Variable slits

X-ray X-press ( SNYDER@edu.alfred.xray )
Mon, 25 Jul 1994 18:02:13 GMT

Just a few words on variable slits and Rietveld refinement:
Using a 1 degree incident slit will cause the incident beam to
splash over the ends of most sample holders at two theta angles somewhat
less than twenty degrees (for example it is 17 degrees in the old vertical
Philips diffractometers). The conventional intensity equation for a
diffractometer assumes a constant volume of sample is illuminated and this
is the equation implemented in all of the Rietveld codes with which I am
familiar. Therefore peaks below the splash over angle will have less
incident beam on them and will fail the constant volume of irradiation
assumption resulting in their observed intensities being systematically
too low. To use these peaks in a Rietveld refinement the pattern must be
run with a narrower incident slit and/or a longer sample holder.
If, on the other hand, you wish to use a variable incident slit to
ensure that the beam never splashes over the sides of your sample holder,
then you must modify the intensity equation in the Rietveld program to
allow for the now variable volume of diffracting material in the beam as a
function of 2-theta. Since it is only relative intensities we care about
you can use a very simple routine like the one that Cam Hubbard and I used
in the AUTO/NBS*QUANT84 system years ago (included below). Or, you can put
in the more formally derived equation of the effect of variable receiving
slit on intensity. Bob Snyder

SUBROUTINE TCSLIT
C**********************************************************************
C
C SUBROUTINE TO CORRECT MEASURED INTENSITIES FOR EFFECTS OF THETA
C COMPENSATING SLITS SUCH THAT THE CORRECTED INTENSITIES CORRESPOND
C TO THOSE FROM A CONSTANT VOLUME OF MATERIAL
C
C CURRENTLY THE ROUTINE USES ONLY A FIRST ORDER CORRECTION-----
C THAT IS VOLUME(THETA COMPENSATING) GOES AS SIN(THETA)
C
C**********************************************************************
C
INCLUDE '[102,2]INP.COM/NOLIST'
INCLUDE '[102,2]ERRBLK.COM/NOLIST'
C
INTEGER*2 KODE, IOVLP, IPK, NPK, NBG, IOPTIM, ILOC
REAL*4 ROONMU, MUREF
COMMON /LINE/BANG,PANG,EANG,DANG,TIMLIM,FIXERR,
* KODE,IOVLP,IPK,NPK,NBG,FREL,STDREL,STD,SIGSTD,WTPRCT,
* ROONMU,WTSPIK,PKTIME,BKGTIM,IOPTIM,ILOC,CONSTD,MUREF,PANG1
C
CHARACTER NAM*4, NAMEOV*4, ELMTPH*2, JKL*6, JKLOV*6
COMMON /LINEC/ NAM, JKL
COMMON /OVLAP/ FRELOV(2),SRELOV(2),WTPCPH(8)
COMMON /OVLAPC/ NAMEOV(2),JKLOV(2),ELMTPH(8)
COMMON/DATA/PEAK,VPEAK,AREA,VAREA,XBKG,VBKG
C
REAL*4 THETA,PANG,RATIO,PEAK,AREA,S2,VPEAK,VAREA,
1 BANG,EANG,DANG,TIMLIM,FIXERR,FREL,STDREL,
2 STD,SIGSTD,WTPRCT,WTSPIK,PKTIME,BKGTIM,
3 CONSTD,PANG1,FRELOV,SRELOV,WTPCPH,XBKG,
4 VBKG
C
C-----CALCULATE SINE OF THETA
C
THETA=PANG/2.0
C
C CORRECT ALL INTENSITIES TO VALUES WHICH WOULD HAVE
C BEEN MEASURED WITH A FIXED ONE DEGREE DIVERGENCE SLIT
C SINCE THETA COMPENSATING SLIT INTENSITIES ARE EQUAL
C TO ONE DEGREE SLIT INTENSITIES AT 30 DEGREES WE
C MULTIPY BY SIN(30)/SIN(THETA)
C
RATIO = 0.5 / SIN(THETA/57.3)
C
C-----CONVERT PEAK AND INTEGRATED AREAS
C
PEAK = PEAK * RATIO
AREA = AREA * RATIO
C
C-----CONVERT VARIANCE OF PEAK AND AREA
C
S2 = RATIO*RATIO
VPEAK=VPEAK/RATIO
VAREA=VAREA/RATIO
RETURN
END