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4.7 The Chopper_simple McXtrace Component
Ideal chopper
Identification
-
Author:
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Origin: Risoe
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Date: August 2011
Description
Ideal model of a chopper situated at Z=0. If a photon arrives at the chopper plane at a time of t = [t0 +-n*T: t0 +-n*T +tau], where T is the period of the chopper, t0 the initial delay and tau the opening time of the chopper, it is left untouched - otherwise it is ABSORBed. If on a continous source the isfirst parameter may be used. In this case the photon time is _defined_ by the chopper. In other words no photons are absorbed, the photon time is merely sampled within the chopper window. t_rise is the rise-time of the chopper opening giving a trapezoidal shape. Limitations: this component does not take chopper geometry into account. If isfirst only samples in the first chopper opening window.
Example: Chopper_simple(
t0 = -0.5/M_C, T = 20e-6, tau = 100e-12, xwidth = 1e-4,
yheight = 1e-4, isfirst = 1)
Input parameters
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
|
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|
t0 |
s |
Initial delay of the opening time |
0 |
|
T |
s |
Period of the chopper |
1 |
|
tau |
s |
Opening time of the chopper |
0.1 |
|
xwidth |
m |
Height of the chopper opening |
0.1 |
|
yheight |
m |
Width of the chopeper opening |
0.1 |
|
isfirst |
0/1 |
Is the chopper the first chopper on a continous source. |
0 |
|
t_rise |
s |
Rise time of the chopper pulse. |
0 |
|
tjit |
1 |
Timing jitter in terms of the opening time tau. For each ray the opening window will be shifted by a random amount within t=[-tjit*.5*tau,tjit*.5*tau] |
0 |
|
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Links
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Component source code found in file Chopper_simple.comp.
An ideal chopper
Chopper_simple is an idelized version of a chopper with a rectangular chopper opening which may open instantly and has no side-scattering etc.
It models the chopper with a blocking infinitely thin aperture which becomes transparent in the time interval \(t\in \left [\mathit {t0},\mathit {t0}+\tau \right ]\). This is an idelized version of a chopper where the chopper opening is rectangular which may open instantly (if t_rise\(=0\), the default). For nonzero rise time the aperture simply becomes gradually less opaque for \(t\in \left [\mathit {t0}-\textit {t\_rise},\mathit {t0}\right ]\).
For correct normalization of intesity a chopper period, T, must also be set.
isfirst is useful when using Chopper_simple with continous sources who inherently have no time-dependence. Thus the emission time of the photon ray is arbitrary, and the chopper defines the temporal signature of the beam, i.e. it simply sets the time-parameter of the photon ray randomly in the opening window of the chopper. Naturally this should only be used for the first chopper element in a simulation.
