The `Source_adapt` Component

Neutron source with adaptive importance sampling

Identification

Author: Kristian Nielsen
Origin: Risoe
Date: 1999
Version: 1.30
Modification history:
- Revised by: Aaron M. Percival
- 2007
- Queen's University Department of Physics
- Added the option of having an initial distribution that is uniform in wavelength

Description

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.

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
alpha	1	Learning cut-off factor (0 < alpha <= 1)	0.25
beta	1	Aggressiveness of adaptive algorithm (0 < beta <= 1)	0.25
filename	string	Optional filename for adaptive distribution output	0
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	2.33
xw	m	Width(x) of target	0.05
yh	m	Height(y) of target	0.1
E0	meV	Mean energy of neutrons	0
dE	meV	Energy spread (energy range is from E0-dE to E0+dE)	0
lambda0	AA	Mean wavelength of neutrons (if energy not specified)	0
dlambda	AA	Wavelength spread	0
flux	1/(cm*2AA**st)	Absolute source flux	1e13

Output parameters

Name	Unit	Description
p_in	-	Internal, holds initial neutron weight
y_0	-	Internal
C	-	Internal
r_0	-	Internal
count	-	Internal, counts neutrons emitted
adpt	-	Internal structure shared with the Adapt_check component

The Source_adapt Component

Identification

Description

Input parameters

Output parameters

Links

The `Source_adapt` Component