Measurement of 26Al in Terrestrial Silicate Rock, Revisited

Abstract

Al (T1/2 = 7.2 × 10 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. searched for Al produced in terrestrial silicate rock (chert) by the interaction of muons with Si, which is the most abundant isotope in the earth’s crust except for O, in order to investigate the secular variation of cosmic radiation intensity. They chemically extracted and purified aluminum from several tens of kg of 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 cm thick. The annihilation γ rays from the decay of Al in these samples 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. Unfortunately, they were not able to obtain the positive results, and reported the Al 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 the sample collected at a depth of 24 meters of water equivalent (mwe). In 1975, Hampel et al. measured again the same sample prepared by Tanaka et al. by means of a low-level γ−γ coincidence spectrometry system consisting of two 15 × 10 cm NaI(Tl) crystals with improved statistics. For the measurement, four original disks of the sample 1A prepared from the surface rock by Tanaka et al. 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 Al/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 their paper, a more quantitative measurement in the future was expected 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 high-sensitive mass spectrometers, accelerator mass spectrometry (AMS). By the advances in AMS, it became possible to apply cosmogenic radionuclides produced in situ in terrestrial samples for studies of erosion and sedimentation rates and surface exposure ages. Recently, quartz, which is geologically abundant mineral, is used as one of the ideal samples for these studies. 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, Al induced by cosmic ray secondaries in terrestrial silicate rock was measured in order to confirm the result reported by Hampel et al. While the large amount of the rock sample is needed for the radioactivity measurement of the cosmogenic radionuclides, only a few grams of the rock is sufficient to determine Al by AMS. In this paper, we report the result for the AMS measurement of Al 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.