Isobaric Background Research Articles

Accelerator mass spectrometry with ANU’s 14 million volt accelerator

Accelerator Mass Spectrometry (AMS) at the Department of Nuclear Physics and Accelerator Applications (Australian National University) is based on a 14UD tandem accelerator. The 14UD has demonstrated exceptional accelerator performance over more than three decades of AMS, e.g. by running regularly above 14 MV. We present the actual performance for the whole range of measured radionuclides and the potential of this specialised high-energy AMS facility - demonstrating the continuing need for such a system. Focus here will be also on the unique setup comprising an Enge split-pole spectrograph used as a gas-filled magnet (GFM). The GFM has seen growing importance at ANU over the years as a means for providing an efficient reduction of isobaric background. We use the Enge now routinely for 10Be, 26Al, 32Si, 53Mn and 60Fe measurements.

Isobar suppression studies for 99Tc detection using Ion-Laser InterAction Mass Spectrometry (ILIAMS)

Ion-Laser InterAction Mass Spectrometry(ILIAMS) has the potential to discriminate 99Tc against isobaric background caused by stable 99Ru to allow for the detection of environmental 99Tc concentrations (≈107atoms/L) at low beam energies. As the electron detachment energies for Tc molecules are not available, Ru and Tc suppression for oxide and fluoride molecules using the available laser systems were determined experimentally for the first time. TcF5− was identified as the most promising anion, as RuF5− was suppressed by a factor 105 with the 2.33eV laser while TcF5− stayed unaffected. Additional Ru suppression in the ion source of another factor 100 was observed. We thus unambiguously identified the 99Tc signal from global fallout in an Austrian peat bog sample. However, no satisfactory suppression of stable 97Mo was achieved, thus hindering the use of the isotopic spike 97Tc for normalizing the detected 99Tc events. Therefore, alternative normalization methods need to be explored.

Developing Accelerator Mass Spectrometry Capabilities for Anthropogenic Radionuclide Analysis to Extend the Set of Oceanographic Tracers

Recent major advances in Accelerator Mass Spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA) regarding detection efficiency and isobar suppression have opened possibilities for the analysis of additional long-lived radionuclides at ultra-low environmental concentrations. These radionuclides, including 233U, 135Cs, 99Tc, and 90Sr, will become important for oceanographic tracer application due to their generally conservative behavior in ocean water. In particular, the isotope ratios 233U/236U and 137Cs/135Cs have proven to be powerful fingerprints for emission source identification as they are not affected by elemental fractionation. Improved detection efficiencies allowed us to analyze all major long-lived actinides, i.e., 236U, 237Np, 239,240Pu, 241Am as well as the very rare 233U, in the same 10 L water samples of a depth profile from the northwest Pacific Ocean. For this purpose, a simplified and very flexible chemical purification procedure based on extraction chromatography (a single UTEVA® column) was implemented which can be extended by a DGA® column for Am purification. The procedure was validated with the reference materials IAEA-381/385. With the additional increase in ionization efficiency expected for the extraction of actinides as fluoride molecules from the AMS ion source, a further reduction of chemical processing may become possible. This method was successfully applied to an exemplary set of air filter samples. In order to determine the quantitative 237Np concentration reliably, a 236Np spike material is being developed in collaboration with the University of Tsukuba, Japan. Ion-Laser Interaction Mass Spectrometry (ILIAMS), a novel technique for the efficient suppression of stable isobaric background, has been developed at VERA and provides unprecedented detection sensitivity for the fission fragments 135Cs, 99Tc, and 90Sr. The corresponding setup is fully operational now and the isobar suppression factors of >105 achieved, in principle, allow for the detection of the mentioned radionuclides in the environment. Especially for 90Sr analysis, this new approach has already been validated for selected reference materials (e.g., IAEA-A-12) and is ready for application in oceanographic studies. We estimate that a sample volume of only (1–3) L ocean water is sufficient for 90Sr as well as for 135Cs analysis, respectively.

Analytical method for the determination of Np and Pu in sea water by AMS with respect to the Fukushima accident

A chemical separation procedure for plutonium (Pu) and neptunium (Np) was developed using extraction chromatography, mass spectrometry and radiometric analysis to determine their concentrations and isotopic ratios in sea water. 241Am, which causes isobaric background to 241Pu in mass spectrometric measurements, was successfully separated from the Pu fraction by this method. Water samples which were spiked with 242Pu and 237Np or 239Np, respectively, were used for chemical yield determination. The chemical yields of Pu and Np, which were determined by alpha and gamma spectrometry at the Radiochemie München (RCM), of more than 85% were obtained. The developed method was applied to analyze the concentration of Pu and Np in the certified reference material, IAEA-443, by Accelerator Mass Spectrometry (AMS) at the Maier–Leibnitz-Laboratory (MLL) to check the applicability of the method to sea water samples. The concentrations of 240Pu, 241Pu and 237Np obtained in this study are in agreement with the certified and literature values within the uncertainties. Due to strong isotopic interference of 239Pu with 238U, it was not possible to analyze the concentration of 239Pu. Some modifications of the chemical separation method to suppress the uranium (U) fraction are under consideration. This method can be used for the analysis of Pu and Np in Pacific Ocean water samples collected after the Fukushima accident.

A complementary laser system for ISOLDE RILIS

The Resonance Ionization Laser Ion Source (RILIS) is a powerful tool for efficient and selective production of radioactive ion beams at Isotope Separator On Line (ISOL) facilities. To avoid isobaric background, highly selective stepwise resonant ionization is applied, using up to three different laser wavelengths. Due to their advantages in terms of stability and reliability, an all solid-state titanium:sapphire (Ti:Sa) system is used or is planned to be installed at the majority of on-line facilities worldwide. Such an all solid-state Ti:Sa laser system is going to be installed at the ISOLDE RILIS at CERN alongside the well-established dye laser system.

A new HVE 6 MV AMS system at the University of Cologne

CologneAMS is the new Centre for Accelerator Mass Spectrometry (AMS) at the University of Cologne. It will operate a dedicated AMS system designed to measure all standard cosmogenic nuclides ( 10Be, 14C, 26Al, 36Cl, 41Ca, 129I) and which uses a 6 MV Tandetron™ accelerator equipped with an all solid-state power supply, foil and gas stripper. The system also enables a sensitive detection of heavy ions up to 239U and 244Pu. The high-energy mass-spectrometer consists of a 90 degree magnet with a radius of 2 m and a mass-energy product of 351 AMU MeV to allow the detection of 244Pu 5+ up to the maximum terminal voltage of 6 MV. This magnet is followed by an electrostatic energy analyzer and a switching magnet that can transport the rare isotope beam into various beamlines. The switching magnet forms a third analyzing element which is needed especially for the sensitive detection of heavy elements. So far two beamlines are equipped with their own detection system. One of these lines is used for suppression of isobaric background in the case of the analysis of e.g. 36Cl. This is accomplished by an absorber foil which generates a Z-dependent energy loss in combination with a momentum/charge-state selection via a 120 degree magnet that features up to 30 mrad acceptance for efficient beam transport. In this contribution we will introduce the new Centre, the layout and specific characteristics of the AMS system as well as the main topics of the future scientific work to be performed at CologneAMS.

Improved isotopic SIMS measurements of uranium particles for nuclear safeguard purposes

The isotopic analysis of particles containing sub-pg to pg levels of uranium, released from nuclear material handling, has been proven as an efficient tool for international safeguard purposes. Precise and accurate measurement of both enrichment and the minor isotopes is, however, a challenging analytical task due to the low levels of material. One of the mainstay techniques for particle measurement is Secondary Ion Mass Spectrometry (SIMS), this study evaluates the analytical benefit of an alternative in the form of large geometry SIMS (LG-SIMS), which combines high transmission with high mass resolution. We report here that LG-SIMS instruments provide a significantly better measurement quality than the small geometry SIMS as almost all isobaric background interferences are removed at a high useful ion yield. Useful yield measurements, performed on uranium oxide particles with calibrated uranium content, showed an overall useful yield of 1.2% for the LG-SIMS at a mass resolution of 3000. These improvements were then demonstrated by comparing results from actual nuclear inspection samples measured on both instruments. Additional benefits include an increased ability to detect particles of interest in a dust matrix while simultaneously reducing the time of sample analysis. An evaluation on the performance of LG-SIMS compared to Thermal Ion Mass Spectrometry (TIMS) is also presented. This evaluation shows that LG-SIMS has an advantage due to its high ion yield but with a limitation in the detection limit of 236U at higher enrichments due to the necessity for a hydrogen correction.

Search for long-lived isomeric states in neutron-deficient thorium isotopes

The discovery of naturally occurring long-lived isomeric states (t_1/2 > 10^8 yr) in the neutron-deficient isotopes 211,213,217,218Th [A. Marinov et al., Phys. Rev. C 76, 021303(R) (2007)] was reexamined using accelerator mass spectrometry (AMS). Because AMS does not suffer from molecular isobaric background in the detection system, it is an extremely sensitive technique. Despite our up to two orders of magnitude higher sensitivity we cannot confirm the discoveries of neutron-deficient thorium isotopes and provide upper limits for their abundances.

Performance of the HVE 5 MV AMS system at CEREGE using an absorber foil for isobar suppression

At CEREGE (Aix-en-Provence, France) a HVE 5MV AMS facility has been put in operation for measuring the long-lived cosmogenic radioisotopes 10Be, 26Al, 36Cl, 41Ca and 129I. Characteristic to the AMS system is that isobaric background can be suppressed by means of the nuclear-charge dependent energy loss in an absorber foil and subsequent energy selection by an electrostatic deflector. In case of 36Cl analysis, this method suppresses 36S by a factor of at least 200. As a consequence, high source output does not overload the detector by isobaric interference. Together with the high-resolution gas ionisation detector 36Cl measurements at an energy as low as 30MeV are supported. This can be readily achieved by gas stripping at 5MV terminal voltage, thereby avoiding the use of cumbersome foil stripping. The performance of the AMS system is reviewed. 36Cl/Cl ratios down to 4×10−16 have been measured.

Laser based 14C counting, an alternative to AMS in biological studies

Accelerator mass spectrometry (AMS) has been developed over the past 25 years as an effective method to count 14C atoms. An alternative 14C measurement technique, suitable for routine laboratory use, combining the optogalvanic effect with laser assisted isotope ratio analysis (LARA) is now being extended from its current use for precision analysis of 13CO2 to similar analysis of 14CO2. The analyzer uses a unique resonance between a specific molecular transition in the molecule 14CO2 and the narrow band output of a laser based on the same molecular transition. Isobaric background is not an issue as in AMS, and the sample is not consumed in analysis so that long averaging times with small samples is possible.

A new AMS setup for nuclear astrophysics experiments

The Nuclear Structure Laboratory (NSL) at the University of Notre Dame installed its Browne–Buechner spectrograph in the early 1970s for highly accurate energy measurements of nuclear reactions. Current renovation and upgrading of this spectrograph will enable operation of the magnet in a gas-filled mode, in particular for the study of nuclear reactions with low cross-sections of interest in nuclear astrophysics. One of the principle issues shared by measurements of extremely low abundances in Accelerator Mass Spectrometry (AMS) and nuclear astrophysics is the discrimination between the nuclei of interest and often very intense isobaric background. Recently the AMS technique of the gas-filled magnet has very successfully been used at Argonne National Laboratory (ANL) to overcome this in the study of both environmental noble gas traces (39Ar) and the measurement of cross-sections of interest in stellar nucleosynthesis i.e. the 62Ni(n,γ)63Ni reaction. We hope to extend these techniques further to the observations of astrophysically important reactions such as 40Ca(α,γ)44Ti and 78Kr(α,γ)82Sr.

High-sensitivity AMS for heavy nuclides at the Munich Tandem accelerator

During the last years we have aimed at utilizing the high energy achievable at the Munich MP Tandem (TV=14 MV) for maximum sensitivity. One of our interests is the measurements of radionuclides around A=60 ( 53Mn, 60Fe, 63Ni) because of their high scientific potentials. However, in this mass region there are usually strong limitations in sensitivity due to a large isobaric background. Its suppression is performed by means of a gas-filled magnet and a multi Δ E ionization chamber. A Wien filter and a time-of-flight path allow a further suppression of (non-isobaric) background. The optimization of the setup as a whole yielded detection limits of 53Mn/Mn ≈2×10 −14 63Ni/Ni ≈2×10 −14 and 60Fe/Fe ≈2×10 −16, which presently cannot be achieved by any other detection method. The overall efficiency (including ion source) is typically 10 −5–10 −4. The gas-filled magnet allows an efficient reduction of the detector counting rate by more than three orders of magnitude. Therefore, relatively high negative ion currents and thus short times for the measurements are possible.

Production of radioactive Ag ion beams with a chemically selective laser ion source

We have developed a chemically selective laser ion source at the CERN-ISOLDE facility in order to study neutron-rich Ag nuclides. A pulsed laser system with high repetition rate has been used based on high-power coppe-vapour pump lasers and dye lasers. With this source significant reductions of the isobaric background has been achieved.

Study of laser interaction with negative ions

Negative ions can be neutralized by detaching their additional electron through interaction with a laser beam. By properly choosing the laser wavelength, the process is highly selective; it can in principle enhance the discrimination power of an accelerator mass spectrometry system for ions of different elements (e.g. isobaric background) by allowing their separation prior to their injection into a tandem accelerator. We demonstrate this process in the case of the 59Ni- 59Co pair with the AMS system based on the Rehovot 14UD Pelletron accelerator and a pulsed Nd-YAG laser at the fundamental wavelength (1064 nm). A photodetachment cross section of (0.6 ± 0.3) × 10 −17 cm 2 was measured for 59Co − and a suppression factor of 125 for the 59Co isobaric background was achieved in a 59Ni AMS measurement. The duty factor due to the pulsed laser was about 10 −4. The laser-AMS system was also applied to the study of rare negative ions in the actinide region. Preliminary results on the laser interaction with uranium negative ions are reported.

A high-temperature laser ion source for trace analysis and other applications

We report here on the development of a high-temperature laser ion source useful for trace analysis and other applications. It consists of a high temperature ionization chamber, three tunable dye lasers pumped by copper vapor lasers for stepwise resonant ionization and a Mattauch-Herzog mass spectrometer for the analysis of photo-ions. The principle of the laser ion source and its theoretical efficiency are discussed, where the efficiency of a laser ion source is the ratio of photo-ions extracted out to the number of atoms introduced into the cavity. Experimentally, an efficiency of 2×10−3 has been achieved for technetium. The scheme of gated detection is described which is used for suppressing isobaric background of molybdenum. The possible improvements are briefly mentioned for achieving isotopic analysis of 108 atoms of 97,98Tc in the presence of 1015 atoms of molybdenum, as separated chemically from molybdenum ore. Such an analysis of technetium isotopes produced in molybdenite ore by a (v,e−) reaction, is expected to yield information about the solar neutrino flux.

Detection of 36Cl with accelerator mass spectrometry

An accelerator mass spectrometry (AMS) system based on the HI-13 tandem accelerator at the Institute of Atomic Energy (IAE) is described, and the first detection of 36Cl with our AMS system is reported. The electrostatic deflector completely rejects isotopic background, 35Cl and 37Cl. The ioinzation chamber distinguishs 36Cl from isobaric background, 36S. The measurement of 36Cl with two samples is presented.

Radioisotope Detection with the Argonne FN Tandem Accelerator

A standard heavy ion nuclear structure facility has been used to detect several long-lived radioisotopes by counting the accelerated ions. The problem of eliminating strong isobaric background beams has been solved by combining the energy loss dispersion through a uniform Al foil stack with the high momentum resolution of an Enge split-pole magnetic spectrograph. Radioisotope concentrations in the following ranges have been measured: 14C/12C = 10–12 to 10–13, 26Al/27Al = 10–10 to 10–12, 32Si/Si = 10–8 to 10–14, 36Cl/Cl = 10–8 to 10–11.