40Ar/39Ar — Alter eines Basits der Ostantarktis

The⁴⁰Ar/³⁹Ar technique is an analytical variation of the K–Ar dating method. A known fraction of the³⁹K in a sample is converted to³⁹Ar by irradiation with fast neutrons. In an incremental heating experiment, the argon is released from the sample fractionally by stepwise heating. The result is a series of apparent ages.⁴⁰Ar/³⁹Ar age spectrum was determined for the sample Oase Vestfold. The minimum of the saddle-shaped age spectrum gives 1.0 billion years.     

Application of neutron activation analysis to the measurement of isotope abundance shifts in calcium and potassium

A simple method for the measurement by neutron activation analysis of the isotopic abundance shifts of⁴⁴Ca and⁴¹K in compounds of low volatility is described. Extraction and measurement procedures for the radioactive argon isotopes (³⁷Ar,³⁹Ar and⁴¹Ar produced by (n, α) and (n, p) reactions are also given. The results obtained demonstrate that the accuracy for the measurement of δ⁴⁴Ca is better than by the mass spectrometric method; in the case of δ⁴¹K measurement the accuracy is the same as that obtained with the mass spectrometer. Interfering reactions which yield other rare gases, and their relative contribution to the error in the measurement of δ values are also discussed.     

Radioargon production through the irradiation of calcium oxide

The production of radioargon through the irradiation of CaO was performed in an in-core facility within the The University of Texas at Austin MARK II TRIGA reactor. The major radioargon isotope produced was ³⁷Ar via the ⁴⁰Ca(n,α)³⁷Ar reaction pathway. The CaO powder was vacuumed sealed in a quartz ampoule. The sample was irradiated in a Cd-lined Al alloy canister for 2 h at 500 kW. After the irradiation, the sample was counted using an HPGe detector five times with increasing count times. ⁴¹Ar, ⁴²K, ⁴³K, and ⁴⁷Ca were detected in the spectra. The induced activities of ³⁷Ar, ³⁹Ar, ⁴¹Ar, ⁴²K, ⁴³K, and ⁴⁷Ca were calculated using a 63-group energy-dependent neutron flux determined utilizing a neutron energy flux profile calculated from a MCNPX model of the TRIGA reactor core. The production ratios generated from the model allowed for the estimation of induced ³⁷Ar and ³⁹Ar activities through the measured induced activities of ⁴¹Ar, ⁴²K, ⁴³K, and ⁴⁷Ca.     

Magmatism and deformation times of the Xidatan rock series,East Kunlun Mountains

The Xidatan rock series consist of mylonite, gneiss and granite. The U-Pb age shows that the granite was formed at 206 Ma, and the⁴⁰Ar/³⁹Ar ages of the biotite display that the mylonite was formed at 145 Ma when the Xidatan ductile shearzone occurred. At about 110—100 Ma, the Xidatan Fault reactivated again. As a result, the biotite and muscovite of Xidatan rock series opened their argon isotope system.     

Magmatism and deformation times of the Xidatan rock series,East Kunlun Mountains

The Xidatan rock series consist of mylonite, gneiss and granite. The U-Pb age shows that the granite was formed at 206 Ma, and the⁴⁰Ar/³⁹Ar ages of the biotite display that the mylonite was formed at 145 Ma when the Xidatan ductile shearzone occurred. At about 110—100 Ma, the Xidatan Fault reactivated again. As a result, the biotite and muscovite of Xidatan rock series opened their argon isotope system.

     

Neutron activation in geochronology: The40Ar/39Ar technique

Fast neutron irradiation of potassium-bearing minerals produces artificial³⁹Ar. Mass spectrometric measurement of the³⁹Ar/⁴⁰Ar ratio allows to calculate a geological age. Stepwise degassing of the sample yields insight into its history. As an example, analysis of a single hornblende from the Valtellina (Italian Alps) reveals an unexpected pre-Alpine history of that region.     

Evaluation of 40K in food by determining total potassium using neutron activation analysis

Summary Potassium is an extremely important major element to the human body. Potassium is made up of three isotopes with abundances of ³⁹K at 93.1%, ⁴⁰K at 0.0118% and ⁴¹K at 6.88%. It is also very well known that ⁴⁰K with its 1.3 ^. 10⁹ years half-life is a major naturally occurring isotope in the body and food. The usual way to determine total ⁴⁰K is to measure the single 1460.8 keV photon from beta-decay. However, this procedure requires a significant amount of sample and typical counting periods of at least a day in well-shielded germanium counting system. Another approach is to determine total potassium via neutron activation analysis using the well known ⁴¹K(n,g)⁴²K (T_1/2 = 12.8 h) reaction and then evaluate ⁴⁰K using the usual activity equation A = lN. In our laboratory we have effectively used epithermal neutron activation analysis and Compton suppression to determine potassium in air filters and other geological material. Upwards to 10-15 samples can be analyzed in one day using only gram quantities of material. In such way one can increase the output of determining ⁴⁰K by at least one order of magnitude. Results of a detailed investigation optimization of the methodology, quality control and detection limits will be presented for reference material and various food samples.     

Fast and precise determination of fluorine in geological samples by 14 MeV neutron activation analysis

A fast and precise method of determining fluorine in geological matrices is proposed. The 0.20 MeV photopeak of¹⁹O, induced by the¹⁹F(n, p)¹⁹O reaction, was used for this assay. Neutron flux monitoring was achieved by adding an internal standard monitor (20 mg Ce) to each sample and counting the activity due to the 0.74 MeV photopeak of^139m Ce, produced in the¹⁴⁰Ce(n, 2n)^139m Ce reaction. This activity was considered to be proportional to the neutron flux during the sample irradiation. This method of fluorine determination was checked on two fluorine geological standards, mica and apatite, containing 1.50 and 2.90% fluorine, respectively. The sensitivity of the method, obviously depending upon the matrix composition, was 1.46 mg for the mica standard.     

Measuring magnesium,calcium and potassium isotope ratios using ICP-QMS with an octopole collision cell in tracer studies of nutrient uptake and translocation in plants

The ability of a quadrupole-based ICP-MS with an octopole collision cell to obtain precise and accurate measurements of isotope ratios of magnesium, calcium and potassium was evaluated. Hydrogen and helium were used as collision/reaction gases for ICP-MS isotope ratio measurements of calcium and potassium in order to avoid isobaric interference with the analyte ions from (mainly) argon ions ⁴⁰Ar⁺ and argon hydride ions ⁴⁰Ar¹H⁺. Mass discrimination factors determined for the isotope ratios ²⁵Mg/²⁴Mg, ⁴⁰Ca/⁴⁴Ca and ³⁹K/⁴¹K under optimized experimental conditions varied between 0.044 and 0.075. The measurement precisions for ²⁵Mg/²⁴Mg, ⁴⁰Ca/⁴⁴Ca and ³⁹K/⁴¹K were found to be 0.09%, 0.43% and 1.4%, respectively. This analytical method that uses ICP-QMS with a collision cell to obtain isotope ratio measurements of magnesium, calcium and potassium was used in routine mode to characterize biological samples (nutrient solution and small amounts of digested plant samples). The mass spectrometric technique was employed to study the dynamics of nutrient uptake and translocation in barley plants at different root temperatures (10 °C and 20 °C) using enriched stable isotopes (²⁵Mg, ⁴⁴Ca and ⁴¹K) as tracers. For instance, the mass spectrometric results of tracer experiments demonstrated enhanced ²⁵Mg and ⁴⁴Ca uptake and translocation into shoots at a root temperature of 20 °C 24 h after isotope spiking. In contrast, results obtained from ⁴¹K tracer experiments showed the highest ⁴¹K contents in plants spiked at a root temperature of 10 °C.     

Evaluation of Compton scattering and self-attenuation coefficient after <Emphasis Type="Italic">γ</Emphasis>-ray analysis of naturally occurring radioactive elements in environmental samples

Comparison of Compton scattering and Compton scattering cross section with self-attenuation coefficient were explained based on the kinematic equation and Klein-Nishina formula. Naturally occurring elements, ²³⁸U (²²⁶Ra), ⁴⁰K, ²³²Th (²²⁸Ra) and ¹³⁷Cs were determined in sediments and water from Ismailia canal in Egypt which were found in the range of permissible level. Self-attenuation coefficients, K, the ratio between photopeak detection efficiency using solid and liquid standards were determined. They fit well comparing to Compton scattering, Compton scattering cross section while inversely fit to energy-absorption Compton scattering cross section based on the Klein-Nishina formula.     

A Penning trap for studying cluster ions

We propose to use a Penning trap for spectroscopy of stored cluster ions. A similar device has been built for the purpose of mass measurements of short-lived nuclei produced at the on-line isotope separator ISOLDE/CERN. A resolving power of 500,000 in a mass measurement of³⁹K and an accuracy of 2 × 10^?7 for the⁸⁵Rb/³⁹K mass ratio were obtained. An efficiency for in-flight capture as high as 70% was achieved. The method provides very high sensitivity since typically only 10 to 100 ions are stored in the trap. We intend to perform laser spectroscopy on trapped Na clusters as a first application of the trap technique.     

Radionuclide concentrations in several seaweeds and their annual variations

To examine the radionuclide concentrations in the environment, seaweeds were collected once a year since 1976 at two different places. A marked change was observed in the number of radionuclides in seaweeds. The increase of radionuclides is attributed to contamination by radioactive fallout from Chinese nuclear explosion tests. The radionuclide concentrations were strongly dependent on the species of seaweeds. A gradual decrease of the¹³⁷Cs/⁴⁰K activity ratio was observed in spite of the input of additional¹³⁷Cs fallout from Chinese nuclear tests. This implies that the amounts of¹³⁷Cs introduced into seawater by these detonations were small in comparison with those already contained in seawater. The cesium content in seawater was estimated by using specific activities of¹³⁷Cs in seaweeds and¹³⁷Cs concentration in seawater. The results show good agreement with the reported values by direct analysis of seawater.     

Production of <Superscript>37</Superscript>Ar in The University of Texas TRIGA reactor facility

The detection of ³⁷Ar is important for On-Site Inspections (OSI) for the Comprehensive Nuclear-Test-Ban Treaty monitoring. In an underground nuclear explosion this radionuclide is produced by ⁴⁰Ca(n,α)³⁷Ar reaction in surrounding soil and rock. With a half-life of 35 days, ³⁷Ar provides a signal useful for confirming the location of an underground nuclear event. An ultra-low-background proportional counter developed by Pacific Northwest National Laboratory is used to detect ³⁷Ar, which decays via electron capture. The irradiation of Ar gas at natural enrichment in the 3L facility within the Mark II TRIGA reactor facility at The University of Texas at Austin provides a source of ³⁷Ar for the calibration of the detector. The ⁴¹Ar activity is measured by the gamma activity using an HPGe detector after the sample is removed from the core. Using the ⁴¹Ar/³⁷Ar production ratio and the ⁴¹Ar activity, the amount of ³⁷Ar created is calculated. The ⁴¹Ar decays quickly (half-life of 109.34 min) leaving a radioactive sample of high purity ³⁷Ar and only trace levels of ³⁹Ar.     

Determination of potassium in beet by 14 MeV neutron activation analysis

The applicability of 14 MeV neutron activation analysis for the determination of K in beet has been investigated by measuring the 2.167 MeV gamma-line from the decay of³⁸gK produced in the³⁹K(n, 2n) reaction. Beet samples were treated in different solutions of KCl and HgCl₂ to study the diffusion of K⁺ ions into the beet cells. The contribution of the gamma-line to the measured peak area from³⁸Cl produced in the³⁷Cl(n, γ) reaction was found to be 1%. Results obtained by fast neutron activation analysis and by flame spectrometry have been compared, and good agreements were found.     

Evolution of 40K in fruit collected in Malaysia by the determination of total potassium using neutron activation analysis

Potassium is an extremely important major to element to the human body. Potassium in made up of three isotopes with abundances of ³⁹K at 93.1%, ⁴⁰K at 0.0118% and ⁴¹K at 6.88%. It is also very well known that ⁴⁰K is to measure the single 1460.8 keV photon from beta-decay. However, this procedure requires a significant amount of sample and typical counting periods of at least a day in well-shielded germanium counting system. Another approach is to determination total potassium via neutron activation analysis using the well know ⁴¹K (n,γ) ⁴²K(T _1/2 = 12.8 h) reaction and then evaluate ⁴⁰K using the usual activity equation A = λN. In our laboratory we have effectively used thermal and epithermal neutron flux for neutron activation analysis to determine potassium in fruits. Upward to 7–9 batches of samples, which each of batch is including 14 samples so can be analysed in 1 day using only gram quantities of material. In such way on can increase the output of determining ⁴⁰K by at least on order of magnitude. Result of a detailed investigation optimization of the methodology, quality control and detection limits will be presented for reference material and various fruits samples.     

KX-ray production cross sections for heavy target elements and14N,40Ar projectiles above the coulomb barrier

KX-ray production cross sections have been measured for beams of¹⁴N at 92, 103, 115 and 180 MeV and⁴⁰Ar at 180 and 300 MeV on targets with atomic numbers ranging from 40 to 90. We find that the measured values can be well reproduced with a plane wave Born calculation, provided that corrections for the Coulomb deflection of the projectile, the increased binding energy of theK-shell electron and relativistic effects of theK-shell wave function are taken properly into account.     

A continuous flow mass spectrometry technique of argon isotope measurement for K/Ar geochronology

A new method for the measurement of argon isotope composition in a continuous flow of helium for potassium/argon geochronology is described. Extraction of argon from geological samples in multiple‐sample holders was carried out in a chamber by heating with a continuous Nd‐YAG laser. The extracted and pre‐concentrated argon is passed through a chromatographic capillary column in a flow of helium. Argon is separated from possible contaminants in the column and is injected through an open split into the ion source of an isotope ratio mass spectrometer. Measurement of the ³⁶Ar, ³⁸Ar and ⁴⁰Ar isotopes was carried out in dynamic mode, using a triple‐collector ion detector. These experiments have shown that continuous flow mass spectrometry can be used for the analysis of radiogenic argon in picogram quantities with an accuracy that is satisfactory for the solution of many geochronological problems. The method of argon isotope measurement in a continuous flow of helium is an alternative to the measurement of argon isotopes in the static mode. The sensitivity and accuracy of argon measurement by this method are comparable with those provided by the classical static method. The measurement of argon isotopes in a continuous flow of helium is simpler and more reliable than measurement in the static mode. Copyright © 2009 John Wiley & Sons, Ltd.     

Tectonic setting discrimination and40Ar/39Ar isotope geochronology of the Miba granite

The Miba granite is located in the juncture of Kang County in eastern Gansu Province and Lueyang County in Shanxi Province. It is considered to be the post-orogenic granite (POG-type) by major element discrimination method of Maniar.⁴⁰Ar/³⁹Ar dating indicates that its emplacement time is about 240—230 Ma and it is the result of the Indo-Sinian magmatism. In the early Yanshan period (about 193.8 Ma), a thermal event resulted in the partial opening of the argon isotope system of the biotite.

     

Tectonic setting discrimination and<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar isotope geochronology of the Miba granite

The Miba granite is located in the juncture of Kang County in eastern Gansu Province and Lueyang County in Shanxi Province. It is considered to be the post-orogenic granite (POG-type) by major element discrimination method of Maniar.⁴⁰Ar/³⁹Ar dating indicates that its emplacement time is about 240—230 Ma and it is the result of the Indo-Sinian magmatism. In the early Yanshan period (about 193.8 Ma), a thermal event resulted in the partial opening of the argon isotope system of the biotite.     

Instrumental neutron activation analysis of geochemical and cosmochemical samples: A fast and reliable method for small sample analysis

We have developed a fast and reliable procedure to routinely measure the abundances of up to about 35 elements even in small (<1 mg) samples. Depending on the type of samples, they are either irradiated for about 8 hours at a flux of about 2·10¹²n·cm^–2·s^–1, or up to 100 hours at a flux of about 6·10¹³n·cm^–2·s^–1. As standards, high-purity synthetic multielement standards and well-characterized geological reference materials are used. Synthetic standards are used as primary standards because they have several advantages over secondary (geological) standards. Three to four counts are done one each sample, starting 1–3 days after the end of the irradiation. We use high-purity germanium (HpGe) detectors with high efficiencies and very good energy resolution (1.6–1.8 keV at 1332 keV). To allow high throughput rates we use fast preamplifiers and gated integrator spectroscopy amplifiers with fast fixed conversion time ADCs. The signals are fed into an acquisition interface module (AIM) and via Ethernet into a Micro VAX. To allow better peak deconvolution, 8k spectra are taken where possible. A specially designed annular NaI(TI) guard detector allows Compton suppression spectrometry. The system uses standard software and was tested with sets of geological standards and has given reliable results for a wide variety of samples, e.g., cosmic spherules in the 30–200 g weight range.