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We have investigated the concept of electronic screening from the standpoint of achieving a uniform and consistent interpretation for the purpose of providing a technique for dealing, in first approximation at least, with the effects of electronic interactions while retaining the relative simplicity of the one-electron picture and, in particular, the plane-wave approximation to the Hartree-Fock solutions. Our study has led us to conjecture that a consistent interpretation of the screening term may be found in its use as a variational function along the lines of the calculation by Macke. Also, we have been led to hypothesize the existence of a uniform screening function to be employed in the calculations using plane-wave determinants of all off-diagonal Coulomb matrix elements occurring in probability coefficients of wave expansions or in transition probabilities.

The large cross section of the reaction H2 + H3 → He4 + n + 17.6 Mev makes it interesting both from the standpoint of its use as a neutron source and because the magnitude of the cross section is essentially the maximum possible predicted by nuclear resonance theory. The published data indicates a maximum cross section at around 200 kev triton energy. Allen and Poole give the maximum cross section as 5.0 barns. The purpose of the present work was to make accurate measurements of the cross section over a wide range of particle energies and to compare results with predictions of a single level resonance theory.

Measurements of total neutron cross sections for nuclei using fast neutrons of various energies have been previously reported. Several of these used neutrons which were not monoenergetic. However, with the advent of monoenergetic neutrons from the reaction T(d,n)He4, investigations have been reported giving total neutron cross sections at 14 Mev using monoenergetic neutrons in "good" geometry to various accuracies. Of these various reports, the data of Coon, Graves, and Barschall point to deviations from the schematic theory of nuclear cross sections proposed by Feshbach and Weisskopf for the lighter elements as well as some of the heavier elements. A systematic deviation from a plot of the equation sT2p =kA13 sT= total neutron cross section A= atomic weight was observed for the elements Be9, B10, B11, and C12. For these four elements the cross section was observed to be a decreasing function of the atomic weight. This effect is consistent with the data of Lasday and Goodman for Be9 and C12. Neither of the latter did the boron isotopes. The data of Lasday and Goodman also represents 14 Mev total neutron cross sections. Of these investigations, no one group carried out cross section measurements at more than one particular neutron energy. It is of interest to see over what energy range these systematic deviations appear. The results of Cook, et al., show that at 90 Mev the total neutron cross section for H, D, L1 7, Be9, C12 are increasing with atomic weight. This indicates that at some point in the energy range 14 to 90 Mev these deviations disappear. The investigations carried out here are measurements of total neutron cross sections for some of the light elements over the energy range 14 to 18 Mev. In addition to checking for a systematic deviation similar to that found by Coon, et al., considerable attention was given to determination of the neutron-proton total cross section over the above energy range.

A study of nuclear relaxation due to paramagnetic impurities has been made, using the system of F19 nuclei in Ca F 2. T1 was measured at helium, hydrogen, and nitrogen temperatures. The field dependence of T1 at helium temperatures consists of three parts: at fields less than a few hundred gauss, the dependence of T1 on the field is parabolic; between a few hundred gauss and about 2.5 Kg, the dependence is linear; and at higher fields, it is linear, but has a steeper slope. Moreover, there is a strong orientation dependence in the helium range for a single crystal, and a size effect appears at higher fields. At temperatures above 14°K, T1 has only a weak orientation dependence. The experimental data indicate that T1 is minimum between 14°K and 63°K. An analysis of the experimental results with existing theories permits a determination of the field and temperature dependence of the relaxation time rho of the spin of the paramagnetic impurity. The analysis gives rho ∼ H-2T-1 for nitrogen and hydrogen temperatures. In the helium range, rho ∼ H-2 T-1 for Ho ≲ 2.5 Kg and rho ∼ H-1T-2 for Ho ≳ 2.5 Kg. rho is calculated to be ∼10-6 sec. at 14°K and the impurity concentration is calculated to be approximately one part in 106, assuming the impurity is iron.

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