David L. Anderson

Facility for non-destructive analysis for major and trace elements using neutron-capture gamma-ray spectrometry

A facility for neutron-capture γ-ray spectroscopy for analytical purposes has been developed and tested at the National Bureau of Standards reactor. The system consists of an internal beam tube with collimators, an external beam tube and irradiation station, a Compton-suppressed Ge(Li) γ-ray detection system, and a minicomputer-based data-collection and-analysis system. Detection limits have been established for many elements and errors arising from neutron self shielding, γ-ray peak overlap, neutron beam variations, and sample matrix evaluated.

Sources and atmospheric distribution of particulate and gas-phase boron

Abstract Atmospheric samples were collected during warm and cold weather conditions at continental, coastal and marine sites. Source sampling was performed at a coal-fired power plant and several volcanic sites. Atmospheric gas-phase and particulate boron concentrations were determined by neutron capture prompt-gamma activation analysis and compared to measurements from other studies. Rain and snow samples collected at one continental site were analysed for soluble and insoluble B. Volcanic deposit and ash samples were also analysed for B. The tropospheric burdens for particulate and gas-phase B were estimated to be 0.6 × 1010 g and (6–11) × 1010 g, respectively, with the latter about a factor of 3 lower than previous estimates. Global anthropogenic particulate and gas-phase B source estimates were consistent with previous estimates, and natural particulate and gas-phase B source estimates agreed reasonably well with previously reported upper limits. About 65–85 % of total B source strength can be attributed to the oceans, and 8–20% to coal, agricultural, fuelwood and refuse burning. Volcanism may contribute 6–15% of the total, but accurate source estimates are difficult. Total global B removal from the atmosphere by wet and dry deposition was estimated to be (530–700) × 1010 g yr−1, a factor of 2 lower than previously reported but in fairly good agreement with the global total B source estimate of (200–530) × 1010 g yr−1.

Simultaneous collection of particles and gases followed by multielement analysis using nuclear techniques

A simple system was constructed for the separation and collection of atmospheric particles and gases. The five-stage sampling train consists of a particle filter followed by four 7LiOH-impregnated filters to collect acidic gases. The treated filters quantitatively collect acidic gas-phase species of B, S, Cl, Br, I, As, Se, Sb and Hg. Simultaneous sampling with activated charcoal-impregnated and 7LiOH-treated filters revealed that B, S, and As species were found only as acidic gases, while the others had both acidic and nonacidic contributions. Concentrations of up to 48 elements on the particle filters and nine elements on the treated filters were determined using instrumental neutron activation analysis and neutron capture prompt γ-ray activation analysis. Minimum sampling volume estimates, average filter blank concentrations, and average atmospheric concentrations at College Park, MD and Wallops Island, VA are presented.

The use of Whatman-41 filters for particle

Abstract Collection efficiencies have been experimentally determined for Whatman-41 cellulose filters during ambient sampling with a stacked-filter train. Efficiencies >2 98% were found for elements associated with coarse-fraction aerosol, but become less for fine-particle elements. By using Whatman prefilters alone, fine-particle penetrations of

Analytical neutron‐capture gamma‐ray spectroscopy: Status and prospects

The use of neutron‐capture gamma rays for elemental analysis has become an established technique, applicable for the measurement of a list of elements which complements conventional delayed‐gamma neutron activation analysis. Three distinct areas of application of the prompt method have been laboratory‐based analysis using reactor neutrons, field measurements (especially borehole logging for mineral exploration), and industrial process stream analysis, the latter two using neutron generators or isotopic neutron sources. Continued improvements in detector systems and the increasing availability of clean, high‐intensity beams from cold‐neutron guide tubes are opening quantitatively and qualitatively new analytical possibilities. Second generation instruments using cold neutrons promise an increase in reaction rate of up to two orders of magnitude. This will increae the productivity of these facilities by a similar factor, and will make the use of capture gamma rays more nearly comparable in sensitivity to co...

NEUTRON CAPTURE PROMPT GAMMA-RAY ANALYSIS OF STRATOSPHERIC AND GROUNDFALL ASH FOLLOWING THE MT. ST. HELENS ERUPTION OF MAY 18, 1980

Neutron-capture prompt γ-ray analysis (PGAA) is an exceptionally good method for the determination of the major and minor elements Na, Mg, Al, Si, K, Ca, Ti, Mn and Fe in most geological matrices. The trace elements H, B, S, Cl, Cd, Sm and Gd can often be readily measured in most geological materials. This technique was applied to volcanic ash samples collected in Washington and Montana by various groups following the May 18 eruption of the Mt. St. Helens volcano, as well as to several particulate samples collected at altitudes between 13 and 18 km by a NASA U-2 research aircraft. Groundfall ash samples show distinct elemental variations versus distance from the volcano. For example, Si/Al ratios vary from 2.85 near the volcano to about 3.70 at≽400 km to the east. Samples collected sequentially at a given location (Almira, Washington) also exhibit variations in the elemental ratios. These sequential samples were also subjected to leaching studies to determine soluble B, Cl and S. On the average, PGAA is able to account for 99.4±1.7% of the mass of each sample when the elements are considered to be in their common oxide forms. The stratospheric samples were collected May 19 on IPC cellulose filters on apparatus in a wing tank of the U-2. The PGAA results indicate that the sample from the greatest altitude resembles ashfall near the volcano, while those at lower altitudes somewhat resemble more distant ashfall.

Neutron-Capture Prompt Gamma-Ray Activation Analysis: Multi-Element Measurements on Various Materials

A beam port was installed at the National Bureau of Standards (NBS) reactor to initiate studies of a new analytical technique: neutron-capture prompt γ-ray activation analysis (PGAA). A detection system based on a large, high resolution Ge(Li) detector surrounded by a NaI crystal is used for measurement of prompt γ rays from 80 keV to 11 MeV. Gamma-ray spectra of many pure elements and simple compounds were collected to select appropriate lines for analysis and to determine interferences. Several NBS Standard Reference Materials and other types of samples have been investigated. About fourteen elements have been determined in most samples, including H, B, C, N, Na, Al, Si, S, Cl, K, Ca, Ti, V, Mn, Fe, Cd, Sm and Gd.