanaka et al.,1 which isalso included in Table 1, while the aluminum contents of rocksamples collected at the 2.5 and 5 mwe of rock are higher thanthose at the surface and 12 mwe.
In the AMS measurement, more than three runs for eachsample were carried out. Examples of the AMS spectra measured for the standard and the surface sample by using a ∆E – E
counter are shown in Figures 1 and 2, respectively. There arealmost no signals induced by impurity ions in the chemicallyseparated sample as shown in Figure 2. For the surface samples, the ratios of 26Al/27Al were measured to be (1 – 7) × 10 -14,
while those for the deeper samples were less than 10 -14. The
ratios for the blank were measured to be (2.2 – 2.6) × 10 -15. In
the case of Figure 2 for the surface sample (1-001201), themeasured ratio of 26Al/27Al was (2.7 ± 0.5) × 10 -14.
From the both results of INAA and AMS, the numbers of
26
Al atoms produced in the rock samples were estimated andsummarized in Table 2. The 26Al atoms determined in threesurface samples agreed with one another within experimental
uncertainties. Thus we estimated to be (5.2 ± 1.0) × 10 6 26Alatoms/g SiO2 as the weighted mean for the surface samples collected in 2000 and 2003. This value was consistent with
0.0068 ± 0.0028 dpm 26Al/kg SiO2 ((3.7 ± 1.5) × 106 26Al atoms/g SiO2) reported more than 30 years ago.4 For the deeper mwe

400

Er ( channel number )In the obtained solution, the hydroxide was precipitated withNaOH solution at pH = 5 – 7 after adding NH4 Cl as a buffersolution. After centrifuging, the precipitate was suspended inNaOH solution. The supernatant solution was adjusted to pH =5 – 7 by adding HCl. Then the white precipitate of aluminumhydroxide was obtained.
The aluminum hydroxide was dissolved in HCl. The aluminum was purified by cation exchange with 1.75 M HCl.5 The
aluminum hydroxide was precipitated again from the eluted
aluminum fraction by adding NaOH solution. The precipitateof aluminum hydroxide was washed by pure water, and dissolved in HCl. The solution was evaporated, and heated to900 ˚C in an electric furnace. The white residue of Al 2 O3
obtained was used for AMS.
The aluminum contents in the rock samples were determinedby instrumental neutron activation analysis (INAA) using
Kyoto University Reactor (KUR). About 2 mg of the rocksamples were irradiated for 30 s at the neutron flux of 2.34 ×1013 n/cm 2 s together with the aluminum standard samples.
After irradiation, 1779 keV γ-ray emitted from 28Al (T 1/2 = 2.25min) produced by 27Al(n,γ) reaction was measured by γ-rayspectrometry. The aluminum contained in the rock sample wasused as a carrier in the chemical separation mentioned above.
The isotopic ratio of 26Al/ 27Al was determined by the AMSsystem using the 5 MV tandem Van de Graaff accelerator at
MALT (Micro Analysis Laboratory, Tandem accelerator), theUniversity of Tokyo.6 Negative ions of 26Al- and 27Al- producedin the ion source by sputtering Cs+ ions were injected alternatively to the accelerator. At the terminal of the tandem accelerator, negative ions were converted into positive ions. The 27Al3+
beam was measured by the off set Faraday cup outside themain course of the accelerator, and the 26Al3+ ions were finallydetected by a ∆E – E counter.7 The measurements were carriedout with a standard, whose ratio of 26Al/ 27Al is 5.1 × 10 -11, forthe normalization of the data, chemically prepared samples
from the rock and a blank sample. The detection limit of26
Al/ 27Al in this AMS system is estimated to be ≲ 5 × 10 -15 atpresent.

300
200
1000260

Al

100200300∆ E ( channel number )400

Figure 1. Two-dimensional plot of 26Al measurement for the standard. (Measurement time: 200 s)

400Er ( channel number )34300
20026Al10000100200300∆ E ( channel number )400

Figure 2. Two-dimensional plot of 26Al measurement for the surfacerock sample (1-001201). (Measurement time: 3100 s) For this run,the measured ratio of 26Al/ 27Al is (2.7 ± 0.5) × 10 -14.

Measurement of 26Al in Terrestrial Silicate Rock, Revisited
26J. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006

27

samples, the measured ratios of Al/ Al were around thedetection limit of this AMS system. Therefore large uncertainties were estimated for those results, and only upper limits ofthe 26Al were obtained in the samples of 2-030620 (2.5 mwe)and 3-030620 (5 mwe).
The production rate of 26Al at the sampling location can becalculated to be 38 ± 3 26Al atoms/g SiO2 y using the altitudedependent scaling factors given by Stone,8 which is consistent
with those calculated by Heisinger et al.9 and Heisinger andNolte.10 From the production rate obtained and the value of26
Al atoms estimated for the surface sample, the surface exposure age was estimated to be (1.4 ± 0.3) × 105 y at the samplinglocation. Based on the result, the erosion rate was calculated tobe (3.7 ± 0.9) × 10 -4 cm/y using the following equation by Lal,11ε = 1/ µ [1/ T ex – λ ],(1)

where ε is the erosion rate, 1/ µ the mean cosmic-ray absorption length, Tex the effective surface exposure age and λ thedecay constant of 26Al. In the calculation, it is assumed thatthe rock surface undergoes steady state erosion and the radionuclide concentration was zero when the rock was deep seatedand shielded from cosmic radiation. The erosion rate calculated is consistent with the geological estimate for the erosion
rate of < 10 m/106 y reported in Tanaka et al.1,24. Conclusion
Cosmogenic 26Al in terrestrial silicate rock collected in thesame sampling place where Tanaka et al.1,2 collected the rocksamples was remeasured by AMS in order to confirm the previous result measured by a low background γ−γ coincidence
spectrometry more than 30 years ago.4 The chemical separation method developed by Tanaka et al.1,2 was improved for thiswork. The AMS result for the surface sample was obtained tobe (5.2 ± 1.0) × 10 6 26Al atoms/g SiO2 , which was consistent
with the previous result by the radioactivity measurement of35(3.7 ± 1.5) × 10 Al atoms/g SiO2. From the result obtained,
the surface exposure age and the erosion rate at the sampling
location were estimated. The erosion rate of 3.7 ± 0.9 m/106 y
was consistent with the geological estimate of < 10 m/106 y.1,26 264Acknowledgements. The authors are indebted to Miss S.
Wakasa, Mr. Y. Maejima, and Mr. T. Aze of MALT (MicroAnalysis Laboratory, Tandem accelerator), the University ofTokyo for their help in the AMS measurements.
References(1) S. Tanaka, K. Sakamoto, J. Takagi, and M. Tsuchimoto, J.
Geophys. Res. 73, 3303 (1968).
(2) S. Tanaka, K. Sakamoto, J. Takagi, M. Tsuchimoto, and T.
Inoue, INS-TCH-3, Institute for Nuclear Study, Univ. ofTokyo (1968) (in Japanese).
(3) S. Tanaka, K. Sakamoto, and J. Takagi, Nucl. Instrum.
Methods 56, 319 (1967).
(4) W. Hampel, J. Takagi, K. Sakamoto, and S. Tanaka, J.
Geophys. Res. 80, 3757 (1975).
(5) Y. Okuda, MS Thesis, Graduate School of Engineering,
Kyoto Univ. (2004) (in Japanese).
(6) H. Matsuzaki, C. Nakano, H. Yamashita, Y. Maejima, Y.
Miyairi, S. Wakasa, and K. Horiuchi, Nucl. Instrum.
Methods Phys. Res. B 223-224, 92 (2004).
(7) H. Matsuzaki, M. Tanikawa, K. Kobayashi, and S. Hatori,Nucl. Instrum. Methods Phys. Res. B172, 218 (2000).
(8) J. O. Stone, J. Geophys. Res. 105, 23,753 (2000).
(9) B. Heisinger, M. Niedermayer, F. J. Hartmann, G.
Korschinek, E. Nolte, G. Morteani, S. Neumaier, C.
Petitjean, P. Kubik, A. Synal, and S. Ivy-Ochs, Nucl.
Instrum. Methods Phys. Res. B123, 341 (1997).
(10) B. Heisinger and E. Nolte, Nucl. Instrum. Methods Phys.
Res. B172, 790 (2000).
(11) D. Lal, Earth Planet. Sci. Lett. 104, 424 (1991).