A holographic technique has been devised for generating a visible display of the effect of exposure of a photorefractive crystal to γ-rays. The technique exploits the space charge that results from trapping of electrons in defects induced by γ-rays.

Figure 1. A Hologram Is Formed in a Crystal to be exposed to γ rays. After exposure to γ rays, the crystal is remounted in this apparatus for reading of the hologram.

The technique involves a three-stage process. In the first stage, one writes a holographic pattern in the crystal by use of the apparatus shown in Figure 1. A laser beam of 532-nm wavelength is collimated and split into signal and reference beams by use of a polarizing beam splitter. On its way to the crystal, the reference beam goes through a two-dimensional optical scanner that contains two pairs of lenses (L1y,L2y and L1x,L2x) and mirrors M1 and M2, which can be rotated by use of micrometer drives to make fine adjustments. The signal beam is sent through a spatial light modulator that imposes the holographic pattern, then through two imaging lenses Limg on its way to the crystal. An aperture is placed at the common focus of lenses Limg to suppress high-order diffraction from the spatial light modulator. The hologram is formed by interference between the signal and reference beams.

Figure 2. This Image Shows the Effect of γ Rays that have passed through a crystal of Fe:LiNbO3.

A camera lens focuses an image of the interior of the crystal onto a chargecoupled device (CCD). If the crystal is illuminated by only the reference beam once the hologram has been formed, then an image of the hologram is formed on the CCD: this phenomenon is exploited to make visible the pattern of γ irradiation of the crystal, as described next.

In the second stage of the process, the crystal is removed from the holographic apparatus and irradiated with γ-rays at a dose of about 100 krad. In the third stage of the process, the crystal is remounted in the holographic apparatus in the same position as in the first stage and illuminated with only the reference beam to obtain the image of the hologram as modified by the effect of the γ-rays. The orientations of M1 and M2 can be adjusted slightly, if necessary, to maximize the intensity of the image. Figure 2 shows such an image that was formed in a crystal of Fe:LiNbO3.

This work was done by Danut Dragoi, Steven McClure, Allan Johnston, and Tien-Hsin Chao of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.

NPO-30622

Topics:
Exteriors Imaging and visualization Lasers Logistics Mirrors Optics Photonics Physical Sciences
This Brief includes a Technical Support Package (TSP).
Document cover
Imaging of y-Irradiated Fegions of a Crystal

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    Photonics Tech Briefs Magazine

    This article first appeared in the September, 2004 issue of Photonics Tech Briefs Magazine (Vol. 28 No. 9).

    Read more articles from the archives here.

    Overview

    The document is a Technical Support Package from NASA's Jet Propulsion Laboratory, focusing on the imaging of gamma-irradiated regions of a crystal, specifically using a photo-refractive crystal like Fe:LiNbO3. It outlines a sophisticated experimental setup designed to visualize radiation patterns induced in the crystal after gamma irradiation.

    The primary apparatus involves a laser beam (Verdi-5, λ=532 nm) that is collimated and split using a polarizing beam splitter (PBS). The reference beam is directed through a 2D optical scanner, which consists of two pairs of lenses and rotary mirrors (M1 and M2) that allow for fine adjustments. An aperture is strategically placed to limit high diffraction orders, ensuring that only the relevant signal beam produced by a spatial light modulator (SLM) device is captured.

    The procedure begins with recording a hologram in the photo-refractive crystal. After the hologram is recorded, the crystal is removed and irradiated with gamma radiation at a dose of approximately 100 krad. The crystal is then repositioned in the same orientation within the optical setup. The reference beam is used to retrieve the image of the hologram, and adjustments to the rotary mirrors are made to enhance the intensity of the resulting image, revealing the radiation pattern.

    The document emphasizes the importance of precise alignment and adjustment of the optical components to achieve high-quality imaging of the radiation patterns. The setup is designed to facilitate the visualization of the effects of gamma radiation on the crystal, which has implications for various scientific and technological applications, including materials science and radiation detection.

    Additionally, the document serves as a resource for those interested in the broader applications of aerospace-related developments, providing contact information for further assistance and access to additional resources from NASA's Scientific and Technical Information Program Office.

    Overall, this Technical Support Package highlights a methodical approach to imaging gamma-irradiated regions in crystals, showcasing the intersection of advanced optical techniques and materials research, with potential applications in both scientific inquiry and commercial technology.