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The Guangxi Zhuang Autonomous Region in southern China is one of the most typical karst relief areas with various forms, and tiankeng is one of the most typical karst relief forms of the Quaternary Period. In this work, accurate measurements of tiankeng exposure ages were made. The exposure ages of carbonate rocks from the five largest tiankengs in the world, the DaShiwei, HuangJing, ChuanDong, DaTuo, and DaCao tiankengs, were determined by accelerator mass spectrometry to be at least 20–400 thousand years, which provides significant proof for activity of new tectonic movement of the Eurasian continent in the Quaternary. [ABSTRACT FROM AUTHOR]

Tiankeng, a typical Karst relief of the Late Quaternary Period, in the south of China, plays an important role when researching the surface shape and evolution of the Karst relief. The accumulation of Chlorine-36 (T 1/2 = 301 ka) through the interaction of cosmic ray particles with Ca, K and Cl in rocks allow it to be used as the ideal nuclide for determination of the exposure age and erosion rate of limestone. In this work, an improved 36Cl sample preparation method for Tiankeng limestone was presented. The Isotope Dilution-AMS technique allows for the simultaneous measurement of 36Cl and Cl with a single AMS sample preparation. The barium introduced during the sample preparation was removed to avoid ionizer poisoning and attain a higher and more stable beam current. The method is particularly suitable for limestone samples, as well as for soil, sediment and water samples. [ABSTRACT FROM AUTHOR]

This study investigates the potential of 36Cl in tracing young groundwater with residence times of up to ∼50 years. Groundwater samples were obtained from 16 irrigation wells in paddy fields located within an upland-lowland system in the Yoro River basin, Central Japan. The 36Cl/Cl ratios were in the range of 17 to 362 x 10-15. Among the samples with higher Cl- concentrations (>10 mg/L), two samples showed high nitrate concentrations as well (>30 mg/L). Except for these samples, the distribution of 36Cl in groundwater was essentially consistent with previous tritium concentration data measured in 1982 and 1994, considering the time that has elapsed since these earlier measurements were performed. 36Cl/Cl values were less than 30 x 10-15 in lowland areas, with higher values in and around upland areas. The results indicate longer residence times in the regional groundwater flow system (>50 years) than those estimated in previous studies, especially in the area west of the Yoro River. The results demonstrate the ability of 36Cl to trace young groundwater flow, particularly because high values of bomb-derived 36Cl/Cl are easily discriminated from pre-bomb water with low 36Cl/Cl values. Because of its very long half-life (3.01 x 105 years), 36Cl remains even after tritium is no longer available. [ABSTRACT FROM AUTHOR]

Abstract: With the current AMS system on the 12UD Pelletron tandem accelerator at the University of Tsukuba, we are able to measure the long-lived radioisotopes of 14C, 26Al, 36Cl and 129I by employing a molecular pilot beam method that stabilizes the terminal voltage with 0.1% accuracy. Several major improvements have been made to increase the sensitivity of the AMS system to 26Al and 36Cl radioisotopes. For 26Al-AMS, the ions extracted from the ion source have been changed from Al− to AlO− by using an Al2O3 target sample. In addition, 26MgO− molecular ions are used as a pilot beam to control the terminal voltage through slit current feedback. Also, a fully-stripping AMS technique is used to achieve an effective detection limit of better than 5×10−15 for the 26Al/27Al ratio. For 36Cl-AMS, the terminal voltage has been increased from 9MV to 10MV, thereby increasing the energy of the 36Cl9+ beam to 100MeV. This energy increment helps us to get a clearer separation between 36Cl and 36S in a new ΔE−E detector. In addition, the yield of 36Cl9+ is increased by 1.5 times as a result of the higher ion energy at the terminal stripper foil. The standard deviation of the fluctuations of the 36Cl/35Cl ratio is within ±2%, and the effective detection limit is better than 2.0×10−14. [Copyright &y& Elsevier]

Abstract: Essential parameters for the applications of 36Cl as a tracer in groundwater studies include the initial 36Cl/Cl ratio, at the time of recharge, and/or the natural background deposition flux of 36Cl in the recharge area. To facilitate the hydrological use of 36Cl in central Japan, this study aimed to obtain a precise estimate of the long-term average local 36Cl flux and to characterize its seasonal variability. The 36Cl in precipitation was continuously monitored in Tsukuba, central Japan over a period of >5 years. The 36Cl flux showed a clear seasonal variation with an annual peak during the spring, which was attributed to the seasonal variability of tropopause height. The long-term average 36Cl flux (32±2atomsm−2 s−1), estimated from the measured data, was consistent with the prediction from the 36Cl latitudinal fallout model scaled using the global mean production rate of 20atomsm−2 s−1. The initial 36Cl/Cl ratio was estimated to be (41±6)×10−15, which is similar to that of pre-bomb groundwater in the Tsukuba Upland. An observation period covering an 11-year solar cycle would yield more accurate estimates of the values, given the increased 36Cl flux during the solar minimum. [Copyright &y& Elsevier]

Abstract A 6 MV tandem accelerator at the University of Tsukuba, Japan, has been working on multi-nuclide (10Be, 14C, 26Al, 36Cl, 41Ca, and 129I) accelerator mass spectrometry (AMS) since 2016. This paper reports improvement of isobar suppression techniques for 36Cl AMS. Previous studies have shown that a Cu sample holder with AgBr backing is the most suitable for 36Cl AMS to decrease isobaric 36S interference. Therefore, the method of AgBr preparation was considered, and we found that preparation under acidic conditions was most appropriate to reduce 36S in 36Cl AMS. Cl charge states and their analysis method were considered to improve peak separation. 36 Cl 8 + was chosen for detection because it allows clear separation of 36S from the 36Cl spectrum. Furthermore, 36S spectrum tails could be removed using a third anode (dE3) signal. As a result, a 36Cl/Cl background of ∼ 3 × 10 - 15 was achieved. [ABSTRACT FROM AUTHOR]