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A Sensitive Recording NMR

Ultrasonic Spectrometer

344

LAWRENCE W. JAMES

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U.S. DEPARTMENT OF COMMERCE

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NBS TECHNICAL NOTE 344

ISSUED SEPTEMBER 7, 1966

A SENSITIVE RECORDING NMR ULTRASONIC SPECTROMETER

LAWRENCE W. JAMES

Radio Standards Laboratory

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

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Contents

Page

Abstract . . . . . . . o . . . . . „ . . » <> . o . . . . . 1

1. Introduction ................o .0.00.0 1

2. Experimental Equipment ................. 2

3. Experimental Technique. . 6

4. Acknowledgements. .................... 12

5. References. ...... ............... ...13

List of Figures

Figure 1: Block Diagram of the Complete System ......

Figure 2: Circuit Diagram of the Boxcar Integrator . . . . . Figure 3: Sequence of Operations. .. ^ .......... . Figure 4: In and In Resonances in InSb „ . . .... . Figure 5: Sb and Ga Resonances in GaSb ....... o 115 Figure 6: Hexadecapole Interaction In Resonance in InAs

Page

, 3

, 5

, 7

. 9

, 10

11

111

A Sensitive Recording NMR Ultrasonic Spectrometer

Lawrence W. James*

Instrumentation is described for automaticallyobtaining a continuous recording of ultrasonic absorption lines. Ultrasonic power is added to the crystal

lattice by a transducer mounted on a single crystal of

the sample. The interaction of the ultrasonics with

the nuclear spin system is recorded by using a pulse

NMR system in conjunction with a boxcar integrator.

This technique is more sensitive and provides more

easily interpretable results than earlier systems.

1. Introduction

The first experimental work in measuring the effect of ultrasonic

power on the nuclear spin system in solids used a pulse NMR system.

The effect of the ultrasonics on the nuclear spin system was determined

by the difference in signal height which was observed on an oscilloscope

when the ultrasonic power was present and when it was absent [ Proctor,

and Tanttila, 1956; Proctor and Robinson, 1956]. Measurements of the

ultrasonic absorption linewidth were obtained by making a number of

separate measurements at frequencies on either side of the transition

frequency.

Direct observation [Menes and Bolef, 1958; Bolef and Menes, 1959]

of the absorption of phonons by the nuclear spin system has been made by

using a transducer bonded to a mechanically resonant sample as the

varying impedance in the tank circuit of a marginal oscillator. A modulated magnetic field, phase detector, and recorder were used to obtain

a recording of the derivative of the absorption line as the nuclear

^Present address: Electrical Engineering Department,

Stanford University, Stanford, California
Larmor frequency was swept through the oscillator frequency. This

technique offers the advantage of a continuous recording, but has the

disadvantages of requiring more refined ultrasonic techniques, of

limiting the voltage which could be applied across the transducer to a

fairly narrow range, and of being considerably less sensitive than the

indirect pulse NMR technique.

This paper describes a system using the pulse NMR technique

which automatically records a continuous plot of the ultrasonic absorption line. From this plot, the line width, the line shape, and any

unusual effects may be easily observed. Other advantages of this

system are its ability to record directly the absorption line rather than

its derivative, its ability to detect an effect too small to be seen with

the normal pulse NMR system, and its ability to observe a saturation

in samples with long spin-lattice relaxation time, T .

2. Experimental Equipment

A block diagram of the entire system is shown in Fig. 1. The

pulse transmitter and receiver form a conventional pulse NMR detection

system. The ultrasonic rf amplifier is a three- stage tuned amplifier

which boosts the low voltage output of the variable frequency oscillator

(VFO) to a variable voltage of up to 500 volts which may be applied

across the transducer. The output frequency is the same as the VFO

frequency; no multiplication is used. An automatic level control is

added to keep the output voltage constant within 1% over a wide frequency

range without the need for retuning. The relay is present to short out

the receiver input while the ultrasonic power is applied.

The heart of this system is the boxcar integrator. This device

contains an RC integration network and a gate which applied the signal

to the integration network only during a sampling pulse. The sampling
Page 3
pulse is timed to occur when the NMR signal is present. When the

sampling pulse is not present, the voltage across the capacitor remains

constant except for a very small drift caused by leakage current.

A circuit diagram of the boxcar integrator developed in this

laboratory is shown in Fig. 2. With the set output switch in the operate

position, the signal is fed through the emitter follower input stage into

the diode bridge gate. When no sampling pulse is present, none of the

diodes in the bridge are conducting. When a pulse is applied to the

sampling pulse input, symmetric pulses of opposite polarity are applied

to terminals 1 and 2 of the diode bridge, causing all four diodes to

conduct and feeding the signal into the RC integration network. The

diode bridge contains matched, ultra-low-leakage diodes. The three

trimmers are adjusted for minimum coupling of the leading and trailing

edges of the sampling pulse to the integration network input. The circuit is designed for a 25-volt sampling pulse and a ±3-volt signal range.

All signal circuits including those in the receiver are dc coupled

and compensated for temperature drift. The dc coupling is necessary

to prevent any signals not occurring at the same time as the sampling

pulse from having an effect on the integrator output.

The integrator time constant, referred to as the "learning time

constant, " is variable by switching both resistors and capacitors in the

integrator. Low leakage switches and mylar capacitors are mounted

in a sealed box to prevent accumulation of dirt. If the learning time

constant is short compared to the length of the sampling pulse, the

voltage across the capacitor after the sampling pulse will correspond

closely to the average voltage of the signal which was present near the

end of the sampling pulse. If the learning time is long compared with

the length of the sampling pulse, the voltage will correspond to the

average signal voltage during the last several sampling pulses. In this

way, variations in signal height caused by noise during each sampling
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pulse will be averaged out. For a discussion of the advantage in signalto-noise ratio that may be obtained by integrating over several sampling

pulses, see the article by Dwight O. North [ 1 9 63 J

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3. Experimental Technique

The sequence of operations is shown in Fig. 3. This sequence is

repeated continuously at time intervals somewhat longer than T . The

boxcar integrator output corresponds to the average signal height during

the sampling pulse. The sampling pulse is timed to occur near the peak

of the signal for the best signal-to-noise ratio. The VFO is set to a frequency on one side of the resonance frequency and swept through the

line by a motor drive. As the NMR signal is reduced by the ultrasonic

phonons, the recorder will follow this reduction and a recording of the

line will be obtained.

Some precautions are necessary to insure the successful operation of this system. The sweep rate must be slow enough so that the

frequency swept between sampling pulses is an incremental part of the

linewidth. Well regulated power supplies must be used to insure stability of the height of the NMR signal during the period of time necessary

to record the line. It is also desirable to synchronize the timing cycle

with the power line frequency so that any 60-Hz noise which is present

will be identical for each sampling pulse. The ultrasonic voltage must

be adjusted so that the NMR signal is not completely saturated when the

ultrasonic frequency is at the resonant frequency to avoid flattening of

the peak of the absorption line.

For use with a good signal-to-noise ratio and a large percentage

of saturation, the learning time constant is adjusted to 30 to 50 per cent

of the sampling pulse length so that changes in average signal height

will be followed on each pulse. Two samples of absorption lines in this
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category are shown in Figs. 5 and 6. Figure 4 shows the In and

113 121

In resonances in InSb. Figure 5 is a dual resonance of Sb and

Ga in GaSb. Both of these curves were obtained using single crystals

at liquid nitrogen temperature. The NMR system was at 7. 5 MHz and

the ultrasonic energy was at 15 MHz to induce Am = ±2 transitions.

Approximately twenty minutes were required to record each trace. In

this period of time, the signal was sampled approximately 600 times.

This system has also been used to record the effect of anharmonically

produced phonons in GaAs [Mahler, et al. , 1 963 J and in NaClO.,

[Mahler and Tanttila, 1963].

The boxcar integrator zero point may be shifted and the gain

increased to obtain a full scale recording of a much smaller effect.

The limit of the effect which may be observed is determined by the

stability of the equipment, the signal-to-noise ratio, and the learning

time constant. Under noisy conditions or conditions where only a small

effect is observed, the integration constant is made several times

longer than the sampling pulse to obtain a less noisy trace. The sweep

time must be correspondingly increased to allow the integrator time to

follow the desired variation in signal height. The effective time constant

which may be used to determine the necessary sweep time is obtained

by dividing the product of the learning time constant and the time between

samples by the length of the sampling pulse. The sweep time should be

greater than 100 times this effective time constant in order to obtain a smooth recording. To avoid waiting for the capacitor to charge to the

correct voltage at the start of a recording, the capacitor voltage may be

set by using the output -voltage -set provision.

One of the main advantages of this system in practice has proven

to be the ability to obtain a recording of an effect too small to be seen

with previous equipment. A recording of an absorption curve
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representing a 2 per cent reduction in height of the NMR signal is

shown in Fig. 6. This figure shows the first experimental evidence of

the hexadecapole interaction [Mahler, et al. , 1966J. The NMR system

was at 5 MHz and the ultrasonic energy at 1 5 MHz to induce Am = ±3

transitions.

4. Acknowledgements

The author is indebted to Dr. W. H. Tanttila for proposing the

principle of operation used, and to Dr. R. J. Mahler for the original

ultrasonic absorption work with the InSb and GaSb crystals.

12
5. References

Bolef, D. I., and M. Menes (1959), Nuclear magnetic resonance

acoustic absorption in KI and KBr, Phys. Rev. 114, No. 6, 1441.

Mahler, R. J., H. P. Mahon, S. C. Miller, and W. H. Tanttila (1963) , Nonlinear acoustic excitation in GaAs, Phys. Rev. Letters 10,

No. 9, 395.

Mahler, R. J., and W. H. Tanttila (1963) , Anharmonically produced

nuclear -transition frequency phonons in NaClO.,, Bull. Am.

Phys. Soc. 8, 468.

Mahler, R. J., L. W. James, and W. H. Tanttila (1966) , Possible

115

observation of In nuclear electric hexadecapole transitions,

Phys. Rev. Letters 1_6, No. 7, 259.

Menes, M. , and D. I. Bolef (1958), Observation of nuclear resonance

115

acoustic absorption on In in InSb, Phys. Rev. 109, No. 1, 218.

North, D. O. (1963), An analysis of the factors which determine signal/

noise discrimination in pulsed-carrier systems, Proc. IEEE 51,

No. 7, 1016.

Proctor, W. G. , and W. Robinson (1956), Ultrasonic excitation of

23

nuclear magnetic energy levels of Na in NaCl, Phys. Rev. 104 , No. 5, 1344.

Proctor, W. G. , and W. H. Tanttila (1956), Influence of ultrasonic

energy on the relaxation of chlorine nuclei in sodium chlorate,

Phys. Rev. 101, No. 6, 1757.

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