Lifetimes of rotational levels in170W

Lifetimes of the first 2⁺, 4⁺ and 6⁺ levels in¹⁷⁰W have been measured by the recoil-distance method based on the Doppler effect, using the¹⁵⁵Gd (²⁰Ne, 5n)¹⁷⁰W reaction. The following conclusions can be drawn from the results: if there are any deviations from the predictions of the rotational model for the lifetimes of the 4⁺ and 6⁺ states, they are smaller than 8.5 %; if this model accurately describes the nucleus¹⁷⁰W, the intrinsic quadrupole moment of this nucleus is: Q₀=5.93±0.06 barns, and its deformation is: β=0.240±0.003. The value of Q₀ is compared with the predictions of various calculations for the tungsten isotopes.     

Finite-size error in many-body simulations with long-range interactions

We discuss the origin of the finite-size error of the energy in many-body simulation of systems of charged particles and we propose a correction based on the random-phase approximation at long wavelengths. The correction is determined mainly by the collective charge oscillations of the interacting system. Finite-size corrections, both on kinetic and potential energy, can be calculated within a single simulation. Results are presented for the electron gas and silicon.     

Bose-Einstein condensation in interacting gases

We study the occurrence of a Bose-Einstein transition in a dilute gas with repulsive interactions, starting from temperatures above the transition temperature. The formalism, based on the use of Ursell operators, allows us to evaluate the one-particle density operator with more flexibility than in mean-field theories, since it does not necessarily coincide with that of an ideal gas with adjustable parameters (chemical potential, etc.). In a first step, a simple approximation is used (Ursell-Dyson approximation), which allow us to recover results which are similar to those of the usual mean-field theories. In a second step, a more precise treatment of the correlations and velocity dependence of the populations in the system is elaborated. This introduces new physical effects, such as a change of the velocity profile just above the transition: the proportion of atoms with low velocities is higher than in an ideal gas. A consequence of this distortion is an increase of the critical temperature (at constant density) of the Bose gas, in agreement with those of recent path integral Monte-Carlo calculations for hard spheres. Received 13 November 1998     

All triples for orthogonal designs of order 40

The use of amicable sets of eight circulant matrices and four negacyclic matrices to generate orthogonal designs is relatively new. We find all 1841 possible orthogonal designs of order 40 in three variables, using only these new techniques.     

Neutral pion production at 350 MeV/u in the system20Ne +27Al

π⁰ -production probability of (9± 2)· 10^{? 3} per²⁰Ne+Al reaction has been measured at 350 MeV/u using the Two Arm Photon Spectrometer TAPS. This yield is consistent with an interpolation of published π-production rates in heavy ion collisions.     

A D-optimal design of order 150

A non-circular Ehlich D-optimal design of order 150 is constructed. Noting that all computer computations so far have not produced a circular D-optimal design of order 150 makes this design quite interesting.     

A characterization of complete matroid base graphs

This study grew from an attempt to give a local analysis of matroid base graphs. A neighborhood-preserving covering of graphs p:G → H is one such that p restricted to every neighborhood in G is an isomorphism. This concept arises naturally when considering graphs with a prescribed set of local properties. A characterization is given of all connected graphs with two local properties: (a) there is a pair of adjacent points, the intersection of whose neighborhoods does not contain three mutually nonadjacent points; (b) the intersection of the neigh-borhoods of points two apart is a 4-cycle. Such graphs have neighborhoods of the form K_n × K_m for fixed n, m and are either complete matroid base graphs or are their images under neighborhood-preserving coverings. If n ≠ m, the graph is unique; if n = m, there are n ? 3 such images which are nontrivial. These examples prove that no set of properties of bounded diameter can characterize matroid base graphs.     

Über die Massenspektroskopische Untersuchung von Diimin und seinen Deuterierungsprodukten

Mass Spectroscopic Investigation of Diimide and its Deutero Products The formation of mono and dideuterodiimide was investigated by spectroscopic means. By high resolution techniques the deuterated products could be identified and their mode of formation ascertained from the isotope-distribution. The stabilities of the molecular and fragment ions were estimated from ionization and appearance potentials. Appearance potentials (electron-impactionizations) were estimated. The field ionization spectra indicate the thermally induced disproportionation of the formed diimide. By measuring the metastable transitions according to the DADI method a fragmentation scheme for diimide can be specified.