Nuclear Data Sheets. Section A最新文献

A compilation is presented of thermal-neutron-capture γ-ray energies and intensities with decay schemes, examples of original data, and fully corrected spectral distributions. A table of stable isotopes with abundances, thermal-neutron radiative-capture cross sections, spins, parities, and other information of value to neutron-capture γ-ray spectroscopy is included.

Stopping-power S and R tables for protons, deuterons, tritons, ³He, ⁴He, and ⁷Li ions of energies between 1- and 200-MeV penetrating silicon absorbers are presented. Auxillary data are included in tables for the coefficents C and α needed for the approximate expression S = C·E^α and in figures giving the energy dependence for the coefficients of R = C_R·E^β. Included is a short review of principles to be considered in detector applications.

The values of the parameters for magnetic substate populations and statistical tensors are computed for levels formed by proton capture. The substate population numbers and the tensor parameters are given for levels with spins ranging from J = 1 to J = 6, J integral, and formed by proton capture in channels with spins ranging from s = 0 to s = 4, s integral. The orbital angular momenta range from l = 0 to l = 7.

The coefficients relating substate populations to tensor parameters are given for spins J = 1 to J = 6, J integral, and for J = 3/2 to 9/2, J half-integral. Extension to J >6 can easily be made with the use of a Clebsch-Gordan subroutine for an electronic computer.

In most cases half-integral spins are formed by capture in the channel with s = 1/2. The substate populations are then constant, P_1/2 = 1. The tensor parameters are then contained in the table of coefficients relating these parameters to the population numbers. The parameters for half-integral spins are not tabulated separately.

A final table gives the coefficients relating the parameters for the second state excited in the (P,γ) process, to those of the initially formed level, when the primary γ-ray is unobserved but of known multipolarity. Primary spins J = 1 to 6, J integral, and J = 3/2 to 9/2, J half-integral, are entered together with the secondary spins reached by dipole and quadrupole transitions.

The table presents, for some 500 stable and unstable targets, computed values of the (n,2n) cross section for three neutron energies in the neighborhood of 14 MeV and for a fission neutron spectrum. Cross sections for (n, 3n) for fission neutrons are also given. The calculations are based on the statistical model and on empirical expressions for inelastic cross sections and level density.

The theory of angular distributions of γ-rays is developed systematically, aiming at a phase consistent derivation of angular distribution formulas for gamma rays emitted in the decay of an aligned initial state. The development starts from first principles, that is, the angular distribution formulas are derived directly from perturbation theory and all quantities introduced are carefully and explicitly defined. In particular the mixing ratios are phase consistently related to reduced matrix elements of interaction multipole operators which again are well defined in phase. Hence the mixing ratios become physical quantities which can be extracted from angular distribution measurements and then compared in both magnitude and sign with the predictions of nuclear models (especially the independent particle model). Critical stages in the theoretical development at which either a choice of phase convention has to be made or transformation properties enter are emphasized.

As a first step, the transition probability for emission of gamma radiation with wave vector k and polarization ε from an initial state ¦λ > to a final state ¦μ > is calculated using time-dependent perturbation theory. This step makes no specification of the angular momentum of the initial and final states and no multipole expansion of the interaction. Particular attention is paid to the relation between emission and absorption. In the second step of the calculation the angular momentum of the initial and final states is specified, the interaction is expanded in a series of multipoles and the final angular distribution formula is derived. In order to describe emission of gamma radiation a definite and well-defined part of the interaction Hamiltonian must be expanded. This part of the Hamiltonian is determined by the order in which the initial and final states are written in transition matrix elements. The expansion yields a set of interaction multipole operators which are well defined in phase and transformation properties. There is no uncertainty in the relative phase of the electric and magnetic interaction multipole operators due to an arbitrariness in the phases of the vector potentials. These interaction multipole operators must be used to define the mixing ratios appearing in angular distribution formulas in this paper. It is shown that care must be taken when using Siegert's theorum and “effective” operators.

In the last section, reduced matrix elements of the interaction multipole operators are given explicitly for one- and two-particle states and one- and two-hole states of the independent-particle model.

Internal conversion coefficients, for threshold values of the gamma-ray energy, are presented for the K-shells, L-subshells, and M-subshells for all values of Z in the range 6 ≤ Z ≤ 95, and for the first five electric and magnetic multipoles. These numbers are based upon the exact analytical results previously obtained by the authors for the internal conversion coefficients for any shell and for any energy for a point nucleus with no screening.