Dominic F. Vecchia

Fast neutron detection with 6Li-loaded liquid scintillator

We report on the development of a fast neutron detector using a liquid scintillator doped with enriched 6 Li. The lithium was introduced in the form of an aqueous LiCl micro-emulsion with a di-isopropylnaphthalene-based liquid scintillator. A 6 Li concentration of 0.15 % by weight was obtained. A 125 mL glass cell was lled with the scintillator and irradiated with ssion-source neutrons. Fast neutrons may produce recoil protons in the scintillator, and those neutrons that thermalize within the detector volume can be captured on the 6 Li. The energy of the neutron may be determined by the light output from recoiling protons, and the capture of the delayed thermal neutron reduces background events. In this paper, we discuss the development of this 6 Li-loaded liquid scintillator, demonstrate the operation of it in a detector, and compare its eciency and capture lifetime with Monte Carlo simulations. Data from a boron-loaded plastic scintillator were acquired for comparison. We also present a pulse-shape discrimination method for dierentiating between electronic and nuclear recoil events based on the Matusita distance between a normalized observed waveform and nuclear and electronic recoil template waveforms. The details of the measurements are discussed along with specics of the data analysis and its comparison with the Monte Carlo simulation.

Consistency tests for key comparison data

Results of International Key Comparisons of National Measurement Standards provide the technical basis for the Mutual Recognition Arrangement (MRA) formulated by Le Comite International des Poids et Mesures (CIPM). With many key comparisons already completed and a number of new key comparison experiments currently under way, we now have a better understanding of the statistical issues that need to be addressed for successfully analysing data from key comparisons and making proper interpretations of the results. There is clearly a need for a systematic approach to statistical analyses of key comparison data that can be implemented routinely by all participating laboratories.The determination of a key comparison reference value (KCRV) and its associated uncertainty and the degrees of equivalence are the central tasks in the evaluation of key comparison data. A satisfactory definition of a KCRV, however, is based on the assumption that all laboratories are estimating the same unknown quantity of the common circulating artefact, that is, the results from the different laboratories are mutually consistent. In this paper, we compare a number of statistical procedures for testing the consistency assumption.

Calibration With Randomly Changing Standard Curves

Changes in calibration curves from one time to the next, caused by drift, often require measuring devices to be recalibrated at frequent intervals. In such situations the usual practice is to estimate the unknown values of test samples using only data from the corresponding calibration period. Under a random coefficient regression model for the different calibration curves, however, it can be shown that it is more efficient to combine the data from all calibration periods to estimate the unknowns. We consider a particular class of point estimators obtained by inverting suitable prediction functions and show that the estimator obtained from a best prediction function is optimal in a sense defined by Godambe (1960) and Durbin (1960) in the context of unbiased estimating equations. We also compare the smallsample performance of this estimator with the usual estimator using the Pitman closeness criterion.

Construction of orthogonal two-level designs of user-specified resolution where N≠2 k

Fractional two-level factorial designs are often used in the early stages of an investigation to screen for important factors. Traditionally, 2 n-k fractional factorial designs of resolution III, IV, or V have been used for this purpose. When the investigator is able to specify the set of nonnegligible factorial effects, it is sometimes possible to obtain an orthogonal design with fewer runs than a standard textbook design by searching within a wider class of designs called parallel-flats designs. The run sizes in this class of designs do not necessarily need to be powers of 2. We discuss an algorithm for constructing orthogonal parallel-flats designs to meet user specifications. Several examples illustrate the use of the algorithm.

Critical Current Measurements on a NbTi Superconducting Wire Standard Reference Material

The National Bureau of Standards is producing a standard reference material (SRM) for the measurement of the superconducting critical current (Ic). This SRM and the recently adopted American Society for Testing and Materials (ASTM) standard test method (B714-82) will aid both the commerce and technology of superconductors through the promotion of more uniform measurements. The SRM will serve as an artifact for interlaboratory comparison to further advance the consensus and evolution of the new test method. The general use and philosophy of an SRM are given in Ref. 1.

PROBLEMS WITH INTERVAL ESTIMATION WHEN DATA ARE ADJUSTED VIA CALIBRATION

The analysis of adjusted data arising from a linear calibration curve is considered. Although it is obvious that adjusted values contain errors due to estimation of the calibration curve, some investigators may be tempted to analyze such data as if they..

A Comparison of Methods for Computing the Residual Resistivity Ratio of High-Purity Niobium.

We compare methods for estimating the residual resistivity ratio (RRR) of high-purity niobium and investigate the effects of using different functional models. RRR is typically defined as the ratio of the electrical resistances measured at 273 K (the ice point) and 4.2 K (the boiling point of helium at standard atmospheric pressure). However, pure niobium is superconducting below about 9.3 K, so the low-temperature resistance is defined as the normal-state (i.e., non-superconducting state) resistance extrapolated to 4.2 K and zero magnetic field. Thus, the estimated value of RRR depends significantly on the model used for extrapolation. We examine three models for extrapolation based on temperature versus resistance, two models for extrapolation based on magnetic field versus resistance, and a new model based on the Kohler relationship that can be applied to combined temperature and field data. We also investigate the possibility of re-defining RRR so that the quantity is not dependent on extrapolation.

Robust regression applied to optical-fiber dimensional quality control

To minimize coupling losses when low-cost connectors are used to mate optical fibers, manufacturers need to maintain tight control of fiber dimensions. The video-microscope, or gray-scale, method is the most frequently used technique on the manufacturing floor for measuring the geometric parameters of the cleaved end of a telecommunications fiber. We present a method for performing optical-fiber dimensional quality control that allows for end face damage and accounts for the special structure of measurement errors in fiber edge points calculated from gray-scale images. The new approach adheres to the industrial standard test procedure by fitting an ellipse to the edge table to obtain geometric measurements. But, to create high-breakdown resistance to outliers, a data filter based on the least median of squares criterion is used.

Measuring Residual Resistivity Ratio of High‐Purity Nb

We compared methods of measuring the residual resistivity ratio (RRR) of high‐purity Nb using transport current. Our experimental study is intended to answer some fundamental questions about the best measurement method for RRR and the biases that may exist among different measurement methods, model equations, and magnetic field orientations. Two common ways to obtain the extrapolated normal‐state resistivity at 4.2 K are (1) measure the normal‐state resistivity as a function of field at 4.2 K and extrapolate to zero field and (2) measure the normal‐state resistivity as a function of temperature in zero field and extrapolate to 4.2 K. Both approaches have their associated difficulties. We also compared data taken with the magnetic field both parallel and transverse to the specimen current and we measured magnetoresistance at various temperatures from 4 to 16 K. We combined all of these data to estimate the RRR using an approach based on the Kohler empirical rule regarding magnetoresistance. The Kohler rule imposes the shape of the magnetoresistance curve at the higher temperatures to better predict the extrapolated normal‐state values at the lower temperatures.We compared methods of measuring the residual resistivity ratio (RRR) of high‐purity Nb using transport current. Our experimental study is intended to answer some fundamental questions about the best measurement method for RRR and the biases that may exist among different measurement methods, model equations, and magnetic field orientations. Two common ways to obtain the extrapolated normal‐state resistivity at 4.2 K are (1) measure the normal‐state resistivity as a function of field at 4.2 K and extrapolate to zero field and (2) measure the normal‐state resistivity as a function of temperature in zero field and extrapolate to 4.2 K. Both approaches have their associated difficulties. We also compared data taken with the magnetic field both parallel and transverse to the specimen current and we measured magnetoresistance at various temperatures from 4 to 16 K. We combined all of these data to estimate the RRR using an approach based on the Kohler empirical rule regarding magnetoresistance. The Kohler rule...

Minimum cost inspection intervals for a two-state process

Any automated process can be said to be in state 0 (operating properly) or, if a malfunction occurs, to be in state 1 (not operating properly). Periodic inspection is necessary to make sure that the process is in state 0. We derive the minimum cost insp..