Controllable variation of the intensity of diffracted X-ray beams and double modulation of such beams for transmission and reception of audio information

M.A. Navasardyan, J. Appl. Crystallogr. (2001). 34, 763–766

Figure 1. Geometry of the experiment for observation of the phenomenon of controllable complete reflectionin the case of a quasi-plane wave in the following settings of a double-crystal spectrometer: (a) (+, –)quartz–quartz, (b) (+, +) quartz–quartz, (c) (+, –)germanium–quartz. 1: the X-ray tube with Mo anode;2: crystal monochromator; 3: studied crystal (crystal analyser); 4: heater; 5: profiles of transmitted Mo Kα₁and Mo Kα₂ beams in the absence (from the top) and in the presence (from below) of a temperature gradient;6: transmitted Kα₁ and Kα₂ beams, when only theKα₁ beam is reflected (from the right-hand side); 7:transmitted (T) and reflected (R) Mo Kβ beams. The thickness of Ge is t = 0.3 mm and of quartz is t ≅ 1 mm.On beam R in position 7, the size of the reflected beam corresponds to the vertical size of the quartz plate.

Communicating acoustic information with the help of Bragg reflections may seem surprising at first sight. Prof. Navasardyan of Yerevan State U., Armenia addresses the topic in a recent issue of the J. Applied Crystallography.

The story starts in the 1930s, when the diffracted X-ray intensity from a quartz crystal was noticed to increase when applying a temperature gradient or ultrasonic oscillations. Such investigations gathered momentum after the discovery (in 1941) of the Borrmann effect, or anomalous transmission, through relatively perfect single crystals. Two régimes were of interest: those with μt ≤ 1 or > 10, where μ is the linear absorption coefficient and t the thickness of the crystal. Explanations for unusual behaviour of the transmitted beams were sought in X-ray standing waves and in a decrease in primary extinction. With a temperature gradient of a certain magnitude and direction (perpendicular to the reflecting atomic planes), the energy of the incident beam was wholly diverted into the reflected beam.

Controllable complete reflection in the Laue case, 'forced' by a temperature gradient or ultrasonic oscillations, is distinguished from the total reflection in Bragg geometry within the angular range of Darwin reflection. Controllable complete reflection has been demonstrated for quasi-plane waves in double-crystal (+,+) and (+,–) geometries (see Fig. 1).

Figure 2. Scheme of a practical device for the transmission and reception of acoustic oscillations (speech) by means of diffracted X-ray beams. 1: source of X-radiation; 2: monochromator; 3: crystal modulator; 4: goniometer; 5, 6, 7: microphone, low-frequency and high-frequency generators; 8: scintillation detectors; 9: integrating circuits; 10: amplifiers; 11a, 11b: oscilloscope with appropriate oscillograms; 12: loudspeakers. A and B are the transmitting and receiving parts of the device, respectively.

Of interest is the time-dependence of the X-ray intensity from the crystal, simultaneously excited by ultrasound resonance (of high frequency ν) and audio oscillations (of low frequency ω): that is, double modulation. In crystals of medium thickness (μt ≤ 1) under conditions of controllable complete reflection, the diffracted (R) beam intensity can equal the initial intensity of the transmitted (T) beam. During one oscillation period, there can be several hundreds to hundreds of thousands of photons in a beam from an X-ray tube (35 kV and 15 mA) and a detector registers a continuous photon count varying by a low frequency (ω ≤ 20 kHz). It is thus possible to transmit and to receive audio information (e.g., speech) by the X-ray beam and a possible scheme is shown in Fig. 2.

The quartz crystal modulator is set for Bragg reflection in Laue geometry and resonance oscillations (ν = 2880/t) so that the condition t = λ/2 is fulfilled, where λ is the wavelength of the acoustic wave in quartz. The diffracted beam intensity increases (by a tenfold factor) and the low-frequency (ω) generator (a microphone or tape recorder) modulates the crystal. The diffracted beam intensity also oscillates with frequency ω, which is registered on the scintillation counter after it has travelled some distance, possibly penetrating barriers inaccessible to other forms of radiation. In this paper, Navasardyan also lists other uses, including the study of fast chemical reactions, the tracking of time-dependent processes in crystals and the characterization of synchrotron and other sources of X-radiation.