of this argument may not be as convincing as has been believed. Hasted 35 measured the detachment cross section for 0~ with rare gas atoms and found this cross section to be independent of the manner in which the negative ions were produced. The two methods of production of 0~ were by electron bombardment of O2 and by presumed thermal dissociation of N 2 0 to N2 and O followed by radiative attachment to O. Recently, it was found, 21,22 however, that 0~ can be produced from N2O by very low-energy electrons (peak at 0.7 eV) and that these ions may well be in the ground state of 0~. Thus, Hasted may have used identical 0~ ions for his two experiments.Recently, Dunn 7 showed that the angular distribution of the particles in dissociative attachment may be far from isotropic. In fact, depending on the intermediate state excited, the distribution could be sharply peaked in either the forward or perpendicular direction. Thus, 35 Relative nuclear magnetic susceptibility, self-diffusion coefficient D, and spin lattice relaxation time Ti (characteristic of wall relaxation) were measured in He 3 gas between 1.7 and 4.2 °K in the range of pressure between 0.08 and 56 atm. These results were derived from measurements of free induction decays and spin echoes at 20.00 Mc/sec. Values for the density of the gas were deduced on the assumption that the gas obeys Curie's law. At moderate densities D was found to be inversely proportional to the density, while Ti was found to increase with the density of the gas. Experimental results obtained for the selfdiffusion coefficient are compared with theoretical values. Details of experimental procedure are described. NUCLEAR MAGNETIC RESONANCE IN GASEOUSHe 3

can also be derived by expanding the exact S of Eq. (16) to second order and dropping the constant terms and those which correspond to the redistributed, steady-state energy, that is, to the second term in Eq. (36).

The magnetic susceptibility of the heart and the intracardiac blood is substantially greater than that of the surrounding lung tissue. The periodic redistribution of blood and tissue during the cardiac cycle will thus alter an externally applied magnetic field. These magnetic field changes can be detected by a magnetometer outside the body, and have been termed magnetic susceptibility plethysmography (MSPG) signals. A differential magnetometer, a liquid helium dewar, and a superconducting persistent-current magnet have been developed to detect the x component of the MSPG signal. The theoretical analysis of this instrument is presented to understand the spatial sensitivity of an MSPG susceptometer with a nonuniform magnetizing field. Preliminary measurements are shown to be in fair agreement with the model calculations. Several possible improvements of the technique are discussed.

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