Neutron Time Download
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The neutron time-of-flight facility n_TOF features a white neutron source produced by spallation through 20GeV/3_ protons impinging on a lead target. The facility, aiming primarily at the measurement of neutron-induced reaction cross sections, was operating at CERN between 2001 and 2004, and then underwent a major upgrade in 2008. This paper presents in detail all the characteristics of the new neutron beam in the currently available configurations, which correspond to two different collimation systems and two choices of neutron moderator. The characteristics discussed include the intensity and energy dependence of the neutron flux, the spatial profile of the beam, the in-beam background components and the energy resolution/broadening. The discussion of these features is based on dedicated measurements and Monte Carlo simulations, and includes estimations of the systematic uncertainties of the mentioned quantities.
Neutron is a free application for Windows that you can use to check and see if your computer's clock is working correctly. If it's not, then you can synchronize the time to match that of a specialized server.
Results of two-dimensional hydra [29] simulations, symmetric about the z axis (hohlraum axis) for (a) the electron density ne, (b) the temperature kBTe, (c) the electron coupling e, and (d) electron degeneracy e at peak energy production in experiment No. N130813. The white curve defines the boundary between the fuel and surrounding carbon ablator. The observed H3(d,n) neutron spectrum is sensitive only to the weakly coupled, nondegenerate plasma inside the white curve.
The STARGazer data-processing software is used for neutron time-of-flight (TOF) single-crystal diffraction data collected using the IBARAKI Biological Crystal Diffractometer (iBIX) at the Japan Proton Accelerator Research Complex (J-PARC). This software creates hkl intensity data from three-dimensional (x, y, TOF) diffraction data. STARGazer is composed of a data-processing component and a data-visualization component. The former is used to calculate the hkl intensity data. The latter displays the three-dimensional diffraction data with searched or predicted peak positions and is used to determine and confirm integration regions. STARGazer has been developed to make it easier to use and to obtain more accurate intensity data. For example, a profile-fitting method for peak integration was developed and the data statistics were improved. STARGazer and its manual, containing installation and data-processing components, have been prepared and provided to iBIX users. This article describes the status of the STARGazer data-processing software and its data-processing algorithms.
The time-of-flight (TOF) method complements the triple-axis spectrometer (TAS) technique which is discussed elsewhere in this volume [6]. The TAS is ideally (but by no means only) suited to the study of excitations in oriented samples at specific points in (_,1) phase space. On the other hand TOF instruments may be used to explore rather large regions of phase space because many detectors simultaneously collect neutrons over a wide range of values of the scattered energy. The price paid for the much larger phase space volume is that the intensity on the sample is significantly reduced because the incident beam is pulsed. In another respect the TOF method complements the very high resolution backscattering and neutron spin echo techniques [7].
where dM is the spacing between reflecting planes in the monochromator, are Bragg reflected in the direction of the sample. The monochromatic beam, characterized by its energy E0 and wave vector 2_0, is then pulsed by a chopper placed at a known distance LCS from the sample. An array of detectors is arranged at a known fixed distance LSD from the sample, and scattered neutrons arrive at the detectors at times determined by their scattered energies E. The time-of-flight of a neutron from the chopper to one of the detectors is simply
Here tCS and tSD are the times-of-flight of the neutron from chopper to sample and from sample to detector, respectively, and 0 and are the reciprocal velocities of the neutron before and after scattering, respectively; relationships between , , k, and E are given in Appendix A. From Eq. (2) it is clear that , E, and the energy transfer
Schematic plan view of a simple time-of-flight spectrometer. The letters R, M, C, S, and D denote the reactor, monochromator, chopper, sample and detectors, respectively. Important distances are indicated. In practice, the slots in the (Fermi) chopper are curved in order to optimize its transmission.
The density of states (DOS) of a silica aerogel synthesized by hydrolysis of tetramethoxysilane without the addition of ammonia to the reaetion water. The open circles are TOF measurements at 160 K using 8 incident neutrons. The dotted curve indieates the DOS that fits neutron spin-eeho data. The solid line is a fit to neutron backscattering data and is extrapolated as shown by the dashes throughout the high-frequency fracton region. The dashed lines indicate the asymptotic phonon as well as the independent bend and stretch eontributions [12].
Quasielastic neutron scattering spectra for -ScH0.16 at several temperatures at 70 eV FWHM elastic energy resolution. The solid lines are the results of least-squares fits to the data; the dotted lines represent the Lorentzian quasielastic component. The increase in the quasielastic linewidth at low temperature is illustrated in the 50 K spectrum, where the length of the horizontal bar is equal to the width of the 70 K Lorentzian component [19].
Each of the disks in the pulsing and monochromating chopper pairs is equipped with three slots of different width, as shown in Fig. 16. The slots in each counter-rotating pair are located such that the width of the slot presented to the neutron beam can be changed by grossly changing the relative phasing of the disks. The choice of slot positions is complicated because of effects resulting from the small separation between the members of a counter-rotating pair [27,28].
Plots of the accessible region in (Q,) space for neutrons of wavelength 5 and 10 (energy 3.272 and 0.818 meV, respectively). The minimum and maximum scattering angles are 5 and 140. There is no (theoretical) limit to the energy transfer in neutron energy gain.
A decrease in E0 also means a proportionate reduction in the maximum possible energy transfer in neutron energy loss. The improved resolution that results as E0 is decreased, i.e., as 0 is increased, is a direct consequence of the contraction in accessible (Q,) space.
In the FCS there are two independent sources of incident beam time spread: the monochromator and the chopper. The chopper contribution is independent of distance but the wavelength spread from the monochromator translates into a time spread which increases with distance. The net result is that the width of the time distribution in the unscattered beam, at a point distant x from the chopper, may be written as follows:
In the disk chopper spectrometer the functions of puiser and monochromator are assumed by a set of disk choppers which rotate about a common axis parallel to (and some distance above) the direction of the beam. The essential concept can be understood by considering the two chopper arrangement illustrated in Fig. 18(a). The pulsing chopper produces short bursts of neutrons of many different energies. These neutrons have different speeds and therefore arrive at the monochromating chopper at many different times. The phasing of the monochromating chopper is chosen to transmit neutrons of the desired energy. The operation of this system can be represented in a distance-time (x,t) diagram as shown in Fig. 19(a).
A series of simplified (x,t) timing diagrams. In each case time is plotted horizontally and distance along the beam direction is plotted vertically; the slope of an inclined line is proportional to the velocity of the corresponding neutrons. The symbols 1, 2, 3, and 4 represent the positions of choppers or chopper pairs; S and D represent the sample position and the detector position, respectively. Breaks in horizontal lines represent time periods when choppers are open to the passage of neutrons. At (a) is shown an idealized timing diagram for a system of two choppers. A more realistic timing diagram for the same system (with counter-rotating choppers) is shown at (b). Several different neutron wavelengths are transmitted and some of those associated with one of the bursts are shown as heavy lines. To stop the unwanted wavelengths order removal choppers are added, as shown at (c). Ambiguities in the analysis of time-of-flight data can arise if the number of bursts at the sample position is too high. This is illustrated at (d); one set of neutrons which arrives at the detector at the same time is shown as heavy lines. A frame removal chopper is used to resolve the problem, as shown at (e); in this example every third burst is transmitted.
Each of the principal choppers in the DCS is actually a counter-rotating pair of disks, as shown in Fig. 18(b). The effective chopping speed for such a device is double that of a single disk which rotates at the same speed as one of the members of the counter-rotating pair [32]. It follows that the intensity per pulse is doubled at constant resolution. This can be understood by considering systems such as the one shown in Fig. 20(a). The burst time of the illustrated system is simply t = (w/u) whereas the transmitted intensity per pulse, Ip, is proportional to (w2/u), where w is the width of the slot and the guide, and u is the chopping speed; the time dependence of the intensity is illustrated in Fig. 20(b). Clearly 5376163bf9
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