Louis D. Roberts

Slowing Down Distribution of U235 Fission Neutrons from a Point Source in Light Water

The results of a measurement of the slowing down distribution in water of neutrons from a point fission source to indium resonance energy, 1.458 ev, are given. The second, fourth, sixth, and eighth moments of the measured distribution are calculated and have the values, r2=184.7 cm2, r4=1.222×105 cm4, r6=2.27×108 cm6, and r8=8.6×1011 cm8.

Slowing Down Distribution to Indium Resonance of U235 Fission Neutrons from a Point Fission Source in Two Aluminum Light Water Mixtures

The mean square slowing down length, r2, has been measured for two aluminum light water mixtures, the aluminum‐to‐water volume ratios being 1:1 and 1:2 by volume. The values of r2 obtained are 460.7 cm2 and 297.4 cm2, respectively.

Study of the Transition of Small Niobium—Zirconium Superconducting Solenoids to the Normal State

A preliminary study has been made of the transition of small superconducting coils to the normal state. This transition was found to be fast compared with velocities found from measurements on short sample lengths of wire. The mode of dissipation of the field energy was studied; a relation between this and the critical current is indicated.

CORRELATION OF THE MOSSBAUER ISOMER SHIFT AND THE RESIDUAL ELECTRICAL RESISTIVITY FOR

In the theoretical discussion of metals, it is often convenient to describe the metallic sample by using a single potential well for the conduction electrons. The Bloch wave functions which extend throughout the entire sample give an approximate solution to the corresponding quantum mechanical problem. When an impurity is dissolved in the metal, the electron wave functions again will, in general, extend throughout the sample. We may then expect a correlation between different physical phenomena associated with the impurity which may each depend predominantly on the different regions of the wave function. For a suitable host and impurity, we should observe a correlation between the charge density at the impurity nucleus, ρ(0), and the transport cross section per impurity atom, σ tr, which the impurity atoms present to the host conduction electrons. For impurity atoms whose nuclei have resonant γ-ray transitions suitable for Mossbauer effect studies,1 a measurement of the isomer shift will give information2 about ρ(0). For suitable dilute alloys, the measurement of the residual electrical resistivity per at. %, ΔR/c, will give σ tr. Thus, we may expect a correlation between the isomer shift, v I, of the resonance γ-ray energy, E γ, of the nucleus of an impurity in a dilute alloy with ΔR/c due to that impurity in the alloy.

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In the study of the hyperfine structure coupling in ferromagnetic or antiferro-magnetic materials by means of the Mossbauer effect the concept of an effective magnetic field H eff has been widely used.1 A precise definition of H eff may be given in terms of the spin Hamiltonian ℋ i associated with the ith magnetic ion of spin S i in the magnetic solid:

HYPERFINE STRUCTURE COUPLING IN FERRIC AMMONIUM SULFATE AS A FUNCTION OF MAGNETIC FIELD AND TEMPERATURE

{H_i} = A{I_i} \cdot {S_i} + {g_n}{\beta _n}{I_i} \cdot H + {g_e}{\beta _e}{S_i} \cdot H + \sum\limits_{j \ne i} {{J_{ij}}{S_i} \cdot {S_j} + {g^2}{\beta ^2}\sum\limits_{j \ne i} {\left[ {\frac{{{S_i} \cdot {S_j}}}{{r_{ij}^3}} - 3\frac{{({S_i} \cdot {r_{ij}})({S_j} \cdot {r_{ij}})}}{{r_{ij}^5}}} \right]} }

SOLID STATE AND NUCLEAR RESULTS FROM A MOSSBAUER-TYPE STUDY OF Au

(1) Here, H is an externally applied magnetic field, I i is the nuclear spin of the ith ion, the fourth term is the exchange interaction, the fifth term is the dipole-dipole interaction, and the remaining symbols have their usual meaning. In materials such as metallic iron and Fe2O3 the exchange interaction is much larger than the other terms in ℋ i , including in particular the hyperfine structure coupling. Correspondingly, the spin-spin relaxation time τ 2 will be very short compared to the nuclear precession time τ L .