The `Source_adapt` component

Neutron source with adaptive importance sampling

Identification

Author: Kristian Nielsen
Origin: McStas release
Date: 1999
Version: 1.14

Description

Rectangular source with flat energy 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.

Focosing 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.

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description
N_E	1	Number of bins in energy dimension
N_xpos	1	Number of bins in horizontal position
N_xdiv	1	Number of bins in horizontal divergence
alpha	1	Learning cut-off factor (0 < alpha <= 1)
beta	1	Aggressiveness of adaptive algorithm (0 < beta <= 1)
filename	string	Optional filename for adaptive distribution output
xmin	m	Left edge of rectangular source
xmax	m	Right edge
ymin	m	Lower edge
ymax	m	Upper edge
dist	m	Distance to target rectangle along z axis
xw	m	Width(x) of target
yh	m	Height(y) of target
E0	meV	Mean energy of neutrons
dE	meV	Energy spread (energy range is from E0-dE to E0+dE)
flux	1/(cm*2AA**st)	Absolute source flux

Output parameters

Name	Unit	Description	Default
p_in
y_0
C
r_0
count
adpt

The Source_adapt component

Identification

Description

Input parameters

Output parameters

Links

The `Source_adapt` component