Paul Falkenstein

Remote optical fiber dosimetry

Abstract Optical fibers offer a unique capability for remote monitoring of radiation in difficult-to-access and/or hazardous locations. Optical fiber sensors can be located in radiation hazardous areas and optically interrogated from a safe distance. A variety of remote optical fiber radiation dosimetry methods have been developed. All of the methods take advantage of some form of radiation-induced change in the optical properties of materials such as: radiation-induced darkening due to defect formation in glasses, luminescence from native defects or radiation-induced defects, or population of metastable charge trapping centers. Optical attenuation techniques are used to measure radiation-induced darkening in fibers. Luminescence techniques include the direct measurement of scintillation or optical excitation of radiation-induced luminescent defects. Optical fiber radiation dosimeters have also been constructed using charge trapping materials that exhibit thermoluminescence or optically stimulated luminescence (OSL).

Characterization of a fiber‐optic‐coupled radioluminescent detector for application in the mammography energy range

Fiber-optic-coupled radioluminescent (FOC) dosimeters are members of a new family of dosimeters that are finding increased clinical applications. This study provides the first characterization of a Cu doped quartz FOC dosimeter at diagnostic energies, specifically across the range of x-ray energies and intensities used in mammographies. We characterize the calibration factors, linearity, angular dependence, and reproducibility of the FOC dosimeters. The sensitive element of each dosimeter was coupled to a photon counting photomultiplier module via 1 m long optical fibers. A computer controlled interface permitted real-time monitoring of the dosimeter output and rapid data acquisition. The axial-angular responses for all dosimeter models show nearly uniform response without any marked decrease in sensitivity. However, the normal-to-axial angular response showed a marked decrease in sensitivity of about 0 degrees C and 180 degrees C. In most clinical applications, appropriate dosimeter positioning can minimize the contributions of the varying normal-to-axial response. The FOC dosimeters having the greatest sensitive length provided the greatest sensitivity, with greatest to lowest sensitivity observed for 4.0, 1.9, 1.6, and 1.1 mm length sensitive elements. The average sensitivity of the dosimeters varies linearly with sensitive volume (R2=95%) and as a function of tube potential and target/filter combinations, generally exhibiting an increased sensitivity for higher energies. The dosimeter sensitivity as a function of tube potential had an average increase of 4.72 +/- 2.04% for dosimeter models and three target-filter combinations tested (Mo/Mo, Mo/Rh, and Rh/Rh) over a range of 25-31 kVp. All dosimeter models exhibited a linear response (R2 > or = 0.997) to exposure for all target-filter combinations, tube potentials, and tube current-time product stations evaluated and demonstrated reproducibility within 2%. All of the dosimeters examined in this study provided a response adequate for the accurate measurement of doses in clinical mammography applications.

Fused preforms for the fabrication of photonic crystal fibers

A novel approach for the fabrication of high-quality preforms for use in the fabrication of photonic crystal fibers is described. The preforms are fabricated in a multistep process that involves stacking a bundle with rods and (or) tubes of two dissimilar glasses, fusing the bundle, and then etching the fused bundle in acid to remove one of the two glasses. The procedure for fabrication of the fused preforms is similar to that used in the fabrication of microchannel plate glass and yields periodically spaced, uniform, round channels that extend through the length of the preform.

Fused array preforms for the fabrication of photonic crystal fibers

A new approach for the fabrication of preforms used in drawing photonic crystal fibers is described. A multi-step process is used that involves stacking a bundle of rods and tubes of dissimilar glasses, fusing the bundle, and then etching the fused bundle to remove one of the two glasses.

Large Area, High Aspect Ratio Microelectrode Arrays

A method is described for the fabrication of arrays of conducting, high aspect-ratio microwires for use as electrodes. The electrode arrays are fabricated by electrochemical deposition of metals, including Ni, Pt, Ag, Au and Rh, in channel glass templates having parallel, uniform, hollow channels with diameters that range from sub-micrometer to over 100 micrometers. The metals completely fill the hollow channels, yielding highly uniform electrodes with aspect ratios on the order of 1000 or more. The glass template electrically insulates the electrodes from one another. The electrode array wafers are cut and polished to a thickness ranging from about 100 to 2000 micrometers. The overall surface area is as large as 1 square centimeter. Alternatively, the wafers can be partially etched with acid to remove some of the glass matrix surrounding the electrodes, exposing an array of bare, solid wire stubs. The high aspect ratio microelectrode arrays were initially fabricated in order to provide the electrical interface for an intraocular retinal prosthesis, but have additional applications including biological and chemical sensing. Arrays with different channel sizes, different electrode spacing (with pitch from approximately 3R to 20R, where R is the electrode radius), and different geometrical arrangement are presented.

Multiple channel optical fiber radiation dosimeter for radiotherapy applications

A seven-channel optical fiber radiation dosimeter system has been developed to monitor doses received by patients undergoing radiation therapy. The dosimeter is based on the use of a copper-doped fused quartz material that exhibits optically stimulated luminescence following exposure to a radiation field. The system has been characterized in a clinical setting and found to provide a linear dose response over the entire range of doses that are commonly encountered during external beam radiation therapy.