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

# The `Source_adapt` Component

Neutron source with adaptive importance sampling

## Identification

- Author: Kristian Nielsen
- Origin: Risoe
- Date: 1999
- Version: 1.28

## 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.

    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 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 | 14.68
dE | meV | Energy spread (energy range is from E0-dE to E0+dE) | 2
flux | 1/(cm**2*AA**st) | Absolute source flux | 1e13

## Output parameters

Name | Unit | Description | Default
--- | --- | --- | ---
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 |

## Links

- Source code for `Source_adapt.comp`.

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