Cynthia J. Zeissler

THE ROLE OF INCLUSIONS IN U-PB AND SM-ND GARNET GEOCHRONOLOGY : STEPWISE DISSOLUTION EXPERIMENTS AND TRACE URANIUM MAPPING BY FISSION TRACK ANALYSIS

The U-Pb and Sm-Nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes. However, the budgets of uranium, lead, samarium, and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon. A combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium, lead, samarium, and neodymium in garnets used for U-Pb and Sm-Nd chronology. Variations of Th/U, U/Nd, and Sm/Nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions. The stepwise dissolution procedure does not induce any apparent artifacts on 207Pb-206Pb and Sm-Nd ages. The technique has been applied to garnets from three metamorphic terranes. Almandine garnet samples from the high grade Pikwitonei Granulite Domain (Manitoba, Canada) and the Wind River Range (Wyoming, USA) have the majority of their uranium, samarium, neodymium, and radiogenic lead (Pb*) hosted by micrometer-scale inclusions of monazite. Fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets. Stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite. In contrast, grossularandradite garnets from Cascade Slide (Adirondack Highlands, NY, USA) show little variation in fission track density and similar isotopic ratios between dissolution steps, indicating that the budgets of uranium, samarium, neodymium, and Pb* are not significantly influenced by inclusions. The demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in U-Pb and Sm-Nd garnet geochronology of metasedimentary rocks.

Radioactive particle analysis by digital autoradiography

We have been exploring ways to evaluate the activity of radioactive particles that have been detected by phosphor plate digital autoradiography based on photostimulated luminescence (PSL). A PSL system with 25 μm pixel digitization has been applied to particle analysis problems, both qualitatively and quantitatively. Two data evaluation methods are currently employed: (1) bulk area signal measurement, and (2) discrete event counting which may include spectral evaluation. The first method is conventional, whereas the second method requires high spatial resolution and is presented here for the first time. The counting methods can discriminate between alpha and background counts. The unshielded background signal accumulation rate was determined by the bulk area method. Using the spectral method of evaluation for α-particle events, the mean signal intensity per recorded α-particle was measured, and the detective quantum efficiency (DQE) was found to be nominally 100%. We present a comparison to gamma-spectrometry for sub-Bq 137Cs activities, and demonstrate an application for the qualitative assay of International Atomic Energy Agency swipe samples collected from uranium enrichment facilities.

Prompt gamma activation analysis enhanced by a neutron focusing capillary lens

Abstract A focusing neutron lens using glass polycapillary fibers has been introduced successfully into a prompt gamma activation analysis (PGAA) instrument placed at the exit of a cold neutron guide. The neutron current density gain of the lens is 80, averaged over the focused beam size of 0.53 mm diameter. PGAA measurements have been made on submillimeter particles of gadolinium and cadmium. The results indicate that elemental sensitivities of measurements are increased by ∼ 60, and that particles of sizes smaller than 0.5 mm can be discerned using the focusing lens. The measured gain in prompt gamma signals for these particles is less than anticipated, probably due to alignment difficulties. Gamma ray background associated with the lens is discussed and improvements are suggested.

High-resolution charged particle and neutron imaging using charge injection devices

A charge injection device (CID) camera and image processing system have been used as a position sensitive detector for energetic charged particles and low energy neutrons. This video radiation detector (VRD) is simple in design but highly effective for real-time radiography and dosimetry with many advantages characteristics. The VRD currently has a dynamic range of 65,000 intensity levels for a 755 X 484 pixel matrix, an active area of 7 mm X 9 mm, a spatial mapping resolution of about 14 micrometers for single detected events (7 micrometers for radiation from a point source), and is sufficiently radiation-hard to be operated in a neutron beam for extended periods of time. Radiation images are updated at a rate of thirty frames per second. The VRD is sensitive to fission fragments, alpha particles, and slow neutrons. Using commercially available image processing hardware and software and an off-the-shelf camera, the system is inexpensive, easy to use with simple interpretation of data, and is capable of performing radiography with only minimal adaptations. Applications in our laboratory include the characterization of focused cold neutron beams, the mapping of uranium and lithium distributions in samples by the detection of neutron absorption reaction products, and the mapping of spontaneous alpha radioactivity from environmental samples. Results provide information on x-y position, counts received, and energy deposited per count, each as a function of time.

Detection and characterization of radioactive particles

Abstract Recent advances in instrumentation provide the capability to measure size, count rate, type of radiation, and the chemical composition of radioactive particles. We present a demonstration of these capabilities, using a laser-scanned phosphor imaging system and a charge injection device system for digital autoradiography, light and electron microscopy, energy dispersive X-ray spectrometry, and conventional radionuclide spectrometry. Response characteristics of the phosphor system that support these efforts are presented.

Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a58Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the γ-count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).

Comparison of semiconductor pixel array, phosphor plate, and track-etch detectors for alpha autoradiography

Abstract The need to locate “hot particles” during environmental radioactivity characterization benefits from autoradiography. Three detectors are compared for the evaluation of point-source alpha emitters. 238 PuO 2 particles were used to compare a detector based on a pixel array, a phosphor imaging plate system, and a track-etch detector. The pixel-array detector is based on a charge injection device (CID) having nominally 14 μm sized pixels in a 7 mm × 9 mm array. Processing at a 30 frames per second rate allows real-time display of results. Background interference is minimal even in long-duration exposures. The phosphor imaging plate system is subject to background accumulation. Track overlap makes single-event discrimination and quantification impractical, but a large exposure area and ease of use are distinct advantages. A system having a pixel size of 50 μm was used. Track-etch detectors have the largest potential exposure area and best spatial resolution, the latter being limited by the amount of material etched, resolving power of the reading system, and the number of tracks recorded per source. Readout, however, can be time-consuming, and track overlap can make quantification difficult. The best apatial resolution, 10 μm, was obtained with the track-etch detector, with a precision in location of 1 μm. The phosphor plate system was capable of detecting 0.002 Bq sources in 2 d, whereas the track-etch and semiconductor pixel array were capable of detecting 0.0002 Bq sources in 2 d. The track-etch method was the most difficult, whereas the most rapid results were obtained with the real-time display of the semiconductor pixel array.

Etched topography in nuclear track detectors and effect on quantitation related to sample composition

Abstract When using the nuclear track technique for multi-element detection and spatial mapping, the presence of moderate to high concentrations of boron or lithium results in a large number of (n,α) reactions when using the high thermal neutron fluences required to map trace amounts of uranium. When chemically etched, tracks generated by a large number of alpha particles cannot be spatially resolved, and are manifested instead by large-scale depressions in the detector surface. High Li and B concentration samples have the most pronounced topographic development in both CR-39 and CN detectors. Samples with low concentrations of Li and B or high concentrations of Gd, Sm or Eu developed moderate depressions. These depressions can affect uranium quantitation, but simultaneously, they provide useful information about the sample. This paper is a preliminary study of observations of etched bulk depressions, variations in track revelation time, and the mapping of samples containing elements with high thermal neutron absorption cross sections such as gadolinium.