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McStas: Source_adapt Component

[ Identification | Description | Input parameters | Output parameters | Links ]

The Source_adapt Component

Neutron source with adaptive importance sampling



Rectangular source with flat energy or wavelength distribution that
uses adaptive importance sampling to improve simulation efficiency.
Works together with the Adapt_check component.

The source divides the three-dimensional phase space of (energy,
horizontal position, horizontal divergence) into a number of
rectangular bins. The probability for selecting neutrons from each
bin is adjusted so that neutrons that reach the Adapt_check
component with high weights are emitted more frequently than those
with low weights. The adjustment is made so as to attemt to make
the weights at the Adapt_check components equal.

Focusing is achieved by only emitting neutrons towards a rectangle
perpendicular to and placed at a certain distance along the Z axis.
Focusing is only approximate (for simplicity); neutrons are also
emitted to pass slightly above and below the focusing rectangle,
more so for wider focusing.

In order to prevent false learning, a parameter beta sets a
fraction of the neutrons that are emitted uniformly, without regard
to the adaptive distribution. The parameter alpha sets an initial
fraction of neutrons that are emitted with low weights; this is
done to prevent early neutrons with rare initial parameters but
high weight to ruin the statistics before the component adapts its
distribution to the problem at hand. Good general-purpose values
for these parameters are alpha = beta = 0.25.

This component is not validated. It does not work properly with MPI.

Input parameters

Parameters in boldface are required; the others are optional.
Name Unit Description Default
N_E 1 Number of bins in energy (or wavelength) dimension 20
N_xpos 1 Number of bins in horizontal position 20
N_xdiv 1 Number of bins in horizontal divergence 20
xmin m Left edge of rectangular source 0
xmax m Right edge 0
ymin m Lower edge 0
ymax m Upper edge 0
xwidth 0
yheight 0
filename string Optional filename for adaptive distribution output 0
dist m Distance to target rectangle along z axis 0
focus_xw m Width of target 0.05
focus_yh m Height of target 0.1
E0 meV Mean energy of neutrons 0
dE energy range is from E0-dE to E0+dE [meV] Energy spread 0
lambda0 if energy not specified [AA] Mean wavelength of neutrons 0
dlambda AA Wavelength spread half width 0
flux 1e13
target_index 1 relative index of component to focus at, e.g. next is +1 this is used to compute 'dist' automatically. +1
alpha 0 < alpha <= 1 [1] Learning cut-off factor 0.25
beta 0 < beta <= 1 [1] Aggressiveness of adaptive algorithm 0.25


[ Identification | Description | Input parameters | Output parameters | Links ]

Generated automatically by McDoc, Peter Willendrup <> / Wed Jan 12 15:13:20 2022

Last Modified: Wednesday, 12-Jan-2022 16:13:20 CET
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