10069365
58Ni+n transmission, differential elastic scattering, and capture measurements and analysis up to 813 keV.
High-resolution neutron cross section measurements for $^{58}\mathrm{enriched}$ targets were made at the Oak Ridge Electron Linear Accelerator of transmission below 20 MeV, of differential elastic scattering from 10 keV to 5 MeV, and of capture from 2.6 keV to 2.5 MeV. The transmission data were analyzed from 0.1 to 813 keV with a multilevel R-matrix code which uses Bayes' theorem for the fitting process. This code provides energies and neutron widths of resonances within the analyzed region, as well as a possible parametrization for resonances external to that region, as a way of describing the smooth cross section over the entire energy range. The differential elastic data at different scattering angles were compared to theoretical calculations from 30 to 813 keV using an R-function code which is based on the Blatt-Biedenharn formalism. Various combinations of spin and parity were tested to predict cross sections for the well-defined lg0 resonances, and comparison with the data then provided spin and parity assignments for most of these resonances.
117298547
Neutron widths and level spacings of Zn 64 + n
High resolution neutron total cross-section measurements on the separated isotope /sup 64/Zn have been made in energy range of few eV to about 370 keV using a pulsed neutron time-of-flight technique with a nominal resolution of 0.07 ns/m. We have analyzed the cross section data up to 370 keV using a multilevel multichannel computer code and obtained values of the resonance parameters (E/sub 0/, J/sup ..pi../, gGAMMA/sub n/). From these results we obtain the following values for the average properties of the resonance parameters: /sub l/=03.59=(plus-or-minus0.16) keV, S/sub 0/( x 10/sup 4/)=1.89 +- 0.26, and S/sub 1/( x 10/sup 4/)=0.59 +- 0.06. We have also investigated the neutron width and nearest neighbor spacing distributions for 103 s-wave resonances up to 370 keV. The neutron width distribution follows a chi-squared distribution with ..nu..=1.15 +- 0.30. We have determined values for Dyson-Mehta's ..delta../sub 3/ statistic for s-wave levels up to 300 keV as a function of energy. These are found to be in excellent agreement with the predicted values as explained in Table V. Similarly the linear coefficient of correlation between nearest neighbor spacings gives a value of rho=-0.35 in agreement with the predicted value of the random matrix model. The valuemore » for the mean p-wave level spacing is larger than expected from the (2J+1) density law, indicative of missed p-wave levels.« less
116539030
Neutron research and facility development at the Oak Ridge Electron Linear Accelerator 1970 to 1995
This report reviews the accomplishments of the first decade of operation of the Oak Ridge Electron Linear Accelerator (ORELA) and discusses the plans for the facility in the coming decade. Motivations for scientific and applied research during the next decade are included. In addition, ORELA is compared with competing facilities, and prospects for ORELA's improvement and even replacement are reported. Development efforts for the next few years are outlined that are consistent with the anticipated research goals. Recommendations for hardware development include improving the electron injection system to give much larger short-pulse currents on a reliable basis, constructing an Electron Beam Injector Laboratory to help make this improvement possible, continuing a study of possibly replacing the electron accelerator with a proton machine, and replacing or upgrading the facility's data-acquistion and immediate-analysis computer systems. Increased operating time and more involvement of nuclear theorists are recommended, and an effective staff size for optimum use of this unique facility is discussed. A bibliography of all ORELA-related publications is included.
39396881
Measurement of the electric polarizability of the neutron.
117328580
New Maxwellian averaged neutron capture cross sections for Cl-35,Cl-37
The Oak Ridge Electron Linear Accelerator (ORELA) was used to measure neutron total and capture cross sections of natural chlorine in the energy range from 100 eV to 600 keV. We performed an R-matrix analysis of our new capture and transmission data up to 500 keV. From these resonance parameters new (n, gamma) astrophysical reaction rates were determined over the entire energy range needed by the latest stellar models of the s process.
119253432
Non‐Statistical Effects in Neutron Capture
There have been many reports of non‐statistical effects in neutron‐capture measurements. However, reports of deviations of reduced‐neutron‐width (Γn0) distributions from the expected Porter‐Thomas (PT) shape largely have been ignored. Most of these deviations have been reported for odd‐A nuclides. Because reliable spin (J) assignments have been absent for most resonances for such nuclides, it is possible that reported deviations from PT might be due to incorrect J assignments. We recently developed a new method for measuring spins of neutron resonances by using the DANCE detector at the Los Alamos Neutron Science Center (LANSCE). Measurements made with a 147Sm sample allowed us to determine spins of almost all known resonances below 1 keV. Furthermore, analysis of these data revealed that the Γn0 distribution was in good agreement with PT for resonances below 350 eV, but in disagreement with PT for resonances between 350 and 700 eV. Our previous (n,α) measurements had revealed that the α strength function...
29173646
Octonionic superstring solitons.
An extended-soliton solution to the low-energy heterotic-field-theory equations of motion is constructed from an eight-dimensional octonionic instanton. The soliton describes a string in ten-dimensional Minkowski space, and preserves only one of the sixteen spacetime supersymmetries.
37191194
Instantons and the spectrum of Bloch electrons in a magnetic field.
A semiclassical path-integral treatment of the spectrum of a charged particle moving in a two- dimensional periodic potential in the presence of a transverse magnetic field is presented. Complex instanton solutions to the Euclidean equations of motion are identified, and it is shown that the dilute-instanton-gas sum reproduces features of the spectrum studied by previous authors. Compar- ison of the path-integral results to a perturbative calculation of the splitting of the lowest Landau level is used to relate the self-similar nature of the spectrum to a type of duality in the parameter a, the number of flux quanta per unit cell. As a byproduct of the instanton calculation the generating functional for a sum over lattice paths of given length and algebraic area modulo a fundamental area determined by the magnetic field is evaluated. The quantum-mechanical system of an electron moving in a two-dimensional periodic potential in a magnetic field transverse to the plane of motion has attracted a great deal of interest. It provided an early example of a physical system whose properties depend crucially on the rationality or irrationality of a physical parameter, in this case the ratio of the flux through a unit cell of the lattice to the flux quantum, a=BR/@o with + =Oh /ce and 2 the area of a unit cell. When this ratio is irrational it also provides a simple model of an incommensurate system with the competing periodicities being the lattice periodi- city and a periodicity determined by a lattice whose unit cell encloses one flux quantum. Recently it has attracted renewed interest because of its connections to the quan- tized Hall effect' and to the mean-field theory of the Hub- bard model. Our own interest in this model was trig- gered by similarities between instantons in this model and instantons in gauge theory in 2+1 dimensions with a Chem-Simons term. In both cases the Euclidean func- tional integral becomes complex and one must search for complex saddle points. In addition, the T violating term in each case modifies the behavior of the solutions at large distances, making perturbation theory suspect. This problem has been studied using a variety of tech- niques in different physical regimes. Although instanton techniques have been used to study simpler problems in- volving tunneling in the presence of a magnetic field, ' to our knowledge they have not been applied to this general class of problems involving a magnetic field and a period- ic potential. We will try to show that path-integral tech- niques can be successfully applied to this problem and that they shed some new light on the intricate recursive structure of the spectrum. In the process of working out the instanton sum in this theory we have had to evaluate a sum over lattice paths of the form All 2D lattice paths w&th fixed endpoints
37326897
Momentum distribution of projectile fragments and nuclear shape.
When one or two neutrons are lost in a collision between a projectile with a diffuse neutron-rich surface and a target nucleus, the transverse momentum distribution of the fragments thus produced can be fit with a double Gaussian curve in which a narrow component is superimposed on a much broader one. It is shown that this phenomenon is also observed for fragments from the normal'' projectile [sup 12]C. A density-dependent nucleon Fermi momentum and fragment recoil with the momentum of the lost nucleon give good agreement with experiment. The two Gaussian components do not require the presence near the nuclear surface of nucleons with two widely different binding energies, as often supposed.
42736184
Nonrenormalization of the superstring tension.
It is argued that the superstring tension is not renormalized in perturbation theory for vacua which preserve N= 1 spacetime supersymmetry. Some implications of this result for macroscopic superstrings are discussed, as well as some analogies between macroscopic superstrings and solitons in supersymmetric theories.
8196433
Reducing S duality to T duality.
The infrared limit of D = 4, N = 4 Yang-Mills theory with compact gauge group G compactified on a two-torus is governed by an effective superconformal field theory. We conjecture that this is a certain orbifold involving the maximal torus of G. Yang-Mills S-duality makes predictions for all correlators of this effective conformal field theory. These predictions are shown to be implied by the standard T-duality of the conformal field theory. Consequently, Montonen-Olive duality between electric and magnetic states reduces to the standard two-dimensional duality between momentum and winding states.