JOURNAL DE PHYSIQUE

mesures. Abstract: A review is given of the methods by which the spin dependent neutron scat ter ing amplitudes can be measured, and of what has been made, up to now,to obtain them. The i n te res t of such measurements is discussed. The i n t e rac t i on between an atom and a thermal neutron may be expressed by a scat ter ing amplitude operator? * which contains three main terms (1) &> = b + 2 mL. T + 2 B T.T The f i r s t and t h i r d terms take account of the i n t e rac t i on between the neutron and the atomic nucleus. The f i r s t one, b, is the i so t rop i c nuclear sca t te r ing amplitude. The t h i r d expresses the i n te rac t i on between the neutron, car ry ing a spin s, and the nucleus,carry ing a spin I. The we l l known second term corresponds to the d ipo la r i n te rac t i on between the e lec t ron ic magnetic moment of the atom, and the magnetic moment of the neutron (v ia i t s spin s). Mj_denotes the component of M normal to the sca t te r ing v e c t o r K. Let us note, i n passing, the formal analoay between the two las t terms (excep