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.     

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.     

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.     

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.     

Direct measurements of small 14C samples after oxidation in quartz tubes

Small ¹⁴C samples gain importance in environmental research and for dating purposes. However, throughput of such samples is limited by the preparation of graphite targets for accelerator mass spectrometry (AMS) measurements. In our approach, oxidation of samples with copper oxide in quartz tubes was applied to form CO₂ which was measured directly with the gas ion source of the small AMS facility MICADAS. The presented method was designed to meet the requirements for fast and easy handling of small samples (<100 μg carbon). As combustion byproducts are likely to interfere with ionisation processes in the gas ion source, we additionally investigated the effects of several gases on C^? currents.     

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.     

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。     

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.     

14C dating of small archaeological samples: neolithic to iron age in the central alpine region

Accelerator mass spectrometry (AMS) radiocarbon ¹⁴C dating will widen enormously the range and scope of archaeological investigations. This is due mainly to 100- to 1000-fold sample size reduction over conventional dating. In order to determine the size and the quality of samples that can be accepted for AMS ¹⁴C dating, we have selected archaeological samples relating to the Neolithic to Iron Age. The basis of our AMS target preparations is the coking (pyrolysis) of organic matter after elimination of impurities by various physical and chemical treatments. The effect of the morphology as well as of the grain size distribution of the charcoal particles was determined in order to achieve optimal conditions for accelerator dating.     

The hardwater effect in AMS 14C dating of food crusts on pottery

The pottery investigated in this study comes from late mesolithic inland sites next to rivers in Northern Germany. The first AMS ¹⁴C datings of food crusts from these sites showed surprisingly high ages, which could be caused by the hardwater effect.Modern samples from the rivers have ages of several hundred ¹⁴C years, and a modern food crust prepared from fish with a certain reservoir age shows the same age as the fish. Surprisingly, there was a large age difference between water samples and fish/mollusc shell from the same river. Associated archaeological samples of terrestrial and fluvial origin show age differences of several hundred and up to 3000 years. These high age differences are only to a limited extent transferred to the archaeological food crusts.     

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     

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.     

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.     

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.     

黄土样品的BeO制备及AMS测量

采用常规方法提取黄土-古土壤样品中的10Be,制备BeO样品时,无法有效降低样品中B含量,导致在10Be AMS测量中本底值偏高.本文对BeO制备方法进行了优化.新的制备流程改进了阳离子树脂交换、Be(OH)2转化为BeO的灼烧方式以及实验所采用的仪器设备.在西安加速器质谱仪(Xi'an-AMS)上测试结果表明,采用该化学流程制备的BeO样品化学本底可以达到2.04×10-14.本文还介绍了Xi'an-AMS测量10Be的方法与测量结果.     

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.     

14C sample preparation for AMS microdosing studies at Lund University using online combustion and septa-sealed vials

The Department of Physics at Lund University is participating in a European Union project called EUMAPP (European Union Microdose AMS Partnership Programme), in which sample preparation and accelerator mass spectrometry (AMS) measurements of biological samples from microdosing studies have been made. This paper describes a simplified method of converting biological samples to solid graphite for ¹⁴C analysis with AMS. The method is based on online combustion of the samples, and reduction of CO₂ in septa-sealed vials. The septa-sealed vials and disposable materials are used to eliminate sample cross-contamination. Measurements of ANU and Ox I standards show deviations of 2% and 3%, respectively, relative to reference values. This level of accuracy is sufficient for biological samples from microdosing studies. Since the method has very few handling steps from sample to graphite, the risk of failure during the sample preparation process is minimized, making the method easy to use in routine preparation of samples.     

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.