Characterization of Capacitance Standards at High Frequency at National Physical Laboratory,India

Four-terminal-pair air dielectric capacitance standards with nominal values of 1000 and 100 pF have been characterized up-to 10 MHz at NPLI. The procedure employed involves the determination of all capacitive and inductive parameters of the simple electrical-equivalent-circuit-model of these capacitance standards. The effective capacitance of each standard has also been computed as a function of frequency from 1 kHz to 10 MHz. The capacitive parameters have been measured at 1 kHz while inductive parameters have been estimated up to 10 MHz using linear regression analysis by employing least-squares-approximation method. The paper highlights the computation procedure of impedance terms which further requires the determination of various capacitive and inductive terms involved in the calculation of effective capacitance. The method employed for the estimation of inductive parameters as a function of frequency is also discussed in detail. The present work will help in the establishment of metrological traceability of capacitance standards at high-frequency at NPLI which will be further used to establish calibration facility for LCR meters and RF impedance analyzers for capacitance parameter up-to 10 MHz.     

Progress in the Realization of the Units of Capacitance, Resistance, and Inductance at the National Physical Laboratory, India

This paper describes the progress made at the National Physical Laboratory (NPL), New Delhi, India, in the realization of the units of capacitance and resistance based on a calculable crosscapacitor. The realization of the unit of inductance in terms of capacitance using the Maxwell-Wein bridge and resonance techniques has also been described.     

Global spectral irradiance variability and material discrimination at Boulder,Colorado

We analyze 7,258 global spectral irradiance functions over 0.4-2.2 microm that were acquired over a wide range of conditions at Boulder, Colorado, during the summer of 1997. We show that low-dimensional linear models can be used to capture the variability in these spectra over both the visible and the 0.4-2.2 microm spectral ranges. Using a linear model, we compare the Boulder data with the previous study of Judd et al. [J. Opt. Soc. Am. 54, 1031 (1964)] over the visible wavelengths. We also examine the agreement of the Boulder data with a spectral database generated by using the MODTRAN 4.0 radiative transfer code. We use a database of 223 minerals to consider the effect of the spectral variability in the global spectral irradiance functions on hyperspectral material identification. We show that the 223 minerals can be discriminated accurately over the variability in the Boulder data with subspace projection techniques.     

Comparison of absolute spectral irradiance responsivity measurement techniques using wavelength-tunable lasers

Independent methods for measuring the absolute spectral irradiance responsivity of detectors have been compared between the calibration facilities at two national metrology institutes, the Helsinki University of Technology (TKK), Finland, and the National Institute of Standards and Technology (NIST). The emphasis is on the comparison of two different techniques for generating a uniform irradiance at a reference plane using wavelength-tunable lasers. At TKK's Laser Scanning Facility (LSF) the irradiance is generated by raster scanning a single collimated laser beam, while at the NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS), lasers are introduced into integrating spheres to generate a uniform irradiance at a reference plane. The laser-based irradiance responsivity results are compared to a traditional lamp-monochromator-based irradiance responsivity calibration obtained at the NIST Spectral Comparator Facility (SCF). A narrowband filter radiometer with a 24 nm bandwidth and an effective band-center wavelength of 801 nm was used as the artifact. The results of the comparison between the different facilities, reported for the first time in the near-infrared wavelength range, demonstrate agreement at the uncertainty level of less than 0.1%. This result has significant implications in radiation thermometry and in photometry as well as in radiometry.     

Realization of the National Institute of Standards and Technology detector-based spectral irradiance scale

A detector-based spectral irradiance scale has been realized at the National Institute of Standards and Technology (NIST). Unlike the previous NIST spectral irradiance scales, the new scale is generated with filter radiometers calibrated for absolute spectral power responsivity traceable to the NIST high-accuracy cryogenic radiometer instead of with the gold freezing-point blackbody. The calibrated filter radiometers are then used to establish the radiance temperature of a high-temperature blackbody (HTBB) operating near 3,000 K The spectral irradiance of the HTBB is then determined with knowledge of the geometric factors and is used to assign the spectral irradiances of a group of 1,000-W free-electron laser lamps. The detector-based spectral irradiance scale results in the reduction of the uncertainties from the previous source-based spectral irradiance scale by at least a factor of 2 in the ultraviolet and visible wavelength regions. The new detector-based spectral irradiance scale also leads to a reduction in the uncertainties in the shortwave infrared wavelength region by at least a factor of 2-10, depending on the wavelength. Following the establishment of the spectral irradiance scale in the early 1960s, the detector-based spectral irradiance scale represents a fundamental change in the way that the NIST spectral irradiance scale is realized.     

An algorithm to evaluate solar irradiance and effective dose rates using spectral UV irradiance at four selected wavelengths

The paper shows a semi-analytical method for environmental and dosimetric applications to evaluate, in clear sky conditions, the solar irradiance and the effective dose rates for some action spectra using only four spectral irradiance values at selected wavelengths in the UV-B and UV-A regions (305, 320, 340 and 380 nm). The method, named WL4UV, is based on the reconstruction of an approximated spectral irradiance that can be integrated, to obtain the solar irradiance, or convoluted with an action spectrum to obtain an effective dose rate. The parameters required in the algorithm are deduced from archived solar spectral irradiance data. This database contains measurements carried out by some Brewer spectrophotometers located in various geographical positions, at similar altitudes, with very different environmental characteristics: Rome (Italy), Ny Alesund (Svalbard Islands, Norway) and Ushuaia (Tierra del Fuego, Argentina). To evaluate the precision of the method, a double test was performed with data not used in developing the model. Archived Brewer measurement data, in clear sky conditions, from Rome and from the National Science Foundation UV data set in San Diego (CA, USA) and Ushuaia, where SUV 100 spectroradiometers operate, were drawn randomly. The comparison of measured and computed irradiance has a relative deviation of about +/-2%. The effective dose rates for action spectra of Erythema, DNA and non-Melanoma skin cancer have a relative deviation of less than approximately 20% for solar zenith angles <50 degrees .     

Development of a detector-based absolute spectral irradiance scale in the 380-900-nm spectral range

A detector-based absolute scale for spectral irradiance in the 380-900-nm wavelength region has been developed and tested at the Helsinki University of Technology (HUT). Derivation of the scale and its use for photometric and colorimetric measurements are described. A thorough characterization of a filter radiometer, constructed from a reflection trap detector, a precision aperture, and a set of seven temperature-controlled bandpass filters, is presented. A detailed uncertainty analysis of the scale indicates a relative standard uncertainty of approximately 0.2% throughout most of the wavelength region. The standard uncertainties obtained in measurements of correlated color temperature and luminous intensity of three Osram Wi41/G tungsten-halogen lamps are 2 K and 0.3%, respectively. The spectral irradiance scale is compared with the HUT luminous intensity scale. The agreement of the results at the 0.1% level is well within the combined standard uncertainty of the two scales.     

Gamma dose measurement in a water phantom irradiated with the BNCT facility at THOR.

It has been proposed that a LiF thermoluminescence dosemeter (TLD) is used as a gamma dosemeter in a water phantom irradiated with the BNCT facility at THOR. Based on the TLD neutron sensitivity and neutron fluxes in the water phantom, which were simulated by the MCNP code, TLD-700 was chosen as a gamma dosemeter in this report. For the correction of the neutron influence on TLD-700, the thermal neutron sensitivity to TLD-700 was investigated with MCNP simulation and the thermal neutron flux was measured with gold foils using the cadmium difference technique. The correction to the neutron influence on the TLD was established on the TLD thermal neutron sensitivity. the thermal neutron flux, and the conversion factor from energy deposition in the TLD to the TLD response. By comparing the experimental data with the thermal neutron influence correction, these data are in very good agreement with the MCNP predictions.     

Radiometic Comparison Between a National Laboratory and an Industrial Laboratory

One of the disciplines that Fluke?CHart Scientific has is radiometric calibration. Part of this program involves use of a radiation thermometer with a pyroelectic detector. It is used as a radiometric transfer standard between a set of liquid-bath variable temperature blackbodies and a flat-plate infrared (IR) calibrator. The flat-plate calibrator is designed for use in the calibration of handheld IR thermometers. The traceability of the variable temperature blackbodies is realized by contact thermometry through the National Institute of Standards and Technology (NIST). A verification of these blackbodies is a comparison between a calibration done by the Radiance Temperature Laboratory at NIST and the blackbodies at Fluke?CHart Scientific. This comparison uses a transfer radiation thermometer (TRT) as a check standard. It would be more desirable to use radiometric traceability as an indication of the blackbodies?? radiometric temperature. However, contact thermometry provides much better uncertainties. These uncertainties are needed for the radiometric transfer from the blackbodies to the flat-plate calibrators. Thus, the NIST radiometric calibration of the TRT is used for verification of normal equivalence. This article discusses Fluke?CHart Scientific??s blackbody traceability. It covers the Fluke?CHart Scientific and the NIST radiometric calibration procedures. It discusses the radiometric uncertainty budgets at both Fluke?CHart Scientific and at NIST. It then discusses the results of this comparison and analyzes the results. The comparison is in the temperature range of ?15 °C to 500 °C. It showed a normal equivalence of less than 1.00 at all points. The article concludes with a set of future actions to ensure quality in Fluke?CHart Scientific??s radiometric calibration program.     

Commissioning of the Rutherford Appleton Laboratory pulsed muon facility

The pulsed muon channel of the ISIS facility at the Rutherford Appleton Laboratory has been successfully commissioned as a tool for μSR research using polarised surface muons and for experiments requiring negative cloud muons. The beam line is described and the present performance given. At the present time, the pulsed muon beam gives 10⁵ μ⁺/s (2000 μ⁺/pulse) with very thin production targets (2.5 mm thick carbon) and 30% of the ISIS design proton current. μSR test spectra demonstrate the capability of the source and instrumentation, the performance relative to the continuous sources at the meson factories, and the potential for new science.     

Comparison of spectral irradiance standards used to calibrate shortwave radiometers and spectroradiometers

Absolute calibration of spectral shortwave radiometers is usually performed with National Institute of Standards and Technology (NIST) or NIST-traceable incandescent lamps. We compare 18 irradiance standards from NIST and three commercial vendors using the same spectrometer to assess their agreement with our working standard. The NIST procedure is followed for the 1000-W FEL lamps from NIST, Optronics, and EG&G. A modified calibration procedure developed by Li-Cor is followed for their 200-W tungsten-halogen lamps. Results are reproducible from one day to the next to approximately 0.1% using the same spectrometer. Measurements taken four months apart using two similar but different spectrometers were reproducible to 0.5%. The comparisons suggest that even NIST standards may disagree with each other beyond their stated accuracy. Some of the 1000-W commercial lamps agreed with the NIST lamps to within their stated accuracy, but not all. Surprisingly, the lowest-cost lamps from Li-Cor agreed much better with the NIST lamps than their stated accuracy of 4%, typically within 2%. An analysis of errors leads us to conclude that we can transfer the calibration from a standard lamp to a secondary standard lamp with approximately 1% added uncertainty. A field spectrometer was calibrated with a secondary standard, producing a responsivity for the spectrometer that was within 5% of the responsivity obtained by Langley calibration using routine field measurements.     

Development of a real-time radiological area monitoring network for emergency response at Lawrence Livermore National Laboratory

A real-time radiological sensor network for emergency response was developed and deployed at the Lawrence Livermore National Laboratory (LLNL). The real-time radiological area monitoring (RTRAM) network comprises 16 Geiger-Mueller sensors positioned on the LLNL Livermore site perimeter to continuously monitor for a radiological condition resulting from a terrorist threat to site security and the health and safety of LLNL personnel. The RTRAM network sensor locations coincide with wind sector directions to provide thorough coverage of the one-square-mile site. These low-power sensors are supported by a central command center (CCC) and transmit measurement data back to the CCC computer through the LLNL telecommunications infrastructure. Alarm conditions are identified by comparing current data with predetermined threshold parameters and are validated by comparison with plausible dispersion modeling scenarios and prevailing meteorological conditions. Emergency response personnel are notified of alarm conditions by automatic radio- and computer-based notifications. A secure intranet provides emergency response personnel with current condition assessment data that enable them to direct field response efforts remotely. The RTRAM network has proven to be a reliable system since initial deployment in August 2001, and it maintains stability during inclement weather conditions.     

Personal neutron dosimetry at a research reactor facility

Individual neutron monitoring presents several difficulties due to the differences in energy response of the dosemeters. In the present study, an individual dosemeter (TLD) calibration approach is attempted for the personnel of a research reactor facility. The neutron energy response function of the dosemeter was derived using the MCNP code. The results were verified by measurements to three different neutron spectra and were found to be in good agreement. Three different calibration curves were defined for thermal, intermediate and fast neutrons. At the different working positions around the reactor, neutron spectra were defined using the Monte Carlo technique and ambient dose rate measurements were performed. An estimation of the neutrons energy is provided by the ratio of the different TLD pellets of each dosemeter in combination with the information concerning the worker's position; then the dose equivalent is deduced according to the appropriate calibration curve.     

New UCSF proton ocular beam facility at the Crocker Nuclear Laboratory Cyclotron (UC Davis)

A new facility has been constructed at the Crocker Nuclear Laboratory at University of California Davis for the purpose of treating ocular tumors using the 67.5 MeV protons from the 76-in. isochronous cyclotron. Beam line design, commissioning, control system, beam characteristics, dosimetry, patient positioner and system performance are discussed.

The unmodulated Bragg peak has a penetration of 29 mm in tissue at the isocenter with a peak to plateau ratio of 3.8:1 and a width of 5 mm (FWHM measured in water). The delivered dose is monitored by two transmission ionization chambers which are calibrated against a thimble ionization chamber with an NIST-traceable ⁶⁰Co calibration factor. The Bragg peak is spread across the target volume by the use of range modulators. The residual range is varied by means of a variable water column. Daily variation in patient dosimetry is within ±3%. The beam penumbra (defined here as the distance between the 90% and 10% isodose levels) is 1.5 mm for a range-modulated beam at a collimator-to-isocenter distance of 50 mm. The beam flatness in a 25 mm diameter beam is within ±2% and the beam symmetry is ±1%.

In the first 18 months, 50 patients have been treated with an average field size of 16.8 × 16.6 mm². The residual range varied between 13.0 mm to 29.2 mm with an average value of 22.4 mm, and the range modulation varied between 16 mm to 24 mm with an average value of 20 mm. The tumor thickness (height) ranged between 1.2 to 11.5 mm with mean of 5.2 mm. The age of the patients ranged from 25 to 88 yr.