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At least 145 records · Page 8

Cross-section measurement of the 82 Kr ($p,γ$) 83 Rb reaction in inverse kinematics

The total cross section of the 82 Kr(p,γ) 83 Rb reaction was measured for the first time at effective center-of-mass energies between 2.4 and 3.0 MeV, within the relevant Gamow window for the astrophysical γ process. The experiment took place at the National Superconducting Cyclotron Laboratory at Michigan State University using the ReA facility. A 82 Kr beam was directed onto a hydrogen gas cell located at the center of the Summing NaI(Tl) (SuN) detector. Here, the obtained spectra were analyzed using the γ-summing technique and the extracted cross section was compared to standard statistical model calculations using the non-smoker and talys codes. The comparison indicates that standard statistical model calculations tend to overproduce the cross section of the 82 Kr(p,γ) 83 Rb reaction relative to the experimentally measured values. Furthermore, the experimental data were used to provide additional constraints on the nuclear level density and the γ-ray strength function used in the statistical model calculations.

59 ≤ A ≤ 89↗

Spin fluctuations, absence of magnetic order, and crystal electric field studies in the Yb 3+ -based triangular lattice antiferromagnet Rb 3 ⁢Yb⁢(VO 4 ) 2

Here, we report a comprehensive experimental investigation of the structural, thermodynamic, static, and dynamic properties of a triangular lattice antiferromagnet Rb 3 ⁢Yb(VO 4 ) 2 . Through the analysis of magnetic susceptibility, magnetization, and specific heat, complemented by crystal electric field (CEF) calculations, we confirm the Kramers' doublet with effective spin 𝐽 eff = 1/2 ground state. Magnetic susceptibility and isothermal magnetization analysis reveal a weak antiferromagnetic interaction among the 𝐽 eff = 1/2 spins, characterized by a small Curie-Weiss temperature (𝜃$^{\textrm{LT}}_{\textrm{CW}}$ ≃−0.26 K) or a reduced exchange coupling (𝐽/𝑘 B ≃ 0.18 K). The 51 V NMR spectra and spin-lattice relaxation rate (1/𝑇 1 ) show no evidence of magnetic long-range-order down to 1.6 K but reflect strong influence of CEF excitations in the intermediate temperatures. At low temperatures, 1/𝑇 1 ⁡(𝑇) shows pronounced frequency dependence and 1/𝑇 1 vs field at different temperatures follows the scaling behavior, highlighting the role of paramagnetic fluctuations. The CEF calculations using the point charge approximation divulge a large energy gap ( ∼18.61 meV) between the lowest and second lowest energy doublets, further establishing Kramers' doublet as the ground state. Our calculations also reproduce the experimental magnetization and specific heat data and indicate an in-plane magnetic anisotropy. These findings position Rb 3⁢ Yb(VO 4 ) 2 as an ideal candidate to explore intrinsic quantum fluctuations and possible quantum spin-liquid physics in a Yb 3+ -based triangular lattice antiferromagnet.

Sebastian, Sebin J. [Indian Institute of Science E↗

Deciphering Microbial Metal Toxicity Responses using RB-TnSeq and Activity-Based Metabolomics

To uncover metal toxicity targets and defense mechanisms of the facultative anaerobe Pantoea sp. strain MT58 (MT58), we used a multiomic strategy combining two global techniques, random bar code transposon site sequencing (RB-TnSeq) and activity-based metabolomics. MT58 is a metal-tolerant Oak Ridge Reservation (ORR) environmental isolate that was enriched in the presence of metals at concentrations measured in contaminated groundwater at an ORR nuclear waste site. The effects of three chemically different metals found at elevated concentrations in the ORR contaminated environment were investigated: the cation Al 3+ , the oxyanion CrO 4 2- , and the oxycation UO 2 2+ . Both global techniques were applied using all three metals under both aerobic and anaerobic conditions to elucidate metal interactions mediated through the activity of metabolites and key genes/proteins. These revealed that Al 3+ binds intracellular arginine, CrO 4 2- enters the cell through sulfate transporters and oxidizes intracellular reduced thiols, and membrane-bound lipopolysaccharides protect the cell from UO 2 2+ toxicity. In addition, the Tol outer membrane system contributed to the protection of cellular integrity from the toxic effects of all three metals. Likewise, we found evidence of regulation of lipid content in membranes under metal stress. Individually, RB-TnSeq and metabolomics are powerful tools to explore the impact various stresses have on biological systems. In this work, we show that together they can be used synergistically to identify the molecular actors and mechanisms of these pertubations to an organism, furthering our understanding of how living systems interact with their environment.

59 BASIC BIOLOGICAL SCIENCES↗

Bi-chromatic intensity squeezing using four-wave mixing in 85 Rb vapor

We experimentally investigate four-wave mixing (FWM) in a diamond interaction scheme using 85 Rb vapor, and identify the optimal conditions for joint amplification and relative intensity squeezing of two optical fields: one near the 85 Rb D1 optical transition (λ = 794.6 nm) and the other in the telecom O-band (λ = 1324 nm). We achieved a reduction of relative intensity noise by up to 2.6 ± 0.4 dB compared with the shot noise level, signifying the non-classical quantum correlations. The observed level of intensity squeezing is primarily limited by the available pump laser power, which constrains the achievable FWM gain. Numerical simulations show good agreement with the experimental results.

74 ATOMIC AND MOLECULAR PHYSICS↗

Third-harmonic generation in phase-matched Rb vapor.

The generation of 0.3547-micron radiation was performed by tripling 1.064-micron radiation in a phase-matched mixture of rubidium vapor and xenon. A third-harmonic nonlinear susceptibility in Rb vapor is confirmed. It is about 100,000 times greater than that of He. This large susceptibility is due to the large oscillator strengths and resonant enhancements resulting from Rb transitions in the near infrared, visible, and UV.

Young, J. F.↗

Rb-Sr systematics of Luna 20 and Apollo 16 samples.

For the soils of two lunar highland sites from which samples were returned by the Luna 20 probe and the Apollo 16 mission, Rb-Sr data are presented. For comparison, the results of some additional analyses of Apollo 15 soils are also given. All these soils indicate that an early lunar differentiation process occurred in the interval of 4.3-4.6 b.y. and resulted in the formation of a crust rich in K, Rb, U, and Th.

Papanastassiou, D. A.↗

Rb-Sr systematics for chemically defined Apollo 14 breccias.

The Rb and Sr systematics were determined for eight lithic fragments from the coarse fines of Apollo 11, Apollo 12, and Apollo 14; for bulk soils from Luna 16, Apollo 14, and Apollo 15 and for 'whole rocks' from Apollo 14 and the Apollo 15 anorthosite 15415. The objective of the studies was to identify important lunar rock types by their major and trace element chemistry and utilize Rb and Sr systematics to obtain a chronology for events which establish or modify rock chemical composition.

Nyquist, L. E.↗

Short-term frequency stability of the Rb-87 maser.

Measurements of the short-term stability of the Rb-87 maser as a function of the maser power output and the receiver cutoff frequency are reported. The experimental data are compared to theoretical results obtained from an approximate theory. In this theory the transfer function of the maser for thermal noise is derived, and the spectral density of the phase fluctuations is calculated. An analytical expression for the 'Allan variance' is also given. A comparison of the stability of the Rb-87 maser with existing frequency standards shows its superiority for averaging times less than 1 sec.

Tetu, M.↗

Light shift effects in the Rb-87 maser

Previous work has shown the possibility to overcome the dependence of the Rb-87 maser frequency on light intensity by tuning the cavity at a proper setting. The conditions for this setting, called the light-independent frequency setting (LIFS), are carefully investigated. The results presented prove the existence of the LIFS and provide a new criterion for an automatic cavity tuning of the Rb maser.

Busca, G.↗

How to lose Rb, K, and change the K/Rb ratio - An experimental study

Thermal volatilization studies on Apollo 16 soils and crystalline rocks have been carried out to measure the depletion of K, Rb, and Na and changes in K/Rb ratios during heating under vacuum. Rubidium can be lost from both soils and rocks at temperatures as low as 1000 C. Because of the greater volatility of Rb as compared to K, the K/Rb ratio can be changed from normal values of 330-360 for the Apollo 16 samples to values as high as 1100 by thermal volatilization. Rapid heating experiments on soil samples were carried out to approximate agglutinate formation conditions in order to measure changes in alkali element abundances. Equal changes in K/Rb ratios can be produced with longer heating at lower temperatures or heating at higher temperatures for shorter times.

Gibson, E. K., Jr.↗

Rb-Sr systematics for chemically defined Apollo 15 and 16 materials

Concentrations of Rb and Sr and the Sr87/86 ratios are determined in a large number of Apollo 15 samples, including KREEP basalts, mare basalts, anorthosite breccia clasts, lithic fragments, a spinel-bearing clast, green glass samples, breccia matrix, and soils. Relative concentrations of Sm, Eu, Rb, and Sr are also examined in the samples, with particular attention to Rb/Sr systematics vs other trace element abundances and total chemical compositions of rocks. Analysis of crystalline KREEP samples rich in trace elements indicates that the differentiation of these rocks has extended to -4.25 AE age, a significantly later time than the presumed time of accretion of the moon.

Nyquist, L. E.↗

Rb, Sr and strontium isotopic composition, K/Ar age and large ion lithophile trace element abundances in rocks and glasses from the Wanapitei Lake impact structure

Shock metamorphosed rocks and shock-produced melt glasses from the Wanapitei Lake impact structure have been examined petrographically and by electron microprobe. Eleven clasts exhibiting varying degrees of shock metamorphism and eight impact-produced glasses have been analyzed for Rb, Sr and Sr isotopic composition. Five clasts and one glass have also been analyzed for large ion lithophile (LIL) trace element abundances including Li, Rb, Sr, and Ba and the REE's. The impact event forming the Wanapitei Lake structure occurred 37 m.y. ago based on K/Ar dating of glass and glassy whole-rock samples. Rb/Sr isotopic dating failed to provide a meaningful whole-rock or internal isochron. The isotopic composition of the glasses can be explained by impact-produced mixing and melting of metasediments.

Winzer, S. R.↗

Rb-Sr and Sm-Nd chronology and genealogy of mare basalts from the Sea of Tranquility

Rb-Sr and Sm-Nd ages of two Apollo 11 mare basalts, high-K basalt 10072 and low-K basalt 10062, are reported. Rb-Sr, Sm-Nd, and Ar-40-Ar-39 ages are in good agreement and indicate an extensive time interval for filling of the Sea of Tranquility, presumably by thin lava flows, in agreement with similar observations for the Ocean of Storms. Initial Sr and Nd isotopic compositions on Apollo 11 basalts reveal at least two parent sources producing basalts. The Sm-Nd isotopic data demonstrate that low-K and high-Ti basalts from Apollo 11 and 17 derived from distinct reservoirs, while low-Ti Apollo 15 mare basalt sources have Sm/Nd similar to the sources of Apollo 11 basalts. Groupings of mare basalt based on Ti content and on isotopic data do not coincide.

Papanastassiou, D. A.↗

More on Rb-Sr in lunar breccia 14321

The reported Rb-Sr studies demonstrate a complex heterogeneity of lunar polymict breccia 14321 and exclude significant thermal metamorphism of 14321 in a hot ejecta blanket. While basalt clast 14321,371 is found to have an age of about 4.0 b.y., corresponding to the previously determined time of internal equilibration for another sample, the results for two microbreccia clasts from 14321,554 and 14321,220 indicate the lack of complete equilibration at that time. Light matrix samples are indistinguishable from the basalt with respect to K, Rb, and Sr, and, in contrast to the matrix from subsample 14321,184, the light matrix samples contain no appreciable KREEP-rich component.

Mark, R. K.↗

Rb-Sr studies of CI and CM chondrites

Rb-Sr whole rock analyses have been performed on 2 CI and 3 CM chondrites. Four of these stones (Ivuna, Orgueil, Cold Bokkeveld and Erakot) were previously studied in this laboratory and were shown to be discordant from a 4.6 Gyr isochron. The fifth, Murchison, was not previously studied. The new data support the discordance of the first four stones, and indicate that Murchison is also discordant. Studies of Sr isotope ratios in unspiked Orgueil show that the discordance is not due to inhomogeneities in the Sr-84/Sr-86 ratio caused by incomplete mixing of nucleosynthesis products. In order to gauge the effects of weathering, two leaching experiments were performed on fresh, interior samples of Murchison; one for a period of 1.5 hr and the other for 117 hr. The results indicate that the relative solubility of nonradiogenic Sr is approximately twice that of Rb and radiogenic Sr is more soluble than the nonradiogenic Sr.

Mittlefehldt, D. W.↗

Rb-Sr age of the Shergotty achondrite and implications for metamorphic resetting of isochron ages

The age of the Shergotty achondrite is determined by Rb-Sr isotope analysis and the metamorphic resetting of isochron ages, which is presumed to have occurred during a shock event in the history of the meteorite, is discussed. The isochron best fitting the Rb-Sr evolution diagram is found to correspond to an age of 165 million years, with an initial Sr-87/Sr-86 value of 0.72260. Different apparent ages obtained by the K-Ar and Sm-Nd methods are interpreted in terms of a model which quantifies the degree of resetting of internal isochron ages by low temperature solid state diffusion. On the basis of these considerations, it is concluded that Shergotty crystallized from a melt 650 million years ago, was shock heated to 300 to 400 C after its parent body was involved in a collision 165 million years ago, and was first exposed to cosmic rays two million years ago.

Nyquist, L. E.↗