Modulation Curve Research Articles

Optical performances of the Ultraviolet Coronagraph Spectrometer of the Solar Heliospheric Observatory

The optical performances of the spectrometer assembly for the Ultraviolet Coronagraph Spectrometer of the Solar and Heliospheric Observatory mission have been tested. The flight unit of the spectrometer assembly, consisting of the structure equipped with the entrance slits assembly, the grating drive mechanisms mounting two toroidal gratings, and the photon-counting detectors, has been integrated and aligned; also the flight unit of the White Light Channel has been integrated and aligned in the spectrometer assembly. Tests with both visible and UV radiation have been performed. Aberration and stray-light measurements have shown that the instrument performs satisfactorily, almost in compliance with the scientific requirements; also some measurements of the polarimeter modulation curve and the relative error have shown performances within the specified requirements.

Thermoluminescence in Proton and Fast Neutron Therapy Beams

In photon radiotherapy, thermoluminescence dosemeters (TLD) are commonly used for in vivo dosimetry. In proton and neutron therapy, their use raises a particular difficulty since their responses vary significantly with the energy spectrum. With reference to 60Co, the response of CaF2:Tm (TLD-300) and 7LiF:Mg,Ti (TLD-700) crystals was investigated in the 85 MeV proton beam and the p(65)+Be neutron beam produced at the cyclotron at Louvain-La-Neuve and used for therapy applications. In the proton beam, TLD dosemeters were irradiated in a phantom, at different positions in the unmodulated and modulated Bragg curve. In the fast neutron beam, TLDs were positioned inside the geometrical limits of the beam at different depths, and outside its limits behind five leaves of the multi-leaf collimator. Peak height variation of glow curves was investigated and correlated with the variation of radiation quality.

Polarization state of the emission of external grating diode lasers

The laser emission of transverse electric (TE) and transverse magnetic (TM) polarization, in alternating successive spectral ranges, that can occur when external grating diode lasers are wavelength tuned, is theoretically and experimentally investigated. The modulated threshold gain curves of the external cavity for the two polarizations can be made to intersect with each other if a grating is used in such a way as to practically eliminate the difference between the laser diode losses and confinement factors for TE and TM polarization. The result is a sequence of wavelength ranges where the threshold gain is alternately lower for the TE resp. TM polarization. Within each range, a wavelength tunable single mode laser emission is obtained. The phenomenon is described, taking into account the characteristics of the key components of the external cavity, i.e., laser diode structure, antireflective (AR) coating, and grating efficiency. Design tools are proposed for the characteristics of the components necessary for the development of such an external cavity. The theoretical and design concepts are experimentally confirmed in a number of external cavities, using diode lasers emitting around 1.5 /spl mu/m, with different types of antireflective coatings, as well as different grating configurations. >

Measurements of low magnetic field microwave absorption in 110-K superconducting Bi-Sr-Ca-Cu-O thin films

Studies on magnetic-field dependent microwave absorption in Bi(Pb)-Sr-Ca-Cu-O thin films superconducting above 100 K are reported. The films were DC magnetron sputtered on MgO and CaNdAlO/sub 4/ single crystals from Pb-rich targets and post-annealed at high temperatures in air for less than 1 h. The measurements were performed as a function of temperature, DC magnetic field, and the film magnetic history. The results show that the microwave absorption measurements are a very sensitive tool for high-T/sub c/ thin-film characterization, since even small variations in the film phase homogeneity, granularity, or crystalline orientation are clearly visible on the modulated absorption curves. It was found that the films characterized by the lowest absorption exhibited the highest critical current densities.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Linear codes and modular curves

Results of recent investigations at the juncture of coding theory, the theory of computability, and algebraic geometry over finite fields are presented. The basic problems of the asymptotic theory of codes and Goppa's construction of codes on the basis of algebraic curves are presented, and a detailed algorithmic analysis is given of the codes arising on the modular curves of elliptic modules of V. G. Drinfel'd.

Neutron study of the incommensurate phase in (N(CD3)4)2ZnCl4crystal

Using the neutron scattering technique, the wavevector of the modulation has been carefully measured as a function of temperature in the incommensurate phase of d-TMATC-Zn. The modulation curve is qualitatively compared with a constant- mu Devil's staircase of the one-dimensional model. A great difference appears between the behaviour of h and d compounds, which is attributed to the isotope effect. Integrated intensities of first-order satellites follow a linear law over a 10 degrees C temperature range below the transition temperature Ti=24.7 degrees C. Odd-order satellites have been observed up to the seventh order.

Calculation of pion dose distributions in water.

Techniques for calculating negative pion beam depth and off-axis dose distributions in a water phantom have been developed at the Clinton P. Anderson Meson Physics Facility in Los Alamos. The superposition of the unmodulated depth-dose curve produced modulated depth-dose curves. The addition of the collimator neutron dose, which has been shown to depend on field size, to the modulated depth-dose curve yields the collimated depth-dose distributions. Off-axis dose distributions under a collimator are produced by calculating the distortion of the uncollimated beam caused by multiple Coulomb scattering and beam phase space. Several comparisons of calculated and measured distributions are shown with agreement of normally +/- 3% of peak dose of +/- 3 mm for a particular dose contour. These distribution are them modified by computerized tomographic data to give patient isodose distributions.

Ratio analysis of electron spin echo modulation envelopes in disordered matrices and application to the structure of solvated electrons in 2-methyltetrahydrofuran glass

A new analysis for two pulse and three pulse electron spin echo modulation data is presented which is independent of the echo decay function. The experimental ratios of maxima to minima of the echo modulation curve are compared with simulated ones to obtain the number of interacting nuclei, their distances, and their isotropic coupling by a least squares fit. From the best simulation, the empirical echo decay function can be determined and the original experimental data can be simulated. This new method is applied to new three pulse and old two pulse [L. Kevan et al., J. Chem. Phys. 63, 409 (1975)] electron spin echo data on solvated electrons in 2-methyltetrahydrofuran (MTHF) glass at 77 K. This analysis together with previous second moment data gives a revised model for electron solvation in this matrix based on a statistical orientation of the MTHF ring carbon planes toward the electron. In this statistical configuration model three equivalent MTHF molecules form the first solvation shell with their ring carbon atom planes perpendicular to the electron and oriented statistically. The closest protons are 3.4 Å from the electron with a proton hyperfine constant of 0.4 MHz which is equivalent to a deuteron hyperfine constant of 0.06 MHz.

Spin-echo envelope modulation study of LaF3:Nd3+

A pulsed-electron-spin-resonance experiment has been performed on a single crystal of lanthanum trifluoride containing 0.1 molar per cent of neodymium impurity. The purpose of the experiment was to measure the superhyperfine coupling between the magnetic moments of the ligand fluorine nuclei and the magnetic moment of the $4f$ electronic spin belonging to the neodymium ion. The experiment employed the technique of spin-echo envelope modulation. Modulation envelopes were obtained at five orientations of the laboratory magnetic field. The interaction parameters were determined by comparing the experimentally derived modulation envelopes with the computed theoretical modulation curves. Under the assumption that all of the near nuclei interact with equal strengths, it was found that the isotropic interaction parameter $A=\ensuremath{-}1.5\ifmmode\pm\else\textpm\fi{}0.5$ MHz and that the dipole-dipole or anisotropic parameter $D=4\ifmmode\pm\else\textpm\fi{}1$ MHz. The negative value of $A$ implies that the $4f$ electronic spin in the vicinity of the ligands is parallel to the net spin of the neodymium ion. The value of $D$ that was measured is greater than what would be expected if the interaction were purely of the classical point-dipole type. Therefore, some covalent bonding involving neodymium $f$ orbitals and fluorine $p$ orbitals must be present.

Theory of spatial-frequency filtering by the human visual system I Performance limited by quantum noise*

A theory and model of the visual system are presented to explain the detection of static sinusoidal gratings near the threshold. The model incorporates a set of independent decision centers and associated photoreceptive fields (PRFs). The decision criterion value at each decision center is proportional to the standard deviation of the excitation current transmitted from a PRF to its associated decision center caused by quantum fluctuations in the absorption of light. It is well known that the spatial-frequency-response (SFR) function and the spatial-impulse-response (SIR) function of a photodetector are a Fourier transform pair. A systematic examination of the SIR and SFR functions of PRF configurations consisting of rectangular regions of alternately excitatory and inhibitory response reveals that modulation sensitivity of the visual system is explained at scotopic and photopic illuminance by a set of PRFs composed of a single excitatory region and a central excitatory region bordered by inhibitory regions, respectively. The complete model is shown to yield a high degree of conformity between theoretical and experimental threshold modulation curves.

Index modulation and spatial harmonic generation in dichromated gelatin films

A model for volume hologram formation in thick dichromated gelatin films is given. Using the model and a few basic film measurements, we can quantify the index modulation vs. exposure curve. The effects of saturation and dynamic absorption increase during exposure are considered in the generation of spatial harmonics. The effects of exposure with non-unity beam-ratio and multiple incoherent exposures are predicted and experimentally verified. The thick transmission grating diffraction efficiencies resulting from first-and higher-order index modulations are calculated and compared to experiments. The methods and conclusions are applicable to other phase materials.

Somatic evoked response components and recovery functions in subjects with mu and alpha rhythms.

Subjects with prominent rolandic mu and alpha rhythms showed similar late somatic evoked response (SER) components but different SER recovery functions. A late P5x component was present when SER were superimposed on mu and alpha rhythms spontaneously recorded from the rolandic region of inattentive individuals. Since P5x was not present in other inattentive subjects unless SER were superimposed on a rhythmic rolandic activity evoked by an attentive situation, it seems reasonable to assume that the P5x component is partially due to spontaneous or evoked rolandic EEG rhythmicity. In addition, single propioceptive stimuli produced blocking of mu rhythm and a flat recovery function of the P5a component. In contrast, similar stimuli produced an enhancement of alpha rhythm and a periodic amplitude modulation curve of the P5a component. These results suggest that the P5a component is mediated by the same neurophysiological mechanisms which block or modulate the spontaneous rolandic EEG rhythms.

Modulation of the Desorbed Ion Current in Bayard–Alpert Gauges

The modulation characteristics of a Bayard–Alpert gauge have been investigated in the presence of a large residual current of desorbed ions, following exposure of the gauge to oxygen. Having observed an increase in the collector current on modulation, when the gas phase ion current was relatively small, modulation factors were measured for the desorbed ion current and were found to vary in magnitude up to 0.2. A sharp peak, which increased the amplitude of the mean residual current by approximately 30%, was also found in the modulation curve between Vmg=−10 and −35 V. Computed potential distributions and ion trajectories have shown that this peak can be attributed to the focussing action of the electrostatic field on energetic ions desorbed from the modulator. Experimental verification of this hypothesis was obtained by selective cleaning of the grid and modulator using electron bombardment. The presence of such a peak would produce errors in the proposed alternative modes of modulation in which a modulator-to-grid potential of −20 V is used.

Time impulse response and time frequency response of optical pupils.:Experimental confirmations and applications

Time impulse response and time frequency response of optical pupils.:Experimental confirmations and applications

Fine-Structure Measurements by the Beam-Foil Technique

We have measured fine-structure separations in some $^{3}P$ states of He I ($n=3, 4, 5$) and Li II ($n=4, 5$) by observation of modulated light-intensity decay curves using the beam-foil technique.

Motoneuron excitability in man: A novel method of evaluation by modulation of tonic muscle activity

The excitability of a population of motoneurons in humans has been investigated classically by the recovery curve of the monosynaptic reflex following a conditioning stimulus. The classical technique is applicable only to a few large muscles with prominent stretch reflexes. The inherent variability of the monosynaptic reflex makes this method both arduous and time-consuming. An alternative means of excitability testing is described. The technique is based upon the observation that during tonic activity, motor unit discharge increased with facilitation of the reflex and decreased with inhibition of the reflex. A system was developed for graphic recording of the modulation of electromyographic activity following a conditioning stimulus. The electromyogram is passed through an absolute-value rectifier circuit and then summed by computer whose sweeps are triggered by the conditioning stimulus. An X-Y plot is then made of the summed potentials (“modulation curve”) representing the graph of activity changes following the conditioning stimulus. The modulation curve resembled but did not reflect precisely changes in central excitability recorded by the classical recovery curve. The modulation curve is rapidly and easily performed. It can be elicited from any muscle, flexor or extensor, and is not restricted to the few large muscles with prominent stretch reflexes as is true in the classical technique. The method is especially adapted to the study of the response characteristics of spontaneous activity in clinical electromyography.

Threshold Modulation Curves for Photographic Films

This paper describes studies of the threshold modulation curve for photographic film. The threshold modulation curve is an empirically derived relation showing the need at any spatial frequency for modulation in the aerial image of a tribar for resolution to be achieved. The paper deals with the history, raison d'être, theory, method of measurement, and accuracy and precision of the threshold modulation curve. The statistical aspects of the threshold modulation curve are emphasized, and the influence of factors that cause variations in the curve are examined. Examples are given of current and potential applications of the threshold modulation curve.

Corrections for Modulation and Time-Constant Effects on Isolated Level-Crossing Signals*

An expression is obtained for the first Fourier coefficient of modulated dispersion curves, which can be used with Wahlquist’s analysis for Lorentz curves to describe any level-crossing signal obtained by modulating magnetic or electric fields. Corrections to the widths and centers of both Δm=1 and Δm=2 signals are tabulated for distortions which arise from modulation, optical misalignment, and the use of large integrating times in phase-sensitive detectors.

The determination of D-region electron densities from observations of cross modulations

Theoretical expressions for the relationship between cross modulation and electron densities and collision frequencies of the D-region are considered. It is found that expressions such as that derived by Bailey and Martin (1934a, b, 1935) from the classical magneto-ionic theory are approximations which are not sufficiently accurate for interpreting cross modulation observations. On the other hand expressions based on Boltzmann's equation and the recent laboratory measurements by Phelps and Pack (1960) of the electron neutral collision frequency predict cross modulation curves which fit the observations fairly well. These new expressions for cross modulation are used along with a trial-and-error method to decide D-region electron densities. It is found that rapid changes in the D-region occur around sunrise, but that there are no striking changes during the rest of the day. Around sunrise a nearly uniform layer extending from about 60 to 80 km is created almost simultaneously at all heights. The implication of this result for theories of the production of D-region ionization are discussed.