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The cross section for the direct radiative capture of protons by 16O has been measured relative to the proton elastic scattering cross section for energies from 800 to 2400 keV (CM). The elastic scattering cross section was normalized to the Rutherford scattering cross section at 385.5 keV. The capture cross section for the reaction 16O(p,γ)17F, which plays a role in hydrogen burning stars, has been extrapolated to stellar energies using a theoretical model which gives a good fit to the measured cross sections. The model involves calculation of electromagnetic matrix elements between initial and final state wave functions evaluated for Saxon–Woods potentials with parameters adjusted to fit both elastic scattering data and binding energies for the ground and first excited states of 17F. Cross sections for capture to the 5/2+ ground and 1/2+ first excited states of 17F in terms of astrophysical S factors valid for energies ≤ 100 keV have been found to be: S5/2+ = (0.317 + 0.0002E) keV b (± 8%); S1/2+ = (8.552 − 0.353E + 0.00013E2) keV b (± 5%).

The critical magnetic field producing spin flop in many antiferromagnets is too small to soften a three-dimensional magnon, i.e., to remove the energy barrier between the equal-energy phases. The barrier can be bypassed, however, via the softening of surface magnons in a smaller field, forming two-dimensional surface-spin-flop states, which broaden with increasing field and which catastrophic ally spread inward across the three-dimensional material as the critical field is approached.It has been suggested that many first-order phase transitions are associated with normal modes which partially but not fully "soften," i.e., which almost go to zero frequency. 1 A nonzero energy barrier between two states of equal free energies provides restoring forces confining the normal-mode oscillations to one side of the barrier and to finite frequencies. In those first-order transitions without superheating or supercooling (i.e., without hysteresis) there arises the problem of how the system manages to hurdle the energy barrier. We exhibit here a possible mechanism for one such system.As first shown theoretically by N6el 2 and experimentally by Poulis and Hardeman, 3 at a critical magnetic field H 3 (see Table I) applied along the preferred axis of an antiferromagnet (AF) there occurs a sudden, nearly 90° rotation of the sublattice vectors into the so-called "spin-flop" (SF) phase. The AF-SF phase transition is first order, there being a finite jump in the magnetic moment, and it can readily be made to take place near 0 K. One's first thought is that it occurs at that field iJ 4 at which the AF resonance frequency goes to zero, i.e., where the restoring torques vanish and a mode softens. Early work either assumed H 3 =H 4 or attempted to derive it. 4 Anderson and Callen 5 identified the three possible critical fields H 2 , H 3 , and H 4 described in Table I. They argued that the AF-SF transition should occur at H 4 in increasing H but at H 2 in decreasing H; and that unless H 2 = H 3 =H 4 , hysteresis should be observed.Experimental evidence of the absence of hystereresis has been accumulating. 6 Furthermore, the AF -SF transition in at least three materials 7 takes place at a field measurably less than H 4 and theoretically evaluated as H 3 , i.e., in the presence of a finite energy barrier.Mills and Saslow 8 have shown that a field H 1

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