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/*******************************************************************************
*
* McStas, neutron ray-tracing package
* Copyright (C) 1997-2008, All rights reserved
* Risoe National Laboratory, Roskilde, Denmark
* Institut Laue Langevin, Grenoble, France
*
* Component: StatisticalChopper
*
* %I
* Written by: C. Monzat/E. Farhi/S. Rozenkranz
* Date: Nov 10th, 2009
* Origin: ILL
*
* Statistical (correlation) Chopper
*
* %D
* This component is a statistical chopper based on a pseudo random sequence that the user
* can specify. Inspired from DiskChopper, it should be used with SatisticalChopper_Monitor component.
*
* Example: StatisticalChopper(nu=1487*60/255) use default sequence
*
* %P
* INPUT PARAMETERS:
*
* yheight: [m] Slit height (if = 0, equal to radius). Auto centering of beam at half height.
* radius: [m] Radius of the disc
* nu: [Hz] Frequency of the Chopper, omega=2*PI*nu
* (algebraic sign defines the direction of rotation)
* sequence: [str] Slit sequence around disk, with 0=closed, 1=open positions.
* NULL or "" will set a default sequence.
*
* Optional parameters:
* isfirst: [0/1] Set it to 1 for the first chopper position in a cw source
* (it then spreads the neutron time distribution)
* n_pulse: [1] Number of pulses (Only if isfirst)
* jitter: [s] Jitter in the time phase
* abs_out: [0/1] Absorb neutrons hitting outside of chopper radius?
* delay: [s] Time 'delay'.
* phase: [deg] Angular 'delay' (overrides delay)
* verbose: [1] Set to 1 to display Disk chopper configuration
*
* %L
* R. Von Jan and R. Scherm. The statistical chopper for neutron time-of-flight spectroscopy. Nuclear Instruments and Methods, 80 (1970) 69-76.
*
* %E
*******************************************************************************/
DEFINE COMPONENT StatisticalChopper
SETTING PARAMETERS (radius=0.5, yheight=0, nu, jitter=0, delay=0, isfirst=0, abs_out=1, phase=0, verbose=0, n_pulse=1,
string sequence="NULL")
/* Neutron parameters: (x,y,z,vx,vy,vz,t,sx,sy,sz,p) */
DECLARE
%{
double delta_y;
double height;
double omega;
int *Sequence; /* Array containing 0=closed and 1=opened*/
int *SequenceIndex; /* index of '1' in the sequence */
int nslit;
int m; /* sum of opened slits ('1' in sequence) */
%}
INITIALIZE
%{
Sequence=NULL;
SequenceIndex=NULL;
nslit=0;
m=0;
/* If slit height 'unset', assume full opening */
if (yheight == 0) {
height=radius;
} else {
height=yheight;
}
delta_y = radius-height/2; /* radius at beam center */
omega=2.0*PI*nu; /* rad/s */
if (!sequence) {
fprintf(stderr,"StatisticalChopper: %s: sequence is NULL. \n", NAME_CURRENT_COMP);
exit(-1);
}
if (strlen(sequence) <=2) {
fprintf(stderr,"StatisticalChopper: %s: Sequence is too short (length=%i). Use conventional DiskChopper instead.\n", NAME_CURRENT_COMP, nslit);
exit(-1);
}
if (!strlen(sequence) || !strcmp(sequence,"NULL"))
strcpy(sequence,
"100000010101001101110010010011110111111001011000011101001010111100101011111011010011011111000100011011100010000010001101101010100001110111001011111011001110001100011011011110101010000000110000011110011010100101011100000001001101000111100010110110001100100"
);
if (yheight && yheight>radius) {
fprintf(stderr,"StatisticalChopper: %s: yheight must be < radius\n", NAME_CURRENT_COMP);
exit(-1); }
if (isfirst && n_pulse <=0) {
fprintf(stderr,"StatisticalChopper: %s: wrong First chopper pulse number (n_pulse=%g)\n", NAME_CURRENT_COMP, n_pulse);
exit(-1); }
if (!omega) {
fprintf(stderr,"StatisticalChopper: %s WARNING: chopper frequency is 0!\n", NAME_CURRENT_COMP);
omega = 1e-15; /* We should actually use machine epsilon here... */
}
if (!abs_out) {
fprintf(stderr,"StatisticalChopper: %s WARNING: chopper will NOT absorb neutrons outside radius %g [m]\n", NAME_CURRENT_COMP, radius);
}
/* Calulate delay from phase and vice versa, 'direction' moderated by sign of omega */
delay *=omega/fabs(omega);
if (phase) {
if (delay) {
fprintf(stderr,"StatisticalChopper: %s WARNING: delay AND phase specified. Using phase setting\n", NAME_CURRENT_COMP);
}
phase*=DEG2RAD;
/* 'Delay' should always be a delay, taking rotation direction into account: */
delay=omega*phase/(omega*omega);
} else {
phase=delay*omega; /* rad */
}
if (verbose && nu) {
printf("StatisticalChopper: %s: frequency=%g [Hz] %g [rpm] time frame=%g [s] phase=%g [deg]\n",
NAME_CURRENT_COMP, nu, nu*60, fabs(1/nu), phase*RAD2DEG);
printf(" height=%g [m], slits centered at radius=%g [m]\n",
height, delta_y);
}
nslit=strlen(sequence);
if (nslit>1) {
int i=0,j=0;
double c=0;
int index=0;
Sequence = malloc(nslit * sizeof(int));
SequenceIndex = malloc(nslit * sizeof(int));
if (!Sequence){
fprintf(stderr,"StatisticalChopper: %s: Memory exhausted when allocating chopper sequence.\n", NAME_CURRENT_COMP);
exit(-1);
}
if (!SequenceIndex){
fprintf(stderr,"StatisticalChopper: %s: Memory exhausted when allocating chopper sequence table lookup.\n", NAME_CURRENT_COMP);
}
/* build sequence index (where sequence==1) */
for (i=0;i<nslit;SequenceIndex[i++]=0);
for (i=0;i<nslit;i++) {
Sequence[i]=(int)sequence[i]-'0';
if (Sequence[i]) {
m++; Sequence[i]=1;
if (SequenceIndex) SequenceIndex[index++] = i;
}
}
c = (double)(m-1)/(double)(nslit-1); /* duty cycle */
/* checking sequence */
for (j=0; j<=2*nslit; j++) {
int A = 0;
for (i=0; i<nslit; i++){
A += Sequence[i]*Sequence[(i+j) % nslit];
}
if (j==0 || j==nslit || j==2*nslit) {
if (A != m)
fprintf(stderr,"StatisticalChopper: %s: Sequence does not satisfy autocorrelation pseudorandom criteria\n"
"WARNING A_0=%i is different from m=%i\n",
NAME_CURRENT_COMP, A, m);
break;
} else {
if (A != m*(m-1)/(nslit-1))
fprintf(stderr,"StatisticalChopper: %s: Sequence does not satisfy autocorrelation pseudorandom\n"
"WARNING A=%i is different from m(m-1)/(N-1)=%i\n",
NAME_CURRENT_COMP, A, m*(m-1)/(nslit-1));
break;
}
} /* for j */
if (verbose && nu)
printf("StatisticalChopper: %s: Sequence length=%i with %i apertures; duty cycle c=%g \n",
NAME_CURRENT_COMP, nslit, m, c);
} /* if nslit */
else {
fprintf(stderr,"StatisticalChopper: %s: Sequence is too short (length=%i). Use conventional DiskChopper instead.\n", NAME_CURRENT_COMP, nslit);
exit(-1);
}
/* end INIT */
%}
TRACE
%{
double yprime;
PROP_Z0;
yprime = y+delta_y;
/* Is neutron outside the vertical slit range and should be absorbed ? */
if (abs_out && (x*x+yprime*yprime)>radius*radius) {
ABSORB;
}
/* Does neutron hit inner solid part of chopper in case of yheight!=radius ? */
if ((x*x+yprime*yprime)<(radius-height)*(radius-height)) {
ABSORB;
}
if (isfirst && SequenceIndex) {
/* choose a slit in the sequence and set time accordingly */
t = atan2(x,yprime)/omega + SequenceIndex[(int)(rand01()*m)]/nslit/nu
-delay +jitter*randnorm()+ (n_pulse > 1 ? floor(n_pulse*rand01()/fabs(nu)) : 0);
p *= (double)m/(double)nslit; /* transmission */
} else {
/* check if we pass through a slit */
int seq;
double angle = 2*PI*nu*(t-atan2(x,yprime)/omega+jitter*randnorm()) - phase;
seq=(int) floor(angle*nslit/(2*PI)) % nslit;
if ( seq < 0 ) seq += nslit;
if (Sequence[seq]) SCATTER;
else ABSORB;
}
%}
FINALLY
%{
free(Sequence); Sequence=NULL;
free(SequenceIndex); SequenceIndex=NULL;
%}
MCDISPLAY
%{
int i;
double radius_min=radius-height;
circle("xy", 0.0, -radius, 0.0, radius);
for (i=0; i<m; i++) {
/* draw a line at each slit center */
double angle=(double)SequenceIndex[i]*nslit/2*PI+phase;
line(radius_min*cos(angle), radius_min*sin(angle)-radius, 0,
radius*cos(angle), radius*sin(angle)-radius, 0);
}
%}
END
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