ULTRASONIC ABSORPTION MICROSCOPE 



scissa of the figure indicates the position of 

 the filament specimen in the plane of move- 

 ment parallel to the crystal face. The or- 

 dinate indicates the relative .acoustic inten- 

 sity detected by the probe, the scale being in 

 units of deflection on the oscilloscope screen. 

 The minimum in the curve corresponds to 

 the position of closest approach of the nylon 

 filament to the probe as the specimen tra- 

 verses the acoustic field. The presence of the 

 filament at the position of closest approach 

 causes a reduction of 30% in the acoustic 

 intensity below the undisturbed level. At 

 half of this reduction, the curve is 0.007 

 inch wide and the width is equal to the fila- 

 ment diameter (0.003 in.) at 0.8 of the total 

 reduction. Not all of the observed reduction 

 is produced, in this case, by absorption of 

 sound within the nylon. Since there is a mis- 

 match in acoustic impedance between the 

 coupling liquid and the filament of at least 

 10%, some of the incident acoustic energy 

 is scattered. In addition, some of the sound 

 energy is converted to heat at the interface 

 between the nylon filament and the coupling 

 liquid as a result of the viscous forces acting 

 there. 



Measurements similar to those shown 

 in Fig. 2, but obtained with copper filaments 

 of 0.001 inch diameter in the field, demon- 

 strate that at an operating frequency of 12 

 mc/sec, model structures of this type with 

 a diameter of 25 microns can be resolved. 



An approximation analysis (3) (based on 

 formulas derived to describe the behaviour 

 of a thermoelectric probe in a pulsed acoustic 

 field) (8, 9, 10) of the operation of the ultra- 

 sonic microscope indicates that a structure 

 with a "radius" of 0.4 micron and having an 

 acoustic intensity absorption coefficient per 

 unit path length differing from the average 

 value of the specimen by 5% should be 

 detectable if the following acoustic and other 

 parameters are employed: frequency — 1,000 

 mc/sec, ultrasonic intensity — 1,000 watts/ 

 cm2, acoustic pulse duration — 0.1 micro- 

 second, and thermocouple lead diameter-0.1 



micron. A convenient pulse repetition rate 

 for rapid assimilation of data would be 1000 

 pps. The "radius" of the structure is defined 

 to be that distance from its center at which 

 the deviation of the absorption coefficient 

 from the average value of the surroundings 

 is down to 0.7 of the maximum deviation. 

 If the absorption coefficient of the structure 

 differs from the average by a greater per- 

 centage, then a smaller structure can be 

 detected. 



Note added in proof: 



Since the preparation of this article, tech- 

 niques have been developed for producing 

 sound fields of appreciable amplitude in high 

 absorption liquids to 500 mc-sec and for 

 fabricating thermoelectric detectors having 

 maximum dimensions in the vicinity of the 

 junction of 5 microns (11). 



REFERENCES 



1. See appropriate sections of this encylopedia 



for comprehensive descriptions of light, 

 phase-contrast and electron microscopy. 



2. Dunn, F., and Fry, W. J., J. Acoust. Soc. 



Am., 31, 632-633 (1959). 



3. Fry, W. J., and Dunn, F., "Physical Tech- 



niques in Biological Research," ed. W. L. 

 Nastuck, Academic Press, New York (to be 

 published, 1961). 



4. Carstensen, E. L., and Schwan, H. P., 



"Ultrasound in Biology and Medicine," 

 ed. E. Kelly, Am. Inst. Biol. Sci., Washing- 

 ton, D. C. (1957). 



5. Schwan, H. P., and Carstensen, E. L., 



WADC Tech. Rept. 56-389, Wright Air De- 

 velopment Center (1956). 



6. Carstensen, E. L., and Schwan, H. P., J. 



Acoust. Soc. Am., 31, 185-189 (1959). 



7. Carstensen, E. L., and Schwan, H. P., J. 



Acoust. Soc. Am., 31, 305-311 (1959). 



8. Fry, W. J., and Fry, R. B., /. Acoust. Soc. 



Am., 26, 294-310 (1954). 



9. Fry, W. J., and Fry, R. B., /. Acoust. Soc. 



Am., 26, 311-317 (1954). 



10. Dunn, F., and Fry, W. J., I.R.E. Trans, on 



Ultrasonic Engineering, PGUE-5, 59-65 

 (1957). 



11. Dunn, F., /. Acoust. Soc. Am., 32 (1960). 



Floyd Dunn and Wm. J. Fry 



547 



