def setMaterial(self,massFraction,polymer):
        """
        Sets the detector material
        """
        M = Materials()
        num = self.material['Detector']['mt']
        if polymer == 'PS':
            self.material['Detector']['matString'] = M.GetPSLiF(massFraction,num)
        elif polymer == 'PEN':
            self.material['Detector']['matString'] = M.GetPENLiF(massFraction,num)
        else:
           raise ValueError('Polymer {} is not in the material database'.format(polymer))