Systematic study of instrumental mass discrimination in multi-collector inductively coupled plasma-mass spectrometry

Multi-collector inductively coupled plasma - mass spectrometry (MC-ICP-MS) has gained substantial importance in isotopic analysis over the last two decades. In the beginning, MC-ICP-MS was almost solely deployed in geo- and cosmochemistry and in nuclear sciences and industry. Nowadays, many other scientific fields make use of the technique, which is mostly based on the high versatility of the ICP ion source and the high sample throughput in comparison to methods with equal or even slightly better precision, such as thermal ionization mass spectrometry (TIMS). The major benefit of MC-ICP-MS is clearly the high ionization power of the ICP, compared to, e.g. that of thermal ionization. A major limitation of MC-ICP-MS is the omnipresent instrumental mass discrimination. It is the effect that light isotopes are discriminated against heavier isotopes during the measurement. The goal of this PhD research project was to identify and possibly quantify the major contributors to instrumental mass discrimination in MC-ICP-MS. Commonly, instrumental mass discrimination is attributed to space-charge effects. Even though this is an easy to comprehend effect at first glance, it becomes more complicated once studied more closely. Firstly, space-charge effects are present in charged particle beams only. Secondly, space-charge effects are most severe for low energetic particle beams with high current. Certainly, the space-charge effects are not the only contributors to mass discrimination. Several other contributors have been identified in the past, namely: collisions, sample introduction and ion formation and energy-selective ion transmission. The effect of the above mentioned processes in terms of mass discrimination were investigated by several strategies. The processes occurring during the ion beam formation were addressed by the Direct Simulation Monte Carlo method. Due to the nature of the ion source, the plasma is extracted from ambient pressure into the vacuum of the mass spectrometer; leading to drastically reduced fluid density. Yet, sufficient collisions between particles take place to possibly contribute to mass discrimination. The modeling results show a significant alteration of the fluid composition after the skimmer cone. Also a radial fractionation of the fluid was found. The ion beam is formed shortly after the plasma is extracted through the skimmer cone, the electrons are lost; a process known as charge-separation. During this phase, the space-charge effects are strongest. Thus a radial dependence of the isotopic composition of the ion beam might occur. This particular effect was investigated by two experiments, one comprising of ion implantation for the subsequent determination of the radial composition of the ion beam, the second experiment with a variable aperture addressing the shortcomings of the ion implantation and provide reliable \emph{in situ} information about the beam composition and diameter. These beam diameters are in contradiction to those expected from typically reported ion beam current. In order to measure the gross beam current, a Faraday cup was placed after the first ion lens of the mass spectrometer. The results reveal a much lower ion current than reported in literature, but are in reasonable agreement with estimations by the Child-Langmuir law for space-charge limited beams. Finally, it has to be pointed out that no dominant contributor to the mass discrimination could be identified. However, the energy-selective transmission can be excluded from the list of contributors, given the low ion beam current with the associated quasi complete beam transport. Since sample introduction a priory can be ruled out, only two contributors remain: collisions and space-charge effects. Both contributors can hardly be separated from one another experimentally, i.e., a higher throughput through the interface will lead to more collisions in the interface and consequently, to a higher ion beam current after charge-separation. Yet, the isolated treatment of both effects in computer simulations might provide a tool to solve this problem. Of course, the same simulator would need to have the capability to model both effects simultaneously, as well as separately, which is not yet possible

Development of a high temperature treatment device for spent nuclear fuel

Novel reprocessing schemes and techniques are the focus of the Euratom FP7 project "Actinide Recycling for Separation and Transmutation” (ACSEPT), where the Paul Scherrer Institute (PSI) is represented in the pyrochemical domain. The subject of investigation is the selective separation of fission products (FPs) from spent nuclear fuel as a head-end step to either classical hydro based or pyro processes which are not yet applied on a large scale. The selective removal of FPs that are major contributors to the overall radiation dose or bear great potentials in terms of radiotoxicity (i.e. cesium or iodine), is advantageous for further processes. At PSI a device was developed to release volatile FPs by means of inductive heating. The heating up to 2,300°C promotes the release of material that is further transported by a carrier gas stream into an inductively coupled plasma mass spectrometer for online detection. The carrier gas can be either inert (Ar) or can contain reducing or oxidizing components like hydrogen or oxygen, respectively. The development of the device by computer aided engineering approaches, the commissioning and evaluation of the device and data from first release experiments on a simulated fuel matrix are discusse

Quantification of 60Fe atoms by MC-ICP-MS for the redetermination of the half-life

In many scientific fields, the half-life of radionuclides plays an important role. The accurate knowledge of this parameter has direct impact on, e.g., age determination of archeological artifacts and of the elemental synthesis in the universe. In order to derive the half-life of a long-lived radionuclide, the activity and the absolute number of atoms have to be analyzed. Whereas conventional radiation measurement methods are typically applied for activity determinations, the latter can be determined with high accuracy by mass spectrometric techniques. Over the past years, the half-lives of several radionuclides have been specified by means of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) complementary to the earlier reported values mainly derived by accelerator mass spectrometry. The present paper discusses all critical aspects (amount of material, radiochemical sample preparation, interference correction, isotope dilution mass spectrometry, calculation of measurement uncertainty) for a precise analysis of the number of atoms by MC-ICP-MS exemplified for the recently published half-life determination of $^{60}$ Fe (Rugel et al, Phys Rev Lett 103:072502, 2009

Characterization of nuclear fuels by ICP mass-spectrometric techniques

Isotopic analyses of radioactive materials such as irradiated nuclear fuel are of major importance for the optimization of the nuclear fuel cycle and for safeguard aspects. Among the mass-spectrometric techniques available, inductively coupled plasma mass spectrometry (ICP-MS) and thermal ionization mass spectrometry are the most frequently applied methods for nuclear applications. Because of the low detection limits, the ability to analyze the isotopic composition of the elements and the applicability of the techniques for measuring stable as well as radioactive nuclides with similar sensitivity, both mass-spectrometric techniques are an excellent amendment to classical radioactivity counting methods. The paper describes selected applications of multicollector ICP-MS in combination with chromatographic separation techniques and laser ablation for the isotopic analysis of irradiated nuclear fuels. The advantages and limitations of the selected analytical technique for the characterization of such a heterogeneous sample matrix are discusse

Measurement of the 244^{244}Cm and 246^{246}Cm Neutron-Induced Cross Sections at the n_TOF Facility

The neutron capture reactions of the $^{244}$Cm and $^{246}$Cm isotopes open the path for the formation of heavier Cm isotopes and of heavier elements such as Bk and Cf in a nuclear reactor. In addition, both isotopes belong to the minor actinides with a large contribution to the decay heat and to the neutron emission in irradiated fuels proposed for the transmutation of nuclear waste and fast critical reactors. The available experimental data for both isotopes are very scarce. We measured the neutron capture cross section with isotopically enriched samples of $^{244}$Cm and $^{246}$Cm provided by JAEA. The measurement covers the range from 1 eV to 250 eV in the n_TOF Experimental Area 2 (EAR-2). In addition, a normalization measurement with the $^{244}$Cm sample was performed at Experimental Area 1 (EAR-1) with the Total Absorption Calorimeter (TAC)

Variable aperture extraction lens for ion beam investigation in inductively coupled plasma-mass spectrometry

A variable aperture was introduced into a commercially available sector field multicollector inductively coupled plasma-mass spectrometer. A diameter-variable aperture allows an in situ study of the radial isotopic composition within the ion beam. Additional information on the intensity distribution could be gained. The elements boron, cadmium and lead, covering a wide mass range, were investigated. In contrast to earlier experiments [Kivel et al., Spectrochimica Acta Part B: Atomic Spectroscopy, 2012, 76, 126-132], the current setup allows for lower element concentration levels in the samples and a drastically reduced measurement time. A significant radial dependence of the isotopic composition within the ion beam was observed for cadmium and lead, whereas for boron, such dependence could not be detected. The beam profiles however show a systematic trend towards smaller beam diameters for higher masses. Even though the beam diameter is dependent upon the mass of the ion, the transmission into the mass spectrometer can be considered almost complete. Thus, a contribution to mass discrimination by space-charge induced beam broadening and energy-selective ion transmission, at least within the boundaries studied here, can be excluded