Magnetic Field Modulation Amplitude Research Articles

The Low-Field Microwave Absorption in EMR Spectra for Ni50-xCoxMn35.5In14.5 Ribbons.

This paper contains a detailed study of low-field microwave absorption, which is observed in EMR spectra registered for a series of () Heusler alloys polycrystalline in situ and annealed at 1173 K ribbons. The LFMA spectra for all ribbons were performed at X-band (∼9.5 GHz), at temperatures below Curie temperature. Additionally, for annealed ribbons, the LFMA signal dependencies of the external magnetic field modulation amplitude, modulation frequency, microwave power and microwave magnetic field phase were registered. These results confirm the resonant character of LFMA. To determine the basic EMR parameters, such as linewidth and resonance field, the experimental data were fitted by the Dyson function. The LFMA signal is satisfactorily matched by the two lines, and the variability of the component lines with temperature is remarkable.

EPR dosimeter material properties of potassium tartrate hemihydrate

The ability of potassium tartrate hemihydrate as a radiation sensitive material for electron paramagnetic resonance (EPR) dosimetry was investigated. The samples were subjected to different doses, in the range of 1–9 Gy of 60Co gamma rays at room temperature. The EPR spectra were investigated through variation of signal intensity with respect to absorbed dose, magnetic field modulation amplitude, microwave power and time stability. The results indicate that the sensitivity of potassium tartrate hemihydrate is about 30% higher than that of alanine. However, the EPR signal is timely less stable within the first two weeks after irradiation.

Reconstruction of the first-derivative EPR spectrum from multiple harmonics of the field-modulated continuous wave signal

Selection of the amplitude of magnetic field modulation for continuous wave electron paramagnetic resonance (EPR) often is a trade-off between sensitivity and resolution. Increasing the modulation amplitude improves the signal-to-noise ratio, S/N, at the expense of broadening the signal. Combining information from multiple harmonics of the field-modulated signal is proposed as a method to obtain the first derivative spectrum with minimal broadening and improved signal-to-noise. The harmonics are obtained by digital phase-sensitive detection of the signal at the modulation frequency and its integer multiples. Reconstruction of the first-derivative EPR line is done in the Fourier conjugate domain where each harmonic can be represented as the product of the Fourier transform of the 1st derivative signal with an analytical function. The analytical function for each harmonic can be viewed as a filter. The Fourier transform of the 1st derivative spectrum can be calculated from all available harmonics by solving an optimization problem with the goal of maximizing the S/N. Inverse Fourier transformation of the result produces the 1st derivative EPR line in the magnetic field domain. The use of modulation amplitude greater than linewidth improves the S/N, but does not broaden the reconstructed spectrum. The method works for an arbitrary EPR line shape, but is limited to the case when magnetization instantaneously follows the modulation field, which is known as the adiabatic approximation.

Use of aspartame-based sweetener tablets in emergency dosimetry using EPR

Accident dosimetry aims to evaluate the unplanned radiation doses delivered to individuals through one of the objects exist in the area of the accident. The gamma dose response of free radicals generated in irradiated aspartame tablets and its usability for emergency dosimetry was studied. EPR spectra of unirradiated and irradiated aspartame-based sweetener were recorded. Two signals arise after irradiating, S(1) at g (S(1)) = 2.00229 +/- 0.00097 and S(2) at g (S(2)) = 2.00262 +/- 0.00088. Some EPR parameters were studied for radiation-induced radicals in aspartame sweeteners tablets, such as the microwave saturation behaviour, the effect of magnetic field modulation amplitude on the peak-to-peak height and peak-to-peak line width for both of S(1) and S(2). Responses of S(1) and S(2) to different radiation doses were studied and resulted in linear relationships, radicals persistence curves were plotted over a 49-d storage period. It was found that Aspartame sweeteners tablets are useful in the range from 0.96 to 39.96 Gy. Radiation-induced radicals possess reasonable stability.

The Third International Intercomparison on EPR Tooth Dosimetry: part 2, final analysis

The objective of the Third International Intercomparison on EPR Tooth Dosimetry was to evaluate laboratories performing tooth enamel dosimetry <300 mGy. Final analysis of results included a correlation analysis between features of laboratory dose reconstruction protocols and dosimetry performance. Applicability of electron paramagnetic resonance (EPR) tooth dosimetry at low dose was shown at two applied dose levels of 79 and 176 mGy. Most (9 of 12) laboratories reported the dose to be within 50 mGy of the delivered dose of 79 mGy, and 10 of 12 laboratories reported the dose to be within 100 mGy of the delivered dose of 176 mGy. At the high-dose tested (704 mGy) agreement within 25% of the delivered dose was found in 10 laboratories. Features of EPR dose reconstruction protocols that affect dosimetry performance were found to be magnetic field modulation amplitude in EPR spectrum recording, EPR signal model in spectrum deconvolution and duration of latency period for tooth enamel samples after preparation.

Origin of the irreversible microwave absorption versus the state of vortex matter in Bi 2Sr 2CaCu 2O x single crystals

We have investigated properties of hysteretic microwave absorption (MWA) by the Bi2Sr2CaCu2Ox single crystals in the temperature range from 10 K to Tc. It was found that the shape of the MWA hysteresis loop and its dependence on the magnetic field modulation amplitude undergo essential changes with temperature. The analysis has shown that near 50 K, the origin of the signal detected by electron spin resonance spectrometer with magnetic field modulation changes. It is due to crossover to another pinning type. At T < 50 K where the bulk pinning is the most effective pinning mechanism, the signal registered by a spectrometer is the magnetically modulated MWA. At high temperatures, 50 K < T < Tc, when vortices are pinned by the surface or the edge barrier, the signal is the derivative of the absorption power with respect to field dP =dH . The shape of the MWA hysteresis loop and its modulation enable one to determine unambiguously the pinning type (bulk or surface) controlling the irreversible properties of a superconductor. 2002 Elsevier Science B.V. All rights reserved.

Optimisation of recording conditions for the electron paramagnetic resonance signal used in dental enamel dosimetry.

Optimisation of the parameters for recording the electron paramagnetic resonance (EPR) spectra of dental enamel for absorbed dose reconstruction was performed for an EMX (Bruker) spectrometer supplied with a high-sensitivity microwave cavity. Dose determination was performed using a previously developed automatic spectra processing procedure, which uses the non-linear fit of a model spectrum. The experimental error was estimated as the standard deviation of the results from the nominal doses for the set of spectra recorded for 10 samples prepared from teeth of different persons and irradiated in the dose range 0-500 mGy. The microwave power and magnetic field modulation amplitude corresponding to the minimum of dependencies of the error on these parameters were adopted as the optimal ones. For the sets of spectra recorded at optimal parameters for sample masses 100, 50 and 30 mg, the errors of dose determination were obtained as 18, 27 and 37 mGy respectively.

Low field microwave absorption and magnetic flux quantization in granular Rb3C60: Observation of nonperiodic sharp line microwave absorption

The low field microwave absorption (LFMA) of granular samples of superconducting Rb3C60 has been studied as a function of the microwave power, magnetic field modulation amplitude and temperature at dc magnetic fields less than 0.5 G. Nonperiodic sharp lines are observed for the first time in these new alkali-metal doped fullerenes. An interpretation is advanced based on a spin-glass model of a set of superconducting current loops with random orientations but uniform areas.

Low-field microwave absorption of the Nd1.85Ce0.15CuO4–ysuperconducting system

The low-field microwave absorption of the Nd1.85Ce0.15CuO4–y electron-carrier superconducting system has been investigated. From the temperature dependence of the absorption signal intensity, it is suggested that superconductivity is achieved in two steps. As the temperature is decreased below the superconducting transition temperature, individual grains in the ceramic sample first become superconducting via intragranular currents. A further temperature decrease then induces improved Josephson couplings between individual superconducting grains to generate intergranular currents which cause the bulk sample to become superconducting and maximize the microwave absorption. Still lower temperature decreases the Josephson coupling efficiency and the microwave absorption signal decreases. The effects of variable microwave power and magnetic-field modulation amplitude on the absorption signal lineshape and intensity are reported and interpreted in terms of a model involving the pinning and depinning of magnetic fluxons at the surface of the granular material. Magnetic-field hysteresis of the absorption signal is found to be weak, even at low modulation amplitudes.

Analysis of the low-field microwave absorption line shape differences in yttrium barium copper oxide (YBa2Cu3Ox) and bismuth strontium calcium copper oxide (Bi2Sr2CaCu2Ox) superconductors

The field-dependent low-field microwave absorption for YBa{sub 2}Cu{sub 3}O{sub x} (123) and Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub x} (2212) superconducting powders and a 2212 crystal was investigated as a function of temperature and magnetic field modulation amplitude. The 2212 samples do not show a significant magnetic field sweep hysteresis while the 123 samples show a hysteresis strongly dependent on temperature and modulation amplitude. The microwave absorption line shapes for all samples, including the signals exhibiting strong hysteretic behavior, can be well simulated by a model for the microwave loss in intergranular Josephson junctions involving boundary and microwave-induced currents. The calculation allow an estimate of the average Josephson junction length to be about half the average grain size observed by scanning electron microscopy. Interpretation based on the assumptions in the line shape model indicates that the Josephson junctions are more weakly coupled in 2212 than in 123 superconductors.

Low field microwave absorption of a superconducting Bi 2Sr 2CaCu 2O x crystal

Abstract Magnetic-field-dependent low-field microwave absorption for a Bi2Sr2CaCu2Ox crystal below the superconducting transition temperature was investigated as a function of temperature, magnetic field modulation amplitude and microwave power. The signal from the crystals stored in vacuum is highly anisotropic and shows a strong temperature dependence. The behavior of the signal versus microwave power is unusual. Above 60 K the signal shows a complex signal shape. The complex signal shape is replaced by a broad, structureless signal when the crystal is stored in air for about one month. The complex signal shape is well simulated by a microwave loss mechanism in Josephson junctions involving boundary and microwave-induced currents. The model simulation explains some aspects of the temperature dependence and microwave power dependence of the signal.

The spin-dependent resonance signal from silicon

The inhomogeneous spin-dependent resonance signal from the depletion region of a p-n junction in silicon is investigated by modulating the external magnetic field as well as the microwave power. The signal is detected at the microwave power modulation frequency using lock-in detection. It is shown that the details of the component lines of the inhomogeneous signal can be obtained from measurements of the dependence of the overall signal on the external magnetic field modulation amplitude and frequency.

Main origin of asymmetric lineshape of the dangling bond ESR spectrum in hydrogenated amorphous Si films

It has been well known that the dangling bond ESR spectrum of hydrogenated amorphous silicon films deposited on quartz substrates has an asymmetric lineshape which is nearly Gaussian for the high magnetic field side, while nearly Lorentzian for the low magnetic field side. A new spectrum with g = 2.0010 .0005 and the linewidth Δ H = 3 G is resolved in this Gaussian line by adjusting the amplitude of magnetic field modulation and is identified as the Si-E′ center. The presence of this center in the near-surface of the quartz substrates is mainly responsible for the asymmetric lineshape.

Modulation scheme for electron–electron double resonance spectroscopy

A modulation scheme for electron–electron double resonance (ELDOR) spectrometers is presented. With this scheme an optimum stabilization signal for locking the pump microwave generator to the pumped electron paramagnetic resonance (EPR) line is generated. A separate pump power level and a separate magnetic field modulation amplitude are used for the purpose of locking. In general, such a modulation scheme introduces false ELDOR lines. These false lines disturb the real ELDOR signals, or introduce an ELDOR signal in the absence of any communication between the observed EPR line and the pumped EPR line. With the described modulation scheme the frequencies of the false ELDOR signals are limited to even multiples of the frequency of the wanted ELDOR signals. This makes a suppression of the false ELDOR lines easy.

Rapid adiabatic passage EPR of ferricytochrome [formula omitted]: Signal enhancement and determination of the spin-lattice relaxation time

The anomalous behaviour of rapid adiabatic passage signals from solids is described. Portis's theory for inhomogeneously broadened resonances is recalled and its predictions reproduced. The dependence of the dispersion signal from a single crystal of ferricytochrome c at 4.2 °K, at 24 GHz, incident microwave power 6 mW, upon magnetic field modulation amplitude (0–3 G) and frequency (10–100 kHz) were found to accord with prediction. From the frequency-dependent phase lag of the EPR signal behind the modulation, the spin-lattice relaxation time (τ) from 4–8 °K was found to be 4 · 10−6 s. Between 8 and 18 °K τ−1 increased as T7. Since the same functional relations were found for rapid adiabatic passage signals from hemoproteins solutions, and from charred dextrose at 300 °K the tested relations are considered to be of wide validity, independent of the source of the inhomogeneous broadening. The optimal signal to noise ratio under slow passage conditions results when the modulation amplitude equals the linewidth. Under rapid adiabatic passage conditions, with the same sample and microwave power, the optimal ratio is the same, but the required modulation amplitude is generally much less. Thus, when linewidths are too large to be matched by the modulation amplitude, detection under rapid adiabatic passage conditions of the dispersion signal yields an enhancement over the largest signal obtainable under slow passage conditions, which may reach several orders of magnitude. With cytochrome c, whose linewidths at 24 GHz exceed 400 G, the enhancement is at least 500 ×.

Absolute Determination of Change in Magnetic Susceptibility Due to Electron Spin Resonance

An automatic compensation method is applied to ESR measurements, and the corresponding addition to a standard spectrometer permits the measurement of the resonant magnetic susceptibility independently of microwave power level, magnetic field modulation amplitude, amplifier gain, etc. A calibration of the instrument is described. Using this, one obtains an accuracy in the measurement of the susceptibility of approximately 0.1%.