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Theoretical and experimental investigation of Sona transitions
No full textIn 1967, P. G. Sona proposed a method to increase nuclear polarization in polarized hydrogen and deuterium ion sources. The technique uses a static magnetic field produced by two opposing solenoid coils. Applying a semi-classical approach, he designed the magnetic field distribution of the device. It consists of a longitudinal (parallel to the beam axis) and a transverse component with axial symmetry. One of the requirements for an efficient performance is a non-adiabatic passage through the low field region. This leads to the zero-crossing requirements, which render Sona transitions inapplicable to thermal sources. For more than 50 years, Sona transitions have been implemented to several experiments involving various types of ground-state and metastable sources (e.g. H, D, \textsuperscript{3}He, etc.). However, an unanticipated behavior of polarization was observed in two of them. The present study provides experimental and theoretical insights into this effect. For this purpose, an experimental setup, consisting of components of a Lamb-shift polarimeter, has been developed. This can be applied to metastable hydrogen and deuterium sources. The recorded signals reveal that there is an interaction between the hyperfine substates of the atoms, which results in transitions between them. The transitions cannot be described by Sona's work and they are observed even while the zero-crossing conditions are fulfilled. Thus, a purely quantum mechanical treatment is employed to explore the spin dynamics in a Sona transition unit. In particular, the theoretical analysis is realized by means of time-dependent perturbation theory applied in the rest frame of the atoms. The system of coupled first order differential equations for the interplay between the states is derived analytically in the hyperfine regime. Its solution is solely feasible with numerical methods. The dependence of the results on the magnetic field configuration and the beam characteristics (kinetic energy, size, position, etc.) is also examined. The simulated and recorded signals are compared and show a very good correlation. Small deviations are noticed, but they are mainly caused by the experimental instabilities. Upgrades to the production of polarized particles are in progress with the intent of improving beam intensity and stability. A new Sona field configuration will be developed with a sinusoidal distribution in order to simplify the study of spin dynamics and the field parameterization in the simulation program. These two improvements, along with the development of data analysis code, can lead to performing precision measurements with such an experimental setup. Eventually, the proposed concept could serve as a spectroscopic tool for testing QED corrections in H or D atoms and more complex atomic/molecular/ionic systems
Theoretical and Experimental Investigation of Sona Transitions
No full textIn 1967, P. G. Sona proposed a method to increase nuclear polarization in polarizedhydrogen and deuterium ion sources. The technique uses a static magnetic field producedby two opposing solenoid coils. Applying a semi-classical approach, he designedthe magnetic field distribution of the device. It consists of a longitudinal (parallel tothe beam axis) and a transverse component with axial symmetry. One of the requirementsfor an efficient performance is a non-adiabatic passage through the low fieldregion. This leads to the zero-crossing requirements which render Sona transitionsinapplicable to thermal sources.For more than 50 years, Sona transitions have been implemented to several experimentsinvolving various types of ground-state and metastable sources (e.g. H, D, 3He,etc.). However, an unanticipated behavior of polarization was observed in two ofthem. The present study provides experimental and theoretical insights into this effect.For this purpose, an experimental setup, consisting of components of a Lambshiftpolarimeter, has been developed. This can be applied to metastable hydrogenand deuterium sources. The recorded signals reveal that there is an interaction betweenthe hyperfine substates of the atoms which results in transitions between them.The transitions cannot be described by Sona’s work and they are observed even whilethe zero-crossing conditions are fulfilled.Thus, a purely quantum mechanical treatment is employed to explore the spin dynamicsin a Sona transition unit. In particular, the theoretical analysis is realized bymeans of time-dependent perturbation theory applied in the rest frame of the atoms.The system of coupled first order differential equations for the interplay between thestates is derived analytically in the hyperfine regime. Its solution is solely feasible withnumerical methods. The dependence of the results on the magnetic field configurationand the beam characteristics (kinetic energy, size, position, etc.) is also examined.The simulated and recorded signals are compared and show a very good correlation.Small deviations are noticed but they are mainly caused by the experimental instabilities.Upgrades to the production of polarized particles are in progress with the intent ofimproving beam intensity and stability. A new Sona field configuration will be developedwith a sinusoidal distribution in order to simplify the study of spin dynamicsand the field parameterization in the simulation program. These two improvements,along with the development of data analysis code, can lead to performing precisionmeasurements with such an experimental setup. Eventually, the proposed conceptcould serve as a spectroscopic tool for testing QED corrections in H or D atoms andmore complex atomic/molecular/ionic systems
Development of polarized sources based on molecular photodissociation
No full textMolecular photodissociation is an innovative method for the preparation of polarized atoms and molecules. It is a fundamental chemical process that involves the absorption of one or more polarized photons by a molecule including its fragmentation into polarized atomic (or molecular) fragments. Recently, T. P. Rakitzis’ group produced high densities of spin polarized hydrogen atoms applying molecular photodissociation to hydrogen halides. The obtained densities (10^19 cm^−3) and short production times (ns timescales) surpass by several orders of magnitude conventional methods such as spin-exchange optical pumping and Stern-Gerlach spin separation. These density and time regimes make it an ideal candidate for a broad range of applications, e.g. laser-induced acceleration from polarized gas targets and polarized five-nucleon fusion reactions (D-T, D-3He). The second has been shown to have an increased cross section by ∼50% compared to the unpolarized cas
A NEW TYPE OF SPECTROSCOPY:Direct observation of hyperfine transitions with energy differences of 10 neV and below
No full textSpectroscopy is a tool commonly used for the study of the energy levels of a sample. In most applications the sample is trapped, however this is not always feasible. An alternative type of spectroscopy includes a static external field and a moving sample. In particular, we use two opposed solenoidal coils which provide a static magnetic field with field direction reversal along the polarization axis. This produces a sinusoidal longitudinal (along the quantization axis) magnetic field component with a zero crossing between the coils. In addition to the longitudinal component, a radial component is also induced which is proportional to the gradient of the first and the distance from the center of the quantization axis.For an atomic beam of metastable hydrogen with a kinetic energy of about 1 keV and a magnetic field configuration with a wavelength lambda ~ 10 cm, the inducedtransitions correspond to an RF frequency f = v/lambda in the MHz range. Equivalently, the energy difference between various levels is of the order of 10^−8 eV andbelow. These can be found between hypefine substates of hydrogen atoms at low magnetic fields in the Breit-Rabi diagram. Here we present first measurements,their interpretation, and possible applications
Macroscopic production of highly nuclear-spin-polarized molecules from IR-excitation and photodissociation of molecular beams
No full textPure, highly nuclear-spin-polarized molecules have only been produced with molecular beam-separation methods, with production rates up to s^-1. Here, we propose the production of spin-polarized molecular photofragments from the IR-excitation and photodissociation of molecular beams, with production rates approaching the tabletop-IR-laser photon fluxes of s^-1. We give details on the production of spin-polarized molecular hydrogen and water isotopes, from formaldehyde and formic acid beams, respectively. Macroscopic quantities of these molecules are important for NMR signal enhancement, and for the needs of a nuclear fusion reactor, to increase the D-T or D-3He unpolarized nuclear fusion cross section by 50%
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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