Improvement of the Spatial Amplitude Isotropy of a $^4$He Magnetometer Using a Modulated Pumping Beam B. Chéron, H. Gilles, J. Hamel, O. Moreau and E. Noël J. Phys. III France, 7 8 (1997) 1735-1740 DOI: 10.1051/jp3:1997215