Small Field Measurements Research Articles

Development of a high-T/sub c/ first-order gradiometer system

The high-T/sub c/ superconducting quantum interference device (SQUID) gradiometer system was developed for operation in unshielded environment. The system consists of the first-order gradiometer and the flux-locked loop electronics. YBa/sub 2/Cu/sub 3/O/sub 7/ single-layer first-order SQUID gradiometers were fabricated on 15 mm/spl times/7.5 mm SrTiO/sub 3/ bicrystal substrates with a baseline of 7 mm. The gradient sensitivities at 77 K and 100 Hz were 0.1 pT/(cm/spl radic/Hz) in magnetic shielding and 1.4 pT/(cm/spl radic/Hz) when operated unshielded in our laboratory. The high-T/sub c/ SQUID gradiometer system demonstrated successful measurement of small magnetic fields in a laboratory environment without any shields.

The value of the PinPoint ion chamber for characterization of small fieldsegments used in intensity-modulated radiotherapy

Volume averaging and lack of electronic equilibrium complicate accuratedosimetry of small photon fields. In this paper the performance of thePinPoint ion chamber for characterizing small fields used inintensity-modulated radiotherapy (IMRT) was investigated and the results were comparedwith those obtained using the Markus ion chamber and a diamond detector. Sharpbeam penumbras were measured for a 5×5 cm field defined using a cerrobendblock mounted on the accelerator head. In addition, output factors weremeasured for a 6 MV photon beam and a variety of small rectangular fieldscollimated widthwise using the multileaf collimator (MLC) in combination withthe back-up jaws. From this study, a reference field of 5×5 cm and a measuringdepth of 5 cm are recommended. This is related to the over-response of thePinPoint chamber to low-energy Compton scattered photons, an effect that wasinvestigated rigorously and turned out to limit the scope of this ionizationchamber. However, taking into account some limitations, the PinPoint chamber isan excellent detector for output measurements in small fields down to 2 cm. Inprofile measurements the chamber causes a broadening of the measured penumbrasbut its spatial resolution is superior to that of the Markus chamber.

Superconducting magnetic shields for SQUID applications

We investigate the shielding of superconducting and μ-metal forms in axial and transverse directed background magnetic noise fields. Analytical expressions are obtained for the improvement in signal-to-noise ratio obtained by placing a superconducting disk in the presence of a dipole source and a uniform noise field. Axial and transverse shielding factors are then compared for identical superconducting and μ-metal cylinders. The signal-to-noise ratio is found to be infinite at certain points inside a superconducting cylinder as well as a superconducting cylinder with a central partition. Shielding factors obtained here are relevant to SQUID measurements of small dipole source fields in the presence of large background noise fields such as those encountered in biomagnetism and nondestructive evaluation.

Superficial and orthovoltage x-ray beam dosimetry.

Output of superficial and orthovoltage x-ray units may be measured with cylindrical or end-window parallel-plate ionization chambers. The air-kerma calibration factors for these chambers are usually determined free in air, and the x-ray machine output is stated as the air-kerma rate free in air, which, when multiplied with the appropriate backscatter factor, gives the air-kerma rate on the surface of a phantom or patient. For end-window chambers, especially when they are used for measurements of small fields or low x-ray energies, the air-kerma calibration factors may also be determined with the chamber embedded in a tissue-equivalent phantom. This results in field size dependent air-kerma in-air calibration factors but obviates the requirement for knowledge of back-scatter factors when determining the air-kerma rate on the surface of a phantom. Since there still is considerable uncertainty in tabulated backscatter factors as a function of field size and x-ray beam energy, the output measurement technique which determines the air-kerma rate on phantom surface with a phantom-embedded end-window ionization chamber offers a clear advantage over the in-air calibration method.

Measurement of the Radiation Dose on Small Field Measurements by Ionization Chambers

Measurement of the Radiation Dose on Small Field Measurements by Ionization Chambers

ライナック X 線の集光照射における小照射野の線量測定

We evaluated and reported the characteristics of flatness, dose distribution, and dose measurements in small radiation fields. The diameter of the small radiation fields were from 0.7cm to 2.9cm at the isocenter and the collimator built in the accelerator head was set for 4×4cm^2 with supplemental collimator placed 56cm from the target. The flatness and the characteristics of dose distribution were measured in these small radiation fields in the narrow beam methods with 6MV and 10MV X-ray beams from the linear accelerator, applying a three dimensional fixed multi-irradiation technique. The sensitive volume effects upon the measurements of field factor and tissue peak dose ratio (TPR) in the small radiation fields were measured by JARP ionization chamber, shallow ionization chamber, and Si-semiconductor detector, because of each own sensitive volume. We concluded that the Si-semiconductor detector with the smallest sesitive volume was the most suitable for the small field dosimeter.

Femtosecond scattering dynamics in magnetic semiconductor spin superlattices.

Spin-dependent dynamics of polarized carriers in a series of ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$SeZnSe superlattice structures are directly observed by time-resolved upconversion photoluminescence spectroscopy at low temperatures. Polarization measurements in small magnetic fields reveal spin-flip scattering of carriers confined within a magnetic quantum well. A marked change in the dynamics is observed with increasing field and subsequent spin superlattice formation. Exciton lifetimes and spin relaxation are seen to be strongly dependent on both the energy and location of spin states in the heterostructure.

Integrated magnetic field sensor

For small magnetic field measurements, Sextant Avionique is developing, in collaboration with the Ecole Supérieure d'Ingénieurs en Electrotechnique et Electronique, a new generation of miniature sensors. The detector is a mechanical resonator using a monocrystalline silicon beam. The excitation of this resonator is achieved through the Laplace force, operating on an alternating current flowing through a conductor deposited on the beam. The oscillation amplitude, which is proportional to the external magnetic field, is detected through a piezoresistive gauge bridge diffused in the silicon beam. The mechanical properties of the silicon and the resulting quality factor of the resonator have allowed a very sensitive device to be designed with a resolution in the nanotesla range. The manufacture of the resonator is possible through extensive use of micromachining and microelectronic technologies. It is then possible to produce a single wafer incorporating hundreds of small transducers at a very low unit cost.

A new probe for measuring small electric fields in plasmas

A dipolar double probe has been developed for in situ measurements of small electric fields in laboratory plasmas. The probe measures dc to ac electric fields (0≤f≲20 MHz) with high sensitivity (Emin≂10 μV/cm) and responds to both space charge electric fields (Esc(Esc= −∇φp) and inductive electric fields (Ei = −∂A/∂t). Using voltage-to-frequency conversion, the probe signal is obtained free of errors and loading effects by a transmission line. Various examples of useful applications for the new probe are presented, such as measurements of (i) dc ambipolar fields, (ii) ac space-charge fields of ion acoustic waves, (iii) ac inductive fields of whistler waves, and (iv) mixed inductive and space-charge electric fields in current-carrying magnetoplasmas.

Spectroscopy of Rydberg atoms at n

We have observed barium Rydberg states with principal quantum numbers up to $n=520$. Such highly excited states allow quantitative in situ measurements of small electric fields on the order of tens of microvolts per centimeter. Because of the effective suppression of stray electric fields we have observed quasi-Landau resonances in external magnetic fields on the order of tens of gauss. The energy dependence of the spacing between neighboring quasi-Landau resonances was found to be in excellent agreement with calculations involving classical trajectories.

A.C. Susceptibility and Specific Heat in Low B.C. Fields of La2-xSrxCuO4 and YBa2Cu3O7 High Temperature Superconductors

We report the giant effect of small d.c. fields on a.c. diamagnetic transitions in La2-xSrxCuO4, and YBa2Cu3O7. An unexpected frequency effect is observed. In order to elucidate the apparent conflict Between the high critical field slope as measured by magnetoresistance experiments and the large depression of a.c. diamagnetic transitions, we locate the bulk transition by high resolution specific heat measurements in small fields. The results, together with d.c. magnetization measurements near Tc and SEM micrographs, suggest a model where superconducting regions with a high critical field are separated by weak links.

Stark spectroscopy of forbidden two-photon transitions: a sensitive probe for the quantitative measurement of small electric fields

The authors have investigated both theoretically and experimentally the intensities of the forbidden two-photon lines 6S1/2 to 41P3/2,61P3/2 in an atomic beam of caesium exposed to an electric field. At low field strengths the line intensities depend quadratically on the electric field and are quantitatively well described by Stark mixing parameters obtained from perturbation theory. At higher field strengths the line intensities exhibit an unexpected linear field dependence which is in good agreement with the results from a more general calculation. The authors applied the results obtained to measure the components of a small spurious electric field in a thermionic ionisation detector.

Peak Height Fluxgate Magnetometer for Measurement of Small Magnetic Fields

In this paper details of peak height fluxgate magnetometer are presented. The instrument can record magnetic fields of the order of one milligauss.

Resistance measurements and vortex fluctuations in two-dimensional superconducting films

Recently theories of vortex fluctuations in two-dimensional superconducting films have been compared to the experimentally determined power law dependence of voltage on current and to the temperature-dependent resistance in small applied fields. Here an alternative interpretation of the power law and a practical difficulty of flux pinning for measurements in small fields are pointed out. To illustrate these points, experimental examples are presented for very homogeneous granular aluminum films.

Ac-susceptibility measurements in small fields on fine superparamagnetic nickel particles

The magnetic behaviour of very fine nickel particles, chemisorbed on a silica carrier, has been studied using susceptibility measurements in small ac magnetic fields (≦0.8 Am−1). A detailed description of the simple ac susceptibility measuring procedure is given, which provides an excellent sensitivity. The temperature ranged from 4.2 K to 700 K. The application of an external magnetic dc field (0≦H ext≦6×104 Am−1) to the sample reduces the temperatureT max where the initial susceptibility has a maximum whereas the peak becomes broader and weaker. Additionally, the measurements determine the shift of the Curie temperatureT c′ which depends on the mean grain size. We discuss the particle interaction in terms of a mean field-like theory. However, as we found some discrepancies between our results and present theories, it appears that a percolation theory would be more appropriate to achieve a better understanding of the complicated interaction phenomena.

Rotatable magnetic anisotropy in epitaxial ferrite layers

Magnetic domain structure and magnetic anisotropy were studied in monocrystalline epilayers of Mg <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</inf> Mn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.8</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> ferrite. The layers, several micrometers thick, were obtained by a CVD method on monocrystalline MgO substrates. Domain observations were performed by the Bitter's method. Magnetic anisotropy measurements were performed by torque and FMR methods. In the demagnetized state, a typical stripe structure of 2.0 to 2.8 μm period was observed. From the domains behavior in the in-plane magnetic fields it was found that in these epilayers the rotatable anisotropy was present. The existence of this anisotropy was confirmed by torque measurements in small in-plane fields. The magnetic parameters characterizing these layers are: 4ΠM = 3500 Gs, K <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> = - 2.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> ergs/cc, K <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</inf> = 2.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> ergs/cc.

Some Comments on the Measurement of Small Scale Strong Magnetic Fields on the Sun

Since Dr. Harvey has already mentioned the main results of my observations, I want to restrict myself to two topics:i)a brief description of the improvements of my method as compared to that used by Stenflo (1973),ii)some puzzeling aspects of my results which might be discussed in this meeting.The main differences between the two methods of simultaneous Zeeman polarization measurements used by Stenflo (1973) and by myself are based on suggestions by Schröter (1973):a)use of three instead of only two differently split lines;b)use of less temperature sensitive lines;c)use of model atmospheres instead of a Milne-Eddington model for the calculation of the ‘calibration curves’;d)observation of selected Ca+K features in order to avoid the statistical method with the scatter-plot diagram.

Rotation during the Implosion of a Theta Pinch

Measurements of fields and pressure gradients during the collapse of a theta pinch allow determination of the ion azimuthal velocity from the radial component of Ohm's law. In hydrogen and nitrogen discharges Vθ was found to change sign across the current sheet. Ions rotated so as to carry an appreciable fraction of the current in the front of the sheet and moved so as to oppose current flow in the rear. The total angular momentum was approximately zero. These results agree well with the Düchs calculations of the influence of small transverse fields. Measurements of small transverse fields in the current sheet are presented.

A Floating Microcompass for Measuring Millioersted Magnetic Fields in Millimeter-Cube Sampling Volumes

A magnetic compass is described which permits the measurement of small magnetic fields with relatively high accuracy and in sample volumes of small linear dimensions. In a particular model, 300 mOe were measured with better than ±2% accuracy and at a distance of less than 1 mm from the source of the magnetic field. Novel features in this compass are its reduced size and the liquid suspension. By the action of surface tension, the magnetic needle floats on the rotationally symmetric meniscus of a water surface in a capillary. The suspension offers centering, damping of motion, and elimination of static friction. The paper gives design view points and technology, discusses the measurement method and error sources, and presents examples of measurements.

Accurate Measurement of Small and/or Inhomogeneous Magnetic Fields using a Set of Transistorized Marginal Oscillators and a Narrow-Linewidth Free Radical

Magnetic flux density measurements have been made at a field intensity of 30mT (300 G) using a single-knob tunable transistorized marginal oscillator, with a narrow linewidth (0.04mT) free radical. The free radical (picryl-n-amino carbazyl) is commercially available. A commercial sample purchased at the end of 1965 was compared with a sample synthesized prior to 1958, and the resonance frequencies of the two samples differed by less than 5×10−4 at 1.2mT (12 G). A complete set of transistorized marginal oscillators has been built to cover the range of field intensities from below 0.5mT to above 30mT. At 30mT, free radical samples can be checked against a commercial proton NMR magnetic flux density meter. This brings accurate calibration of small and/or inhomogeneous magnetic fields within the range of most university and industrial laboratories. A free radical sample116 in. in diameter and116 in. long was used to check the calibration of a solenoid at a magnetic flux density of 1mT (10 G). The repeatability of the measurement was better than 1×10−3. It was possible to obtain a clear resonance signal at 1.3mT (13 G) with a sample measuring 0.011 in. in diameter and 0.150 in. long, without the use of enhancement techniques.