Publications

Abstract: We investigated the uniformity of electron transit times for two 5-in photomultiplier tubes: the Hamamatsu R1250 and the Adit B133D01S. We focused a highly attenuated short-pulse laser on the tubes while they were mounted on a programmable stage. The stage translated the tubes relative to the incident beam so that measurements could be made with light focused at points along a grid covering the entire photocathodes. A portion of the incident light was split from the incident beam and measured and recorded by a fast photodiode. Electron transit times were measured by computing the time delay between the recorded photodiode signal and photomultiplier signal using software constant-fraction discrimination. The Hamamatsu tube exhibited a uniform timing response that varied by no more than 1.7 ns. The Adit tube was much less uniform, with transit times that varied by as much as 57 ns. The Adit response also exhibited a spatially varying double-peak structure in its response. The technique described in this paper could be usefully employed by photomultiplier tube manufacturers to characterize the performance of their products.

Abstract: Using the combination of a neutron-sensitive ⁶Li glass scintillator detector with a neutron-insensitive ⁷Li glass scintillator detector, we are able to make an accurate measurement of the capture rate of fission neutrons on ⁶Li. We used this detector with a ²⁵²Cf neutron source to measure the effects of both non-borated polyethylene and 5% borated polyethylene shielding on detection rates over a range of shielding thicknesses. Both of these measurements were compared with MCNP calculations to determine how well the calculations reproduced the measurements. When the source is highly shielded, the number of interactions experienced by each neutron prior to arriving at the detector is large, so it is important to compare Monte Carlo modeling with actual experimental measurements. MCNP reproduces the data fairly well, but it does generally underestimate detector efficiency both with and without polyethylene shielding. For non-borated polyethylene it underestimates the measured value by an average of 8%. This increases to an average of 11% for borated polyethylene.

Abstract: The response of a He-3 neutron detector is highly dependent on the amount of moderator incorporated into the detector system. If there is too little moderation, neutrons will not react with the He-3. If there is too much moderation, neutrons will not reach the He-3. In applications for portal or border monitors where He-3 detectors are used to interdict illicit importation of plutonium, the fission source is always shielded to some extent. Since the energy distribution of neutrons emitted from the source depends on the amount and type of shielding present, the optimum placement of moderating material around He-3 tubes is a function of shielding. In this paper, we use Monte Carlo techniques to model the response of He-3 tubes placed in polyethylene boxes for moderation. To model the shielded fission neutron source, we use a point Cf-252 source placed in the center of polyethylene spheres of varying radius. Detector efficiency as a function of box geometry and shielding is explored. We find that increasing the amount of moderator behind and to the sides of the detector generally improves the detector response, but that incremental benefits are minimal if the thickness of the polyethylene moderator is greater than about 5-7 cm. The thickness of the moderator in front of the He-3 tubes, however, is very important. For bare sources, about 4-5 cm of moderator is optimum, but as the shielding increases, the optimum thickness of this moderator decreases to 0.5-1 cm. Similar conclusions can be applied to polyethylene boxes employing two He-3 tubes. Two-tube boxes with front moderators of non-uniform thickness may be useful for detecting neutrons over a wide energy range. (c) 2012 Elsevier B.V. All rights reserved.

Abstract: Lichen samples were collected and observations about lichen communities were recorded at sites in the intermountain western United States. Specifically the states of Idaho, Montana, Wyoming, Nevada, Utah, Colorado, Arizona, and New Mexico were included in this study. The minor and trace-element concentrations in many of the lichens collected were determined by proton induced X-ray emission (PIXE). These data are part of a base line assessment of current air pollution conditions in this region. These data also provide an opportunity to study some aspects of lichen physiology. Using the data from 508 foliose and fruticose lichens, frequency distributions for concentrations of phosphorus and calcium are considered. Phosphorus has a closely-spaced, bimodal distribution: one mode for foliose lichens and one mode for fruticose lichens. This suggests that all lichen genera in this study have similar requirements and absorption mechanisms for phosphorus. Calcium has a complex frequency distribution and concentrations that range from 450 mg/kg to 14 % dry weight. Contributions to this complex distribution pattern can be understood if the data are resolved into growth form, genera within each growth form, and in some cases species within a given genus. This complex dependence on calcium is strong evidence that lichens develop specific calcium-related adaptations in order to accommodate various habitat conditions

Abstract: (14)/C ratios in samples from radial transects across individual thalli of Caloplaca trachyphylla collected at two sites were measured and the results used to investigate whether C-14/C data might provide some insight into the magnitude of carbon turnover in this lichen species, The C-14/C data suggest that significant internal recycling/translocation of carbon is unlikely in the sampled thalli, However, converting the C-14/C data for the larger intact thalli sampled at each site to calendar years, using the atmospheric C-14 record, does not yield constant or even monotonically varying growth rates. Since crustose lichen growth rates are constant or decrease with thallus size, and since the C-14/C data from these larger thalli show a relatively small spread in C-14/C data values compared to the Northern Hemisphere atmospheric C-14 record over the past 50 years, the C-14/C data suggest that carbon turnover may be occurring. Carbon turnover was modelled starting with the atmospheric C-14 record. Turnover was incorporated so that for each year in the record a constant percentage of the total carbon was lost annually and replaced by new photosynthetically fixed carbon with a C-14/C ratio equal to that of the contemporary atmosphere. The C-14/C data from the radial samples were then converted to a calendar year using the model record. Constant annual carbon turnover values of 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5,5, 6, 7, 8, 9, 10, 15, 20, 25 and 50% were modelled. Carbon turnover values between 3 and 6% created C-14 model records that when applied to C-14/C data from the thalli produced constant radial growth rates that were: (1) identical for all lichens at a given site, and (2) independent of lichen size at a given site. The C-14/C data further indicate that annual carbon turnover in this species of lichen is < 10%, independent of the nature of thallus radial growth. The data and modelling suggest that carbon turnover might provide a simple explanation for the C-14/C data from the thalli and might explain the discrepancies between the standard atmospheric C-14 record and the C-14/C ratios observed in G. trachyphylla. (C) 2002 The British Lichen Society Published by Elsevier Science Ltd. All rights reserved.

Abstract: Radial C-14/C profiles across three individuals of the crustose lichen Rhizocarpon geographicum (L.) DC. have been determined using accelerator mass spectrometry. These data were used to assess whether lifespan estimates can be determined in this species using C-14/C isotope ratio measurements. C-14/C profiles are relatively flat with Delta C-14 values (deviations from the modern radiocarbon standard) for the radial samples displaying a small spread ranging from 130 to 200 per mil. The data are consistent with carbon cycling based on growth patterns involving replacement and fusion of areoles within the thallus as well as or instead of cellular or molecular replacement. Consequently, lifespan estimates cannot be obtained from C-14/C measurements of this species and the Delta C-14 profiles provide no insights into whether the relationship between size and age is linear or curvilinear in this species. (C) 2001 The British Lichen Society.

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A cross section of the vagrant soil lichen Xanthoparmelia chlorochroa was analyzed using proton microprobe PIXE. Data were used to generate quantitative, two-dimensional element distribution maps for Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, and Sr. Element maps show differential element partitioning between the stratified layers of the thallus. These data document transfer of inorganic nutrients across the thallus to the algal layer. Inorganic particle entrapment was also evident in the element maps. Dense accumulations of calcium oxalate at the junction of the medulla and the algal layer on the order of 10% by dry mass were discovered. Scanning electron microscopy and thermogravimetric analyses were used to characterize the calcium oxalate region. These data provide evidence for possible functional roles of the calcium oxalate layer, including regulation of water and light. Data also provide support for a mutualistic interpretation of the lichen association.

Abstract: Sections of the crustose saxicolous lichen, Caloplaca trachyphylla, were dared using C-14 accelerator mass spectrometry (AMS). The data show a strong linear dependence of radial position on time (r=0.993), suggesting a constant radial growth rate. This specimen had averaged a marginal growth rare of 1.48 mm/year. Extrapolation of the growth curve yields a thallus age of 20 years. These data demonstrate the feasibility of using AMS technology to precisely date lichen tissues and determine growth races of lichen thalli. (C) 2000 The British Lichen society.

Abstract: In order to better understand the distribution pattern of mineral elements in lichen tissues, thin sections (15 μm) of the foliose, vagrant soil lichen Xanthoparmelia chlorochroa were examined using proton microprobe Particle induced X-ray emission (PIXE). This technique was used to make two-dimensional scans, with 5 μm resolution, across tissue cross sections of the test species. Element maps for Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, and As have been prepared. Several elements are strongly localized in the element maps. PIXE data are complimented with STIM, light micrographs, and SEM images. Preliminary data suggest that nuclear microprobe techniques may be useful in elucidating element absorption and transport mechanisms in lichens.

Abstract: Lichens have been shown to be effective biomonitors of air quality. They are currently being used to characterize background element levels and to identify air pollution effects on federally administered lands in the Rocky Mountain region of the western United States. PIXE analysis for twenty elements has been performed on over two hundred lichen specimens collected from various national forests, national monuments, and national parks in the region. This paper reports on patterns of iron and titanium accumulation in lichen tissues. Data show a strong relationship between concentrations of iron and titanium. The Fe/Ti ratios agree well with values reported in similar lichen studies; however, our values for both iron and titanium concentrations are ten times greater than other reports. A distribution function for the log of iron concentrations is distinctly bimodal. The lower concentration mode contains fruticose lichens from bark substrates and the higher concentration mode contains foliose lichens from rock substrates. High iron concentrations in fruticose lichens along the Wasatch Front suggest air pollution impact from a local steel plant.

Abstract: Total fluoride (gas plus fine particle), spherical alumino silicate particles, particulate selenium, arsenic, lead, bromine, and absorption of light by fine particles have been used to characterize chemical profiles for sources of sulfur oxides impacting the Grand Canyon National Park Class I Visibility Region. During the Project MOHAVE (Measurement of Haze and Visual Effects) winter and summer intensive studies in 1992, these various species were determined at seven sampling sites in and around the Grand Canyon. Extensive upper air and surface-based meteorological measurements were examined to determine probable geographical origins of the air mass present during a given sampling period for each sampling site. Samples corresponding to air masses dominated by transport from a single major source region were used to determine a source profile for each region. Source regions which have been characterized by this analysis include the San Joaquin Valley area, the southern California coastal urban area, the Baja, California- Imperial Valley area, the Arizona and Mexico area (including major smelters) south of the Grand Canyon, the area southeast of the Grand Canyon, the Colorado Plateau area, the Wasatch Front in Utah, and the area in Nevada to the west and northwest of the Grand Canyon. Source profiles giving the ratio of each endemic marker to SO_x for each identified regional source in these areas have been determined. The source profiles for the various regional sources are all statistically different and distinguishable from those for other geographically adjoining sources.

Abstract: Air pollutants were collected in Logan, Cache County, UT, in February 1993 during two periods of atmospheric inversion accompanied by fog. The following atmospheric species were determined: (1) gaseous SO₂, NO₂ (semi-quantitatively),HNO3, NH3, and HF; (2) fine particulate SO₄ ⁼, NO₃ ^-, NH₄ ⁺, F–, H⁺, C, Si, S, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Pb, Se, Br, and Sr, and; (3) fine particulate mass, which was calculated. The major components of fine particulate matter were carbonaceous material, ammonium nitrate, and ammonium sulfate, while the soil component was small. Calculated, fine particulate mass averaged 80 μg/m³ and reached concentrations as high as 120 μg/m³. SO₂/So_x and NO₂/NO_y mole ratios generally varied between 0.2 and 0.1 during inversions. These ratios also showed moderate but consistent diurnal patterns. The emission inventory for Cache County indicates sources of SO₂ and NO_x but not significant amounts of primary sulfate and nitrate. The observations reported here indicate there is significant conversion of SO₂ and NO_x in the presence of excess oxidants to sulfuric and nitric acid that are neutralized by excess ammonia.

Abstract: Hair samples were obtained from ten mummies of the archaic and formative cultures of the north coast of Chile. Each sample was divided into two portions. One portion was washed in acetone and then water and the second portion remained unwashed. All samples were then prepared by an acid digestion procedure and analyzed by PIXE. The washed and unwashed samples were compared to determine what elements were present in the material on the hair surface. Concentrations of the elements Ca, Ti, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, and Pb are reported. Comparison was made to previously published elemental analyses of mummy hair and modem hair. One sample, which had a reddish hue, was found to contain unusually large quantities of Fe and As. Since the unwashed sample had much higher concentrations of these elements than the washed sample, it is believed that the reddish color originated in a mineral dye applied to the hair.

Abstract: Thirty-six lichen samples that were collected from five wilderness areas and a national monument in the intermountain western United States have been analyzed by PIXE for the determination of 21 elements: P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Ba, and Pb. Samples were powdered and spiked with Y as an internal standard. The powder was analyzed directly as a sample of moderate thickness. Elemental concentration means and ranges were generally consistent with concentration ranges from other studies of unimpacted lichens. Some samples, however, had high concentrations of S, Cu, As, and a few other elements. There are interpretations of the data that imply some wilderness areas are being impacted to some degree by industrial-plant atmospheric emissions.

Abstract: Self-supporting foils of aluminum oxide in the thickness range 100 to 300 nm were prepared by anodic oxidation of aluminum foil and subsequent dissolution of the unreacted aluminum. The foil thicknesses were measured by scanning electron microscopy. Using these foils, stopping cross sections of aluminum oxide for protons and deuterons were measured for particles in the energy range 0.90 to 250 MeV. These measured stopping cross sections are compared to other literature results and to the theoretical values as predicted by Bragg's Rule.

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Abstract: One of the primary components of the background in PIXE spectra is secondary electron bremsstrahlung (SEB). However, in order to evaluate the integral usually used to calculate SEB cross sections, certain simplifications must be made, such as assuming an infinite target thickness. In order to more realistically model the physical processes involved in SEB, a Monte Carlo approach is useful. SEB cross sections are calculated by Monte Carlo techniques and the results are compared with the work of Ishii, Morita, and Tarawa [Phys. Rev. A13 (1976) 131; Int, J. PIXE 1 (1990) 1]. To demonstrate the flexibility of the Monte Carlo method, effects due to the target thickness and the proton electron cross section are examined.

Abstract: Materials containing a significant silicate matrix are often considered for element analysis by the PIXE method. In some circumstances such materials may not be conveniently prepared for sample analysis. It has been found that a lithium metaborate fusion-acid dissolution procedure works well for converting siliceous materials into solution from which thin-target PIXE samples may be made. We have demonstrated that this method will allow rapid and accurate element analysis. The lithium metaborate, however, creates a matrix residue leading to the requirement for sample thickness corrections and an increase in detection limits. We have found this to be a useful sample-preparation method.

Abstract: Six samples of fossilized dinosaur bone and surrounding rock were analyzed by PIXE to investigate general relations between the bone and rock. The powdered samples were brought into an acid solution by a lithium metaborate fusion process. The data were analyzed with chemometric methods to successfully differentiate the bone samples from the complementary rock samples. The Sr/Ca ratio is consistently higher in fossilized bone than the surrounding rock. Ba, Y and U, when found in the rock, appear in significantly higher concentrations in the bone. S and Ti, elements commonly found in rock samples, are not found in the bone samples.

Abstract:

Considering the experiments which have been listed as convincing in Fig. 1, two groups have provided evidence for the existence of an unidentified small heat source. The low level of heat that is produced in the Texas A&M and Stanford work, can most plausibly be explained by a chemical explanation. The absence of helium and neutrons is consistent with this explanation, however, the high tritium level observed by Texas A&M is not.

In the dry cell tests two groups present good evidence for an unidentified small neutron source. The best guess for an explanation may be hot fusion cascade. In our opinion, there is really no convincing case yet for nuclear fusion, certainly not of any practical value, but there seems to be a real effect and it has to yet be identified.

There are far more groups with good equipment who found no effect at all. There may be some possible reasons for that, but there is certainly no clear reason for it. We think that the evidence suggests that more work is appropriate. We think that funding should be commensurate with the understanding of this phenomenon and of its possible usefulness.

Abstract: It has long been recognized that PIXE (particle-induced X-ray emission) spectra from thick targets need to be modified with respect to the thin target spectra used for calibration. This is due to the degradation of the energy of the protons entering the sample and the attenuation of the X-rays emerging from the sample. Thick-target corrections typically assume the target to be composed of a layer of sample material having uniform thickness. Because many environmental samples, however, are composed of particles averaging several mu-m in diameter, the usual thick-target corrections are inappropriate. It has previously been shown that size corrections for spherical particles of homogeneous composition can be significant. In the current work a method is presented which employs Monte Carlo techniques to calculate X-ray intensity corrections for particles of arbitrary shape, composition, orientation and size distribution. Empirical equations for proton stopping power and X-ray production cross sections are used in conjunction with X-ray attenuation coefficients to calculate the intensity of the emergent beam. The uncertainty associated with the Monte Carlo calculation is also explored. It is shown that the spherical particle corrections are approximately correct for particles of near-spherical shape; however, they are inadequate for highly elongated or flattened particles or for particles of nonuniform composition.

Abstract: Both (αμ)⁺ and α particles have been observed in coincidence with fusion neutrons in a gaseous D−T target at 2.8×10^{− 3} liquid‐hydrogen density. The initial muon sticking probability in muon‐catalyzed d−t fusion, measured directly for the first time, is (0.80±0.15±0.12 systematic)% in agreement with ‘standard’ theoretical calculations. However, this measured value does not support those theories that invoke special mechanisms to alter the initial sticking value.

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Cross sections, analyzing powers, and spin-flip cross sections have been measured at small angles for the ⁵¹V(→p,→p′) ⁵¹V^∗ reaction at 319 MeV. In disagreement with electron scattering results but in confirmation of proton scattering data, a clear M1 resonance is observed. Its strength is comparable to that seen in ⁴⁸Ca and ⁵⁴Fe. Significant spin excitation strength is observed also above the resonance.

Abstract: We have measured the angular and momentum distributions of the scattered pions from the reaction 12C(π+, π+p)11B and 6Li(π+, π+ p)5He in a coincidence experiment. We compare our results with the plane and distorted wave impulse approximations.

Abstract: A complete set of polarization transfer observables was measured for inclusive 500 MeV proton scattering from H2, Ca(nat.), and Pb(nat.) at thetalab=18.5° (q=1.75 fm-1). The excitation energy ranged over the entire quasielastic peak from 20 to 100 MeV. Longitudinal and transverse spin-flip probabilities were extracted from the data. These have simple model-dependent connections to the spin-longitudinal and transverse response functions for the heavy targets. Detailed analysis of the data reveals no evidence for collective enhancement in the spin-longitudinal response function. The relation of this analysis to the interpretation of the European Muon Collaboration effect in terms of excess pions is discussed in detail.

Abstract: The continuum cross sections and analyzing powers from the H2, 3,4He(p→,p’) and 3,4He(p→,d’) reactions were measured for 98.7 and 149.3 MeV polarized protons in the angular range 17.5° to 60°. The H2 and He3 data show strong quasifree contributions with more ambiguous results for He4. These data are compared with distorted-wave impulse approximation calculations based on quasifree nucleon and deuteron knockout. Overall, the agreement between theory and experiment is moderately good. However, some significant discrepancies are observed and corrections to the simple model are discussed.

Abstract: We have measured a complete set of polarization-transfer observables in the inclusive scattering of 500-MeV protons from H2 and Pb at q=1.75 fm−1. Axial longitudinal and transverse response functions derived from these data show no differences between Pb and H2. This implies no enhancement of the nuclear pion field in heavy nuclei and consequently that models of the low-x A dependence of certain nuclear structure functions requiring such an enhancement are unlikely to be correct.

Abstract: Elastic scattering cross sections for π± scattering from C12, C13, and C14 are presented for pion energies of 65 and 80 MeV and for scattering angles from 20° to 120°. Energy dependent isospin effects are observed. The cross sections were fit with the Kisslinger potential, and the potential strength parameters show an (N−Z)A dependence similar to that predicted by the impulse approximation at 65 MeV, but not at 80 MeV. Calculations using the Siciliano potential, which contains the explicit isospin dependence of the Lorentz-Lorenz-Ericson effect plus isoscalar, isovector, and isotensor terms, indicate the importance and energy dependence of absorption effects. Finally a "model independent" parametrization of the neutron density suggests a neutron excess at the surface of C13 but not C14.

Abstract: Distorted wave impulse approximation calculations for the (π, πp) quasifree knockout reaction on O16 and Ca40 have been carried out for a variety of bombarding energies and angle pairs. In general, the distortion effects arising from the interaction of the pions and protons with the target core are strong and vary with bombarding energy. The results emphasize the need to include these interactions properly in any analysis of experimental data. NUCLEAR REACTIONS DWIA reaction theory of proton knockout from nuclei O16, Ca40(π+, π+p), Tπ=100−300 MeV, calculate σ(θ1, θ2, E1).

Abstract: A comparison of distorted-wave impulse-approximation calculations with recent C12(π+, π+p)B11 data shows excellent agreement and demonstrates the applicability of the quasifree knockout reaction model. Both the shape and the magnitude of the energy-sharing distributions are well described by the calculations.

Abstract: A formalism for distorted wave impulse approximation calculations of the A(π+,2p)B reaction to discrete states in nucleus B is described. Assuming that the π+ capture occurs on a deuteronlike structure in the target nucleus, calculations are presented covering a range of bombarding energies from 25 to 350 MeV and target nuclei from A=16 to 90. In most cases, distortion effects arising from the interaction of the pion and two outgoing protons with the core are large, but primarily affect the magnitude of the cross section. Based on these calculations it is concluded that any more realistic calculation of pion absorption will require the inclusion of the interaction of the incoming and outgoing particles with the core.

Abstract: In recent years, a great deal of attention has been given to the passive detection of fissile material by detecting spontaneously emitted neutrons. Neutron detection schemes generally rely on low-density detection media with high neutron-gamma discrimination capability. However, such detectors generally suffer from low efficiency for high energy (>1.00 MeV) neutrons and usually are constructed with moderating material built into the detector package to compensate for that. But when the source is highly shielded by hydrogenous materials, the moderating material surrounding the detector actually decreases the detector efficiency (Rees and Czirr, 2012). This problem is further compounded if the source is shielded with borated material, significantly reducing the number of emerging neutrons.
We have built, tested, and modeled a simple neutron detector that uses a large block of plastic scintillator fitted with a removable cadmium foil. This detector is sensitive to fission gammas, gammas emitted from neutron capture in shielding, gammas emitted from neutron capture in the plastic scintillator, and gammas produced by neutron capture in the cadmium foil. We demonstrate that by applying suitable data analysis, we can detect a shielded californium source with a very small chance of false positives from typical gamma sources. When boron is added to the shielding, we also use the 477 keV gammas emitted when neutrons capture in boron. This process is enhanced by using an auxiliary NaI gamma detector.
Although the detection method flies in the face of currently promoted detectors, it is a simple technique that could be applied to a variety of detection applications.

Abstract:

Background

Researchers have examined the relationship between mental health and weather/pollution with mixed results. The current study aimed to examine a range of weather and atmospheric phenomena and their association with time-bound mental health data.

Methods

Nineteen different weather/pollution variables were examined in connection with an archive of self-reported mental health data for university students participating in mental health treatment (n=16,452) using the Outcome Questionnaire 45.2 (OQ-45). Statistical approach involved randomly selecting 500 subjects from the sample 1000 different times and testing each variable of interest using mixed models analyses.

Results

Seasonal changes in sun time were found to best account for relationships between weather variables and variability in mental health distress. Increased mental health distress was found during periods of reduced sun time hours. A separate analysis examining subjects’ endorsement of a suicidality item, though not statistically significant, demonstrated a similar pattern. Initial results showed a relationship between pollution and changes in mental health distress; however, this was mediated by sun time.

Limitations

This study examined a relatively homogenous, predominantly European American, and religious sample of college counseling clients from an area that is subject to inversions and is at a high altitude and a latitude where sun time vacillates significantly more than locations closer to the equator.

Conclusions

Seasonal increases in sun time were associated with decreased mental health distress. This suggests the need for institutions and public health entities to plan for intervention and prevention resources and strategies during periods of reduced sun time.