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A multichannel silicon pad detector for EXAFS (Extended X-ray Absorption Fine Structure) applications has been designed and built. The X-ray spectroscopic measurements demonstrate that an adequate energy resolution of 230 eV FWHM (corresponding to 27 rms electrons in silicon) can be achieved reliably at -35 °C. A resolution of 190 eV FWHM (corresponding to 22 rms electrons) has been obtained from individual pads at -35 °C. At room temperature (25 °C) an average energy resolution of 380 eV FWHM is achieved and a resolution of 350 eV FWHM (41 rms electrons) is the best performance. A simple cooling system constituted of Peltier cells is sufficient to reduce the reverse currents of the pads and their related shot noise contribution, in order to achieve resolutions better than 300 eV FWHM which is adequate for the EXAFS applications.

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X-ray absorption studies of dilute samples require fluorescence detection techniques. Since signal-to-noise ratios are governed by the ratio of fluorescent to scattered photons counted by a detector, solid state detectors which can discriminate between fluorescence and scattered photons have become the instruments of choice for trace element measurements. Commercially available 13 element Ge array detectors permitting total count rates < 500000 counts per second are now in routine use. Since X-ray absorption beamlines at high brightness synchrotron sources can already illuminate most dilute samples with enough flux to saturate the current generation of solid state detectors, the development of next-generation instruments with significantly higher total count rates is essential. We present the design and current status of the 100 element Si array detector being developed in a collaboration between the NSLS and the Instrumentation Division at Brookhaven National Laboratory. The detecting array consists of a 10×10 matrix of 4 mm×4 mm elements laid out on a single piece of ultrahigh purity silicon mounted at the front end of a liquid nitrogen dewar assembly. A matrix of charge sensitive integrating preamplifiers feed signals to an array of shaping amplifiers, single channel analyzers, and scalers. An electronic switch, delay amplifier, linear gate, digital scope, peak sensing A/D converter, and histogramining memory module provide for complete diagnostics and channel calibration. The entire instrument is controlled by a LabView 2 application on a MacII ci; the software also provides full control over beamline hardware and performs the data collection.

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Human ocular tissues from 50 donor eyes were elementally and morphologically analyzed in order to correlate the elemental content and distribution of Ca, Ba, Cr, Cu, Zn, and Se with ocular morphology, sex, race, irideal pigmentation, age, time of death, birth weight, presence and severity of diabetes, and other pathologies noted at autopsy. Initially, to facilitate the transport of donor tissue to the laboratory, the eyes were fixed in glutaraldehyde. Because our preliminary data revealed alterations in elemental content following chemical fixation of ocular tissues, all of the subsequent samples were analyzed in their fresh, hydrated (unfixed) condition as soon after enucleation as possible. Samples were elementally analyzed by X-ray fluorescence spectrometry (XRF) and proton-induced X-ray emission spectrometry (PIXE) using high resolution Si(Li) X-ray detectors. Tissue was morphologiscally examined by scanning electron microscopy and light microscopy histoichemistry.

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Bone cortical thickness and density were measured in rat femurs in vitro by a photon absorptiometry technique. A scan perpendicular to the long bone axis using photons from highly collimated 109Cd or 125I sources yielded the bone wall thickness and attenuation at mid-shaft. A second scan after the bone was rotated axially 90 degrees was taken to measure the wall or shaft thickness identically responsible for the mid-shaft attenuation of the first scan. The mid-shaft attenuation of the second scan was due to the bone thickness derived from the first scan; thus two complementary measurements are derived by this technique. The bone density is then directly calculated with an estimated accuracy of 10% (coefficient of variation) using empirically determined mass attenuation coefficients. Measurements of mass attenuation coefficients of ashed and dried samples were compared with calculated coefficients for estimated bone composition. Results indicate that the mass attenuation coefficient for bone in vivo can be well estimated such that bone density can be accurately derived from this technique.

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A technique has been developed to micromap deuterium by using the D(T,n)(4)He reaction and plastic track detectors. Labeling of cells with subpicogram quantities of deuterium was demonstrated. The technique was used to localize human lymphocytes transformed in vitro.

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Diffusion of radon and thoron from the Ilunar surface provides a mechanism for production of a radioactive surface layer on the moon. If the radon and thoron flux from the lunar surface is equal to that measured at the earth's surface, the equilibrium activity of this surface layet is estimated as approximately 1 microcurie per square meter, due to radon and its decay products. This activity consists of alpha particles and gamnmna rays at well-defined energies and of beta rays.

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Lithium-drifted germanium detectors for high-resolution gamma-ray spectroscopy reduce the need for wet chemistry in neutron-activation analysis. Problems in fields as diverse as geochemistry and the history of 15th-century printing have proved susceptible to this analytic technique.