ADAS Subroutine sigma

      SUBROUTINE SIGMA

      IMPLICIT REAL*8 (A-H,O-Z)
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C
C     PURPOSE: Calculates <SIGMA*V> or SIGMA*V rates for collisional
C              excitation by proton or electron impact
C
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C
C ADAS305 version. Developed from JETSHP.STARK.FORT (H P Summers).
C
C VERSION  : 1.1
C DATE     : 24-02-2005
C MODIFIED : Martin O'Mullane
C             - First version.
C
C VERSION  : 1.2
C DATE     : 24-10-2005
C MODIFIED : Martin O'Mullane
C             - Change MK(66) to BK(66) to maintain maning and
C               size of ADAT common block.
C
C VERSION  : 1.3
C DATE     : 16-05-2007
C MODIFIED : Allan Whiteford
C             - Updated comments as part of subroutine documentation
C               procedure.
C
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