Helicity of Neutrinos

first approximation. The resonant scattered gamma-ray distribution is shown in Fig. 2 for a source strength of 20 millicuries. The ratio of the 837-kev to 961-kev photopeaks is 1.8 which is in agreement with the ratio of the p rays before scattering, 1.4, corrected for detection e%ciencies and for the different angular distributions of the scattered gamma rays at 100'—the mean angle of scattering. The cross section measured with the solid source is (5&1)X10 's cm', the major uncertainties being in the geometry and source strength determinations. The liquid source gave a 20% higher cross section; the diGerence may be due to a slowing down of the recoiling nucleus in the solid or a change in its effective mass. In order to calculate the lifetime of the transition one should take into account the natural width of the emitting level, the Doppler broadening of the emitted gamma ray due to the neutrino recoil, the possible slowing down of the recoiling nucleus, the effective mass, and the temperature broadening of the emi.ssion and absorption lines. If the recoil and temperature effects are neglected, a lower limit on the mean life can be set as 1.7X10 '4 sec. An upper limit is certainly the slowing-down time in the solid, approximately 2&(10 " sec, ~ since a stationary nucleus with a level of this mean life will give a cross section less than 1/100 that observed. Conversely, therefore, the gamma ray is emitted in general before the recoil slows down so that the Doppler broadening due to the neutrino emission must be taken into account, and the resonance scattering becomes a sensitive detector of the direction of neutrino emission. Taking into account the width of the emitting line and the Doppler shift due to the recoiling nucleus, assumed to have an effec;tive mass number equal to 152, the mean life of the 1 level becomes r= (3&1)X10 "sec. The effect of the temperature of the source and scatterer has been neglected. The measured lifetime is thus approximately 150 times longer than the single proton estimate. This mean life corresponds, according to the formulation of Bohr and Mottelson, ' to an octupole deformation parameter, Ps~0.07, for the 1 state of Sm'". I would like to thank M. Goldhaber, A. W. Sunyar, and J. Weneser for many valuable discussions.