Search

A technique for establishing the total neutron rate of a highly-collimated monochromatic cold neutron beam was demonstrated using a method of an alpha-gamma counter. The method involves only the counting of measured rates and is independent of neutron cross sections, decay chain branching ratios, and neutron beam energy. For the measurement, a target of 10B-enriched boron carbide totally absorbed the neutrons in a monochromatic beam, and the rate of absorbed neutrons was determined by counting 478keV gamma rays from neutron capture on 10B with calibrated high-purity germanium detectors. A second measurement based on Bragg diffraction from a perfect silicon crystal was performed to determine the mean de Broglie wavelength of the beam to a precision of 0.024 %. With these measurements, the detection efficiency of a neutron monitor based on neutron absorption on 6Li was determined to an overall uncertainty of 0.058 %. We discuss the principle of the alpha-gamma method and present details of how the measurement was performed including the systematic effects. We also describe how this method may be used for applications in neutron dosimetry and metrology, fundamental neutron physics, and neutron cross section measurements., Comment: 44 pages

The Alpha-Gamma device at the National Institute of Standards and Technology (NIST) utilizes neutron capture on a totally absorbing 10B deposit to measure the absolute neutron flux of a monochromatic cold neutron beam. Gammas produced by the boron capture are counted using high purity germanium detectors, which are calibrated using a well-measured 239Pu alpha source and the alpha-to-gamma ratio from neutron capture on a thin 10B target. This device has been successfully operated and used to calibrate the neutron flux monitor for the BL2 neutron lifetime experiment at NIST. It is also being used for a measurement of the 6Li(n,t)4He cross section. We shall present its principle of operation along with the current and planned projects involving the Alpha-Gamma device, including the recalibration of the U.S. national neutron standard NBS-1 and (n,f) cross section measurements of 235U.

A precise value of the neutron lifetime is important in several areas of physics, including determinations of the quark-mixing matrix element |Vud|, related tests of the Standard Model, and predictions of light element abundances in Big Bang Nucleosynthesis models. We report the progress on a new measurement of the neutron lifetime utilizing the cold neutron beam technique. Several experimental improvements in both neutron and proton counting that have been developed over the last decade are presented. This new effort should yield a final uncertainty on the lifetime of 1 s with an improved understanding of the systematic effects.

In a paper presented in the Proceedings of the 19th World Conference of the INTDS, 1998, the results of a blind comparison between groups at NIST and the IRMM showed excellent agreement in characterization of actinide targets by alpha-particle counting with low solid-angle geometry. At that time the most accurate NIST results estimated an uncertainty of 0.03 % for the solid-angle determination. The NIST Neutron Physics Group now has improved the accuracy of its solid-angle determination by about a factor of 4, based on an improved aperture design, improved metrology, and comparisons among independent metrology techniques at NIST. Applications of this improved technology are expected to include improved NIST measurements of the free neutron lifetime and a recalibration of the NIST standard for neutron emission rate, NBS-I.

Over the 50-year history of Section III (Neutron Measurements) of the Consultative Committee for Ionizing Radiation of the International Committee for Weights and Measures, only a single interlaboratory comparison of thermal neutron measurements has been completed, although a new comparison of this kind is in progress at the time of this writing. The initial comparison was perhaps considered to have demonstrated a sufficient degree of agreement that its repetition was not of particularly urgent priority. Thermal neutron measurements are indeed an old and well-developed field, as is seen in the many references cited in this paper. This paper describes briefly the production of thermal neutron fields from fast neutron sources, the moderation of neutron spectra towards thermal equilibrium, various conventions for describing thermal neutron fields, corrections for self-absorption and scattering in detector materials and the selection of detectors based on established thermal neutron cross-section standards.

We report on the development of a fast neutron detector using a liquid scintillator doped with enriched Li-6. The lithium was introduced in the form of an aqueous LiCl micro-emulsion with a di-isopropylnaphthalene-based liquid scintillator. A Li-6 concentration of 0.15 % by weight was obtained. A 125 mL glass cell was filled with the scintillator and irradiated with fission-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 Li-6. 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 6Li-loaded liquid scintillator, demonstrate the operation of it in a detector, and compare its efficiency 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 differentiating 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 specifics of the data analysis and its comparison with the Monte Carlo simulation.

It is demonstrated that a thin polyimide foil can be employed to prevent contamination from the self-sputtering of a 252 Cf source under vacuum, with small energy loss of the emitted fission fragments, with very small effect on the efficiency of counting the fission fragments, and with a long lifetime of the plastic foils.

The National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) is a national center that provides thermal and cold neutrons beams for activities such as condensed matter physics, materials science, nuclear chemistry, and biological science. Four beams are currently in use for fundamental physics experiments. These include a high-intensity polychromatic beam, a 0.496 nm monochromatic beam, a 0.89 nm monochromatic beam, and a neutron interferometer and optics facility. Experiments focus primarily on studies of the weak interaction using neutrons. This paper provides a general overview of the facilities and highlights some current experiments.

The 10 B(α,n) 13 N reaction is studied as an activation process in a variety of solid boron-containing neutron shielding materials. The source of α-particles is the neutron capture reaction 10 B(n,α) 7 Li. Samples of boron carbide, boron oxide, and boron nitride are irradiated with thermal neutrons and the rate of 13 N production is determined. 13 N promptly decays, emitting a positron. This positron efficiently annihilates with electrons in the material and the resultant 511 keV gamma ray is detected. For each of the above-mentioned materials, the rate of 13 N production is (1–2) × 10 −10 per captured neutron.

We detail the design and operation of an electrical substitution radiometer, which measures the absolute fluence of thermal neutron beams based on neutron absorption in a 6Li-rich metallic alloy. We demonstrate that the device possesses a linear response to electrical power in the 50–500 nW range with better than 0.1% absolute accuracy and negligible zero offset. We show the response of the radiometer to a 3 meV neutron beam. We compare the noise of the radiometer to the noise observed upstream in a thin rate monitor and show that the radiometer noise is understood quantitatively. We establish that the radiometer measures neutron rates above 105 s−1 to an absolute accuracy of 0.1%.

Neutron response calibrations have been performed for miniature SiC semiconductor detectors based on a Schottky diode design in neutron fields maintained at the National Institute of Standards and Technology (NIST). The SiC diodes detect neutrons via neutron-induced charged particles (tritons) produced by the 6 Li(n,α) 3 H reaction. The neutron response was calibrated at fluence rates from 1.76×10 4 to 3.59×10 10 cm −2 s −1 in NIST neutron fields. The maximum deviation from linearity of the fit of the neutron response of the SiC detectors to the NIST-measured fluence rates is much less than 5%, which is the estimated uncertainty in the measured fluence rates. A direct comparison of the SiC count rates to count rates obtained with a NIST double fission chamber over limited ranges where the detector positions and configurations remain unchanged shows a relative precision of ±0.6%. In a separate set of calibrations, a SiC neutron detector that had been previously irradiated with a fast ( E >1 MeV) neutron fluence of 1.3×10 16 cm −2 was found to have an absolute neutron fluence rate response that was indistinguishable from that of a previously unirradiated detector.

The achievable uncertainty in neutron lifetime measurements using the beam technique has been limited by the uncertainty in the determination of the neutron density in the decay volume. In the Sussex-ILL-NIST series of beam lifetime experiments, the density was determined with a neutron fluence mon itor that detected the charged particle products from neutron absorption in a thin layer of 6Li or lOB. In each of the experiments, the absolute detection efficiency of the neutron monitor was determined from the measured density of the neutron absorber, the thermal neutron cross section for the absorbing ma terial, and the solid angle of the charged particle detectors. The efficiency of the neutron monitor used in the most recent beam lifetime experiment has since been measured directly by operating it on a monochromatic neutron beam in which the total neutron rate is determined with a totally absorbing neutron detector. The absolute nature of this technique does not rely on any knowl edge of neutron absorption cross sections or a measurement of the density of the neutron absorbing deposit. This technique has been used to measure the neutron monitor efficiency to 0.06% uncertainty. VVe show that a new monitor and absolute neutron detector employingmore » the same technique would be capable of achieving determining neutron fluence to an uncertainty of 0.01%.« less

A measurement of the neutron lifetime τn performed by trapping and counting decay protons from in-beam neutron decays in a Penning trap is in progress at the National Institute of Standards and Technology (NIST). A description of the measurement technique, the status of the data analysis, and prospects for improvements in the measurement are discussed.

We report progress towards magnetic trapping of ultra-cold neutrons (UCN) in preparation for a neutron lifetime measurement. UCN will be produced by inelastic scattering of cold (0.89 nm) neutrons in a reservoir of super#uid 4He and con"ned in a three-dimensional magnetic trap. As the trapped neutrons decay, recoil electrons will generate scintillations in the liquid He, which should be detectable with nearly 100% e$ciency. This direct measure of the number of UCN decays vs. time can be used to determine the neutron beta-decay lifetime. ( 2000 Elsevier Science B.V. All rights reserved.

Actinide samples were characterized in an interlaboratory comparison between IRMM and NIST, including alpha-particle counting at defined low solid angle and counting in a 2π proportional gas counter. For this comparison, nine 233 UF 4 samples with high uniformity in the layer thickness were prepared at IRMM by deposition under vacuum. Polished silicon wafers were used as source substrates, and these were rotated during the deposition using a planetary rotation system. The estimated uncertainties 1 for the defined low solid-angle methods were about 0.1% at both NIST and IRMM. The agreement of reported α-particle emission rates in the energy range 2.5–5.09 MeV was better than or equal to 0.02% for the defined solid-angle methods. When comparing total α-particle emission rates over the larger energy range 0–9 MeV (which includes all emissions from the daughter nuclides and the impurities), the agreement of the defined solid-angle methods was better than or equal to 0.05%. The 2π proportional gas counter results were about 0.75% higher than those of the defined solid-angle methods. A re-examination of the correction for scattering from the polished silicon wafer substrates was undertaken to explain this discrepancy. Preliminary results of the recalculated 2π proportional gas counter results are also given.

Prenatal drug exposure may affect postnatal response to the reinforcing effects of a broad array of drugs. This study investigated the effects of prenatal cocaine exposure on operant self-administration of ethanol. Eighteen male, C57BL/6J mice were divided into three groups. The first had been prenatally exposed to 30 mg/kg of cocaine twice per day while the other groups were offspring of mothers which were either pair fed and saline intubated or untreated. Once adults, the mice were trained and subsequently tested for reinforcement from etha-nol. The prenatal cocaine group responded less than the two control groups, with the largest decrease during a progressive ratio schedule of reinforcement. There were no differences in responding as a function of ethanol concentrations. These findings suggest that prenatal exposure to cocaine may not affect reinforcement per se but may decrease motivation, drive state or propensity to work for ethanol.

An animal model was used to see if maternal genetic factors contribute to ethanol-induced fetal malformations. Susceptible C57BL/6J (B6) and resistant Short-Sleep (SS) mice were used in a reciprocal cross-breeding design. This design produced four fetal genotypes: true-bred B6B6 and SSSS liters and hybrid B6SS and SSB6 litters. Dams were intubated with either 5.8 g/kg of ethanol or an isocaloric amount of maltose-dextrin on day 9 of pregnancy. Fetuses were removed on day 18 of pregnancy and assessed for soft tissue and gross malformations. Results show different prenatal treatment effects on malformations depending on the fetal genotype. Mean percentage of total malformations in ethanol-treated groups were 61% (B6B6), 28% (B6SS), 3% (SSB6), and 16% (SSSS), respectively. Litters exposed to maltose-dextrin showed low (< or = 4%) malformation rates regardless of fetal genotype. The difference in teratogenic response between genetically identical hybrid litters (B6SS and SSB6) suggests a maternal genetic contribution to susceptibility. Of the two major types of malformations, forelimb but not kidney malformations showed a pattern characteristic of a maternal genetic effect. It was concluded that maternal genetic effects can be distinguished from fetal genetic effects responsible for ethanol teratogenesis. Genetically based physiological responses to alcohol in the mother may confer varying degrees of risk for prenatal alcohol effects in the child.

PET isotopes like 11 C and 18 F may be produced in deuteron-induced reactions using linear deuteron accelerators for energies 2–2.5 MeV. It is shown that the yield of PET isotopes in reactions 10 B(d,n) 11 C and 20 Ne(d,a) 18 F may be higher than the yield of the same isotopes in proton-induced reactions at higher energies, where protons are accelerated by a cyclotron. The lower cost and smaller size of linear accelerators make deuteron-induced reactions very promising for industrial PET production.

The most precise determination of the neutron lifetime using the beam method was completed in 2005 and reported a result of $\tau_n = (886.3 \pm 1.2 [\textrm{stat}] \pm 3.2 [\textrm{syst}])$ s. The dominant uncertainties were attributed to the absolute determination of the fluence of the neutron beam (2.7 s). The fluence was measured with a neutron monitor that counted the neutron-induced charged particles from absorption in a thin, well-characterized 6Li deposit. The detection efficiency of the monitor was calculated from the areal density of the deposit, the detector solid angle, and the evaluated nuclear data file, ENDF/B-VI 6Li(n,t)4He thermal neutron cross section. In the current work, we have measured the detection efficiency of the same monitor used in the neutron lifetime measurement with a second, totally-absorbing neutron detector. This direct approach does not rely on the 6Li(n,t)4He cross section or any other nuclear data. The detection efficiency is consistent with the value used in 2005 but was measured with a precision of 0.057 %, which represents a five-fold improvement in the uncertainty. We have verified the temporal stability of the neutron monitor through ancillary measurements, allowing us to apply the measured neutron monitor efficiency to the lifetime result from the 2005 experiment. The updated lifetime is $\tau_n = (887.7 \pm 1.2 [\textrm{stat}] \pm 1.9 [\textrm{syst}])$ s., Comment: v2 - Updated article with revisions made during editorial process, journal citation and DOI added. v1 - Draft article as submitted to Phys. Rev. Lett. (plus a few changes from institution review)

The preparation and characterisation of a new series of 10B and 6LiF reference deposits for accurate neutron fluence rate measurements are described and compared to those of an earlier campaign. Important improvements have been made by designing and building a new planetary rotating substrate holder and by using an optimized vacuum deposition set-up. Both deposition homogeneity and edge sharpness were significantly improved. However, a hitherto unrecognized source of border effect, i.e. mask expansion during the vacuum deposition process, was observed and discussed.

This paper describes the determination of the relative masses or relative areal densities of two new sets of reference deposits, prior to destructive analysis of subsets of these deposits by isotope dilution mass spectrometry and nondestructive analyses by other means. This work is a repeat of a previous effort of about five years ago. The goal of the present work is to improve the accuracy of the results to the level of one part per thousand. Improvements were attempted in several aspects of the measurements. The most significant improvement was realized simply by the increase in the number of repetitions of each measurement. In addition, a more extensive and elaborate data acquisition system was used, and an additional run-to-run monitor was employed to avoid possible problems related to the dependence of the 235 U monitor on neutron temperature variations.

Contraband detection systems for homeland security applications are typically tested for probability of detection (PD) and probability of false alarm (PFA) using pass‐fail testing protocols. Test protocols usually require specified values for PD and PFA to be demonstrated at a specified level of statistical confidence CL. Based on a recent more theoretical treatment of this subject [1], this summary reviews the definition of CL and provides formulas and spreadsheet functions for constructing tables of general test requirements and for determining the minimum number of tests required. The formulas and tables in this article may be generally applied to many other applications of pass‐fail testing, in addition to testing of contraband detection systems.

The areal densities (g/cm2) of enriched boron and lithium fluoride targets were compared to those of reference deposits from the Central Bureau for Nuclear Measurements (CBNM, Geel, Belgium) by charged-particle counting at the NIST Research Reactor. The uniformity of the target thickness was also investigated by scans with a small-diameter, intense neutron beam. The neutron uniformity measurements agree satisfactorily with previously reported interferometric results and no pattern of uniformity variation except the expected, small radial dependence was found. In a separate series of measurements, the boron areal density ratios of a number of the deposits were determined using a new double ionization chamber. The new double ionization chamber permits a comparison of two targets directly without a beam monitor and without sensitivity to the exact placement of the samples. The new device needs only a very simple data acquisition system and requires less neutron beam intensity than the previously employed systems with arrays of surface barrier detectors. The ionization chamber counting losses from the combined effects of self-absorption and sub-threshold pulse-height formation were determined by comparison with ratio measurements made by the conventional surface barrier detector technique.

We propose a standard method of establishing the absolute neutron output from small, D-T, 14 MeV neutron generators. This method uses a copper activation measurement in a configuration that we have calibrated with fission ionization chambers from NIST. The absolute uncertainty in this calibration is less than ± 7%. The copper activation method is insensitive to backgrounds from low energy scattered neutrons because it uses the 63 Cu(n, 2n) 62 Cu reaction which has a 12 MeV threshold. With this calibration method, measurements of absolute neutron output are possible under a variety of experimental conditions, including those simulating nuclear well logging. In addition, the configuration of the copper samples that we propose gives high counting rates so that the statistical precision of the measurement of neutron output, depending upon the generator voltage and beam current, is on the order of 1%.

The Albany subcolony of the selectively bred Long- and Short-Sleep mice was directly compared to the original Colorado colony. As expected from the additional selection applied to the Colorado colony, small differences in the selection phenotype, loss of the righting reflex duration following ethanol treatment, were observed in the Short-Sleep line. However, no colony differences existed in three other indices of ethanol effects. Clear line differences in the shape of the locomotor activity dose-response curve, thermoregulatory effects of ethanol, and ethanol elimination rate replicated earlier findings, but these differences were similar in the two colonies. These data argue for stability of the polygenic control system and provide a picture of remarkable similarity of the two sublines, which were separated by more than 30 generations.

This paper describes part of a project whose overall aim was the fabrication of a set of 10 B and 6 LiF deposits of known areal density for use in a measurement of the free neutron lifetime. (n, α) rates were measured in a neutron beam in a reproducible geometry and were related to the masses of several of the deposits (determined by isotope dilution mass spectrometry) so that the masses of the remaining deposits, and hence their areal densities, could be established. Details are given here of the detection arrangement and of the corrections which have to be made to the charged particle counting rates in order that the masses and counting rates be properly correlated.

In beam experiments, the achievement of an accurate neutron lifetime measurements depends heavily on the use of well characterised boron and/or lithium reference deposits to assess the neutron density. Sets of 6 LiF and 10 B reference deposits on silicon wafers were prepared by vacuum deposition using planetary rotating substrate holders. The uniformity of deposition was investigated using visible light spectrophotometry and accurate dimensional characterisation was carried out on the basis of travelling microscope and Talistep measurements. The results obtained agreed well with theoretical calculations based on the geometry of the evaporation system. The deposits prepared in successive evaporation runs were then characterised by neutron induced charged particle measurements, calibrated by isotope dilution mass spectrometry. 6 Li and 10 B surface densities are certified with accuracies of 0.35–0.50% and 0.30–0.41%, respectively. An intercomparison of the 6 Li and 10 B calibrations confirms the consistency of the measurements.

The accurate characterization of the surface density (nuclei/cm 2 ) of thin isotopic deposits is of highest importance in certain experiments. If accuracies better than ± 0.5% are to be quoted, careful consideration of seemingly minor effects is necessary, as these effects may generate serious errors. The radial surface density distribution in the central region of the target disks, the thickness profile at the edges, and the evaluation of effective deposit diameters are discussed for the case of measurements and observations made during the preparation and characterization of 6 LiF and 10 B reference deposits, which were employed in a recent determination of the free neutron lifetime. Theoretical calculations are in excellent agreement with the experimental results.

Alcohol sensitivity may influence the severity of alcohol-related birth defects (ARBD). To examine this hypothesis, pregnancy outcome and offspring development were examined in alcohol-sensitive Long-Sleep (LS) mice and alcohol-resistant Short-Sleep (SS) mice following prenatal ethanol exposure. Dams were intragastrically intubated twice per day (6 hr apart) with either 4.5 g/kg (20% w/v) ethanol (E) or an isocaloric amount of sucrose (S) on days 7 through 18 of pregnancy. An untreated control group (C) was maintained for each line. Results showed litter mortality at 10 days of age was greater for LS-E litters compared to both LS-S and LS-C litters. Litter mortality for SS-E litters did not differ from either SS-S or SS-C litters. Maternal weight gain, blood ethanol levels, and birth weight deficits were similar for ethanol-exposed LS and SS groups. These results suggest genetically based alcohol sensitivity influences the severity of ARBD.

Individual sensitivity to alcohol may influence the severity of functional deficits due to prenatal alcohol exposure. To examine this hypothesis, Long-Sleep (LS) and Short-Sleep (SS) mice, selectively bred for differences in ethanol-induced narcosis, were intubated with either 2.9 g/kg ethanol (E) or an isocaloric amount of sucrose (S) twice per day on days 7 through 15 of pregnancy. An untreated control group (C) was maintained for each line. Offspring were fostered to lactating Rockland-Swiss mice at birth. Males and females from each litter were challenged with an acute dose of ethanol (3.8 g/kg) at 30 days of age. Measures of sleep time duration, waking blood ethanol concentrations (BEC), rectal temperatures, heart rate, and ethanol clearance were obtained to examine whether the acute effects of ethanol are altered by prenatal alcohol exposure. Prenatal alcohol exposure did not differentially affect responses to ethanol challenge within either genotype. Ethanol-induced hypothermia, heart-rate depression, and sleep time did differ between genotypes, with LS more affected than SS mice. Ethanol clearance rates were faster for SS than LS mice. These results suggest postnatal pharmacological responses to acute ethanol challenge are not altered by prenatal alcohol exposure in LS and SS mice. Prenatal alcohol-exposed offspring of both mouse genotypes showed lower average heart rate responses than controls, suggesting this measure may be a sensitive indicator of prenatal alcohol effects in mice.

Photofission measurements have been made in {sup 238}U, {sup 232}Th, and {sup 237}Np in iron and cadmium capture gamma-ray spectra in cylindrical neutron-driven gamma-ray sources in the thermal column of the National Bureau of Standards (NBS) Reactor. The gamma-ray source strength was measured with neutron activation foils and by direct counting of activations produced in the metal cylinders. Photofission measurements were made with NBS miniature fission chambers. The integral photofission cross sections were compared with differential measurements by integrating the capture gamma-ray spectra with measured cross-section shapes. The integral cross sections measured in the capture gamma-ray fields are lower than the cross sections calculated from measured differential data.

A measurement of the neutron lifetime $\tau_{n}$ performed by the absolute counting of in-beam neutrons and their decay protons has been completed. Protons confined in a quasi-Penning trap were accelerated onto a silicon detector held at a high potential and counted with nearly unit efficiency. The neutrons were counted by a device with an efficiency inversely proportional to neutron velocity, which cancels the dwell time of the neutron beam in the trap. The result is $\tau_{n} = (886.6\pm1.2{\rm [stat]}\pm3.2{\rm [sys]})$ s, which is the most precise measurement of the lifetime using an in-beam method. The systematic uncertainty is dominated by neutron counting, in particular the mass of the deposit and the $^{6}$Li({\it{n,t}}) cross section. The measurement technique and apparatus, data analysis, and investigation of systematic uncertainties are discussed in detail., Comment: 71 pages, 20 figures, 9 tables; submitted to PRC

The program in fundamental neutron physics at the National Institute of Standards and Technology (NIST) began nearly two decades ago. The Neutron Interactions and Dosimetry Group currently maintains four neutron beam lines dedicated to studies of fundamental neutron interactions. The neutrons are provided by the NIST Center for Neutron Research, a national user facility for studies that include condensed matter physics, materials science, nuclear chemistry, and biological science. The beam lines for fundamental physics experiments include a high-intensity polychromatic beam, a 0.496 nm monochromatic beam, a 0.89 nm monochromatic beam, and a neutron interferometer and optics facility. This paper discusses some of the parameters of the beam lines along with brief presentations of some of the experiments performed at the facilities.

Spent fuel from nuclear reactors, even weapons‐grade plutonium, and certain weapons‐initiator materials emit large numbers of neutrons from spontaneous fission or (alpha, n) reactions. These are materials which terrorist groups could fairly easily obtain for construction of radiation dispersion weapons or fission weapons. Because the U.S. borders are fairly easily crossed in many wilderness areas and because these materials could be obtained from sites within the U.S., it is not sufficient to inspect baggage and cargo at points of entry. Very sensitive neutron detectors coupled with surveillance cameras could be positioned at highway choke points on the outskirts of major cities or near other sensitive locations to detect neutrons from contraband nuclear components and provide warnings to security personnel to try to prevent such an attack. The major advantage of neutron detectors over beta and gamma detectors is that the background rates and incidences of false positives would be much lower. There is onl...

Genetically based alcohol sensitivity may influence the severity of alcohol-related birth defects. To examine this question, measures of growth and survival were examined in offspring of the alcohol sensitive Long-Sleep (LS) and alcohol-resistant Short-Sleep (SS) mouse lines following prenatal ethanol exposure. Pregnant LS and SS mice received an ethanol dose of either 6 or 8 g/kg/day from days 7 through 18 of pregnancy. Control groups received a maltose-dextrin solution made isocaloric to the 8 g/kg/day dose. Ethanol and maltose-dextrin solutions were administered as split doses, 6 h apart, via gavage. Nonintubated lab chow control groups were also included for both mouse lines. Offspring were fostered at birth to lactating mice of an outbred stock. Pregnancy was longer for ethanol-treated LS dams compared to maltose-dextrin and lab chow LS control groups, whereas pregnancy length for ethanol-treated SS dams was similar to SS controls. Prenatal ethanol exposure resulted in dose-related growth deficits in LS but not in SS litters. Line differences in postnatal growth deficits in response to prenatal alcohol exposure suggest maternal or fetal alcohol sensitivity influence alcohol-related birth defects.

Prenatal cocaine exposure leads to multiple abnormalities in the mature offspring. We explored the effects of gestational exposure to cocaine on neurotransmitter systems of adult mice. The subjects were the mature offspring of mice (a) prenatally fed cocaine between gestational day (G) 8 and G19, (b) pair-fed chow and water, or fed chow and water ad libitum. The forebrains of the mature offspring were assayed for monoamines and amino acids. Cocaine exposure particularly affected the dopaminergic system and in a sex-specific manner. In males dopamine concentrations were decreased and dopamine turnover was increased, whereas in females dopamine concentrations were increased and turnover was decreased. Neither norepinephrine, the serotonergic system, nor neuroactive amino acids (or their precursors) were affected by cocaine. Thus, in utero exposure to cocaine produces long-lasting, specific defects in the dopaminergic system.

Sensitivity to alcohol may influence the severity of prenatal alcohol effects. To examine this hypothesis, the ontogeny of thermoregulation was measured in prenatal ethanol exposed offspring of mice selected for differences in alcohol sensitivity. Pregnant long-sleep (LS) and short-sleep (SS) mice were exposed to 3 or 4 g/kg ethanol or an isocaloric amount of maltose-dextrin twice per day from day 7 through 18 of pregnancy. Doses were given six hours apart via gavage. Nonintubated lab chow controls were included for both genotypes. Offspring were fostered at birth to lactating mice of an outbred strain. Offspring temperatures were measured at 0, 60, and 120 min away from the nest on alternating days from 7 through 21 days of age. LS and SS offspring prenatally exposed to the high ethanol dose showed lower temperatures at the 60 and 120 min time points on each day of testing compared to all other treatment groups. Temperatures of offspring prenatally exposed to the low ethanol dose did not differ from controls. These results suggest a relatively steep dose-response curve for thermoregulatory deficits in LS and SS offspring prenatally exposed to alcohol. Genetically-based alcohol sensitivity did not influence the effects of prenatal alcohol exposure on this response © 1992 John Wiley & Sons, Inc.

Performance standards for detector systems often include requirements for probability of detection and probability of false alarm at a specified level of statistical confidence. This paper reviews the accepted definitions of confidence level and of critical value. It describes the testing requirements for establishing either of these probabilities at a desired confidence level. These requirements are computable in terms of functions that are readily available in statistical software packages and general spreadsheet applications. The statistical interpretations of the critical values are discussed. A table is included for illustration, and a plot is presented showing the minimum required numbers of pass-fail tests. The results given here are applicable to one-sided testing of any system with performance characteristics conforming to a binomial distribution.

The neutron interferometry and optics facility at the National Institute of Standards and Technology (NIST) has been in full operation since September 1996 as both a NIST research facility and as a national user facility. Unprecedented performance characteristics have been achieved in interferometry. A very busy schedule of experiments in fundamental physics, applied physics, and materials science has developed.

Maternal genetic factors and/or fetal genetic factors contribute to variations in response to prenatal alcohol exposure. To assess the contribution of maternal genotype to ethanol teratogenesis, a reciprocal cross study was conducted in an animal model using C57BL/6J (B6) and long-sleep (LS) mice. B6 mice are more susceptible than LS mice to prenatal ethanol-induced malformations but both mouse stocks are susceptible to fetal weight deficits following in utero alcohol exposure. B6 and LS dams were reciprocally mated to B6 or LS males producing four embryonic genotype groups: the true-bred B6B6 and LSLS genotypes, and the genetically similar B6LS and LSB6 genotypes (the F1 genotype). Dams were intubated with either 5.8 g/kg ethanol (E) or an isocaloric amount of sucrose (S) on day 9 of pregnancy. Fetuses were removed on gestation day 18, weighed, and assessed for soft tissue or skeletal malformations. Results showed a greater litter weight deficit and increased total malformation rate in ethanol-exposed F1 litters carried by B6 mothers compared to ethanol-exposed F1 litters carried by LS mothers. This result would be expected only if maternal genetic factors contribute significantly towards susceptibility to ethanol teratogenesis. The influence of the LS mother was to decrease susceptibility to ethanol teratogenesis compared to the B6 mother while the influence of the B6 mother was to increase susceptibility to ethanol teratogenesis compared to the LS mother. The average malformation rate for F1 litters was significantly less than the predicted midparental value. This shows that the F1 genotype exhibited dominance towards resistance to prenatal alcohol effects. There was no differential effect of prenatal treatment on male and female fetuses for any embryonic genotype, indicating that the differential teratogenic response between F1 litters was not due to sex-linked traits. Likewise, weight gain due to pregnancy and maternal blood ethanol levels did not differ among genotypes. It was concluded that genetic factors differentially influence maternal physiological response to ethanol, which in turn affects teratogenic responses in F1 litters. Separation of maternal physiological mechanisms from fetal mechanisms responsible for ethanol teratogenic susceptibility is needed for targeting women and infants at risk. Such knowledge will help in formulating effective therapeutic intervention strategies.

The neutron lifetime ${\mathrm{\ensuremath{\tau}}}_{\mathit{n}}$ has been measured by counting decay protons stored in a Penning trap whose magnetic axis coincided with a neutron-beam axis. The result of the measurement is ${\mathrm{\ensuremath{\tau}}}_{\mathit{n}}$=893.6\ifmmode\pm\else\textpm\fi{}5.3 s, which agrees well with the value predicted by precise measurements of the \ensuremath{\beta}-decay asymmetry parameter A and the standard model.

To ensure the validity of their national standards, National Metrology Institutes, NMIs, participate regularly in international comparisons. In the area of neutron metrology, Section III of the Consultative Committee for Ionizing Radiation is in charge of the organization of these comparisons. From September 2011 to October 2012, the eleventh key comparison, named CCRI(III)-K11, took place at the AMANDE facility of the LNE-IRSN, in France. Participants from nine NMIs came with their own primary reference instruments, or instruments traceable to primary standards, with the aim of determining the neutron fluence, at 1 m distance from the target in vacuum, per monitor count at four monoenergetic neutron fields: 27 keV, 565 keV, 2.5 MeV and 17 MeV. The key comparison reference values (KCRV) were evaluated as the weighted mean values of the results provided by seven participants. The uncertainties of each KCRV are between 0.9 % and 1.7 %. The degree of equivalence (DoE), defined as the deviation of the result reported by the laboratories for each energy from the corresponding KCRV, and the associated expanded uncertainty are also reported and discussed.

We measured the neutron decay lifetime by counting in-beam neutron decay recoil protons trapped in a quasi-Penning trap. The absolute neutron beam fluence was measured by capture in a thin (6)LiF foil detector with known efficiency. The combination of these measurements gives the neutron lifetime: τ n = (886.8 ± 1.2 ± 3.2) s, where the first (second) uncertainty is statistical (systematic) in nature. This is the most precise neutron lifetime determination to date using an in-beam method.

The characterisation of a new set of 10 B deposits has revealed that the calibration of the 10 B deposits used in the neutron-lifetime experiment of Byrne et al. [1] was in error and it is therefore recommended that this result be excluded from future evaluations of τ n .