Fit to field map

        A phenomenological fit by Ryan Bayes (Glasgow) to a field map
        generated by Bob Wands (FNAL).  It assumes a 1 cm plate.  This is dated
        January 30th, 2012.  Not defined for r <= 0

def PhenomModel(self, r):
        """Fit to field map

        A phenomenological fit by Ryan Bayes (Glasgow) to a field map
        generated by Bob Wands (FNAL).  It assumes a 1 cm plate.  This is dated
        January 30th, 2012.  Not defined for r <= 0"""
        if r <= 0:
            raise ValueError
        field = self.B0 + self.B1 * G4.m / r + self.B2 * math.exp(-1 * self.H * r / G4.m)
        return field