Possible Sources Of Systematic Errors Research Articles

Monte Carlo results for three-dimensional self-avoiding walks

We discuss possible sources of systematic errors in the computation of critical exponents by renormalization-group methods, extrapolations from exact enumerations and Monte Carlo simulations. A careful Monte Carlo determination of the susceptibility exponent γ for three-dimensional self-avoiding walks has been used to test the claimed accuracy of the various methods.

Hybrid ion sources: Radiocarbon measurements from microgram to milligram

Gas ion sources provide a very efficient way of dealing with very small samples and, by injecting samples in a stream of helium carrier gas, measurements can be obtained from as little as 1 μg carbon. This is particularly useful for measurements on high activity samples and for the study of sample contaminants in relation to high precision AMS measurements. Another benefit of gas based measurements is that they can be made in real time allowing a range of possible applications such as GC-AMS and LC-AMS which could make AMS more widely applicable in the life sciences. Graphite ion sources provide a more efficient use of accelerator time for the measurement of large samples because of the higher carbon currents attainable. The precisions attainable is this way are so good that re-evaluation of possible sources of systematic error (such as low level contamination and sample size effects) is essential. Hybrid ion sources capable of operating on either gas or graphite allow the advantages of both techniques to be exploited. With minor modifications to the gas ion source at Oxford we are now able to operate with both sample types. Currents from gas are typically 10–12 μA and those from graphite 40–50 μA while maximum currents from graphite exceed 300 μA. We discuss the implications of this for the future development of radiocarbon AMS facilities designed for a wide variety measurements and research.

An Alternate Calculation of the Distance to M87 Using the Whitmore et al. Luminosity Function for Its Globular Clusters: [ITAL]H[/ITAL][TINF]0[/TINF] Therefrom

An alternate calculation to that made by Whitmore et al. (WSLMB) is set out for the distance to M87 using their globular cluster luminosity function (GCLF) for M87 in the Virgo Cluster. They have concluded from these data that H0 = 78 ± 11 km s-1 Mpc-1. Our purpose is to show how a different set of precepts leads to a substantially smaller value of H0 that is in statistical agreement with other analyses that favor the long distance scale. Adopting the WSLMB observed turnover apparent magnitude for the GCLF of V = 23.79 ± 0.2 (external), together with AV = 0.00 mag for the foreground Galactic absorption, and MV0 = -7.62 ± 0.2 (external) as the calibration of the turnover luminosity gives (m - M)0 = 31.41 ± 0.28 for the distance modulus of M87. Using the cosmic velocity of the E galaxy Virgo core as v(cosmic) = 1179 ± 17 km s-1 gives H0 = 62 ± 9 km s-1 Mpc-1. The procedure of WSLMB and others in stepping a Virgo distance to the Coma Cluster and then using a Coma velocity that still contains a random component to determine H0 is argued. The cosmic velocity for Virgo, calculated by reading a Hubble diagram of relative distances of 17 remote clusters versus cosmic microwave background velocities at zero modulus difference to Virgo (Jerjen & Tammann), overcomes this error by circumventing the problem of accounting for any random and/or streaming motions of the Virgo Cluster and/or of the Coma Cluster about the ideal Hubble flow. This procedure cuts to the core of the distance scale problem, eliminating the major uncertainty concerning local random motions. Four sources of possible systematic errors in the Virgo Cluster modulus using the GCLF method, as applied to M87, are also set out, suggesting that the value of H0 = 62 is an upper limit.

Mutual electron-ion interactions in electron impact ionization of a Ca beam

We evaluate the role of electron-ion mutual interactions as a possible source of systematic errors in an experiment of electron impact ionization and excitation of an effusive Ca beam. By monitoring photons resulting from the impact excitation of a Ca resonant transition, the transmitted electron beam current by a sectored Faraday cup and total ion yields, we extract information about elementary processes responsible for ionization. We show that our diagnosis allows us to monitor the influence of the produced ionization on the elementary processes and on the density profile of the electro beam.

Globular Clusters in Coma Galaxy NGC 4881

view Abstract Citations (48) References (64) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Globular Clusters in Coma Galaxy NGC 4881 Baum, W. A. ; Hammergren, M. ; Groth, E. J. ; Ajhar, E. A. ; Lauer, T. R. ; O'Neil, E. J., Jr. ; Lynds, C. R. ; Faber, S. M. ; Grillmair, C. J. ; Holtzman, J. A. ; Light, R. M. Abstract We report HST-WFPC2 observations of the bright E0 galaxy NGC 4881, located about 18' north of the center of the Coma Cluster. Counts of globular clusters (GCs) were made both within the PC 1(f/28) frame and within the three WFC (f/13) frames. Attention was focused mainly on the PC1 data, in which faint GCs could be distinguished from noise spikes by the use of a PSF fitting test. Within the 34"X 34" field of the PC1 detector (minus a circle of 3" radius around the NGC 4881 nucleus), 88 GCs have been identified to a threshold of F555W = 27.6 mag~V. Noise spikes are only beginning to appear near that threshold, and they have been excluded. Extended sources, such as distant background galaxies, have also been identified and excluded. If corrected for incompleteness in the faintest bins, the numbers of GCs per 0.4 mag bin in NGC 4881 are rising monotonically to our threshold and therefore imply that we probably have not yet passed the turnover of the globular cluster luminosity function (GCLF). Alternatively, instead of applying completeness corrections, we can compare uncorrected NGC 4881 counts with a model GCLF transported to various assumed Coma Cluster distances. The relative likelihoods of various distances can then be calculated using Poisson statistics to quantify the fit. For a model we use the GCLFs of the Milky Way and M31. We find the likelihood of the GCLF fit to rise with increasing Coma distance until it crests at ~108 Mpc, and then levels off. Therefore, if the model fairly represents the GCs in NGC 4881, our estimate of the minimum Coma distance is 108 Mpc with a formal statistical error of (-11,+{infinity}) Mpc. A comparison of three independent calibrations indicates that any zero-point error in the photometry should be < 5%. Three other possible sources of systematic error include the RR Lyrae distance scale for the Milky Way GCs, the distance modulus of M31, and any intrinsic difference between NGC 4881 and the model in the M_V_ of the GCLF turnover. In all three cases, there are envisaged adjustments which, if applied, would have the effect of increasing our estimate of the minimum distance to Coma. A distance of 108(-11,+{infinity}) Mpc implies a Hubble Constant H_0_ = 67(+7, -[67]) km/s per Mpc. We cannot set an upper limit on the distance, nor a lower limit on H_0_, until observations of Coma galaxies reach deep enough to show the turnover of the GCLF, and we also caution that the results above are based on only one Coma galaxy. If the GCs detected on all four CCDs are fitted with the GCLF model and if counts are adjusted for incomplete areal coverage, the GC system in NGC 4881 is found to have a specific frequency of only about 1.0, which is lower than in the majority of bright ellipticals but not the lowest known. The GC system in NGC 4881 is also found to be only a little more spatially extended than the luminosity profile of the galaxy itself. Those findings are somewhat sensitive to any nonuniformity in the distribution of background objects in the WFC frames. Even so, the specific frequency is almost certainly less than 2.0. The low specific frequency in NGC 4881 may have implications concerning the formation, migration, and fate of GCs in a dense environment like that of Coma, where very large numbers of GCs and dwarf galaxies have been reported near the center of the cluster. There is no evidence that the GCs of NGC 4881 are abnormal: they have a mean color index (V - I) = 0.97 mag with relatively small dispersion, and the implied metallicity, [Fe/H] ~ -1.2, is within the range of GC metallicities found in the Milky Way, though a little more metal rich than the mean. Moreover, the NGC 4881 GCLF has the same slope as the bright side of the GCLF in the Milky Way. Publication: The Astronomical Journal Pub Date: December 1995 DOI: 10.1086/117710 Bibcode: 1995AJ....110.2537B Keywords: GALAXIES: INDIVIDUAL: NGC 4881; GLOBULAR CLUSTERS: GENERAL full text sources ADS | data products SIMBAD (9) NED (3) MAST (1) ESA (1)

On the seismic discontinuities in the upper mantle

Mode conversions and reflections at upper-mantle seismic discontinuities may be contained in earthquake seismograms as weak secondary phases that often become visible only after special signal processing techniques are applied to the data. To extract fully the information these secondary phases carry about the three-dimensional structure of the Earth, new observational and interpretational methods have to be developed. However, new sources of possible systematic errors may lead to conflicting results. Studies carried out by various research groups on the thickness of the upper-mantle transition zone, the sharpness of upper-mantle discontinuities and the global existence of a 520 km discontinuity are examples where such discrepancies did arise. Although there is a general consensus that the depths to the 410 km and 660 km discontinuities vary by a few tens of kilometres at most, the question of whether the depth variations of the 410 km and 660 km discontinuities are correlated or anticorrelated is still unresolved. Similarly, different data sets and methods yielded different answers on the sharpness of the upper-mantle discontinuities at 410 km and 660 km depth. Finally, data apparently supporting the global existence of a seismic discontinuity at 520 km depth can be equally well explained by models that do not contain this discontinuity.

Angle resolved XPS study of inhomogeneous specimens of polycrystalline silver covered with uniform graphite overlayers

The depth dependence of the XPS peak shape parameter is determined experimentally for inhomogeneous specimens of the type f( x) = 0 for x 0 > x > 0 and f( x) = constant for x > x 0, where f( x) is the electron emitter concentration as a function of depth ( x) and x 0 is the overlayer thickness. In order to do this, specimens with sharp interfaces are prepared by depositing uniform overlayers of graphite on flat surfaces of polycrystalline silver. The peak shape parameter thus determined is found to be a function of the overlayer thickness. The observed dependence has been explained in terms of the existing first order analytical expression for this parameter. Angle resolved studies have also been done to explore the possible contributions arising out of surface excitation losses. Such studies clearly pointed out the presence of such an effect. Experimental results are discussed with a special emphasis on the possible sources of systematic errors which may contribute to the measured values of this parameter.

Application of the transient hot-wire method to gases at low pressures

The need to determine the thermal conductivity of non-ozone-depleting refrigerants implies measurements at pressures below 10 bar in the gaseous phase. In order to apply the transient hot-wire method with proven accuracy to this state, possible sources of systematic errors in the measurements have been carefully assessed theoretically and experimentally. The influence of the finite heat capacity of the hot wire and of the isothermal outer wall of the cell have been identified to affect the measurements substantially. An improved correction method to account for the wire heat capacity is presented, as well as criteria to choose the parameters in the experiments in order to avoid errors due to the outer boundary and due to the finite wire length. The results are presented in dimensionless quantities, and as an example, they are discussed for argon.

Simultaneous determination of the birefringence, optical activity and rotation of the indicatrix of triglicine sulphate

The birefringence, rotation of the optical indicatrix and optical activity of triglicine sulphate (TGS) have been measured in the paraelectric and ferroelectric phases using a high-accuracy universal polarimeter (HAUP). The present measurements are compared with results already published in the literature and, surprisingly, many of them have been found to be in clear disagreement. Possible sources of systematic error are pointed out in the case of the optical activity measurements. The spontaneous parts of the different optical properties in the ferroelectric phase have been studied. These parts can be considered to develop under the exclusive influence of the spontaneous polarization. Simple relationships between the spontaneous optical quantities and the spontaneous polarization (quadratic for birefringence and indicatrix angle, and linear for gyration) are observed. Several electro-optic and electrogyration coefficients referred to the polarization are evaluated.

Lack of optical activity in the incommensurate phases of Rb2ZnBr4 and

The optical activity of ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ and [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{CuCl}}_{4}$ has been measured in their centrosymmetric incommensurate phases using a high-accuracy universal polarimeter (HAUP). The results obtained show a zero value of this quantity along all the measured directions B(Rtwo in the case of ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ and one for the [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{CuCl}}_{4}$ crystalB)R. This behavior differs from results published previously for these or other similar incommensurate materials. The present measurements are compared with those reported in these previous studies, analyzing, in view of the differences, the possible sources of systematic errors in the HAUP technique. Our experimental results are in accordance with the idea that gyration tensors in incommensurate structures must be symmetry restricted, in a conventional manner, by the point group associated to their superspace group. Recent models allowing the appearance of gyration effects in incommensurate crystals with inversion symmetry should be reconsidered in the light of our results.

The space frequency and origin of the runaway O and B stars

Determinations of space frequency for runaway O and B stars are in very poor agreement, and many possible sources of systematic errors have been put forward. This paper investigates these sources, as well as others, and presents means for determining unbiased (true) space frequencies. When corrected for major sources of systematic error, space frequency determinations for the runaway O stars are in very good agreement but less so for the B-type runaways. The true space frequencies of runaway O and B stars are, respectively, 46 percent and 4 percent, as discussed in this paper. The former value is very supportive, along with other arguments presented in this paper, for runaway O stars being produced from supernova explosions in massive binaries. The origin of the B-type runaways is less clear. 89 refs.

Variable isotopic composition of lead as a source of systematic error in proton activation analysis

This paper considers the variable isotopic composition of lead as a possible source of systematic errors in proton activation analysis. The importance of this source of error was evaluated for environmental and geological samples. As proton activation analysis is frequently used for the certification of lead, the error was determined for three sediment reference materials. Inductively coupled plasma-mass spectrometry was used for the determination of the isotopic composition of lead. The systematic error amounted for the sediments to approximately 1%.

Absolute Isotopic Abundance Ratios and Atomic Weight of a Reference Sample of Nickel.

Absolute values have been obtained for the isotopic abundance ratios of a reference sample of nickel (Standard Reference Material 986), using thermal ionization mass spectrometry. Samples of known isotopic composition, prepared from nearly isotopically pure separated nickel isotopes, were used to calibrate the mass spectrometers. The resulting absolute isotopic ratios are: 58Ni/60Ni=2.596061±0.000728, 61Ni/60Ni=0.043469±0.000015,62Ni/60Ni=0.138600±0.000045, and 64Ni/60Ni=0.035295±0.000024, which yield atom percents of 58Ni=68.076886 ±0.005919, 60Ni = 26.223146±0.005144,61Ni=1.139894±0.000433, 62Ni =3.634528±0.001142, and 64Ni =0.925546±0.000599. The atomic weight calculated from this isotopic composition is 58.693353 ±0.000147. The indicated uncertainties are overall limits of error based on two standard deviations of the mean and allowances for the effects of known sources of possible systematic error.

The isotopic composition of antimony

The isotopic composition of antimony in synthetic mixtures prepared from highly enriched 121Sb and 123Sb tracers has been measured by solid source mass spectrometry. This has enabled the isotopic composition of natural antimony to be determined to give a 121Sb/ 123Sb ratio of 1.3453 ± 0.0014, which yields atom percents of 121Sb = 57.362 ± 0.026 and 123Sb = 42.638 ± 0.026. The atomic weight calculated from this isotopic composition is 121.757 ± 0.003. The indicated uncertainty in the atomic weight is an overall limit of error based on two standard deviations of the mean of the measured ratios and an allowance for the effects of known sources of possible systematic error. This value for the atomic weight is in good agreement with the presently accepted value of 121.75 ± 0.03 as set by the International Union of Pure and Applied Chemistry.

Absolute Isotopic Abundance Ratio And Atomic Weight Of a Reference Sample of Gallium.

An absolute value has been obtained for the isotopic abundance ratio of a reference sample of gallium (Standard Reference Material 994), using thermal ionization mass spectrometry. Samples of known isotopic composition, prepared from nearly isotopically pure separated gallium isotopes, were used to calibrate the mass spectrometers. The resulting absolute 69Ga/71Ga ratio is 1.50676±0.00039, which yields atom percents of 69Ga=60.1079±0.0062 and 71Ga=39.8921±0.0062. The atomic weight calculated from this isotopic composition is 69.72307±0.00013. The indicated uncertainties are overall limits of error based on two standard deviations of the mean and allowances for the effects of known sources of possible systematic error.

Accuracy and Linearity in UV Spectrophotometry with a Liquid Absorbance Standard

Estimations of the uncertainty of eight significant values (λ = 235, 257, 313, 322, 323, 345, 346, and 350 nm) of apparent molar absorptivities in the UV region have been obtained for acidic potassium dichromate solutions at pH 2.21–2.25, recommended as Standard Reference Materials (SRMs). The uncertainties include all the usual sources of possible systematic error in a commercial microprocessor-controlled spectrophotometer. The comparison of these values with those provided by the National Bureau of Standards (U.S.A.) indicates that a precision of ±0.40% has been obtained throughout the absorptivity range. Likewise, the uncertainty in the linearity of experimental absorbances for the eight wavelengths has been estimated, and a table has been drawn up of the absorptivity values and their uncertainties at 10-nm increments over the wavelength range of 200 to 500 nm.

Lyman-α revisited

The cross-section for excitation of Lyman- alpha radiation following electron impact of H2 has been measured at 100 eV incident energy. The data were made absolute by comparison with the signal obtained from electron impact on H at the same energy making use of the Long, Cox and Smith value (1968) for this cross-section. Identical excitation and detection geometries were used, thus automatically removing many possible sources of systematic error. The value obtained, 7.13+or-0.59*10-18 cm2, is 42% lower than the earlier measurement of Mumma and Zipf (1971) and is in good agreement with very recent measurements obtained using less direct calibration techniques. The accuracy of the present measurement is a considerable improvement over most previous measurements.

Evidence for unusual dynamical effects in the halo of NGC 5128 (Centaurus A)

view Abstract Citations (44) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evidence for Unusual Dynamical Effects in the Halo of NGC 5128 (Centaurus A) Hesser, J. E. ; Harris, H. C. ; Harris, G. L. H. Abstract New observations bring to 35 the number of spectroscopically confirmed globular clusters in NGC 5128. At radii less than about 11 arcmin the ensemble-average cluster velocity appears to exceed by about 4 simga the systemic velocity of the galaxy, while at larger radii the two values agree. Several sources of possible systematic error are eliminated by the new data, suggesting that this peculiar result is not likely to have arisen from a statistical fluctuation. These inner clusters lie beyond the obvious bulge light but within the faintest outer isophotes, and generally have redder colors than more distant ones. It is suggested that whatever phenomenon produced the warped dust lane and/or the faint optical 'shells' around NGC 5128 may also be related to the unexpected cluster velocities. If the phenomenon involved a merger, the cluster velocity observations would appear to suggest that the last merger occurred more recently than a dynamical crossing time. Publication: The Astrophysical Journal Pub Date: April 1986 DOI: 10.1086/184651 Bibcode: 1986ApJ...303L..51H Keywords: Elliptical Galaxies; Globular Clusters; Stellar Motions; Astronomical Spectroscopy; Radial Velocity; Velocity Distribution; Astrophysics; CLUSTERS: GLOBULAR; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 5128; GALAXIES: INDIVIDUAL NAME: CENTAURUS A; GALAXIES: INTERNAL MOTIONS full text sources ADS | data products SIMBAD (35) NED (14)

The luminosity scale of Cepheid variable stars - A revison

The period-luminosity color (PLC) relation for classical Cepheids is discussed in terms of a recent redetermination of the distance moduli of open clusters with Cepheid members. The distance moduli of eight clusters with well-established Cepheid members were determined from Stromgren four-color and H-beta photometry of the B stars. Possible sources of systematic errors in these distance moduli are discussed. Zero points are derived from these data for the PLC relation on the UBV system and the period-luminosity relations for near-infrared magnitudes. The present relations yield absolute magnitudes which are fainter by 0.4-0.6 mag than those now commonly used. These lower luminosities cause the pulsational masses of the Cepheids to be less than the evolutionary masses by up to a factor of 2.5 but bring them into good agreement with the masses of beat and bump Cepheids.

Parity nonconservation in proton-nucleus scattering at 6 GeV/c

A parity-nonconserving asymmetry in the total cross section for 6-GeV/c polarized protons on a water target has been measured. The experiment used two independent detector systems to measure the helicity dependence of the transmission through the target. Special attention was paid to possible sources of systematic errors. The result is ${A}_{L}=(2.65\ifmmode\pm\else\textpm\fi{}0.60\ifmmode\pm\else\textpm\fi{}0.36)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ where ${A}_{L}=\frac{({\ensuremath{\sigma}}_{+}\ensuremath{-}{\ensuremath{\sigma}}_{\ensuremath{-}})}{({\ensuremath{\sigma}}_{+}+{\ensuremath{\sigma}}_{\ensuremath{-}})}$ is the fractional difference of the total cross sections for positive- and negative-helicity protons on an unpolarized target. The first error is statistical and the second is an estimate of systematic uncertainties. This result is much larger than predictions based on meson-exchange calculations but consistent with a recent QCD calculation.