Magnetic Field Calibration Research Articles

Traceable magnetic field strength measurements to 120 kHz

With the advent of a series of reports on studies of the effects of magnetic fields on human health there has been a demand for traceable calibrations of the plethora of instruments produced for this type of measurement. To meet this and ongoing requirements, a facility has been established at the National Physical Laboratory (NPL) for low AC magnetic field calibrations up to a frequency of 120 kHz and for improved DC calibrations, made possible by an ambient field cancellation system. A prime consideration for low magnetic field calibrations is the level of the ambient field and the field gradient. The paper describes the surveys carried out which highlight the significant effect of transient disturbances from the electric train service, choice of materials for fitting out the laboratory, and the means of cancelling the ambient magnetic field using a triaxial Helmholtz coil and servo system. Details of the design, construction and calibration of a Helmholtz coil system for producing magnetic field strengths at high frequencies are given together with the requirements of the key circuit elements (power supply, current measurement system). An outline of the design, construction and calibration of a NPL triaxial Helmholtz coil system is also given.

Comparison of transient magnetic field calibrations for 108Pd nuclei Coulomb-excited by 32S and 58Ni beams

The calibration of the transient magnetic field has been checked for Pd in Fe at an average velocity of 2υ 0, using beams of 32S and 58Ni to Coulomb-excite 108Pd. The calibration was independent of the type and intensity of the beam. The dependence of the calibration on velocity is discussed in view of the application to g-factor measurements.

G factors of the lowest (5/2)+ and (7/2)+ states in 197Au and calibration of the transient magnetic field in Gd.

The angular precessions of the lowest ${(5/2)}^{+}$ and ${(7/2)}^{+}$ states in $^{197}\mathrm{Au}$ have been measured employing the transient field method. The $^{197}\mathrm{Au}$ nuclei, Coulomb-excited by a 180 MeV $^{63}\mathrm{Cu}$ beam, traversed the Gd foil with velocities between 5${v}_{0}$ and 2${v}_{0}$ (${v}_{0}$=c/137). We have obtained the values g${((5/2)}^{+}$)=0.21(2) and g${((7/2)}^{+}$)=0.15(2), which are consistent with the predictions of the particle-core weak-coupling model. The field has been calibrated with the Chalk River parametrization, which has been checked using the known g factors of the lowest ${2}^{+}$ states in $^{184}\mathrm{W}$, $^{186}\mathrm{W}$, $^{194}\mathrm{Pt}$, and $^{196}\mathrm{Pt}$ as probes.

High precision floating wire measurements of large dipole magnets at LAMPF

The current-carrying, floating wire technique was used to measure the particle momentum versus magnetic field calibration of the EPICS pion spectrometer at LAMPF. Wire orbits were measured through a pair of dipoles bending through 120° in the vertical plane. Measurements were made in situ and corrected for gravitational and frictional effects. The specific manner of correcting for gravitational effects is given. Magnetic field versus particle momentum calibrations were derived with an accuracy of 2 parts in 10 4.

Calibration of the transient magnetic field for rare-earth nuclei in iron: Application to g-factors of high spin states in 152, 154Sm

A calibration for the transient magnetic field interaction for rare-earth nuclei recoiling through thin iron has been obtained. Time integral spin rotations for states of known g-factors in 169Tm were measured for the range of velocities, 1.5 ≦ υ/ υ 0 ≦ 6.5. A recent measurement for 156Dy at a very high recoil velocity ( υ/ υ 0 = 10.5) has also been incorporated into the calibration. These data indicate that the field becomes nearly constant with velocity for υ/ υ 0 ≳ 7. For a fixed range of velocity ( υ/ υ 0) in; ( υ/ υ 0) out = 5.0; 2.0. measurements were made on the 4 + states of 156,158,160Gd, 160,162Dy and 174Yb. Arguments are presented to relate the 4 + g-factors to the previously measured 2 + values, so that these data provide an additional calibration of the field over the rare-earth region. By comparing the spin rotation for low- and high-spin states in 152,154Sm, the trend of their g-factors with spin has been derived. We find no strong reduction, at least to spin 10. The 154Sm results are in good agreement with theoretical predictions.

Pulsed magnetic field calibration for transducers for rapidly changing pressures

Pulsed magnetic field calibration for transducers for rapidly changing pressures

Zeeman effect measurements in SO 2: An internal magnetic field calibrant

It is proposed that the Zeeman effect of the 2 02 → 1 11 transition in sulfur dioxide be used as a magnetic field calibration standard. The recent accurate beam maser measurements on this transition by Ellenbroek and Dymanus have been confirmed by electric resonance measurements in the 1 11 level. The Zeeman splittings most useful for calibration purposes depend linearly on the magnetic field. For perpendicular selection rules the Zeeman coefficient is 0.5282(3) MHz/kG while parallel transitions have a 0.3722(3) MHz/kG Zeeman coefficient. The sign of the rotational magnetic moment of SO 2 has been determined to be negative.

A new field-tracking NMR magnetometer system

An automatic NMR magnetometer covering the magnetic field range 0.1-3.2 T is described. Instead of continuous tracking of the magnetic field by a conventional feedback loop technique, a new approach to triangular frequency modulation of the marginal oscillator is used in this instrument, ensuring symmetrical frequency deviation about the resonance position. Direct digital display of the magnetic field value is obtained by digital counting of the swept frequency with a suitable counting gate time. The particular advantage of this system is that the restrictions characteristic of automatic frequency control feedback loop systems, as well as for adiabatic passage conditions, can be omitted. The magnetometer is a data-sampling system with a sampling frequency of 43 Hz and a typical tracking rate of about 0.12 T s-1. The maximum search time is 0.5 s. Because of the good dynamic properties and measurement accuracy approaching 10-6 T and lack of modulation coils in the probe, the instrument is particularly suited for the measurement and calibration of magnetic fields in ESR and NMR spectrometers.

The low temperature enhancement of the critical field for a Nb 3Sn superconducting magnet

The maximum quenching field for a Nb 3Sn superconducting magnet was increased from 137 kG to above 150 kG by cooling the magnet from 4.2 K to 1.9 K. A method of magnetic field calibration was employed using the Raman spin-flip signal in ZnSe:In.

An automatic self-locking n.m.r. gaussmeter

An all solid-state automatic self-locking n.m.r. gaussmeter is described covering a magnetic field range of 2700-4000 G. The introduction of integral control reduces the static error and avoids the use of mechanical links. Sweep rates of up to 100 G s-1 can be followed. An automatic search-lock circuit is also provided. The instrument has been used in precise magnetic field calibration for e.s.r. experiments.

Precision Measurements of〈111〉de Haas-van Alphen Frequencies in Copper, Silver, and Gold

Precise measurements have been made of $〈111〉$ neck de Haas-van Alphen frequencies and belly/neck ratios in copper, silver, and gold, using deuterium resonance for magnetic field calibration. All three metals were studied in a superconducting solenoid using a field-modulation technique; the neck in gold was also measured by the torque method in a conventional electromagnet. The results are believed to have an absolute accuracy of better than \ifmmode\pm\else\textpm\fi{}0.2%, and show discrepancies of up to 1.6% when compared with earlier published figures.

Calibration and Differential Field Measurement of High Magnetic Fields by Means of a Periodic Size Effect in Gallium

The periodic size effect in pure single crystal gallium plates is applied to high field calibration and particularly to accurate differential measurements of high magnetic fields. A simple rf probe is described which is used to measure the characteristics of the size effect and to demonstrate the applicability of the method to such measurements up to about 100 kG to an accuracy of at least 1%. By combining aluminum powder and Mylar tape as markers in the rf probe, NMR calibration of the unit can be made during a single field sweep. The rf probe is small and readily adapted to a wide variety of field geometries. In addition to a brief description of the probe, the characteristics of the periodic size effect in gallium are summarized and the rf and dc dependence of the oscillatory amplitude of the periodic size effect are also discussed.

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.

A simple magnetic field calibration system

A simple system is described for obtaining a magnetic field calibration over a range of ± 9 G or ± 14 G about field values in the region of 12000 gauss. A conventional proton resonance oscillator is frequency modulated and proton resonance signalsare obtained from the sidebands thus generated.

Automatic method of magnetic field calibration using proton resonance

A circuit is described which enables the automatic calibration of a magnetic field to be carried out. A marginal oscillator, frequency controlled by one of eleven quartz crystals mounted in a motorized turret tuner, detects nuclear magnetic resonance signals. These signals activate a pen recorder and also trigger a control circuit which advances the turret tuner to the next channel.

Fine Structure of the Hydrogen Atom. Part II

In the first paper of this series, the shift of the $2^{2}S_{\frac{1}{2}}$ level of hydrogen was determined to be 1000 Mc/sec. A new apparatus differing from the original one in details, but not in principle, has been built in order to improve the accuracy of the above result. This provides a greater yield of metastable hydrogen atoms, a more homogeneous magnetic field, and more accurate means of measurement of magnetic field and frequency. With these improvements, preliminary measurements of considerably increased accuracy have been made on both hydrogen and deuterium. The transitions observed were $2^{2}S_{\frac{1}{2}}$, $m=\frac{1}{2}$, to $2^{2}S_{\frac{1}{2}}$, $m=\ensuremath{-}\frac{1}{2}$, as well as to $2^{2}P_{\frac{1}{2}}$, $m=\frac{1}{2}$ and $m=\ensuremath{-}\frac{1}{2}$. The first transition permits observation of the hyperfine structure of $2^{2}S_{\frac{1}{2}}$, as well as an accurate calibration of magnetic field. Hyperfine structure was also resolved for the last transition in hydrogen. There was no observable difference between the level shifts for hydrogen and deuterium which may be taken as 1062\ifmmode\pm\else\textpm\fi{}5 Mc/sec. Later papers of this series will deal with the numerous experimental and theoretical corrections necessary to obtain a level shift accurate to 1 Mc/sec.