SUBROUTINE SUSDKL( IErr )
C --------------------------------------------------------------------------
C   Purpose: Determine how far a particle of given decay length
C            travels before decaying.
C
C            The equation governing the decay is:
C               N = N_o * exp( -x/decay_length)
C            Thus probabilty of decay in interval x -> x + dx is:
C               abs(dN/N_o)  =  exp( -x/decay_length) * dx/decay_length
C
C       IErr = 1  -----> too many loops. gave up. In this case a random length
C                        within the decay length is generated.
C
C    Author: JMB
C --------------------------------------------------------------------------
C
      IMPLICIT NONE
C
      INCLUDE 'SUSANA.INC'
C
      REAL randum, rn32, gone
      REAL prob
      INTEGER nloop, IErr
C
C --  Initialise
C
      ierr     = 0
      nloop    = 0
      distance = 0.0
C
      gone = rn32(randum) 
C
 1    CONTINUE
C
        IF (nloop .GT. 100) THEN
          IErr = 1
          GOTO 7900
        ENDIF
C
        nloop = nloop + 1
        distance = distance + dx*decay_length
C
        prob = dx * exp(-1.0*distance/decay_length)
C
      IF ( gone .GT. prob) GOTO 1 
C
 7900 CONTINUE
      distance = distance + rn32(randum)*dx*decay_length
C
 8900 CONTINUE
      RETURN
      END