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.     

Some Heretical Thoughts on Fusion and Climate

This article presents and explores three heretical thoughts regarding fusion and climate. First, the only way that fusion can contribute to midcentury power is by switching its goal from pure fusion, to fusion breeding. Doing so could lead to a sustainable, carbon free, environmentally and economically viable, midcentury infrastructure, with little or no proliferation risk, which could provide terawatts of power for the world. Second, while CO₂ input to the atmosphere may, at some point, become a concern to the earth’s climate, an Internet search shows that there is no evidence that we are anywhere near that point now and likely will not be before midcentury at the earliest. Third, those who insist on a nearly immediate end to CO₂ input into the atmosphere, are little different from others who have caused panics at various times in American history. The timing could be serendipitous; the time necessary to develop fusion breeding could well match up to the time when it is needed so as to avoid harm to the earth’s climate and/or depletion of finite energy resources.     

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.     

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.     

Heterogeneity versus homogeneity effects in neutron absorbers used in transport packages

Abstract

Many fissile material transport packages incorporate boron as a neutron poison [predominantly as a boron–metal matrix composite (MMC)] to maintain criticality safety. The MMC contains a prespecified proportion of Boron ‘homogeneously’ distributed throughout the metal matrix. Uncertainty arises as to the meaning of ‘homogeneous’, in the context of providing the neutron absorbing properties assumed in the safety case for a package and the potential effect on criticality. During criticality analyses, it is usual for homogeneous materials to be assumed as uniform, without irregularities, with equal properties in all directions, at the atomic level. Since the boronated materials are ‘alloys’, the constituents are not chemically combined but finely mixed, with the boron particles, of various sizes, viewable via a microscope. This signifies that at the atomic level, the material is not homogeneous, but a heterogeneous mixture with size and distribution of boron within the MMC being not strictly uniform.

Depending on variation in boron size and distribution, the neutron absorption capability of the MMC could be reduced, with consequential reduction in criticality safety margins. During the recent manufacture of a MOX fuel transport package, which included a boron carbide (B₄C)–aluminium alloy, material quality tests were performed to examine the structure of the material. Although the tests confirmed the size and distribution of B₄C within the MMC to be such that it could be classified as ‘homogeneous’, supporting calculations were completed to determine potential effects on criticality of a heterogeneous versus homogeneous neutron absorbing material. To determine the change in neutron absorbing properties of the MMC due to atomic versus microscopic assumptions, the Monte Carlo neutronics code MONK was used to extensively examine the effects of a heterogeneous MMC with a boron particulate of various sizes and proportions compared to a homogeneous boron distribution within the material. The paper presents, from a criticality viewpoint, the effects of heterogeneity versus homogeneity for boronated poisons in a particular fissile material transport package. It also emphasises the benefits of the utilisation of software/computing developments during the calculational process, which enable wide ranging surveys over many variables to be completed quickly and efficiently.     

A technique for measuring the density of small fragments of high-density materials both before and after irradiation

A method for determining the density of small fragments of unirradiated and irradiated UO₂ to an absolute accuracy of ± 0.6 % using a density gradient column is described. The specimens are suspended in PTFE ‘carriers’ and the combined specimen plus ‘carrier’ density measured. Changes in UO₂ density of 0.08 % can be detected. The method is, in principle, applicable to any solid that does not react with the liquids in the density column. The main error comes from measuring the mass of the specimen.     

Development of a fraction collector for coupling gas chromatography with an AMS facility

It has been shown that microscale ¹⁴C measurements are possible by using a gas handling system and a gas ion source [T. Uhl, W. Kretschmer, W. Luppold, A. Scharf, AMS measurements from microgram to milligram, Nucl. Instr. and Meth. (2005) 474 (240th ed.), T. Uhl, W. Luppold, A. Rottenbach, A. Scharf, K. Kritzler, W. Kretschmer, Development of an automatic gas handling system for microscale AMS (¹⁴C) measurements, Nucl. Instr. and Meth. (2007) 303 (259th ed.)]. In Erlangen a gas handling system was especially developed for environmental and biomedical investigations. For the separation of the compound of interest a standard gas chromatograph (GC) is used. To minimize the sample contamination and sample loss we have designed a fraction collector that connects a GC and an elemental analyzer (EA) directly. The selected compound is combusted in the EA and the resulting CO₂ is then transferred into the gas handling system for AMS measurements. From the beginning of GC preparation up to the AMS measurement the sample is in a closed line. All operations are fully automated, so no manual operations are necessary. This guarantees high cleanness and maximum sample yield. Preliminary measurements are done using modern and old ethyl alcohol (from fermentation and of petrochemical origin, respectively). The results are consistent with their expected values although cross contamination and background signal increased as the sample mass was decreased.     

Isobar separation at very low energy for AMS

The separation of atomic and molecular isobars, prior to injection into a tandem accelerator for Accelerator Mass Spectrometry (AMS), is discussed. To accomplish this separation, the anions from a standard sputter ion source are retarded to eV energy. The advantages of using very low energy (eV) for this purpose are twofold. The ionic reactions in gases can be isobar specific and the multiple scattering of the eV ions, unlike that at higher energy, can be controlled in linear radio-frequency multipoles. An example of current interest to AMS practice, the suppression of the S⁻ isobar ions from negative ion sources generating mainly Cl⁻ ions, will be described. It will be argued that this is a universal method for isobar separation prior to AMS, which is applicable to atomic anions and cations as well as their molecular counterparts. This procedure should be applicable to the AMS analysis of most rare radioactive species, as atomic or molecular ions, starting with either anions or cations, with appropriate charge changing. In some cases the ions may be analysable without AMS.     

The minimization of uncertainties in reactor theory

This paper deals with the question of the requirements associated with the uncertainties in nuclear data. This question arises when given target accuracies are required for selected performance parameters of a nuclear reactor. This problem is also known as the ‘inverse problem’. There are two ways to deal with the inverse problem as presented in this paper. The first way is to consider the nuclear data only in what we call the ‘differential approach’. The second way is to consider the nuclear data as well as integral experiments in what we call the ‘integro-differential approach’. When there are no specific target accuracies, minimization of the uncertainties in the nuclear data can be considered by minimization of the reactor input parameters' entropy.     

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.     

The new sample preparation line for radiocarbon measurements on atmospheric aerosol at LABEC

Radiocarbon measurements on the carbonaceous aerosol fractions have been demonstrated as an effective tool for aerosol source apportionment. For these measurements, a new sample preparation facility was installed at the INFN-LABEC laboratory of Florence (Italy). The line was designed to allow the preparation of samples from different carbonaceous fractions: the combustion of the aerosol samples can be performed in helium or oxygen flows, according to thermal sequences. The evolved CO₂ is cryogenically trapped and reduced to graphite, which is the target material for following Accelerator Mass Spectrometry (AMS) ¹⁴C measurements. This preparation line is described in detail in the paper. As a first step, the line was tested by means of AMS measurements performed on standards to check the reproducibility and the accuracy of the system; moreover, preliminary measurements on the total carbon fraction in aerosol samples were made. Results of these measurements are also reported.     

Application of the Transport System Concept to the Transport of LSA Waste

Abstract

Transport Regulations rely essentially on the packaging and do not take into account the contribution to safety which may be made by other features of the operation. In some situations, mainly for routine transports not fully complying with the Regulations, it would be beneficial to envisage the possibility of using a package which does not meet all the Type B requirements, complemented by additional safety measures put in place to compensate for these shortfalls. The ‘Transport System’ concept will take into account the contributions to safety from these additional measures. It will ensure that the proposed system is at least as safe as a reference operation complying fully with the Regulations. If this equivalent safety level can be properly demonstrated, the Competent Authority will provide a ‘Transport System Approval’ for well defined shipments over a specific period. Two examples are presented. In the first case, a thermally insulated ISO container is envisaged for the transport of drums containing combustible LSA material having a total activity per conveyance up to 600 A₂. In the second one, two dedicated trucks transporting conditioned waste in drums has been shielded so as to comply with the regulatory dose rate limits. These examples show the benefits of the TS concept. Nevertheless, the full requirements of the Regulations should be implemented as far as reasonably practicable, and the TS concept should be applied only to particular difficulties and is not suitable to all situations. Therefore, some general restrictions (applicable to every TS) have to be set by IAEA. Depending on the case, complementary ones may be required by the CA. Bearing in mind possible restrictions presented in this paper, the TS concept will be useful in solving some of the current problems of the transport of waste without needing a fundamental change in the Regulations.     

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.     

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 Si₃N_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 BeF₂ and BaBeF₄ measurements at our system. Several metal matrices and cathode preparation procedures for BeO samples were investigated to maximize current and cathode lifetime.     

Plutonium measurements on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA)

Plutonium isotopes have been recently added to the list of radionuclides that can be measured with the new generation of compact AMS facilities. In this paper we present first experimental results concerning the development of the plutonium AMS technique at 680 kV on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA) in Sevilla, Spain. This is the first compact AMS machine designed and manufactured by High Voltage Engineering Europa. As we demonstrate, the obtained backgrounds for ^239,240Pu, of about 10⁶ atoms, and the ²³⁹Pu/²³⁸U mass suppression factor, in the range of 10⁻⁹, compare to the ones achieved on other AMS facilities. With the measurement of reference materials provided by the International Atomic Energy Agency (IAEA-375, IAEA-Soil-6, IAEA-381) and samples already studied on the 600 kV compact ETH/PSI AMS system at Zürich, we show that the CNA system can be perfectly used for the routine measurement of plutonium isotopes at environmental levels.     

Software development for continuous-gas-flow AMS

The National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility at Woods Hole Oceanographic Institution is presently completing installation of a novel continuous-flow AMS system. A multi-year development of an AMS microwave gas ion source in collaboration with Atomic Energy Canada Limited (AECL), Chalk River, has preceded this final step of an implementation that is expected to add a new dimension to ¹⁴C AMS. National Instruments, NIM, and CAMAC modules have been programmed with LabVIEW on a Windows XP platform to form the basis for data acquisition. In this paper we discuss possible applications and include simulations of expected data acquisition scenarios like real-time AMS analysis of chromatograms. Particular attention will have to be given to issues of synchronization between rapidly changing input amplitudes and signal processing cycles in hardware and software.     

Yield Optimization of Helium and Lyman Emissions in Low Energy Plasma Focus Operated with Argon

Numerical experiments have been investigated on UNU/ICTP PFF low energy plasma focus device with argon filling gas. In these experiments, the temperature window of 1.4–5?keV has been used as suitable temperature range for argon soft X-rays. The Lee model was applied to characterize the UNU/ICTP PFF plasma focus. The optimum Ysxr was found to be 0.039?J, with the corresponding efficiency being 0.0014% at pressure of 0.673?Torr and the end axial speed was to be V_a?=?8.6?cm/μs. The optimum combination of p₀, z₀ and ‘a’ for argon Ysxr was found to be as 1.31?Torr, 12?cm and 0.7?cm respectively, with the outer radius b?=?2.37?cm. This combination gives Ysxr?=?0.045?J, with the corresponding efficiency is about 0.0016% and the end axial speed is of 8?cm/μs. Thus we expect to increase the argon Ysxr of UNU/ICTP PFF, without changing the capacitor bank, merely by changing the electrode configuration and operating pressure. The Lee model code was also used to run numerical experiments on UNU/ICTP PFF with argon gas for optimizing soft X-ray yield with reducing L₀, varying z₀ and ‘a’. From these numerical experiments we expect to increase the argon Ysxr of UNU/ICTP PFF with reducing L₀, from the present computed 0.039?J at L₀?=?110 nH to maximum value of near 0.266?J, with the corresponding efficiency is about 0.01%, at an achievable L₀?=?10 nH.     

Estimation of distribution algorithms for nuclear reactor fuel management optimisation

In this paper, estimation of distribution algorithms (EDAs) are used to solve nuclear reactor fuel management optimisation (NRFMO) problems. Similar to typical population based optimisation algorithms, e.g. genetic algorithms (GAs), EDAs maintain a population of solutions and evolve them during the optimisation process. Unlike GAs, new solutions are suggested by sampling the distribution estimated from all the solutions evaluated so far. We have developed new algorithms based on the EDAs approach, which are applied to maximize the effective multiplication factor (K_eff) of the CONSORT research reactor of Imperial College London. In the new algorithms, a new ‘elite-guided’ strategy and the ‘stand-alone’ K_eff with fuel coupling is used as heuristic information to improve the optimisation. A detailed comparison study between the EDAs and GAs with previously published crossover operators is presented. A trained three-layer feed-forward artificial neural network (ANN) was used as a fast approximate model to replace the three-dimensional finite element reactor simulation code EVENT in predicting the K_eff. Results from the numerical experiments have shown that the EDAs used provide accurate, efficient and robust algorithms for the test case studied here. This encourages further investigation of the performance of EDAs on more realistic problems.     

Progress with a gas-accepting ion source for Accelerator Mass Spectrometry

The National Ocean Sciences AMS (NOSAMS) facility at Woods Hole Oceanographic Institution has developed a novel, gas-accepting microwave-plasma ion-source. The source is a key component of a compact Accelerator Mass Spectrometry (AMS) system built for the analysis of ¹⁴C in a continuously flowing gas stream. The gas source produces carbon currents from a stream of CO₂ with currents typical of a traditional graphite source. Details of the gas source, including ion current achieved, optimal flow rate, efficiency, and memory are presented. Additionally, data obtained from coupling a gas chromatograph to the source shown. Details about ion optics are presented separately [1].     

Simulation of experimental spectra for medium-heavy nuclides in accelerator mass spectrometry

Some interferences are often encountered in accelerator mass spectrometry （AMS） measurements, especially for medium-heavy nuclide measurement, It is difficult for online discrimination of the nuclide of interest from the interfering ones. In order to solve this problem, we developed a method to simulate the experimental spectra of medium-heavy nuclides in AMS measurements. The results obtained from this method are in good agreement with experimental values.