Proton Magnetometer and Stable Oscillator for Remote Measurement of StrongMagnetic FieldsR. L. Garwin and A. M. Patlach Citation: Review of Scientific Instruments 36, 741 (1965); doi: 10.1063/1.1719689 View online: http://dx.doi.org/10.1063/1.1719689 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/36/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Torsional oscillator magnetometer for high magnetic fields Rev. Sci. Instrum. 67, 4161 (1996); 10.1063/1.1147562 Diaphragm magnetometer for dc measurements in high magnetic fields Rev. Sci. Instrum. 61, 848 (1990); 10.1063/1.1141452 Rotating Field Magnetometer for Measurement of Anisotropic Magnetic Materials Rev. Sci. Instrum. 38, 591 (1967); 10.1063/1.1720773 Rotating Coil Magnetometer for the Measurement of the Earth's Magnetic Field Am. J. Phys. 29, 333 (1961); 10.1119/1.1937766 Oscillations of a Finite Cold Plasma in a Strong Magnetic Field Phys. Fluids 2, 103 (1959); 10.1063/1.1705899

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 36, NUMBER 6 JUNE 1965

Proton Magnetometer and Stable Oscillator for Remote Measurement of Strong Magnetic Fields

R. L. GARWIN AND A. M. P ATLACH

IBM Watson Laboratory, Columbia University, New York, New York

(Received 28 January 1965; and in final form, 26 February 1965)

A simple apparatus for proton-resonance magnetic-field measurement permits remote location of the rf source and indicator. A high stability variable-frequency oscillator is also described.

PROTON-RESONANCE field measurements in large magnets have in the past presented several difficulties.

In many existing circuits the measuring probe is part of the tank circuit of an oscillator. Line losses and multiple reso­nant frequencies preclude the use of long cables to connect such probes to their associated electronics, making it neces­sary to keep the oscillator close to the magnet. In some cases this is undesirable because the oscillator may cease to function in the fringing field. Ferromagnetic materials in the apparatus may distort the field, or even worse, cause the apparatus to be drawn into the magnet unless it is well anchored. These problems are worse in the measure­ment of high fields.

We have used a simple probe circuit that permits the rf source to be kept at any distance desired. Unlike most ordinary magnetometers, which become more difficult to operate as the measured field increases, the signal-to-noise ratio of this circuit improves and its tuning becomes com­pletely noncritical. The circuit is also easy to build and reliable in operation. All that is required in addition to the probe is an oscillator having a stable output and an oscilloscope. A suitable oscillator was also designed and is described.

Probes of this type have been successfully used at the Columbia University Nevis Cyclotron and at CERN­Geneva.

PROBE

In the circuit of Fig. 1, rf excitation is introduced through a transmission line terminated by a resistor which matches the line impedance. The line may be as long as desired. A constant rf current is fed to the sample coil by Rl, a high resistance. The probe is tuned to the incoming

FIG. 1. Cl and C2 tune the model 154-A "Yellow" probe to about 60 Mc. Because of the sample relaxation time it is inadvisable to modulate at more than 60 cps.

741

frequency by Cl and C2. Voltage developed across Cl, Ll, C2, is coupled to the lN58 diode detector by C3. R2 iso­lates the signal output line from the sample coil. It is essential that this output line not load the rf circuit lest the over-all Q be lowered and the output signal be reduced. R3 provides a discharge path for C3. The probe output is fed into an amplifier with a high input impedance (over 500 kQ).

As the resonance is traversed, the absorption of energy by the nuclear spins causes a decrease in rf voltage across the tuned circuit. This appears as a dip in the dc output of the detector. It should be noted that at frequencies above SO Me, Rl and R2 do not appear as strictly resistive components but rather as an ill-defined RLC circuit. It was found after some experimentation that Allen-Bradley ! W carbon resistors work well. Although not critical, the detector diode should have a low shunt capacity in order that maximum signal be available.

Resonance can be obtained either by frequency modu­lating the driving oscillator or by sweeping the local field around the probe. Field modulation is achieved by intro­ducing a modulating current into L2, a set of Helmholtz coils wound at right angles to the sample coil.

If FM is used, the frequency deviation should be kept below '" 100 kc, else the resonant response of the probe produces envelope distortion that may obscure the resonance.

Initially a home-made probe was used. Later, however,

FIG. 2. Probe assembly. All components are rigidly mounted be­tween stand-offs to assure mechanical integrity and electrical stability. Because of the high frequencies involved, it is necessary to keep leads as short as possible. This illustration shows a desirable component layout.

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742 R. L. GARWIN AND A. M. PATLACH

a Perkin-Elmer! Numar probe was substituted with excel­lent results and was used in all subsequent construction. Because of the commercial probe's small size, it was pos­sible to build the circuit into a brass tube 2.5 cm in diam­eter, 15 cm long, shown in Fig. 2. This mounting scheme provides a magnetometer of unparalleled immunity to rough handling.

DRIVING OSCILLATOR

The signal-to-noise ratio obtained from the probe may be limited by the amplitude stability of the driving oscil­lator. The possibility of field modulation implies that the oscillator be frequency stable. A transistor oscillator was designed that fulfills these requirements and has been du­plicated successfully by several experimenters, in many different frequency ranges. The oscillator lends itself readily to frequency modulation using a Varicap in place of Cl, and in one case was used in a very-high-precision servo 100p.2

As a fixed tuned oscillator, the short-term random fre­quency drift is about 1 part per million over 30 min. No temperature control is used, the oscillator case being ex­posed to the laboratory ambient.

The oscillator circuit (Fig. 3) is a transistor version of the highly stable "Clapp" oscillator and with the transis­tors shown is probably useful to over 300 Me. Its good stability is due in part to the very large capacity shunting the tank. Changes of transistor characteristics due to tem­perature variation or aging thus have relatively little in­fluence on the oscillator frequency.

To operate at a frequency F times the 60 Mc at which the circuit of Fig. 3 works, one scales the reactive compo-

el

1-10 pI

12.6V

6V ..,

.pl - +

FIL. XFRMR ~~~:;:;

FIG. 3. 60 Me oscillator. The tank coil is 12.7 mm of B & W 3003 Miniductor (0.55 ~H).

1 Numar probe manufactured by Perkin-Elmer Corporation, Nor­walk, Connecticut, for their model 192 gaussmeter.

2 R. J. Blume, Rev. Sci. Instr. 34, 1400 (1964).

4

3 u

~ 2

o VOLTS

FIG. 4. Voltage/frequency curve of a typical oscillator.

nents a factor l/F. If this is done no problem should be encountered in making the circuit oscillate. It does, how­ever, have two peculiarities.

One of these is a continuous almost linear decrease of frequency with time. After turn-on, there is a rapid de­crease of frequency for the first 10 min of operation. This is followed by a 10 cps/h decrease, as far as we know, indefinitely. The same behavior was exhibited by several different transistors and might be due to water-vapor absorption by the materials of the circuit.

The second peculiarity, which is turned to good use, is a region of zero-slope in the voltage/frequency character­istic (see Fig. 4). In every oscillator thus far built there was found a portion of this curve where the Af / AV = O. This region is different for each transistor, but it is quite marked, occurring somewhere between 4 and 7 V. Once this voltage has been determined one selects a Zener diode of the same voltage and inserts it in the simple but highly effective voltage regulator shown in Fig. 3. No other com­ponent changes are necessary for this regulator.

The oscillator circuitry is built on a 6 mm thick alu­minum subchassis, and is completely enclosed in a box made of 6 mm aluminum. This is necessary to achieve the quoted frequency stability. Silver-mica condensers are used throughout. For fine frequency control, a brass plate from a small variable capacitor is made to rotate in close prox­imity to the tank coil (asterisk in Fig. 3). Depending on the spacing, frequency variation per turn may be made as small as a few tens of cycles.

The linewidth of the resonance is dependent only on the sample size and the homogeneity of the field being meas­ured. Neither the rf level nor probe tuning were found to be critical. In a 15 kG field homogeneous to 1 part in 104, a 200 ,.N signal was obtained for 100 m V excitation to the Numar probe.

In actual use the driving oscillator was located some 300 m from the probe. A preamplifier was used to raise the audio signal level for return to the monitoring area.

This scheme has been used by several experimenters and found to be reliable and easy to duplicate.

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