Neutronic Simulations for Source-Instrument Matching at the Lujan. The neutronics design of pulsed spallation neutron sources can be difficult when

Transcription

1 . - W--iTR ~ aflf-qsii66-- Neutronic Simulations for Source-Instrument Matching at the Lujan Center J. D.Court, P. D. Ferguson, and B. P.Schoenbom (LANL) The neutronics design of pulsed spallation neutron sources can be difficult when considering the fact that multiple instruments view the same moderator. Typically, the moderator design is a compromise between the required resolution for one instrument and the maximum intensity desires of another instrument. At the Manuel Lujan, Jr., Neutron Scattering Center (MLNSC), a recent target redesign has offered the opportunity to design a moderator for a Laue diffiactometer with few other requirements. Detailed time and energy spectra were calculated for a variety of moderator decoupling options. The purpose of this summary is to document the neutronics calculations required for-this source-instrument matching process.. Each proposed target configuration was precisely modeled in three-dimensions using the LAHET Code System (LCS)"I. Examples of two of the target decoupling options are shown in Figure 1. LAHET' models the 800-MeV proton beam from the LANSCE linac and the charged particle interactions that take place in the target/moderator/reflector system (TMRS). All charged particle transport, as well as neutron transport above the energy of 20 MeV, is done in LAHET. When a neutron's energy falls below 20 MeV, the information is written to a source file to be used by MCNP2. MCNP does all subsequent transport of the neutrons, and provides tally information at the detector point. LAHET and MCNP are trademarks of the Regents of the University of California, Los Alamos National Laboratory. N

2 This report was prepared as an account of work sponsorai by an agency of the Unitai States Government Neither the United States Gomnmwt nor any agency thereof, nor any of their employas, makes any warranty, express or implied, or l p u m e s any i@ liabity or rcsponsiiity for the -cy, camplncnets, or usefulness of any infomation, apparatus, product, or process disdoscd, or nprrs~ts that its usc would not infringe privately owned rights. Rcfuwce herein to any spe. cific commtfcial pmduct, process, or service by trade name, traduwk in~ufacturn, or othcxwisc does not necessarily constimte or imply its tndorxmcnt, reammendation. or favoring by the United States Gowrnment or any agency thtnof. The views and opinions of authors cxprrssed herein do not a d y state or nfiectthose of the United States Government or any agency thereof..

3 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. -

4 Cadmium Liner Around Be Cadmium Liner on Be and Pb (4 (b) Figure 1. Plan views through the moderator tier showing two decoupling schemes: (a) Cd along the Pb portion of the flight path and around the Be reflecter and (b) Cd along the entire flight path. The MCNP point detector tally allows a characterization of the neutron pulse at many different detector positions and TMRS configurations. The tally is termed a 'nextevent estimator' because the contribution fiom each neutron interaction in the moderator is calculated and added to the tally. To achieve the required resolution for detailed pulse analysis, the tally uses extensive energy and time binning. As many as 550 energy bins were used to correspond to 0.02 A to 11 A (204.5 ev to 6.76e-4 ev). 152 time bins from 0 to 68 msec were used. Figure 2 shows a neutron pulse for a partially coupled target system. Figure 2(a) is the complete Time-Wavelength plot of the neutron pulse. The wavelength (energy) of the neutrons arriving at the detector at any give time can be extracted fiom this data, and is shown in Figure 2(b). A wavelength slice, Figure 2(c), of the pulse at the maximum

5 wavelength in (b) shows the time dependence of the neutrons of this energy, and allows a pulse-decay constant to be determined. Integral analysis of wavelength- and time-slices gives intensity and intensity-to-background data, which allows the determination of the most favorable TMRS configuration and detector placement. Details of the instrument analysis and the results of the target configuration studies can be found in the work of Schoenborn, et al3 Figure 2. Neutron spectra developed from the Monte Carlo data: (a) Time-Wavelength neutron spectrum; (b) a time slice of the spectrum at msec; (c) the decay of the neutron pulse at 1.64 A. A more detailed modeling of instrument performance using a Monte Carlo approach has been accomplished for other instruments by Seeger, et al. using MCLIB4 in conjunction with the detail time and energy spectra calculated by LCS. A detailed Monte Carlo analysis allows identical experiments to be run numerically to determine instrument resolution, time required for experiment completion, etc., as a function of target decoupling options. As instrument Monte Carlo modeling continues to evolve,

6 detailed analyses for source-instrument matching will become a standard spallation source design technique. Acknowledgments This work was supported by the U. S. Department of Energy, BES-DMZ, under contract No. W-7405-Eng-36 with the University of California. References 1) R. E. Prael and H. Lichtenstein, User Guide to LCS: The LAHET Code System, Los Alamos National Laboratory report LA-UR (September 1989). 2) J.F. Briesmeister, Ed., MCNP- A General Monte Carlo N-Particle Transport Code, * Los Alamos National Laboratory Report LA M, Version 4B (March 1997). 3) B. P. Schoenborn, J. D. Court, A. C. Larson, and P. D. Ferguson, Moderator Decoupling Options for Structural Biology at Spallation Neutron Sources, accepted for publication in Journal of Neutron Research (1 998). 4) P. A. Seeger, me MCLIB Library: Monte Carlo Simulation of Neutron Scattering Instruments, in ICANS-Xm: Proceedings of the 13th Meeting of the International Collaboration on Advanced Neutron Sources, G. S. Bauer and R. Bercher, Eds., Paul Scherrer Institut report PSI 95-02, ISSN , pp (November 1995).