Xenon saturation spectroscopy by diode laser

Summary Commercial diode lasers have emission wavelengths which overlap some absorption xenon lines. We have performed saturation spectroscopy of four lines, leaving from the first excited atomic configuration 5p ⁵6s, at 823.16 nm, at 828.01 nm, at 834.68 nm and at 820.63 nm, in a weak glow discharge of natural xenon. Natural xenon is a mixture of several isotopes, and the two even isotopes, Xe(129) (26% of relative abundance) and Xe(131) (21%), have a nuclear spin (1/2 and 3/2, respectively) that produces a hyperfine structure. The complex resulting spectra have been resolved and the results are compared with the available literature data. The authors of this paper have agreed to not receive the proofs for correction.     

Messung einiger (n, γ)-Spektren mit einem Paarspektrometer hoher Empfindlichkeit

A pairspectrometer of the 180°-focussing type with high efficiency and good resolution is described. The efficiency has been determined in function of the energy, both theoretically and experimentally and it is possible, therefore, to use this spectrometer for absolute calibration of gamma rays above 2·5 MeV. The evaluation methods of the pair spectra allow the single line to be determined with an error as little as ±8 keV (from 2·5 to 9 MeV). We have measured the neutron capture gamma spectrum of J¹²⁷, Cs¹³³, Ir^191,193, Bi²⁰⁰ and given the energy and intensity of the resolved gamma rays. From these results it is possible to compute the binding energy of the last neutron and also some unknown levels. The experiments, which have been performed in connection with the pairspectrometer, have shown an interesting possibility for research work with reactors. With an appropriate experimental arrangement and some special neutron capture gamma spectra, monochromatic gamma rays in the energy range between 3 and 11 MeV are obtained, which are better and much more intense as corresponding gamma rays from (p, γ) reactions.     

Excitation and neutron decay of xenon studied by forward-angle scattering of high-energy 17O ions

Excitation energy spectra and neutron decay of xenon have been measured in extreme forward angle scattering of 250A and 400A MeV ¹⁷O ions, using one quadrant of the CELSIUS storage ring as a magnetic spectrometer. The observed excitation energy spectrum, ranging from 12 to 36 MeV, has been compared with Coulomb cross section calculations. Neutron and missing energy spectra are compared with statistical decay calculations using the code CASCADE. In addition to statistical decay from giant resonances and other continuum excitations, a large fraction of forward peaked fast neutrons was observed throughout the entire excitation energy region. A possible reaction mechanism behind these fast neutrons is discussed.     

The determination of theK-shell fluorescence yield of xenon by use of the proportional counter method

The wall-less proportional counter filled with propane with a small admixture of xenon was used as a spectrometer of electrons emitted by xenon atoms when photoelectrically excited by americium-241γ-rays and erbiumKα rays. TheK-shell fluorescence yield of xenon as extracted from spectra ammounts toω _k=0.889(1±0.01).     

Calculation of the parameters of vacuum-ultraviolet emission of excimers in the plasma of a barrier discharge in a krypton-xenon mixture

The intensities of vacuum-ultraviolet emission of excimers in a krypton-xenon mixture are calculated in terms of the homogeneous model of a barrier discharge. The mechanisms of the formation of excimer molecules are investigated at different xenon concentrations. The obtained dependences of the efficiency of vacuum-ultraviolet emission on the reduced electric field E/N (where E is the electric field strength and N is the particle concentration of the gas) and the xenon content exhibit a plateau and a subsequent steep decline. The efficiency of vacuum-ultraviolet emission reaches a maximum at the plateau when the xenon content δ is approximately equal to 40%. The concentrations of xenon excimers can be as high as 10¹⁴–10¹⁵ cm^?3. This is comparable to the concentrations in a barrier discharge in pure xenon.     

Die Reaktion54Xe(n, α)52Te mit Neutronen der Energie 12,5 bis 18,0 MeV

The reaction₅₄Xe(n, α)₅₂Te was investigated in the neutron energy range 12.5 to 18.0 MeV. A high pressure gas scintillator filled either with pure xenon of natural isotopic abundance or with a mixture of xenon and helium was used as a target and as a detector simultaneously. The helium served as a monitor, (n, α)-spectra were measured and analyzed by evaporation theory. Large components of direct processes were found. The level density parameter of⁵²Te was determined as (17.0±1.7) MeV^?1. The cross section excitation function is given as well as the branching ratios for evaporation and direct processes as a function of neutron energy.     

VUV spectrum of the barrier discharge in a krypton-xenon mixture

VUV spectra of a barrier discharge in a krypton-xenon mixture at the pressure P=400 hPa were experimentally studied. The xenon addition was varied in a range of 0.01–1%. The supply voltage frequency was 20 kHz. The form of spectra and their evolution at different xenon impurity concentrations were analyzed by solving the system of nonlinear differential equations simulating the main processes taking place in the active phase of a discharge and in the afterglow. The analysis supports the results obtained earlier and qualitatively describes the experimental spectra.     

The KrXe molecule: Modification of potential curves of the ground and first excited electronic states

Potential curves of the ground state of the KrXe molecule and its excited states that converge to the Kr(4p ^{6 1} S ₀) + Xe(5p ⁵6s ³ P ₁) atomic states are corrected and tested using the results of modeling published vacuum ultraviolet spectra of a gas-discharge plasma of a krypton/xenon mixture.     

Simulation of emission spectra of the gas-discharge plasma of a krypton-xenon mixture

The narrow-band radiation observed in the range of the resonance line of xenon at 147 nm in the VUV emission spectra of the gas-discharge plasma of a krypton-xenon mixture is proposed to interpret as a manifestation of bound-bound transitions between the vibrational levels of the excited electronic states 0⁺(³ P ₁) and 1(³ P ₁) and the ground electronic state 0⁺(¹ S ₀) in the KrXe* molecule. A correction of the potential curves of the electronic states under consideration is proposed from a comparison of the calculated and experimental spectra.     

A Superconducting Quantum Interference Device Measurement System for Ultra Low-Field Nuclear Magnetic Resonance

We describe a superconducting quantum interference device (SQUID)-based nuclear magnetic resonance (NMR) spectrometer operating at ultralow magnetic fields far below the Earth’s field. The spectrometer consists of a helium-cooled magnetic sensor system and two Helmholtz coils, one for pre-polarizing the sample by fields of up to 5 mT, and one for the detection in fields of the nanotesla and microtesla range. The spectrometer represents the current state of the art in ultralow-field NMR and enables the observation of phenomena that are difficult or impossible to achieve by a conventional NMR setting. In particular, one can obtain broad band spectra covering different nuclei, such as ¹H and ³¹P, with a frequency resolution in the millihertz range, observe the variation of their heteronuclear coupling with the detection field strength, and investigate relaxation processes that reflect molecular dynamics in the millisecond range.     

Diode-laser spectroscopy of supersonic free jets

A supersonic-free-jet infrared spectrometer has been constructed for investigation of molecular vibrational spectra at low rotational and vibrational temperatures. The sensitivity of measurement in a pulsed jet is increased by employing a phase-sensitive detection method synchronized with the pulse frequency. The performance of the spectrometer is examined for the absorption lines of the NH₃ v ₂ band. A rotational temperature as low as 16K is attained when seeded in He. Cold-jet spectra are demonstrated for thev ₃ bands of PF₅,³⁴SF₆, and¹⁸²WF₆.     

Gamma ray and π0 production from hydrogen and complex nuclei by 270–375 MeV gamma rays

Measurements are reported on the differential cross section for secondary gamma ray production on hydrogen and nuclear targets at 90° in the laboratory through the interactions of primary gamma rays in the energy range 270–375 MeV. A difference method using bremsstrahlung beams at different end point energies was employed. The gamma ray detector was a high resolution Nal(Tl) spectrometer and time of flight discrimination was used to reject neutrons. For incident gamma rays in the range 270–375 MeV the first pion nucleon resonance is strongly excited and the observed gamma ray yields are consistent with those expected due to an admixture of coherent and incoherent π⁰ photoproduction in the resonance region. The experiment marks the first successful use of a large Nal(Tl) crystals as a gamma ray spectrometer at a high energy electron linear accelerator, despite the low duty cycle and its accompanying difficulties for such detectors.     

Luminescence spectra of solid xenon excited by volume discharge

The luminescence spectra of solid xenon excited by a volume discharge are studied for the first time. The majority of the energy is emitted in the VUV spectral region due to the radiative decay of excitons. A specific feature of the spectra in the UV and visible regions is the complete absence of lines belonging to neutral excited particles (one-center Xe* and two-center Xe ₂ ^* excitons) and to diatomic Xe ₂ ⁺ ions. Numerous lines of the Xe⁺ ion are shifted by 0.1–0.3 nm with respect to their positions in the gas; the larger the shift, the stronger the distortion of the line shape. Based on the data obtained, a mechanism by which solid xenon is excited and ionized by fast drifting electrons is proposed.     

Millimeter-wave spectrum of BrCN produced by dc discharge

Ground state (v=0) and first excited state (v=1) millimeter-wave rotational absorption spectra of cyanogen bromide (BrCN) and some of its isotopic species, have been investigated in the frequency region: 40.0-75.0 GHz using a source-modulated millimeter-wave spectrometer. Millimeter-wave radiation has been produced using a frequency multiplier, the fundamental radiation source being klystrons. BrCN has been produced by applying a dc glow discharge through a mixture of 3-bromobenzonitrile and trifluoromethylbromide (CF₃Br) at low pressure. The quadrupole hyperfine structure of ⁸¹Br and ⁷⁹Br have been resolved, measured, and analyzed. Finally, internuclear distances of BrCN have been determined.     

Influence of 60Co gamma irradiation on the structural and optical properties of 2-aminopyridinium 4-nitrophenolate 4-nitrophenol crystals

In this article, effect of gamma irradiation on the structural and optical properties of 2-aminopyridinium 4-nitrophenolate 4-nitrophenol (2AP4N) has been reported. The grown crystals of 2AP4N were exposed to ⁶⁰Co gamma rays with a dose of 50 kGy and 100 kGy. The radiation-induced effects were analyzed using X-ray diffraction, FT-IR, UV–visible, photoluminescence techniques. The refractive index was determined using a long arm spectrometer. The structural properties of the pristine and irradiated crystals were studied using powder XRD. The peak intensity decrease after irradiation may be attributed to the formation of point defects. The UV visible study reveals that the energy gap has decreased after irradiation and then has increased for the higher dose. The intensity variation in the PL spectra is due to colour center mechanism. The SHG efficiency of 2AP4N crystals was found to be unaffected by gamma irradiation.     

Pulsed-Field-Gradient Measurements of Time-Dependent Gas Diffusion

Pulsed-field-gradient NMR techniques are demonstrated for measurements of time-dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O₂, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 × 10⁻⁶m²s⁻¹for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-field-gradient NMR techniques is demonstrated by the first measurement of time-dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo, or PGE, and the Pulsed Gradient Multiple Spin Echo, or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times.     

On the magnetization process in an ising-spin lattice

Gamma ray spectra of a mixture of the isotopes⁸⁶Br and⁸⁷Br have been measured by means of a Ge(Li) detector. Both manual and automated techniques were used to separate bromine from other fission products of natural uranium. A total of 13 gamma rays were observed. The most intense of the⁸⁶Br gamma peaks (1,565 keV) was found to grow in with a half-life of 4.5±1 s. This is explained to be due to an isomeric state in⁸⁶Br decaying to the ground state.     

Positron spectra from internal pair conversion observed in 238U + 181Ta collisions

We present new results from measurements and simulations of positron spectra, originating from ²³⁸U + ¹⁸¹Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e⁺-energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed γ-transitions ( E _γ∼ 1250-1600 keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed. Received: 27 April 2000 / Accepted: 14 September 2000     

Intravascular delivery of hyperpolarized129Xenon forin vivo MRI

The nuclear polarization of¹²⁹Xe and³He can be enhanced by several orders of magnitude by using optical pumping techniques, thus allowing NMR detection of xenon and helium in very low concentrations. The benefits of optically enhanced magnetic resonance (MR) are already exploited in MR imaging of the lungs using optically polarized³He. The high solubility of xenon in blood and lipids suggests a variety ofin vivo MR applications, for instance perfusion measurements or functional MR studies. This article reviews some current work directed towards delivery of optically polarized xenon forin vivo MR applications.