subroutine pitch (b, theta, phi, alpha)
c
c------------------------------------------------------------------------
c     This routine calculates the pitch angle of a particle from a unit
c     direction defined by a theta (polar ang) and a phi (azimuthal ang)
c------------------------------------------------------------------------
c
      dimension  b(3)
c 
c------------------------------------------------------------------------
c     all input and output angles are in degrees
c------------------------------------------------------------------------
c
      radeg = 3.14159 / 180.0
      thetar = theta * radeg
      phir = phi * radeg
c
c------------------------------------------------------------------------
c     strip out the b field components
c------------------------------------------------------------------------
c
      bx = b(1)
      by = b(2)
      bz = b(3)
      bmag = sqrt(bx * bx + by * by + bz * bz)
c
c------------------------------------------------------------------------
c     first solve for cos(alpha)
c------------------------------------------------------------------------
c
      uniti = bx * sin(thetar) * cos(phir)
      unitj = by * sin(thetar) * sin(phir)
      unitk = bz * cos(thetar)
      cosalp = (uniti + unitj + unitk) / bmag
c
c------------------------------------------------------------------------
c     calculate the pitch angle in degrees
c     alpdr is the pitch angle of the detector in radians
c------------------------------------------------------------------------
c
      alpdr = acos(cosalp)
      alpdd = alpdr / radeg
      alpha = 180.0 - alpdd
c
c------------------------------------------------------------------------
c------------------------------------------------------------------------
c
      return
      end