Compact AMS System Research Articles

A Study of Variation of the 11-yr Solar Cycle before the onset of the Spoerer Minimum based on Annually measured 14C Content in tree Rings

ABSTRACTProxy-based observations of solar activity in the past have revealed long-term variations, such as the Gleissberg cycle (~88 yr), de Vries cycle (~200 yr), and the Hallstatt cycle (~2000 yr). Such long-term variations of solar activity sometimes cause the disappearance of sunspots for several decades. Currently, solar activity is becoming weaker, and there is a possibility that another long-term sunspot minimum could occur. However, the detailed mechanism of the weakening in solar activity is unknown, and the prediction of solar activity is ambiguous. In this study, we investigate the transitions of solar cycle length before the onset of the Spoerer Minimum, the longest grand minimum in the past 2000 yr. We measured the 14C content in an asunaro tree (Thujopsis dolabrata) excavated at Shimokita Peninsula from 1368–1420 CE using the compact AMS system at Yamagata University. It is found that the solar cycle lengthened to be 14–16 yr from 2 cycles before the onset of the Spoerer Minimum.

AMS radiocarbon measurement with high accuracy

A project of 14C trace on deep water in South China Sea requires an accuracy of AMS radiocarbon measurement better than 2.5‰, which is higher than the accuracy for routine measurements of the NEC compact AMS system at Peking University. A series of modifications have enhanced the stability of the AMS system, minimizing fractionation effects and improving the accuracy and quality control of measurements. The ripple of terminal voltage has been reduced from 100 V to less than 50 V. The variation of beam currents from different samples is within 20% and a special tool is designed for proper installation of sample wheels. Measurements on standards and secondary samples demonstrate an improvement in accuracy to <2.5‰.

AixMICADAS, the accelerator mass spectrometer dedicated to 14C recently installed in Aix-en-Provence, France

A compact AMS system dedicated to measuring 14C in ultra-small samples was installed at the CEREGE in Aix-en-Provence at the end of March 2014, together with an automated graphitization system. AixMICADAS operates at around 200kV with carbon ion stripping in helium leading to a transmission of about 47%. The hybrid ion source works with graphite targets and CO2 gas. It is coupled to a versatile gas interface system that ensures stable gas measurements from different sources: a cracker for CO2 in glass ampoules, an elemental analyzer for combusting organic matter and an automated system to handle carbonate by wet chemistry.The analyses performed during the first half-year of operation show that a precision of about 2‰ is reached on modern samples of about 1mg of carbon. Measurements of IAEA reference materials of various 14C ages show a good agreement with consensus values. Direct measurements of geological graphites indicate a machine background equivalent to an age of 68,000years BP. AixMICADAS is thus limited solely by the 14C contamination of samples in the field and in the laboratory.The performances of the gas ion source and its gas interface system were tested with two CO2 production units: the elemental analyzer and the automated carbonate hydrolysis unit. These tests show that samples ranging between 10 and 100μgC can produce a 12C− ion beam of the order of 10–15μA during time spans ranging from 3 to 30min depending on the sample mass.Coupling the automated hydrolysis system to the gas ion source of AixMICADAS, enables us to develop a method involving sequential leaching of carbonate samples with direct 14C measurements of the leached fractions and the residual sample. The main advantage is that all of steps leaching and hydrolysis are performed in the same vial for a particular sample.A sequential leaching was applied to a young carbonate sample (ca. 6600years BP) whose 14C age agrees with previous determination and which shows no sign of significant surface contamination. By contrast, the tests also show that the leached CO2 from very old carbonates is significantly “younger” than the residual sample. This study, though preliminary, already confirms that pretreatment by acid leaching of old carbonates is essential if we are to accurately measure the true age of such samples.

Δ14C level of annual plants and fossil fuel derived CO2 distribution across different regions of China

The 14C level in annual plants is a sensitive tracer for monitoring fossil fuel derived CO2 in the atmosphere. Corn leave samples were selected from different regions of China, including high mountains in the Tibetan Plateau, grassland in Inner Mongolia, and inland and coastal cities during the summer of 2010. The 14C/12C ratio of the samples was measured with the NEC compact AMS system at the Institute of Heavy Ion Physics, Peking University. The fossil fuel derived CO2 was estimated by comparing the measured Δ14C values of corn leave samples to background atmospheric Δ14C level. The influences of topography, meteorological conditions and carbon cycling processes on the fossil fuel derived CO2 concentration are considered when interpreting the data. Our results show a clear association of the low Δ14C values with regions where human activities are intensive.

129I measurements on the 1MV AMS facility at the Centro Nacional de Aceleradores (CNA, Spain)

The AMS system at CNA has been the first 1MV compact AMS system designed and manufactured by HVE. In this paper we present the experimental set-up for (129)I measurements in this facility. Charge state +3 has been selected at high-energy side and an optimum stripper pressure of 6×10(-3)mbar of argon (mass thickness of about 0.15μgcm(-2)) has been reached to obtain lowest blank values ((129)I/(127)I≅3×10(-13)). The measurements of the reference materials provided by the IAEA have demonstrated the viability of this facility to make routine measurements of (129)I at environmental levels. This blank value obtained is enough for the measurement of most environmental samples and comparable with other reported backgrounds obtained in facilities working at higher energies and higher charge states.

Development of a preparation system for the radiocarbon analysis of organic carbon in carbonaceous aerosols in China

Carbonaceous aerosols comprising a large fraction of elemental carbon (EC) and organic carbon (OC) are considered to affect both global climate and human health. Radiocarbon measurements have been proved to be a useful isotopic tracer for distinguishing contemporary and fossil emissions. An optimized system of a two-step thermal preparation system for radiocarbon ( 14C) measurement of OC/TC is firstly established in China. In this system, OC/TC are converted into carbon dioxide under a pure oxygen flow at 340 °C/650 °C and then reduced to graphite for AMS target using the method of zinc reduction. Afterwards, radiocarbon measurements of the targets performed by the NEC Compact AMS System at the Institute of Heavy Ion Physics, Peking University. The measured results for estimated reference martial including HOx I, HOx II and IAEA-C6 are consistent with internationally accepted values. The radiocarbon-based source appointment of carbonaceous aerosols in China would be much more convenient and faster with the preparation system developed in this work.

Comparison of 41Ca analysis on 0.5 MV and 5 MV-AMS systems

A comparative study was made between the compact AMS system at the PSI/ETH Laboratory of Ion Beam Physics in Zurich with 0.5 MV terminal voltage and the 5 MV-AMS system at the Scottish Universities Environmental Research Centre (SUERC), Glasgow. Overall 34 urinary samples with 41Ca/ 40Ca ratios in the range from 4 × 10 −11 to 3 × 10 −10 were processed to CaF 2 and aliquots of the same material were measured on both instruments. Measurements on the compact AMS system were performed in charge state 3+ achieving a transmission of 4% at 1.7 MeV beam energy. Under these conditions a suppression of the interference 41K is virtually impossible. However, samples with an excess of potassium can be identified by a shift of the 41Ca/ 41K peak in the Δ E − E histogram of the gas ionization detector employed and a criterion for data rejection can be defined. An overall precision of ∼4% and a 41Ca/ 40Ca background level of 5 × 10 −12 have been reached. For studies with higher demands on the detection limit AMS systems like the one at SUERC are attractive: in charge state 5+ and using a gas stripper beam energy of 27 MeV, a transmission of 5%, a 41K suppression factor of ∼500 and a 41Ca/ 40Ca background level of 3 × 10 −14 are achieved. We demonstrate that both systems are well suited for large-scale 41Ca biomedical applications.

On the measurement of 10Be on the 1 MV compact AMS system at the Centro Nacional de Aceleradores (Spain)

In this work we present the most recent improvements performed by our group on 10Be measurements on the 1 MV AMS system recently set up at the CNA (Centro Nacional de Aceleradores), in Seville (Spain). Our efforts have been focused on the study of the viability of our system for BeO and BeF − measurements. To achieve this, different standard materials have been measured to demonstrate the reliability of the system for BeO measurements in a wide 10Be/ 9Be atomic ratio range and several environmental samples have been studied both at the 1 MV AMS CNA facility and at the 6 MV AMS ETH-PSI facility of Zurich to validate our measurements. The results show a good agreement between laboratories. New experiments also have been carried out selecting 1+ and 2+ charge states at the exit of the accelerator and inserting Si 3N 3.1 foils with different thicknesses to separate 10Be from its isobar, 10B. The influence of each foil on the overall transmission (detected 10Be compared to BeO − injected into the accelerator) and background level was also assessed. In addition some tests were also done to assess the viability of BeF 2 and BaBeF 4 measurements at our system. Several metal matrices and cathode preparation procedures for BeO samples were investigated to maximize current and cathode lifetime.

Status of the AMS facility at the University of Georgia

Since 2001, the Center for Applied Isotope Studies at the University of Georgia has analyzed more than 10,000 samples using a compact AMS system for carbon isotope measurement. The system, manufactured by National Electrostatics Corporation, utilizes a Model 1.5SDH-1 Pelletron accelerator with a maximum terminal voltage of 0.5 MV. The source has recently been modified, doubling the count rate and improving efficiency more than 50%. Background values have reached 0.12 pMC and the precision is below 0.4 pMC for modern samples. The control and data acquisition system has also been upgraded.

A new compact AMS system at Peking University

A compact 14C AMS system manufactured by the National Electrostatics Corporation has been installed at the Institute of Heavy Ion Physics, Peking University. The system is based on a Model 1.5SDH-1 Pelletron accelerator with a maximum terminal voltage of 0.6MV. The 14C measurement accuracy with this system is better than 0.4% and the machine background is lower than 0.03pMC. The performance of the new system, especially the background and the δ13C measurements, is presented. Several important applications are also described briefly.

Ultra small-mass AMS 14C sample preparation and analyses at KCCAMS/UCI Facility

We have developed techniques for accurately and precisely measuring samples containing less than a few hundred micrograms of carbon, using a compact AMS system (NEC 0.5MV 1.5SDH-1). Detailed discussions of the sample preparation, measurement setup, data analysis and background corrections for a variety of standard samples ranging from 0.002 to 1mgC are reported. Multiple aliquots of small amounts of CO2 were reduced to graphite with H2 over pre-baked iron powder catalyst. A reduction reaction temperature of 450°C was adopted for graphite samples below 0.05mgC, rather than the usual 550°C used on samples of 0.1–1mgC. In our regular reactors (∼3.1cm3), this reduction in temperature improved the graphite yield from ∼60 to 90–100% for samples ranging from 0.006–0.02mgC. The combination of lower reaction temperature with a reduced reactor volume (∼1.6cm3) gave yields as high as 100% on graphite samples <0.006mgC. High performance measurements on ultra-small samples are possible also due to a modified NEC MC-SNIC ion-source that generates C− currents of 1μA per μg of carbon for samples in the 0.002 to 0.010mgC range, combined with on-line measurement of 12C and 13C (AMS δ13C) to correct machine-induced isotopic fractionation. Source efficiencies are in excess of 10%, which enables 4–5% of the radiocarbon atoms in 0.005–0.010mgC samples to be measured. Examination of the background samples revealed two components: (a) 0.2–1μg of modern carbon and (b) 0.1–0.5μg of dead carbon. The latter component can be ignored when measuring unknown samples paired to small standards of precisely identical size (matching size normalizing standard method). Otherwise, one must make corrections for both background components. Ultra-small samples from 0.002 to 0.01mgC can be measured with accuracy and precision of a few percent, based on scatter in results for multiple aliquots of a primary standard and deviations of secondary standards from their known values.

New concepts of 10Be AMS at low energies

Isotopic ratios of 10Be/9Be with a background sensitivity of 2×10−14 can now be measured with the 0.6MV Tandem accelerator of PSI/ETH Zurich using BeF2 instead of BeO as sample material and injecting BeF− to suppress the boron detector count rate. A new, high-resolution gas ionization detector with a thin silicon nitride entrance window and special preamplifier design are essential in order to achieve this sensitivity. A particle transmission of 50% is possible for charge state 1+. Due to the maximum count rate that can be accepted by the detector the BeF− currents are limited to a few hundred nA.Measurements of 10Be in rainwater samples performed under these conditions at the low terminal voltage of 0.6MV with the new compact AMS system show good agreement with results using the conventional 10Be measurement setup on the 6MV facility. Thus, this type of small scale equipment can now also be applied for 10Be AMS as long as 10Be/9Be ratios are sufficiently high and measurement time is not a limiting factor.Instead, using BeO and the degrader foil technique with a silicon nitride membrane the boron count rate can also be suppressed sufficiently but the 10Be/9Be background level is limited to 10−13 by scattered 9Be and the overall transmission is less than 5%.

The compact 14C AMS facility of Paleo Labo Co., Ltd., Japan

A compact AMS system was installed by a private company in Japan for a 14C dating business. Since this system was designed to be very compact with a low voltage accelerator, it has unique features compared to larger AMS systems. For example, the stripper gas pressure has to be maintained at a high level to dissociate molecular ions. However, it cannot be too high because ion transmission decreases at higher stripper gas pressures due to increased angular straggling of the ions scattered by the high density stripper gas.The optimum pressure of the stripper gas for the lowest background and the highest transmission was investigated.With all the features of the compact system, the produced measurements can be of quite high precision.In the ten months after the installation of the equipment in November 2004, we have measured more than 1400 dating samples.