Papers by james christian
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, 2003
The tracking of radiation contamination and distribution has become a high-priority US DOE task. ... more The tracking of radiation contamination and distribution has become a high-priority US DOE task. To support DOE needs, Radiation Monitoring Devices Inc. has been actively carrying out research and development on a gamma-radiation imager, RadCam 2000TM. The imager is based upon a position-sensitive PMT coupled to a scintillator near a MURA coded aperture. The modulated gamma flux detected by the PSPMT is mathematically decoded to produce images that are computer displayed in near real time. Additionally, we have developed a data-manipulation scheme which allows a multi-dimensional data array, comprised of x position, y position, and energy, to be used in the imaging process. In the imager software a gate can be set on a specific isotope energy to reveal where in the field of view the gated data lies or, conversely, a gate can be set on an area in the field of view to examine what isotopes are present in that area. This process is complicated by the FFT decoding process used with the coded aperture; however, we have achieved excellent performance and results are presented here.
IEEE Transactions on Electron Devices, 2006
This paper examines the performance of CMOS avalanche photodiode pixels operated in a Geiger mode... more This paper examines the performance of CMOS avalanche photodiode pixels operated in a Geiger mode. The pixels, called Geiger photodiode (GPD) pixels, convert an incident analog photon flux into a digital count rate. The maximum detection efficiency of the characterized GPD pixel for 632-nm light is 22%. The passively quenched GPD pixel exhibits an after pulsing at excess bias voltages above 2 V, and a minimum in the after-pulsing correction factor, of 0.53, occurs at an excess bias of 5.8 V. The after pulsing increases the fluctuations, or noise, in the count-rate signal. The following expression accurately describes the noise in the characterized GPD pixel, which exhibits a relatively low after-pulsing probability: σ2=n~dp+n~ap+2·[n~dp·n~ap]12/, where σ represents the count-rate fluctuations, the "dp" subscript stands for "detected photons," the "ap" subscript stands for "after pulses," and the n~ represents the "average count rate of" dp, or ap. The noise-equivalent illumination exhibits a minimum of 300 Hz at an operating voltage of 28 V. The best operating voltage for the GPD pixel increases from 28 V with increasing signal intensity.
Advancing nuclear and high-energy physics often requires experiments conducted in harsh environme... more Advancing nuclear and high-energy physics often requires experiments conducted in harsh environments, such as a liquid helium bath and a superconducting magnet at several Tesla. These experiments need improved sensors that operate in these conditions. Improvements in detector technology used in extreme environments can improve the data quality and allow new designs for experiments that operate under these conditions. Solid-State Photomultipliers (SSPM), a device built from a monolithic array of photodiodes, can be used in these environments where traditional PMTs may not operate. Measurements of the diode properties at low temperatures down to 5 K are used to determine the potential of CMOS SSPMs in these environments. At temperatures below 60 K, extensive after pulsing is observed, which renders the Geiger photodiodes in the SSPM nonfunctional for biases above breakdown. In proportional mode operation, below the reverse bias breakdown, the photodiodes show a linear response to incident light with a relatively large gain and can be used at temperatures near 5 K.
IEEE Transactions on Nuclear Science, 2000
ABSTRACT The space environment is inherently complicated with multiple sources of radiation, and ... more ABSTRACT The space environment is inherently complicated with multiple sources of radiation, and within a spacecraft, these radiation fields are further complicated by the production of secondary particles (i.e., γ, e±, n, π±, π0), where the generation of neutrons represent a significant contribution to the dose received by astronauts. The signals in most detectors resulting from neutron interactions are difficult to discriminate from other types of interactions such as the incident energetic protons and gamma rays, making it difficult to provide accurate dose equivalent information. The results presented here demonstrate the capability of Diphenylanthracene (DPA) scintillation materials to detect and discriminate fast neutrons from gamma rays using pulse shape discrimination (PSD) techniques. The new scintillation sensors generate amplitude and emission-time signatures that provide information regarding the neutron dose and linear energy transfer (LET). This information can then be used to determine appropriate quality factors and the dose equivalent or biological effect. Considerations for a DPA based dosimeter design will be presented along with optimization of the detector signal processing steps for discriminating neutrons from gamma rays. The emission time and amplitude signatures from a new scintillation material, crystalline DPA, are characterized for proton, neutron, and electron (from gamma-ray irradiation) irradiation. An estimation of Birk's parameters for DPA, which is necessary to describe the light yield as a function of LET, is presented.
Journal of Geophysical Research, 1994
Modeling biogeochemical fluxes in the marine plankton requires the application of factors for ext... more Modeling biogeochemical fluxes in the marine plankton requires the application of factors for extrapolation of biomass indicators measured in the field (chlorophyll a, adenosine triphosphate, bacterial counts) to biomass carbon or nitrogen. These are often inferred from culture studies and are poorly constrained for natural populations. A least squares inverse method with a simple linear model constrains the values of several common indicator ratios, giving self-consistent solutions that provide useful information about the structure of the microbial community at our North Pacific Ocean study site (Station ALOHA (A Long-term Oligotrophic Habitat Assessment)). These results indicate that the fraction of the microbial biomass that is autotrophic (pigmented) is greater in the mixed layer than at the deep chlorophyll maximum layer and that heterotrophic bacteria are a significant but not necessarily predominant component of the microbial community in the euphotic zone. 14,269 14,270 CHRISTIAN
Journal of Geophysical Research, 1997
International Journal of Remote Sensing, 2004
... Authors: Raghu Murtugudde 1 ; Liping Wang 1 ; Eric Hackert 2 ; James Beauchamp 2 ; James Chri... more ... Authors: Raghu Murtugudde 1 ; Liping Wang 1 ; Eric Hackert 2 ; James Beauchamp 2 ; James Christian 3 ; Antonio Busalacchi 4. ... The 1997-1998 ENSO (El Niño-Southern Oscillation) was not only the largest event of the century but also the most comprehensively observed. ...
Global Biogeochemical Cycles, 1995
AbstractTime series measurements of in situ fluorescence, extracted particulate chlorophyll a, pr... more AbstractTime series measurements of in situ fluorescence, extracted particulate chlorophyll a, primary productivity, extracted adenosine 5′-triphosphate, and fluorescence per cell, as measured by flow cytometry, demonstrate seasonal cycles in fluorescence and chlorophyll concentrations in the North Pacific Subtropical Gyre (22° 45′N, 158° 00′W). Two opposing cycles are evident. In the upper euphotic zone (0–50 m), chlorophyll a concentrations increase in winter, with a maximum in December, and decrease each summer, with a minimum in June or July. In contrast, chlorophyll a concentrations in the lower euphotic zone (100–175 m) increase in spring, with a maximum in May, and decline in fall, with a minimum in October or November. The winter increase in chlorophyll a concentration in the upper 50 m of the water column appears to be a consequence of photoadaptation in response to decreased average mixed-layer light intensity rather than a change in phytoplankton biomass. In the lower euphotic zone, however, the seasonal cycle in pigment concentration does reflect a change in the rate of primary production and in phytoplankton biomass as a consequence of increased light intensity in summer. These observations have important implications for phytoplankton dynamics in the subtropical oceans and for remote sensing of phytoplankton biomass.
Deep-sea Research Part Ii-topical Studies in Oceanography, 2004
The subtropical gyres of the world are extensive, coherent regions that occupy about 40% of the s... more The subtropical gyres of the world are extensive, coherent regions that occupy about 40% of the surface of the earth. Once thought to be homogeneous and static habitats, there is increasing evidence that mid-latitude gyres exhibit substantial physical and biological variability on a variety of time scales. While biological productivity within these oligotrophic regions may be relatively small, their immense size makes their total contribution significant. Global distributions of dynamic height derived from satellite altimeter data, and chlorophyll concentration derived from satellite ocean-color data, show that the dynamic center of the gyres, the region of maximum dynamic height where the pycnocline is deepest, does not coincide with the region of minimum chlorophyll concentration. The physical and biological processes by which this distribution of ocean properties is maintained, and the spatial and temporal scales of variability associated with these processes, are analyzed using global surface chlorophyll-a concentrations, sea-surface height, sea-surface temperature, surface winds from operational satellite and meteorological sources, and hydrographic data from climatologies and individual surveys. Seasonal and interannual variability in the areal extent of the subtropical gyres are examined using 8 months (November 1996–June 1997) of Ocean-color and Temperature Sensor (OCTS) and 6 years (September 1997–October 03) of Sea-viewing Wide Field-of-View Sensor (SeaWiFS) ocean-color data. The ocean-color data are interpreted in the context of climate variability and measured changes in other ocean properties (i.e. wind forcing, surface currents, Ekman pumping, and vertical mixing). The oligotrophic waters of the North Pacific and North Atlantic gyres are observed to be expanding over this period, while those of the South Pacific, South Atlantic, and South Indian Ocean gyres show much weaker and less consistent tendencies.
Journal of Geophysical Research, 1999
High-quality ocean color data (chlorophyll) provided by the Sea-viewing Wide Field of view Sensor... more High-quality ocean color data (chlorophyll) provided by the Sea-viewing Wide Field of view Sensor (SeaWiFS) satellite were analyzed for the first complete year of coverage (October 1997 to September 1998) in the tropical Indo-Pacific basin. This period coincides with the peak of one of the strongest E1 Nifio events during December 1997 and the La Nifia of 1998 that appeared dramatically in less than a month as a sea surface temperature (SST) change of over 6øC in the central equatorial Pacific during June 1998. The tropical Indian Ocean also underwent a highly anomalous series of events with negative SST anomalies (SSTA) of over 3øC in the eastern equatorial and coastal regions during October-December 1997 and warm SSTA in the west that peaked at over 2øC during February 1998. The ocean color variability is interpreted using other satellite data such as sea level from TOPEX/Poseidon and also in terms of the dynamics and thermodynamics of the region from simulations with an ocean general circulation model. The E1 Nifio-related reductions in equatorial production and the off-equatorial increase in biological activity, and their basin scale evolution is clearly seen for the first time. Persistent northerly wind anomalies resulted in a northward shift of the equatorial divergence and the upwelling Kelvin wave which signalled the end of the 1997-1998 E1 Nifio. The anomalous surface chlorophyll associated with this Kelvin wave was also clearly shifted north of the equator by nearly 300 km and appeared more than a month before the negative sea level anomalies seen by TOPEX/Poseidon. On the equator near 165øE, the disappearance of the barrier layer appeared to coincide with a localized bloom that occurred in response to the easterly wind bursts over the western Pacific that lasted from December 1997 through the boreal summer. The ecosystem response to the cold La Nifia conditions is clearly seen as elevated chlorophyll during the boreal summer of 1998 in the equatorial Pacific cold tongue region. In the Indian Ocean, an anomalous phytoplankton bloom was observed by SeaWiFS during October-December 1997 coincident with the anomalous upwelling in the eastern equatorial region and off the coast of Sumatra. A stronger than normal northeast monsoon is seen as higher than climatological values of surface chlorophyll. The open ocean Ekman pumping and the shoaling of the thermocline near 60øE and 10øS and the eastward extension of mixed layer entrainment in the same latitude band is seen as a region of higher biological activity during the boreal summer.
Journal of Geophysical Research, 2007
Abstract[1] Application of biogeochemical models to the study of marine ecosystems is pervasive, ... more Abstract[1] Application of biogeochemical models to the study of marine ecosystems is pervasive, yet objective quantification of these models' performance is rare. Here, 12 lower trophic level models of varying complexity are objectively assessed in two distinct regions (equatorial Pacific and Arabian Sea). Each model was run within an identical one-dimensional physical framework. A consistent variational adjoint implementation assimilating chlorophyll-a, nitrate, export, and primary productivity was applied and the same metrics were used to assess model skill. Experiments were performed in which data were assimilated from each site individually and from both sites simultaneously. A cross-validation experiment was also conducted whereby data were assimilated from one site and the resulting optimal parameters were used to generate a simulation for the second site. When a single pelagic regime is considered, the simplest models fit the data as well as those with multiple phytoplankton functional groups. However, those with multiple phytoplankton functional groups produced lower misfits when the models are required to simulate both regimes using identical parameter values. The cross-validation experiments revealed that as long as only a few key biogeochemical parameters were optimized, the models with greater phytoplankton complexity were generally more portable. Furthermore, models with multiple zooplankton compartments did not necessarily outperform models with single zooplankton compartments, even when zooplankton biomass data are assimilated. Finally, even when different models produced similar least squares model-data misfits, they often did so via very different element flow pathways, highlighting the need for more comprehensive data sets that uniquely constrain these pathways.
Deep-sea Research Part Ii-topical Studies in Oceanography, 2002
The Coastal Zone Color Scanner (CZCS) data set provided some insights into biological processes i... more The Coastal Zone Color Scanner (CZCS) data set provided some insights into biological processes in the equatorial Pacific, but the sampling was too sparse to address questions on temporal and spatial variability. Since late 1996, the Ocean Color–Temperature Sensor (OCTS), the Polarization Detection Environmental Radiometer (POLDER), the Sea-viewing Wide Field-of-View Sensor (SeaWiFS), and the Moderate-Resolution Imaging Spectroradiometer (MODIS) have provided a nearly continuous record of biological processes in this region for the first time. This study summarizes the SeaWiFS observations of the tropical Pacific from September 1997 through March 2000, with particular emphasis on equatorial and mesoscale variability, the influence of biological processes on penetrating irradiance, and the performance of primary production algorithms in this region.Specific mesoscale phenomena described are the phytoplankton blooms along the west coast of Central America, in the vicinity of the Costa Rica dome, and south of the equator. The coastal Central American and Costa Rica dome blooms result from orographically steered coastal winds and Ekman divergence, respectively. An unusual bloom event occurred south of the equator and persisted for several months in 1999; specific mechanisms that would have sustained the bloom could not be identified. Also, the time-evolution of the equatorial bloom during the May–August 1998 transition from El Niño to La Niña is discussed. Again, no concise and broadly accepted explanation of the bloom's genesis and migration has yet emerged. During this transition, the monthly mean diffuse attenuation coefficient decreased by a factor of 3 at some locations along the equator. This change in water transparency, coupled with large changes in mixed-layer depth, resulted in significant changes in surface layer heating rates that were substantiated with field observations. Finally, certain primary production algorithms designed to use remotely sensed chlorophyll-a concentrations are evaluated. None of the algorithms capture the observed variability in primary production, and all appear to underestimate the total primary production of the tropical Pacific.
Limnology and Oceanography, 1995
Ectoenzymatic hydrolysis is a crucial first step in bacterial utilization of polymeric dissolved ... more Ectoenzymatic hydrolysis is a crucial first step in bacterial utilization of polymeric dissolved organic matter (DOM). Variation in the relative activities of different enzymes can indicate seasonal and geographic variation in the mode of bacterioplankton nutrition. We found that relative activities of leucine aminopeptidase and P-glucosidase in seawater varied significantly among three oceanic regions: the subtropical North Pacific, the equatorial Pacific, and the Southern Ocean. The temperature responses of these enzymes also vary significantly among these three regions, suggesting distinct bacterial phenotypes with distinct isozymes. Our results suggest a latitudinal trend in bacterial carbon and nitrogen utilization, with significant synthesis of cell constituents from glucose and ammonium in equatorial waters but little such de novo synthesis in Antarctic waters. The observed patterns have important implications for the parameterization of secondary production and nutrient regeneration in global production models and for understanding the role of DOM in global carbon and nitrogen fluxes.
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Papers by james christian