共查询到20条相似文献,搜索用时 31 毫秒
1.
S. I. Penttila J. D. Bowman 《Journal of research of the National Institute of Standards and Technology》2005,110(3):309-313
The abBA collaboration is developing a new type of field-expansion spectrometer for a measurement of the three correlation coefficients a, A, and B and the shape parameter b. The measurement of A and B requires precision neutron polarimetry. We will polarize a pulsed cold neutron beam from the SNS using a 3He neutron spin filter. The well-known polarizing cross section for n-3He has a 1/v dependence, where v is the neutron velocity, which is used to determine the absolute beam polarization through a time-of-flight (TOF) measurement. We show that by measuring the TOF dependence of A and B, the coefficients and the neutron polarization can be determined with a small loss of the statistical precision and with negligible systematic error. We conclude that it is possible to determine the neutron polarization averaged over a long run in the neutron beta decay experiment with a statistical error less than 10−4. We discuss various sources of systematic uncertainty in the measurement of A and B and conclude that the fractional systematic errors are less than 2 × 10−4. 相似文献
2.
在中子周围剂量当量仪校准过程中,使用D-D中子源代替传统同位素中子源会使校准过程具有更高的安全性。为推动D-D中子源在中子周围剂量当量仪校准过程中的应用,提出了应用蒙特卡罗(MC)法的能量截断法代替影锥法的散射中子研究方法;分析了在不同类型房间中,不同内部空间尺寸对散射中子的影响。计算结果表明随着房间内部空间的增大,入射探测器的散射中子所占比例会逐渐减小。若使用D-D中子源进行中子周围剂量当量仪校准,在探测距离75cm,探测器直径20cm的情况下,所需的最小正方体房间的内部空间棱长为332cm;所需最小长方体房间的内部空间的长、宽、高分别为410cm、410cm、205cm。 相似文献
3.
Geoffrey Greene Vince Cianciolo Paul Koehler Richard Allen William Michael Snow Paul Huffman Chris Gould David Bowman Martin Cooper John Doyle 《Journal of research of the National Institute of Standards and Technology》2005,110(3):149-152
The Spallation Neutron Source (SNS), currently under construction at Oak Ridge National Laboratory with an anticipated start-up in early 2006, will provide the most intense pulsed beams of cold neutrons in the world. At a projected power of 1.4 MW, the time averaged fluxes and fluences of the SNS will approach those of high flux reactors. One of the flight paths on the cold, coupled moderator will be devoted to fundamental neutron physics. The fundamental neutron physics beamline is anticipated to include two beam-lines; a broad band cold beam, and a monochromatic beam of 0.89 nm neutrons for ultracold neutron (UCN) experiments. The fundamental neutron physics beamline will be operated as a user facility with experiment selection based on a peer reviewed proposal process. An initial program of five experiments in neutron decay, hadronic weak interaction and time reversal symmetry violation have been proposed. 相似文献
4.
F. E. Wietfeldt M. S. Dewey D. M. Gilliam J. S. Nico X. Fei W. M. Snow G. L. Greene J. Pauwels R. Eykens A. Lamberty J. Van Gestel 《Journal of research of the National Institute of Standards and Technology》2005,110(4):327-331
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 6LiF foil detector with known efficiency. The combination of these measurements gives the neutron lifetime: τn = (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. 相似文献
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6.
M. Arif M. S. Dewey G. L. Greene W. M. Snow 《Journal of research of the National Institute of Standards and Technology》1993,98(1):135-144
The features of two fundamental neutron physics research stations at the NIST cold neutron research facility are described in some detail. A list of proposed initial experimental programs for these two stations is also given. 相似文献
7.
Neutron imaging techniques were investigated at Peking University based on a 4.5 MV Van de Graaff accelerator. The thermal neutron radiography, fast neutron radiography and fast neutron resonance radiography were tested. The low neutron flux limits the image quality. A new radio frequency quadrupole (RFQ) accelerator based neutron source with a yield of 1012 n/s is being set up. 相似文献
8.
P. R. Huffman G. L. Greene R. R. Allen V. Cianciolo R. R. Huerto P. Koehler D. Desai R. Mahurin A. Yue G. R. Palmquist W. M. Snow 《Journal of research of the National Institute of Standards and Technology》2005,110(3):161-168
Monte Carlo simulations are being performed to design and characterize the neutron optics components for the two fundamental neutron physics beamlines at the Spallation Neutron Source. Optimization of the cold beamline includes characterization of the guides and benders, the neutron transmission through the 0.89 nm monochromator, and the expected performance of the four time-of-flight choppers. The locations and opening angles of the choppers have been studied using a simple spreadsheet-based analysis that was developed for other SNS chopper instruments. The spreadsheet parameters are then optimized using Monte Carlo techniques to obtain the results presented in this paper. Optimization of the 0.89 nm beamline includes characterizing the double crystal monochromator and the downstream guides. The simulations continue to be refined as components are ordered and their exact size and performance specifications are determined. 相似文献
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10.
Craig Heimbach 《Journal of research of the National Institute of Standards and Technology》2006,111(6):419-428
A neutron spectrometer was acquired for use in the measurement of National Institute of Standards and Technology neutron fields. The spectrometer included options for the measurement of low and high energy neutrons, for a total measurement range from 0.01 eV up to 17 MeV. The spectrometer was evaluated in calibration fields and was used to determine the neutron spectrum of an Americium-Beryllium neutron source. The calibration fields used included bare and moderated 252Cf, monoenergetic neutron fields of 2.5 MeV and 14 MeV, and a thermal-neutron beam. Using the calibration values determined in this exercise, the spectrometer gives a good approximation of the neutron spectrum, and excellent values for neutron fluence, for all NIST calibration fields. The spectrometer also measured an Americium-Beryllium neutron field in a NIST exposure facility and determined the field quite well. The spectrometer measured scattering effects in neutron spectra which previously could be determined only by calculation or integral measurements. 相似文献
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12.
S. N. Dzhosyuk A. Copete J. M. Doyle L. Yang K. J. Coakley R. Golub E. Korobkina T. Kreft S. K. Lamoreaux A. K. Thompson G. L. Yang P. R. Huffman 《Journal of research of the National Institute of Standards and Technology》2005,110(4):339-343
We report progress on an experiment to measure the neutron lifetime using magnetically trapped neutrons. Neutrons are loaded into a 1.1 T deep superconducting Ioffe-type trap by scattering 0.89 nm neutrons in isotopically pure superfluid 4He. Neutron decays are detected in real time using the scintillation light produced in the helium by the beta-decay electrons. The measured trap lifetime at a helium temperature of 300 mK and with no ameliorative magnetic ramping is substantially shorter than the free neutron lifetime. This is attributed to the presence of neutrons with energies higher than the magnetic potential of the trap. Magnetic field ramping is implemented to eliminate these neutrons, resulting in an
trap lifetime, consistent with the currently accepted value of the free neutron lifetime. 相似文献
13.
D. A. Neumann B. Hammouda 《Journal of research of the National Institute of Standards and Technology》1993,98(1):89-108
Two types of ultra high energy resolution neutron scattering instruments, the backscattering spectrometer and the spin echo spectrometer, are described. Examples of the types of research which can be done with these instruments are given and plans for a cold neutron backscattering spectrometer which will be built in the NIST Cold Neutron Research Facility (CNRF) are discussed. It is hoped that this information will be of use to researchers considering neutron scattering experiments at NIST. 相似文献
14.
K. Schoen W. M. Snow H. Kaiser S. A. Werner 《Journal of research of the National Institute of Standards and Technology》2005,110(3):259-262
The neutron index of refraction is generally derived theoretically in the Fermi approximation. However, the Fermi approximation neglects the effects of the binding of the nuclei of a material as well as multiple scattering. Calculations by Nowak introduced correction terms to the neutron index of refraction that are quadratic in the scattering length and of order 10−3 fm for hydrogen and deuterium. These correction terms produce a small shift in the final value for the coherent scattering length of H2 in a recent neutron interferometry experiment. 相似文献
15.
Measurement of the Neutron Lifetime Using a Gravitational Trap and a Low-Temperature Fomblin Coating
A. Serebrov V. Varlamov A. Kharitonov A. Fomin Yu. Pokotilovski P. Geltenbort J. Butterworth I. Krasnoschekova M. Lasakov R. Tal’daev A. Vassiljev O. Zherebtsov 《Journal of research of the National Institute of Standards and Technology》2005,110(4):333-338
We present a new value for the neutron lifetime of 878.5 ± 0.7stat. ± 0.3syst. This result differs from the world average value by 6.5 standard deviations and by 5.6 standard deviations from the previous most precise result. However, this new value for the neutron lifetime together with a β-asymmetry in neutron decay, A0, of −0.1189(7) is in a good agreement with the Standard Model. 相似文献
16.
概述了近年探则快中子的新技术,它们分别是:抑制γ射线、热中子和带电粒子的符合谱仪;高分辨、宽能量范围(0.1~15.0MeV)带正比计数器的3He夹心谱仪;含氢的纤维闪烁作用于抑制γ射线、中子位置分布和中子能谱测量,及合锂的纤维玻璃闪烁体用于长中子计数器测平均中子能量;中子的直接探测;用于中高能和重离子核物理的多元件阵列快中子探测器和极化仪;用于核核查的中子源影像探测器;高入射氚核能量和高能中子的伴随粒子技术等七个方面。它们对中子计量学的发展是重要的。 相似文献
17.
J. David Bowman S. I. Penttila 《Journal of research of the National Institute of Standards and Technology》2005,110(4):361-366
We present a conceptual design for an experiment to measure the neutron lifetime (~886 s) with an accuracy of 10−4. The lifetime will be measured by observing the decay rate of a sample of ultracold neutrons (UCN) confined in vacuum in a magnetic trap. The UCN collaboration at Los Alamos National Laboratory has developed a prototype UCN source that is expected to produce a bottled UCN density of more than 100/cm3 [1]. The availability of such an intense source makes it possible to approach the measurement of the neutron lifetime in a new way. We argue below that it is possible to measure the neutron lifetime to 10−4 in a vacuum magnetic trap. The measurement involves no new technology beyond the expected UCN density. If even higher densities are available, the experiment can be made better and/or less expensive. We present the design and methodology for the measurement. The slow loss of neutrons that have stable orbits, but are not energetically trapped would produce a systematic uncertainty in the measurement. We discuss a new approach, chaotic cleaning, to the elimination of quasi-neutrons from the trap by breaking the rotational symmetry of the quadrupole trap. The neutron orbits take on a chaotic character and mode mixing causes the neutrons on the quasi-bound orbits to leave the trap. 相似文献
18.
W. S. Wilburn J. D. Bowman G. S. Mitchell J. M. O’Donnell S. I. Penttila P.-N. Seo J. R. Calarco F. W. Hersmann T. E. Chupp T. V. Cianciolo K. P. Rykaczewski G. R. Young R. T. De Souza W. M. Snow D. Desai G. L. Greene R. K. Grzywacz E. Frlez D. Pocanic T. R. Gentile V. Gudkov G. L. Jones 《Journal of research of the National Institute of Standards and Technology》2005,110(4):389-393
We are developing an experiment to measure the correlations a, A, and B, and the Fierz interference term b in neutron decay, with a precision of approximately 10−4. The experiment uses an electromagnetic spectrometer in combination with two large-area segmented silicon detectors to detect the proton and electron from the decay in coincidence, with 4π acceptance for both particles. For the neutron-polarization-dependent observables A and B, precision neutron polarimetry is achieved through the combination of a pulsed neutron beam, under construction at the SNS, and a polarized 3He neutron polarizer. Measuring a and A in the same apparatus provides a redundant determination of λ = gA/gV. Uncertainty in λ dominates the uncertainty of CKM unitarity tests. 相似文献
19.
Neutron response function for a detector with He counters for the 0.39–1.54 MeV neutron energy range
J. erný Z. Dlouhý Z. Doleal M.P. Ivanov P. Kubik E.A. Kuznetsova Yu.E. Penionzkevich E.A. Sokol J. vejda I. Wilhelm 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》2005,540(2-3):430-436
An experimental study was carried out on the neutron response functions of two neutron detector arrays consisting of 39 3He proportional counters with a polyethylene moderator for monoenergetic neutrons within the 0.39–1.54 MeV neutron energy range. Experimental data on the sensitivity of neutron counting to a change in neutron energy and the influence of the thickness of polyethylene moderator were obtained. The experimental efficiency curves were compared with the calculated response functions generated by a neutron transport code. 相似文献
20.
Diane M. Markoff 《Journal of research of the National Institute of Standards and Technology》2005,110(3):209-213
Our understanding of hadronic parity violation is far from clear despite nearly 50 years of theoretical and experimental progress. Measurements of low-energy parity-violating observables in nuclear systems are the only accessible means to study the flavor-conserving weak hadronic interaction. To reduce the uncertainties from nuclear effects, experiments in the few and two-body system are essential. The parity-violating rotation of the transverse neutron polarization vector about the momentum axis as the neutrons traverse a target material has been measured in heavy nuclei and few nucleon systems using reactor cold neutron sources. We describe here an experiment to measure the neutron spin-rotation in a parahydrogen target (n-p system) using pulsed cold-neutrons from the fundamental symmetries beam line at the Spallation Neutron Source under construction at the Oak Ridge National Laboratory. 相似文献