Abstract
This paper demonstrates the feasibility of an Electret Sokolov Camera (ESC) which employs electron-beam scanning of a subdivided-backplate foil-electret array. The mechanism of electron-beam reading is described and the different stable operating modes are identified. In one operating mode, the threshold ultrasound sensitivity of the ESC is estimated as better than 2×10-6 watts/cm2 for operation in water at 1MHz. This compares favorably with other real-time imaging systems such as the subdivided-foil-subdivided-backplate array and the conventional Sokolov camera (which employs a piezoelectricplate array). However, unlike the conventional Sokolov camera, the ESC does not suffer from limitations of physical and angular aperture. Furthermore, the transducing sensitivity of the ESC can be optimized independently of the material of the backplate. This added flexibility offers promise for further enhancement of the sensitivity.
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References
A. K. Nigam, K. J. Taylor and G. M. Sessler, in “Acoustical Holography, Vol. IV,” Springer Science+Business Media New York (1972) p. 173;
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An assessment of the complexity of interconnections for the N2 array is provided in References 1 and 20. See also M. G. Maginness, J. D. Plummer and J. D. Meindl, this volume.
Another drawback of the 2N array appears to be higher interelement crosstalk due to residual sensitivity of elements. The results of references 1 and 2 above, indicate that this drawback is eliminated by suitable choice of foil-material having the appropriate bulkresistivity and/or by applying opposite polarity biasvoltages during alternate scanning frames.
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At the point V=V c1 the equality I=I’ is also satisfied, however, this point does not correspond to stable equilibrium for small perturbations in the target potential V.
The signal may also be detected at the front electrode of the foil (Fig. 1) , however, it appears that this scheme may suffer from crosstalk if large stray capacitance loading is evident on the backplate elements.
The results of a more detailed analysis (see Ref. 19 below), in which several other parameters as well as space charge smoothening-effects have been considered, indicate that for certain target (backplate) materials VT has another minimum (for a second value of I) which may correspond to its lowest value.
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Nigam, A.K., French, J.C. (1974). Modified Sokolov Camera Utilizing Condenser-Microphone Arrays of the Foil-Electret Type. In: Green, P.S. (eds) Acoustical Holography. Springer, Boston, MA. https://doi.org/10.1007/978-1-4757-0827-1_41
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DOI: https://doi.org/10.1007/978-1-4757-0827-1_41
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