z-distribution

From: Ed Six (edsix@spinspin.lns.mit.edu)
Date: Thu Oct 18 2001 - 11:43:20 EDT


Hey,

Here is just a very short routine to give you the correct vertex in a
generator. It's quicker than the rejection method. I took a look at
several generators in cvs and could not find how the vertex was
calculated in those programs. The x and y parts of the vertex mostly
depend on the size of the electron beam which is small (1mm) and mostly
gaussian but has a long tail. For now, it is probably best to generate
events on axis since going off axis does not change the acceptance much.

      TAR(3) = targetlength/2.

C**** The distribution along the target cell (z axis) is triangular.
C**** Used the inversion/transformation technique.
C**** (See Numerical Recipes 7.2)
      VERTEX(1) = 0.0
      VERTEX(2) = 0.0
      CALL GRNDM (RAN,1)
      IF (RAN(1).LT.0.5) THEN
        VERTEX(3) = -TAR(3) +TAR(3)*SQRT(2.0*RAN(1))
      ELSE
        VERTEX(3) = +TAR(3) -TAR(3)*SQRT(2.0*(1.0 -RAN(1)))
      ENDIF

I hope my last minute comments are helpful.
Ed

-- 
 ___________________________________________________________________
 Ed Six     		                          	     Postdoc
 MIT-Bates Linear Accelerator Center                   edsix@mit.edu
 21 Manning Ave.                                tel:  (617) 253-9217
 Middleton, MA 01949                            fax:  (617) 253-9599



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