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ESTIMATING THE PERFORMANCE OF NEUTRON TIME-OF-FLIGHT SPECTROMETERS

Description: Analogy with optical instruments is used to develop formulas for estimating the counting rate achieved by a neutron time-of-flight system. lnput data consist of the flux at the spectrometer entrance and some of the physical characteristics of the spectrometer. Convenient relations for the comparison of different systems are presented. (auth)
Date: November 1, 1961
Creator: Stanford, G.S.
Partner: UNT Libraries Government Documents Department

Lunar prospector measurements of the distribution of incompatible elements gadolinium, samarium and thorium

Description: Lunar Prospector neutron spectrometer (NS) and gamma ray spectrometer (GRS) observations have been used to map out the distribution of incompatible elements on the lunar surface. Specifically, the GRS data provide maps of the distribution of thorium and potassium while the NS data provide information on the distribution of iron and titanium, and the rare earth elements gadolinium and samarium. Using results of analysis of Celementine spectral reflectance (CSR) data, the Fe- and Ti-contributions to the NS data can be removed, leaving primarily rare earth element contributions from Gd and Sm. The Th and K maps correlate with the inferred Gd and Sm maps (r {approximately} 0.93), but there are regions of significant disagreement. One of these is in the KREEP-rich circum-Imbrium ring. No clear explanation has emerged for this disagreement, though Th, K, Gd and Sm have differing degrees of incompatibility. These results clearly are important to discussions of the geochemistry of the Procellarum-Imbrium Th-rich Terrane and the South-Pole-Aitken Terrane.
Date: April 1, 1999
Creator: Elphic, R.C.; Lawrence, D.J.; Feldman, W.C.; Barraclough, B.L.; Maurice, S.; Binder, A.B. et al.
Partner: UNT Libraries Government Documents Department

Closed-loop step motor control using absolute encoders

Description: A multi-axis, step motor control system was developed to accurately position and control the operation of a triple axis spectrometer at the High Flux Isotope Reactor (HFIR) located at Oak Ridge National Laboratory. Triple axis spectrometers are used in neutron scattering and diffraction experiments and require highly accurate positioning. This motion control system can handle up to 16 axes of motion. Four of these axes are outfitted with 17-bit absolute encoders. These four axes are controlled with a software feedback loop that terminates the move based on real-time position information from the absolute encoders. Because the final position of the actuator is used to stop the motion of the step motors, the moves can be made accurately in spite of the large amount of mechanical backlash from a chain drive between the motors and the spectrometer arms. A modified trapezoidal profile, custom C software, and an industrial PC, were used to achieve a positioning accuracy of 0.00275 degrees of rotation. A form of active position maintenance ensures that the angles are maintained with zero error or drift.
Date: August 1, 1997
Creator: Hicks, J.S. & Wright, M.C.
Partner: UNT Libraries Government Documents Department

Predicted Performance of Neutron Spectrometers Using Scintillating Fibers

Description: One class of neutron spectrometers is devices that provide a measure of the neutron spectrum by using moderating and absorbing materials together with thermal-neutron detectors. Pacific Northwest National Laboratory has developed scintillating fibers that are sensitive to thermal neutrons. Because these fibers are thin, they present an enabling technology for several applications, including highly efficient neutron spectroscopy. The underlying concept is to arrange the fibers in an array of layers separated by materials whose characteristics have been chosen to optimize the instrument function for the application. Monte Carlo experiments have been performed to characterize the conceptual design and to determine the value of the concept as a tool for research and other applications.
Date: February 14, 2000
Creator: Craig, Richard A & Bliss, Mary
Partner: UNT Libraries Government Documents Department

Predicted performance of neutron spectrometers using scintillating fibers

Description: A variety of needs exists for knowing the energy spectral content of a neutron flux. Among these needs are arms-control and national-security applications, which arise because different neutron sources produce different neutron energy spectra. This work is primarily directed at these applications. The concept described herein is a spectrometer in the same sense as a Bonner sphere. The instrument response reflects a statistical average of the energy spectrum. The Bonner sphere is an early rendition of this class. In this, a neutron detector is placed at the center of a moderating (and absorbing) sphere (of varying thickness and composition). Spectral unfolding is required, and the resolution and efficiency are, typically, poor, although the potential bandwidth is very large. A recent variation on the Bonner-sphere approach uses {sup 3}He gas proportional counters with resistive wires to locate the position of the event (Toyokawa et al 1996). The spectrometer concept investigated here has the potential for better resolution and much improved neutron efficiency compared to Bonner spheres and similar devices. These improvements are possible because of the development of neutron-sensitive, scintillating-glass fibers. These fibers can be precisely located in space, which allows a corresponding precision in energy resolution. Also, they can be fabricated into arrays that intercept a large fraction of incident thermal neutrons, providing the improvement in neutron economy.
Date: February 14, 2000
Creator: Craig, RA & Bliss, M
Partner: UNT Libraries Government Documents Department

AN ENERGY MEASUREMENT OF PuF$sub 4$ NEUTRONS AND THE NEUTRON DOSE RATE FROM PuF$sub 4$ PROCESSING EQUIPMENT

Description: By use of the multisphere neutron spectrometer, the neutrons emitted from a sample of PuF4 were analyzed for average neutron energy. The results indicated an average neutron energy of 1.25 plus or minus 0.25 Mev. The room background had a fast neutron energy of 1.00 plus or minus 0.25 Mev, with a reasonably large contribution from scattered neutrons. A 110 gram sample of PuF/ sub 4/ gave a dose rate reading of about 9 mrem/hr at a distance of 30 cm, which corresponds to a neutron yield of 8.0 x 10/sup 5/ n/sec. The room background was about 0.5 mrem/hr. Neutrons originating in the PuF/sub 4/ processing equipment were measured with the 10 inch sphere neutron survey instrument. Comparable readings were obtained with the converted PeeWee neutron survey instrument by using a correction factor of 30, or a fast neutron energy of 0.5 or 0.4 Mev. Either of these factors enabled a monitor to obtain a more accurate dose rate with the PeeWee than was previously possible. (auth)
Date: May 1, 1962
Creator: Hankins, D.E.
Partner: UNT Libraries Government Documents Department

THE PRELIMINARY CHOICE OF DESIGN PARAMETERS FOR NEUTRON CHOPPERS

Description: Some of the factors affecting the counting rate achieved by a neutron chopper spectrometer are investigated. It is shown that in a chopper of optimum design the counting rate per channel at any given energy is proportional both to the slit width and to the square of the resolution in mu sec/m, as well as to the cutoff velocity vc of the rotor; this last is because a larger number of bursts per second become possible as vc is increased. For an idealized cutoff function, it is shown that the total running time for measuring a spectrum is minimum when the ratio of cutoff velocity to peripheral velocity of the rotor is such that approximately 1.67 runs per energy decade are required; this means that the chopper speed and the time-channel width are changed by a factor of two from one run to the next. (auth)
Date: November 1, 1961
Creator: Stanford, G.S.
Partner: UNT Libraries Government Documents Department

Higher Resolution Neutron Velocity Spectrometer Measurements of Enriched Uranium

Description: The slow neutron transmission of a sample of enriched uranium containing 3.193 gm/cm{sup2} has been investigated with a resolution width of 1 microsec/m (1 microsec/m is the full width of the triangular resolution function at the base) and with points spaced 1/3 microsec/m. The results of these transmission measurements are shown in Fig. 1. The solid line represents the new measurements while the dashed line represents the previous measurements (CUD-4) on this sample using a resolution width of 1.4 microsec/m and with points spaced 2/3 microsec/m.
Date: August 9, 1950
Creator: Havens, W. W., Jr. & Rainwater, L. J.
Partner: UNT Libraries Government Documents Department

Development of an ultracompact neutron spectrometer for identifying near-surface water on mars.

Description: One of the major goals of the Mars science program is to identify exact locations of near-surface water or hydrated minerals on Mars. Evidence is mounting that Mars may have contemporary near-surface groundwater activity. Though very water-poor by terrestrial igneous standards, the SNC meteorivtes were found to contain evaporite minerals suggestive of groundwater activity within the past 1.3 Ga. More recently, the Mars Surveyor camera recorded images of geologically young seepage and outflow channels attributed to liquid water. The sources of these channels were suggested to be only a few hundred meters or less below the surface. If these channels are truly geologically young, thinly buried ice may still exist at the termination of these channels.
Date: January 1, 2001
Creator: Lawrence, David J. (David Jeffery); Wiens, R. C. (Roger C.); Moore, K. R. (Kurt R.) & Prettyman, T. H. (Thomas H.)
Partner: UNT Libraries Government Documents Department

A MECHANICAL NEUTRON FILTER FOR A CRYSTAL SPECTROMETER

Description: A velocity selector of the Dash-Sommers type with straight slits was constructed for use with the MTR crystal spectrometer. The rotor, designed to remove undesirable higher order contaninants from the Bragg beam, is built of magnesium and fiberglas-epoxy laminate to minimize weight and is driven directly by a 1/8-hp synchronous motor. Vanes of fiberglas laminate are set into 235 slots milled around the periphery of a 4'' diameter magnesium core. High strength is attained by winding the assembled rotor with glass filaments impregnated with epoxy resin. Effective helical pitch is obtained by setting the rotor axis at an angle to the Bragg beam. Delow 0.25 ev, the rotor gave good suppression, but the transmission was low. (auth)
Date: June 14, 1963
Creator: Smith, J.R. & Miller, H.G.
Partner: UNT Libraries Government Documents Department

Neutron sensors for locating sites of planetary water deposits

Description: This is the final report of a six-month, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). This project helped in exploration of the value and feasibility of use of collimated neutron detection methods for improving the sensitivity of neutron spectrometers specifically designed for deep-space missions to detect and identify both present-day deposits of near-surface water ice. The authors believed that this result helped enable a decision to include a Los Alamos-designed neutron sensor as a component of the NASA Mars Global Surveyor-01 Gamma-Ray/Neutron Spectrometer.
Date: December 31, 1998
Creator: Feldman, W.C.; Fenimore, E.E.; Byrd, R.C. & Wiens, R.C.
Partner: UNT Libraries Government Documents Department

Low energy magnetic fluctuations in the TSDW phase of chromium

Description: A polarized neutron study of chromium carried out in a field of 6T applied to a single-domain single-Q crystal indicates that the inelastic intensity observed close to the transverse spin density wave (TSDW) satellite positions (1 {+-} {delta}, 0,0) does not behave as expected for spin-wave scattering. In particular, the signal corresponds to magnetization fluctuations of almost equal magnitude both parallel and perpendicular to the ordered moments in the TSDW phase.
Date: December 1, 1997
Creator: Azuah, R.T.; Kulda, J.; Pynn, R. & Stirling, W.G.
Partner: UNT Libraries Government Documents Department