Moderators
Detailed design of scattering instruments requires detailed knowledge of the neutron beam emitted from the moderator in question. We are currently providing moderator performance estimates based on the result of detailed Monte Carlo simulations of the entire target system. These simulations are being performed for both the High Power Target Station (HPTS) and the Long Wavelength Target Station (LWTS), and have been performed for the Intense Pulsed Neutron Source (IPNS) in the same manner. We make the IPNS source files available as a means to benchmark instrument simulation codes. These files follow a particular "source file format" which will adapt to the changing needs of instrument simulation as necessary. We also provide worked examples showing how to use the information in the source files analytically. Any questions regarding moderator performance should be directed to E. B. Iverson.
- HPTS Design as of 7/31/02 (SCT series) CURRENT
- HPTS Design as of 2/21/01 (HL2 series)
- HPTS Design as of 9/13/00 (POI series)
- LWTS Design as of 10/3/00 (LW1K32 series)
- IPNS Configuration as of 7/98 (IPNS71 series)
HPTS Optimization Studies
"Comparison of Composite and Water Moderator Performance at SNS," TSR-202.
This document discusses the comparison of a composite moderator, as in the base case for the HPTS listed above, with that of an ordinary water moderator in the same position. Partially as a result of this comparison, we have decided to replace the composite moderator with a decoupled, poisoned water moderator. The upstream water moderator source file used for the equivalent-basis comparison is poi264_bu.
Parametric studies of poison depths in decoupled moderators.
These studies examine the effects of varying poison depths in decoupled water and parahydrogen moderators. The comparisons are made with the SCT base case.
Source File Format
The source file format we use is an ASCII text file. The first section of this file gives the absolutely normalized spectral intensity, in terms of neutrons per steradian per pulse per unit energy (wavelength) as a function of energy - wavelength. The binning is logarithmically spaced in energy, with the energy given corresponding to the logarithmic center of the bin. The intensity given per unit energy should correspond to that given per unit wavelength. These results are raw output from simulations. The precision given for a particular bin only represents the Monte Carlo precision of the value of the tally within that bin, and should be used to check that said bin is adequately sampled. If the uncertainty is greater than 5%, it may be suspect.
The second section of the file gives the emission time distribution (i.e., pulse shape) as a function of energy. For more recent source files, version 0.96 and later, the intensity given is absolutely normalized, that is, the emission time distribution should integrate to the spectral intensity for that energy. For older source files, the emission time distribution is arbitrarly normalized, and should be used only as a shape function. The energy grid for the spectral intensity and for the emission time distributions is not guaranteed to be the same.
How to Use Source Files
Example 1: Calculating Instantaneous Counting Rates in Beam Monitors
