Ngày 5 tháng 11, Đoàn khai quật địa điểm Thành Dền đã gửi 02 mẫu - 02 hạt gạo cháy sang Nhật Bản (qua TS. Mariko YAMAGATA) để phân tích niên đại AMS. Mẫu phân tích được gửi tới Viện AMS IAA ( Institute of Accelerator Analysis Ltd) Địa chỉ: 129-1 Noborito-shinmachi, Tama-ku, Kawasaki, Kanagawa 214-0013 JAPAN), nơi trước đây Đoàn đã gửi phân tích niên đại AMS của một số hạt thóc nảy mầm và vỏ trấu.
Dưới đây là kết quả phân tích niên đại AMS của 01 hạt gạo cháy tìm thấy cùng chỗ với những hạt thóc nảy mầm đợt I tại hố rác bếp số 3 hố khai quật 2 (ký hiệu 10TD.F3.H2).
Toàn văn báo cáo kết quả của IAA gửi
Institute of Accelerator Analysis Ltd.
To: Professor Mariko Yamagata, Showa Women’s University
AMS Radiocarbon Dating Report of Plant Remains Conserved at Hanoi National University Museum (Museum of Anthropology)
December 1, 2010
Institute of Accelerator Analysis Ltd.
129-1 Noborito-shinmachi, Tama-ku
Kawasaki, Kanagawa 214-0013 JAPAN
Description of Analytical Processes Used in Testing Your Sample
(Mô tả quá trình phân tích và xử lý mẫu)
1) Rootlets and granules were removed using tweezers.
2) The Acid-Alkali-Acid (AAA) pretreatment process was used for eliminating carbonates and secondary organic acids. After the treatment, the sample was neutralized with ultra pure water, and dried. In the acid treatments of the AAA, the sample is treated with HCl (1M). In the alkaline treatment, the sample is normally treated with NaOH, by gradually raising the concentration level from 0.001M to 1M. If the alkaline concentration reaches 1M during the treatment, the treatment is described as “AAA” in the table, while “AaA” if the concentration does not reach 1M.
3) The sample was oxidized by heating to produce CO2 gas.
4) The produced CO2 gas was purified in a vacuum line.
5) The purified CO2 gas sample was reduced to graphite by hydrogen using iron as a catalyst.
6) The produced graphite was pressed into a target holder with a hole of 1 mm diameter for the AMS 14C dating, using a hand-press machine.
The graphite sample was measured against a standard of Oxalic acid (HOxII) provided by the National Institute of Standards and Technology (USA), using a 14C-AMS (accelerator mass spectrometry) system based on the tandem accelerator. A blank for the background check was also measured.
Calculation of Age
1) The Libby half-life of 5568 years was used for the calculation of 14C age (Stuiver and Polach 1977).
2) 14C age (conventional 14C age: yrBP) refers to an age going back from the year 1950 (= 0 yrBP), assuming a constant 14C concentration in the past atmosphere. The value was corrected for its isotopic fractionation using the δ13C value. 14C age and error are rounded off to tens place. The one σ error term means that the 14C age lies within ±1σ range at the 68.2% confidence level.
3) The δ13C value refers to a difference of the 13C concentration (13C/12C) of a sample from that of a standard. The value is expressed by ‰ for the difference from the standard value. The ratios are measured using an accelerator mass spectrometer, and it is noted as (AMS) in the table.
4) pMC (percent Modern Carbon) refers to a ratio of the 14C concentration in the sample relative to that of the year 1950.
5) Calibrated calendar age is a range of age corresponding to 14C age via a calibration curve, which was produced from the 14C concentration of samples of known age. It is expressed by one σ error range (68.2% probability) or two σ error range (95.4% probability). The 14C age without rounding off was used for calibration. Calibration curve and calibration program can be updated by the database revision, and therefore, the calibrated age may vary depending on the program used. The calibration in this report was conducted by OxCal v.4.1 (Bronk Ramsey 2009) based on IntCal09 database (Reimer et al. 2009).
Bronk Ramsey C. 2009 Bayesian analysis of radiocarbon dates, Radiocarbon 51(1), 337-360
Reimer, P.J. et al. 2009 IntCal09 and Marine09 radiocarbon age calibration curves, 0-50,000 years cal BP, Radiocarbon 51(4), 1111-1150
Stuiver, M. and Polach, H.A. 1977 Discussion: Reporting of 14C data, Radiocarbon 19, 355-363
Results of the Analysis
Client: Prof. Mariko Yamagata, Showa Women’s University
Như vậy, niên đại của hạt gạo cháy nằm trong khoảng 3000 năm cách ngày nay, phù hợp với tư liệu địa tầng, loại hình di tích hố rác bếp và những di vật thuộc giai đoạn văn hóa Đồng Đậu trong hố rác bếp cũng như trong hố khai quật.