Frank S. Dietrich

Cross sections for neutron capture from surrogate measurements: An examination of Weisskopf-Ewing and ratio approximations

Motivated by the renewed interest in the surrogate nuclear reactions approach, an indirect method for determining compound-nuclear reaction cross sections, the prospects for determining (n,{gamma}) cross sections for deformed rare-earth and actinide nuclei are investigated. A nuclear reaction model is employed to simulate physical quantities that are typically measured in surrogate experiments and used to assess the validity of the Weisskopf-Ewing and ratio approximations, which are typically employed in the analysis of surrogate reactions. The expected accuracy of (n,{gamma}) cross sections extracted from typical surrogate measurements is discussed and limitations of the approximate methods are illustrated. Suggestions for moving beyond presently employed approximations are made.

Development of high-energy neutron imaging for use in NDE applications

We are currently developing a high-energy (10–15 MeV) neutron imaging system for use in NDE applications. Our goal is to develop an imaging system capable of detecting cubic-mm-scale voids or other structural defects in heavily-shielded low-Z materials within thick sealed objects. The system will be relatively compact (suitable for use in a small laboratory) and capable of acquiring tomographic image data sets. The design of a prototype imaging detector and multi-axis staging system will be discussed and selected results from recent imaging experiments will be presented. The development of an intense, accelerator-driven neutron source suitable for use with the imaging system will also be discussed.

Coupled-Channel Calculation of Nonelastic Cross Sections Using a Density-Functional Structure Model

A microscopic calculation of reaction cross sections for nucleon-nucleus scattering was performed by coupling the elastic channel to all particle-hole excitations in the target and one-nucleon pickup channels. The particle-hole states may be regarded as doorway states through which the flux flows to more complicated configurations, and subsequently to long-lived compound nucleus resonances. Target excitations for (40,48)Ca, 58Ni, 90Zr, and 144Sm were described in a random-phase framework using a Skyrme functional. Reaction cross sections obtained agreed very well with experimental data and predictions of a fitted optical potential. Couplings between inelastic states were found to be negligible, while the pickup channels contribute significantly. For the first time observed absorptions are completely accounted for by explicit channel coupling, for incident energies between 10 and 40 MeV.

Recent results in the development of fast neutron imaging techniques

We are proceeding with the development of fast (≈12 MeV) neutron imaging techniques for use in NDE applications. Our goal is to develop a neutron imaging system capable of detecting sub-mm-scale cracks, cubic-mm-scale voids and other structural defects in heavily-shielded low-Z materials within thick sealed objects. The final system will be relatively compact (suitable for use in a small laboratory) and capable of acquiring both radiographic and full tomographic image sets. The design of a prototype imaging detector will be briefly reviewed and results from several recent imaging experiments will be presented. The concurrent development of an intense, accelerator-driven neutron source suitable for use with the final production imaging system will also be briefly discussed.

Conceptual design for a neutron imaging system for thick target analysis operating in the 10–15 MeV energy range

Fast neutron imaging offers the potential to be a powerful non- destructive inspection tool for evaluating the integrity of thick sealed targets. This is particularly true in cases where one is interested in detecting voids, cracks or other defects in low-Z materials (e.g. plastics, ceramics, salts, etc.) which are shielded by thick, high-Z parts. In this paper we present the conceptual design for a neutron imaging system for use in the 10 - 15 MeV energy range and discuss potential applications in the area of nuclear stockpile steward- ship. The background of this project, currently under development at LLNL, will be outlined and computer simulations will be presented which predict system performance. Efforts to assess technical risks involved in the development of the system will be discussed and the results of a recent experiment designed to evaluate background radiation levels will also be presented.

An anticoincidence-shielded NaI spectrometer designed to minimize neutron backgrounds

Abstract A 25 × 25 cm2 NaI gamma spectrometer with a plastiv anticoincidence shield has been constructed with particular regard to minimizing neutron-associated backgrounds. The spectrometer is used with the Lawrence Livermore Laboratory cyclograaff accelerator. The unique feature of the spectrometer is the use of thick shields of 6LiH completely surrounding both the central NaI crystal and the anticoincedence plastic. It is found that under normal conditions the count-rate limitation of the spectrometer is due to gammas in the central crystal emitted from accelerator target, and not to neutron-induced events.