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The total 'Be(n, p)'Li cross section has been measured from 25 meV to 13.5 keV. These energies correspond to temperatures of T =2.9)& 10 ' to 0.16 GK. For thermal neutrons the cross sections to the ground state (po) and the first excited state (p, ) of 'Li are 38400+800 b and 4202120 b, respectively. This result for the total 'Be(n, p)'Li thermal cross section is about 25%%uo lower, and is approximately a factor of 10 more precise than previous published measurements. For energies above 100 eV, a significant departure from a 1/u shape for the total cross section is observed. The data were analyzed using a single-level approximation, and were also analyzed together with other data using multilevel-multichannel R-matrix theory. Results are presented for the properties of the 2 threshold state and for a possible nearby 2 state. The astrophysical reaction rate, N~(nv ), was calculated from the measured cross sections for the combined po and pl transitions. The resulting reaction rate is approximately 60-80% of the rate currently in use. This reduction in the Be(n,p) Li reaction rate could result in a calculated increase in the production of 'Li during the big bang by as much as 20%.

The renewed interest since 1990 in accelerator-driven subcritical systems for transmutation of commercial nuclear waste has evolved to focus on the issue of whether fast-or thermal-spectrum systems offer greater promise. This review addresses the issue by comparing the performance of the more completely developed thermal-and fast-spectrum designs. Substantial design information is included to allow an assessment of the viability of the systems compared. The performance criteria considered most important are (a) the rapidity of reduction of the current inventory of plutonium and minor actinide from commercial spent fuel, (b) the cost, and (c) the complexity. The liquid-fueled thermal spectrum appears to offer major advantages over the solid-fueled fast-spectrum system, making waste reduction possible with about half the capital requirement on a substantially shorter time scale and with smaller separations requirements.

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The degree of parity mixing in the 0.734-eV neutron resonance in " La was measured in a twosample transmission experiment. An unpolarized neutron beam was weakly polarized in the longitudinal direction at energies near the resonance by transmission through a La&03 sample. The beam was passed through a spin flipper and then through a second sample of La203. The difference in transmission for the two spin directions was then measured to determine the longitudinal asymmetry. We obtain 9.2+1.7% s-wave mixing into this p-wave resonance, which is consistent with earlier measurements using polarized beams in single transmission experiments. Because parity mixing is an attribute of the weak force, the experiment demonstrates the remarkable enhancement in the weak force in neutron resonancesan effect large enough to allow polarization of a neutron beam. Experiments of this class are described which should permit the first systematic study of the manifestation of the weak force in complex nuclei.

vestigation of the p-ray strength function as a function of A. Therefore, it should be possible, in a series of experiments such as this, to differentiate between the Asls and A'~d ependen. ce of (I'~p/D) predicted by the single-particle and giant-resonance models, respectively.The data-taking rates for this experiment can be improved by a factor of at least j.000 with the best equipment available with existing technology. '8 It is therefore reasonable to expect this technique to become a fruitful source of information for many other properties of nuclei other than those investigated here. for his encouragement. We also wish to thank Capt. R. L. Van Hemert for assistance in the reduction of the data; E.M. Lent for calculations of the bremsstrahlung energy dependence and intensity; J. Nutter and the mechanical technicians for construction and assembly of the equipment required for this experiment; and E. Dante, Jr. and the accelerator operators for the good performance of the machine under unusual operating conditions. We also are indebted to Dr. R. C. Block of the Rensselaer Polytechnic Institute for sending us his high-resolution neutroncapture cross-section data on Fe" prior to publication. PHYSICAL REVIEWThe Beg (y,n) Be' cross section was measured as a function of photon energy from less than 1 to about 40 keV threshold, by the threshold photoneutron technique. The cross section rises sharply to a maximum of 1.6 mb at 6 keV above threshold and then decreases slowly to 1.2 mb at 40 keV. An attempt was made to fit these data, and earlier data obtained from monoenergetic y-ray source measurements, with a curve of the Breit -Wigner form in order to examine the possibility that the shape of the cross section is influenced primarily by a single level near threshold. The results show that such a curve is inadequate to explain the data, and thus indicate that a more complex theory and even better data are needed to understand fully the nature of the cross section near threshold for the Be&(y,n) reaction.