Rare Gas Mass Research Articles

Atom trap trace analysis of krypton in xenon for the XENON Dark Matter Project

We report on the first atom trap trace analysis (ATTA) measurements of Kr in Xe samples with Kr/Xe concentrations of \U0001d4aa(10 ppt) and \U0001d4aa(100 ppt). The ATTA results were independently confirmed by measurements of the same samples with a rare gas mass spectrometer. Measurements of samples with Kr/Xe concentrations below these levels were inhibited by a residual Kr background in the ATTA apparatus, attributed to a cumulative cross sample contamination due to the radio frequency plasma discharge source region from tests with Kr-rich gas samples.

The effects of collision energy, vibrational mode, and vibrational angular momentum on energy transfer and dissociation in NO2+–rare gas collisions: An experimental and trajectory study

A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by approximately 4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2+ + Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+ -NO2 charge state.

Measurement of Helium Isotopes in Soil Gas as an Indicator of Tritium Groundwater Contamination

The focus of this study was to define the shape and extent of tritium groundwater contamination emanating from a legacy burial ground and to identify vadose zone sources of tritium using helium isotopes (3He and 4He) in soil gas. Helium isotopes were measured in soil-gas samples collected from 70 sampling points around the perimeter and downgradient of a burial ground that contains buried radioactive solid waste. The soil-gas samples were analyzed for helium isotopes using rare gas mass spectrometry. 3He/4He ratios, reported as normalized to the air ratio (RA), were used to locate the tritium groundwater plume emanating from the burial ground. The 3He (excess) suggested that the general location of the tritium source is within the burial ground. This study clearly demonstrated the efficacy of the 3He method for application to similar sites elsewhere within the DOE weapons complex.

Endogenous Lithium and Boron Red Cell–Plasma Ratios: Normal Subjects versus Bipolar Patients Not on Lithium Therapy

This study was undertaken to compare endogenous lithium concentrations in human blood and its components from normal donors versus bipolar patients. The patients were not on lithium therapy at the time that the blood samples were donated and had not received any lithium therapy for at least 2 yr. Blood components were separated by centrifugation. The analytical method for lithium as developed in this laboratory consists of thermal-neutron activation of freeze-dried samples. 3H is produced via the reaction 6Li + n = 3H + 4He, and high-sensitivity rare gas mass spectrometry is used to measure 3He formed from beta-decay of 3H. Boron measurements are made concurrently using 4He from the reaction 10B + n = 4He + 7Li. Seven normal donors and seven patients with a diagnosis of bipolar disorder participated in this study. Measurements of lithium and boron were made in whole blood, plasma, and red cells. Red cell-plasma ratios R(Li) and R(B) were calculated after corrections were made for trapped plasma in the red cells. The results show that bipolar patients may have higher concentrations of lithium in blood, plasma, and red cells (p = 0.08, 0.02, and 0.02, respectively) and may have higher R(Li) values than normal donors (p = 0.01). No evidence was found for bipolar-normal differences in these four parameters for boron. Although our sample size is admittedly very small, the results clearly show that the endogenous red cell ratio R(Li) and plasma or red cell lithium concentrations may become useful diagnostic indicators for bipolar illness if the analytical methods are further developed.

Simulation of the interaction of galactic cosmic‐ray protons with meteoroids: On the production of radionuclides in thick gabbro and iron targets irradiated isotropically with 1.6 GeV protons

Abstract— Thick spherical targets made of gabbro (R = 25 cm) and of steel (R = 10 cm) were irradiated isotropically with 1.6 GeV protons at the Saturne synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay in order to simulate the interaction in space of galactic cosmic‐ray (GCR) protons with stony and iron meteoroids. Proton fluences of 1.32 × 1014 cm−2 and 2.45 × 1014 cm−2 were received by the gabbro and iron sphere, respectively, which corresponds to cosmic‐ray exposure ages of about 1.6 and 3.0 Ma. Both artificial meteoroids contained large numbers of high‐purity target foils of up to 28 elements at different depths. In these individual target foils, elementary production rates of radionuclides and rare gas isotopes were measured by x‐ and γ‐spectrometry, by low‐level counting, accelerator mass spectrometry (AMS), and by conventional rare gas mass spectrometry. Also samples of the gabbro itself were analyzed. Up to now, for each of the experiments, ∼500 target‐product combinations were investigated of which the results for radionuclides are presented here. The experimental production rates show a wide range of depth profiles reflecting the differences between low‐, medium‐, and high‐energy products. The influence of the stony and iron matrices on the production of secondary particles and on particle transport, in general, and consequently on the production rates is clearly exhibited by the phenomenology of the production rates as well as by a detailed theoretical analysis. Theoretical production rates were calculated in an a priori way by folding depth‐dependent spectra of primary and secondary protons and secondary neutrons calculated by Monte Carlo techniques with the excitation functions of the underlying nuclear reactions. Discrepancies of up to a factor of 2 between the experimental and a priori calculated depth profiles are attributed to the poor quality of the mostly theoretical neutron excitation functions. Improved neutron excitation functions were obtained by least‐squares deconvolution techniques from experimental thick‐target production rates of up to five thick‐target experiments in which isotropic irradiations were performed. A posteriori calculations using the adjusted neutron cross sections describe the measured depth profiles of all these simulation experiments within 9%. The thus validated model calculations provide a basis for reliable physical model calculations of the production rates of cosmogenic nuclides in stony and iron meteorites as well as in lunar samples and terrestrial materials.

Cosmogenic 3He Accumulation in Alluvial Garnets in Bedload from Central Nepal Rivers: Implications for Cosmic-Ray Exposure Dating of Himalayan Erosional Processes

Cosmogenic isotopes, such as 3He, l~ 26A1, are continuously producted at the Earth's surface in rocks and soils by cosmic ray spallation of light isotopes, providing a powerful tool for dating young geomorphological processes. The datation method based on the accumulation of cosmogenic 3He is particulary appealing because: 3Hec (where suffix c refers to 3He produced by cosmic rays) is a stable isotope, its production rate is high and its natural background is very low. Moreover, its analysis is sensitive, relatively simple and rapid. However, the applicability of this method is limited by the high diffusivity of 3He. In practice, 3He~ is quantitatively retained in olivine phenocrysts, allowing datation of young basaltic flows. Dung/ and Roselieb (1996) have argued that the diffusivity of helium is lower in garnet than in olivine. The use of garnet for 3He c dating presents the interesting advantage to extend the field of this method to crustal metamorphic domains, and therefore to the case of major orogens. We have measured3Hec in alluvial garnets sampled in bedload from Central Nepal rivers (Fig. 1). They represent 3 different monolithologic catchments. By analysing fractions of several tenths of grains, we integer the complex history of each individual grain and the results obtained are mostly meaningful for comparison between different positions in a river basin. The garnets were formed in gneisses of the High Himalaya Crystalline (HHC) and schists of the Lesser Himalaya (LH) during high grade metamorphism resulting from the Main Central Thrusting (MCT) during Miocene. In this system, uplift rates estimated using the Ar/Ar method on micas, feldspars and monazite in garnet are high between 2 and 4 mm/yr depending on the position with regard to the MCT (Copeland et al., 1991). For 3Heo dating purpose, pyropes and almandines are more appropriate than grossularites et spessartites (Dung/and Roselieb, 1996). Electron probe analysis showed that the mean composition of the garnets was Aim. 70%, Pyr. 15%, Gros. 7%, Spes. 5%, Andr. 3% except for sample MO38 wich showed a larger contribution of the grossular end-member (Alm. 54%, Pyr. 12%, Gros. 26%, Spes. 6%, Andr. 2%). Helium was extracted using a CO2 laser from mgsized pure garnet fractions and its amount and isotopic ratio measured with a high sensitivity rare gas mass spectrometer in Nancy (SHe = 3.5 • 10 -4 A/Torr, He blank = 2 x 10 15 mol). Helimn-3 present in minerals is the sum of the following potential contributions: 3Her 3He c + 3Hea + 3Hen + 3He m (where suffixes t, a, n and m refer to total, atmospheric, nucleogenic, and mantle-derived). The parent rocks of metamorphic garnets are thought to be marine sediments, mostly turbidites, which makes the possible contribution of mantle-derived 3He m unlikely. In agreement with that, He isotopic ratios measured in hot springs along the MCT are show typical radiogenic values (Marty et al., 1996). 3Hea was estimated from the measured 2~ content. The contribution of nucleogenic 3He in garnet, produced by activation of Li by natural thermal neutrons, was computed from ion probe analysis of Li in garnet and from U and Th analysis &garnets (1CP-MS), or from estimate of U, Th content of parent rocks. In all cases the atmospheric and nucleogenic 3He contributions were found to be small with respect to the total 3He content of the samples.

Rate constants for the de-excitation of the bending vibrational levels of NCO(X) by helium, neon, argon, krypton and xenon

Rate constants at room temperature (ca. 295 K) are reported for the de-excitation of the (0, 1, 0) and (0, 2, 0) bending vibrational levels of the NCO radical in its ground electronic state. Radicals were produced by infrared multiple-photon dissociation of phenyl isocyanate and their relaxation as a function of time after the photolysis pulse was followed by laser-induced fluorescence. Rate constants were found to be independent of the vibronic components of each level. For the (0, 1, 0) level, values, in units of 10–13 cm3 molecule–1 s–1, were found to be 5.40 ± 0.7, 1.2 ± 0.13, 4.7 ± 0.9, 9.8 ± 1.4 and 21.7 ± 6.1 for the collision partners He, Ne, Ar, Kr and Xe, respectively. For the (0, 2, 0) level, de-excitation rate constants were approximately a factor of 2 higher than these, showing the same trend with mass of rare gas. The ordering of the rate constants, Ne < He ≈ Ar < Kr < Xe, is not that expected from either Schwartz–Slawsky–Herzfeld (SSH) or vibrational close coupling infinite order sudden approximation (VCC-IOSA) theories, and a non-adiabatic mechanism is proposed to explain the results.

Triply charged argon clusters: production and stability (appearance energy and appearance size)

We report the first well resolved rare gas mass spectrum showing singly, doubly and triply charged cluster ions. The critical size deduced for triply charged argon ions is Ar 3+ 226. This result is compared with theoretical predictions. We have also studied the threshold electron energy dependence of doubly and triply charged cluster ions giving insight into the electron ionization mechanism. A comparison between mass spectra of Ar n + and Ar n 2+ (with 100 ⩽ n ⩽ 160) gives new evidence that magic numbers are only due to the intrinsic stability of the respective ion.

Improved charcoal trap for rare gas mass spectrometry

A charcoal trap for progressive desorption of rare gases, including helium and neon, is described and performance data given. Cooling is by a commercial helium expansion refrigerator. Temperatures are readily controlled down to about 20 K. No cold, condensed gases are used.

The excitation functions of Ba (p, X) M Xe (M — 124-136) in the energy range 38-600 MeV; the use of ‘cosmogenic’ xenon for estimating ‘burial’ depths and ‘real’ exposure ages

Thin target experiments were performed to obtain the excitation functions of the reactions Ba (p, X) MX.e. The abundances of all stable Xe isotopes and of the radionuclides 127Xe and 131Ba were determined by means of rare gas mass spectroscopy and y -counting, respectively. The excitation functions show marked characteristics leading to strong variations in the proton-induced Xe-ratios as functions of energy. The 131Xe/126Xe ratio - the special lunar anomaly-was found to vary from 1.14 ±0.4 (600 MeV) to 248.8 ± 4.0 (75 MeV). The Reproduction rates and the MXe/126Xe ratios as functions of depths were estimated for 271 geometry utilizing the depth dependent galactic-cosmic-ray (g.c.r.) fluxes of Reedy &amp; Arnold (1972). Substantial isotopic variations for all of the proton-induced Xe ratios were found, sufficient to explain most of the cosmogenic Xe ratios (exceptions are 130Xe, and 132Xe) measured yet in lunar samples with proton-induced reactions on Ba at different depths in the Moon. Actual lunar samples are used to check the validity of the results.