超灵敏小型回旋加速器质谱计14C测年的样品制备和制样系统

样品制备是加速器测量不可缺少的一部分.不同类型的样品经过化学前处理后,其中的有机碳在密闭的真空系统里被CuO氧化为C02.以Fe作催化剂、Zn作还原剂,CO2最后转化为石墨碳.整个制备流程在真空系统内完成,操作方便,流程短,污染减少.     

Properties of milligram size samples prepared for AMS 14C dating at ETH

We have investigated the ion beam characteristics (intensities, yields, duration and stability) of a variety of targets prepared for radiocarbon dating by AMS. The target preparation techniques include: organic matter pyrolysis products mixed with silver, high pressure temperature ‘graphites’, CO reduction by cracking, magnesium reduction of CO₂ and direct dating of extractives. Values for minimum sample amounts, sample preparation times, contamination as judged by background and precision of determination are given and the overall performance is evaluated.     

Correlation of the paleoclimatic record in lacustrine sediment sequences: 14C dating by AMS

We have determined ¹⁴C ages by accelerator mass spectrometry (AMS) for small amounts (5–20 mg) of terrestrially derived organic remains taken from a sediment core drilled from Lake Zürich, Switzerland. The continuous core relates in part to Late-Glacial and Holocene glacial-to-lacustrine sediments.Samples were selected after careful consideration of sedimentological criteria, and prepared for AMS using the high-pressure-temperature method of graphitization. Precise ages at seven levels in the sequence, combined with sedimentologic interpretation of the core, establish a new chronology for deglaciation and subsequent climatically induced events within the catchment. Evidence indicates a later disappearance of glacial ice from the Lake Zürich basin than previously reported.     

A new graphite preparation line for AMS 14C dating in the Zagreb Radiocarbon Laboratory

The new line for preparation of graphite samples for ¹⁴C dating by AMS has been constructed in the Zagreb Radiocarbon Laboratory. The performance of the rig and sample preparation procedure has been validated by preparing graphites from various reference materials of known ¹⁴C activity. The yield of the graphitization was good and the measured fraction of modern carbon (F_m) values have not significantly deviated from the expected ones. Detailed analysis of measured F_m values indicates a slight bias to more positive values and should be carefully investigated.     

Developments in micro-sample 14C AMS at the ANTARES AMS facility

We continue development of micro-sample radiocarbon sample preparation and AMS measurement at the ANTARES AMS facility. We routinely prepare samples containing 10–200 μg of carbon using an iron catalyst with an excess of hydrogen in ～2.5 mL graphitisation reactors. These use a tube furnace to heat the catalyst to 600 °C and a Peltier-cooled water trap. Samples containing just a few micrograms of carbon can be prepared. We describe progress with a 0.5 mL laser-heated ‘microfurnace’ we are developing for the rapid and efficient graphitisation of ～5 μg samples. Following operating experience with a prototype unit, work has commenced on the development of a second-generation device with the goal of fully automated operation with minimal introduction of extraneous carbon.Key to development of micro-sample ¹⁴C AMS is the ability to reliably handle the graphite/iron sample and to mount it in the ion source target holder. We have developed a target holder that permits the sample to be loaded in a 1 mm diameter recess and rear pressed, ensuring a high quality surface finish, at a reproducible depth. Additionally we have developed a method for systematically aligning the sample stage with the cesium beam following ion source servicing.     

14C dating of polar ice

We have dated ice core samples by ¹⁴C AMS to show that this method extends other ice dating methods and enables direct comparison with ¹⁴C related climatic events found elsewhere. For the measurement we use the CO₂ that was occluded together with the air at the time of ice formation. To extract the CO₂ we crush the samples with a milling cutter. For a ¹⁴C AMS measurement the CO₂ content of about 10 kg of cleaned ice (0.25 cm³) has to be converted to amorphous carbon. First ¹⁴C results on the Dye 3 (Greenland) ice core are in good agreement with other age determinations.     

基于小型单极加速器质谱测量~(14)C的样品制备技术研究

为满足新研发的200kV单极静电加速器质谱仪测量14 C的需要,开展了14 C样品制备技术研究。在自主设计的14C制样装置上优化了H2-Fe法、Zn-Fe法、Zn-H-Fe法制备石墨样品的实验条件,制备的样品经离子源引出12C-的束流均可达15~40μA,稳定性好于0.2%。在加速器质谱仪的调试中发现压低制备样品12CH+13C和12C的粒子个数比更有利于实现14 C-SE-AMS小型化及高灵敏测量。~(14)C-SE-AMS的测试样品中碳含量小于1mg时,采用Zn-Fe法有利于压低~(12)CH+~(13)C和~(12)C的粒子个数比;样品碳含量为1~5mg时,采用Zn-Fe法和Zn-H-Fe法效果相当。建立的制样方法对含碳量为100μg~5mg的样品,其产率均能达到95%以上,且空白样品测试结果表明此系统能有效避免样品制备中的交叉污染。     

The AMS 14C dating of Iron Age rice chaff ceramic temper from Ban Non Wat,Thailand: First results and its interpretation

Pottery tempered with rice chaff from the early Iron Age cemetery of Ban Non Wat site, northeast Thailand, has been subjected to direct AMS ¹⁴C dating, using low temperature combustion with oxygen as originally developed by authors. The carbon yield (0.2–0.5%) testifies the suitability of this pottery for dating. However, not all the results are in agreement with expected archaeological ages and other ¹⁴C dates from the studied site and neighboring site of Noen U-Loke. This calls for a thorough analysis and interpretation of pottery temper dates from the region.     

Precision measurements of 14C in AMS — some results and prospects

Some of the factors affecting the precision in AMS measurements will be discussed and the specific developments undertaken to reduce the errors at the ETH facility are described. Based on a large number of ¹⁴C measurements we show the present limitations of our system and consider what improvements might be possible. Further, a comparison is made between high precision measurements of ¹⁴C and current achievements relating to Be, Al and Cl.     

Archaeological and geological interest in applying 14C AMS to small samples

One aspect of the accelerator mass spectrometry technique is that samples can be dated which are essentially smaller than required for conventional ¹⁴C dating. This two orders of magnitude expansion offers applications of ¹⁴C which are new and in some fields of great importance. This paper attempts to list and group the various types of sample materials, with special emphasis on their role in archaeology and geology.     

Progress in 14C and 10Be dating at SFU

Since early 1982, a substantial upgrading of the Simon Fraser University radioisotope dating system at the McMaster FN accelerator has taken place. The accelerator itself was equipped with a new charging system and beam tubes, resulting in excellent stability and beam transmission. A new isotope filtering system of two magnets and a Wien filter has removed previously troublesome backgrounds: our carbon background from graphite is now ～ 47 ka BP, and is due to real ¹⁴C rather than scattered ¹³C. Measurement of ¹⁰Be in natural samples is now routine, with several hundred samples processed. Other work on ¹⁰Be has included studies which showed for the first time that ¹⁰Be could be measured at low energies (3 MV). We have also started work on ²⁶Al detection as a preliminary to the investigation of ²⁶Al/¹⁰Be dating. Finally, we present examples of our applications of this facility to studies in oceanography and archaeology.     

ZSS单板机在~(14)C年代测量中的应用

一、14C年代测定单板机的功能1.数据处理 1)被测样品的在线数据处理 14C年代测量时间较长,一般一个样品要测够1000分钟,每10分钟为一段,测量到预置的时间后,单板机就用“3σ”作判据对各段计数进行统计检验,对偏离值超过“3σ”的数据可剔除掉,若要求测量时间一致,还可再补充新的测量值,经过剔除和补充后,将得到“σ1”值,再用“3σ1”作判据,重新进行统计检验,直至全部偏离值没有超过“3σn”为止。仪器及电网负载工作状态正常时,偏离值超过“3σn”的机率只有3‰,所以一般不需要剔除和补充。当测量结束时,打印机打出样品的平均计数率、标准误差、试验误     

AMS measurements of 10Be and 14C in loess profile at Donglingshan,Beijing

A loess profile in Donglingshan site (40°02′N, 115°27′E) near Beijing was chosen to study the loess formation process and paleo-climate variation. Thirty eight samples were collected and analyzed for ¹⁴C, ¹⁰Be as well as MS, TOC and δ¹³C. Based on ¹⁴C measurements, we established a time scale for this loess profile during Holocene. The averaged ¹⁰Be deposition flux was found to be 4.87 × 10⁶ atoms/cm² year. This is similar to the flux of 4.2 × 10⁶ atoms/cm² year estimated for Chinese Loess Plateau in central China. High ¹⁰Be concentrations of 3.85–5.66 × 10⁸ atoms/g for the samples in layer 23–39 cm from 2965 to 528 years BP suggest a warm and humid weather during this period. MS values have similar variation with ¹⁰Be and reflect the similar paleo-climate information. TOC and δ¹³C suggest that the vegetation around Donlingshan area was C₃ type plants during entire Holocene.     

Impact of AMS 14C determinations on considerations of the antiquity of Homo sapiens in the western hemisphere

The ability of AMS facilities to obtain direct ¹⁴C determinations on milligram amounts of organic extracts of bone has significantly advanced efforts to reexamine the validity of the dating evidence for a number of allegedly Pleistocene Homo sapiens skeletons from the Western Hemisphere previously assigned ages of from about 20000 to 70000 years. AMS ¹⁴C analysis has indicated that four of these skeletons are actually of Holocene age, i.e., less than 10000 years old. Holocene ages have previously been documented on the basis of conventional ¹⁴C analysis for six other purported Pleistocene human skeletons from the New World. These data point to the danger of accepting pre-Holocene age assignments for such skeletal materials in the absence of direct ¹⁴C evidence.     

Development AMS Device for ~(14)C Measurement and Its Applications

<正>Carbon is the main component in the aerosol.In order to study the source of carbon,it isnecessary to analyze the ~(14)C in the particles.Accelerator mass spectrometry(AMS)is the mosteffective analytical instrument for measuring ~(14)C.Therefore,the purpose of this work is to develop an AMS device,so as to analyze the source of     

Low level gas multicounter for 14C dating of small samples

A compact gas multicounter has been constructed and is undergoing tests. Up to 14 methane samples can be counted simultaneously in an array of 10 ml (at NTP) copper counters at a pressure of 1–8 bar. The gas filling, application of high voltage and decay energy monitoring are microprocessor controlled. Multichannel analyses (256 ch. sample and cosmic detector energy data), sample identity, counting time and critical validity parameters are stored on disc. Numerical discrimination and manipulations of counting parameters can be performed without destroying the original data set. Statistical quality control is based on chi-square and Poisson distribution of countrates around their mean in user defined energy regions as well as “ time of arrival” (TA) of pulses mode. TA analysis offers the user an early means of recognition of some types of system malfunction that otherwise might remain undetected for long periods of time. Pulse shape analysis is used to discriminate sample beta from environmental radiation pulses resulting in a low background with compact and relatively inexpensive shielding.     

Progress in AMS measurement of natural 32Si for glacier ice dating

AMS measurement of ³²Si can allow for ice core dating over the last thousand years. Technique developments are reported. Necessary negative-ion yields of 20–30% can now be consistently achieved, and permit an overall efficiency from ice sample to detector of ～1%. A ³⁰Si-spike technique has overcome the problem of extremely low intrinsic silicon concentration, with the added benefit of allowing determination of ppb-level silicon via isotope dilution. Improvements have also been made to the ionization detector in the gas-filled magnet that separates the accelerated ³²Si ions from the intense flux of ³²S ions. Preliminary ³²Si AMS results of snow and ice samples from Mt. Cook National Park, New Zealand, are reproducible, and with ³²Si concentrations 1.2–7.2 mBq/m³ comparable to results from mid-latitude snow samples measured previously via the radiometric technique, demonstrating the feasibility of the method. With these developments, the potential of ³²Si as ice core dating tool is close to being realized, and attempts to determine chronologies for both alpine and Antarctic glaciers are underway.     

Small-mass graphite preparation by sealed tube zinc reduction method for AMS 14C measurements

Previous work has demonstrated that the sealed tube Zn reduction method for converting CO₂ to graphite for AMS ¹⁴C measurements produces targets that can be measured with high precision and low background for samples of about 1 mg C down to approximately 0.1 mg C at the Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS). Now a modified method has been developed to prepare small-mass samples ranging from 0.015 to 0.1 mg C. In this modified method, the volume of the sealed reactor tube is reduced to ～1.9 cm³, and the amounts of Zn and TiH₂ reagents are reduced proportionally. The amount of Fe catalyst used remains the same to ensure a long lasting current in the AMS. Small-mass samples prepared by this method generally yield ¹²C⁺¹ currents of about 0.5 μA per 1 μg C. An in situ simultaneous AMS δ¹³C measurement allows for correction of both graphitization and machine-induced isotopic fractionation, and is a prerequisite for high precision and accurate measurements using the Zn reducing method. Corrections for modern-carbon and dead-carbon background components are applied to samples based on small-mass samples of a ¹⁴C free material and of a modern standard covering the sample size range. It was discovered during additional investigation into lowering the modern-carbon background component that baking assembled reactor tubes at 300 °C for 1 h prior to use resulted in significantly lower modern-carbon background values. The accuracy and precision of small-mass samples prepared by this method are size dependent, but is usually ±10–15‰ for the smallest samples (0.015–0.02 mg C), based on duplicate measurements of primary and secondary standards.     

Target preparation for milligram sized 14C samples and data evaluation for AMS measurements

Our preparation technique produces in a glow-discharge an amorphous carbon deposit on a copper substrate. The process starts with 1.6 cm³ CO₂ STP (900 μg carbon) which is reduced over hot zinc to CO and subsequently cracked in the discharge. The yield of the process is typically 80%. With these targets in the Zürich ion source ion currents up to 20 μA are obtained. The background of samples prepared with this technique is presently around 30 ka (2.5% MODERN). The precision after half an hour measuring time for a modern sample is 0.7% and 2.7% for a three half-lives old sample, including the errors of the background and the NBS oxalic acid measurement. The method we use to correct for the background of the preparation and the accelerator as well as for the fractionation in the accelerator is presented.