R.A. Macri

Neutron Stimulated Emission Computed Tomography of Stable Isotopes

Here we report on the development of a new molecular imaging technique using inelastic scattering of fast neutrons. Earlier studies demonstrated a significant difference in trace element concentrations between benign and malignant tissue for several cancers including breast, lung, and colon. Unfortunately, the measurement techniques were not compatible with living organisms and this discovery did not translate into diagnostic techniques. Recently we have developed a tomographic approach to measuring the trace element concentrations using neutrons to stimulate characteristic gamma emission from atomic nuclei in the body. Spatial projections of the emitted energy spectra allow tomographic image reconstruction of the elemental concentrations. In preliminary experiments, spectra have been acquired using a 7.5MeV neutron beam incident on several multi-element phantoms. These experiments demonstrate our ability to determine the presence of Oxygen, Carbon, Copper, Iron, and Calcium. We describe the experimental technique and present acquired spectra.

Coulomb excitation of the 242mAm isomer

The 242mAm isomer, a well-known candidate for photodepopulation research, has been studied in this first ever Coulomb excitation of a nearly pure (≈98%) isomer target. Thirty new states, including a new rotational band built on a Kπ = 6− state, have been identified. Strong K-mixing results in nearly equal populations of the Kπ=5− and 6− states. Newly identified states have been assigned to the Kπ=3− rotational band, the lowest states of which are known to decay into the ground-state band. Implications regarding K-mixing and Coulomb excitation paths to the ground state are discussed.

ELECTROMAGNETIC EFFECTS AND THE LONG-STANDING THREE-NUCLEON ANALYZING POWER PUZZLE

New results for the neutron-deuteron analyzing power Ay(θ) at En = 1.2 and 1.9 MeV and their comparison to proton-deuteron data reveal a sizeable and unexpected difference which increases with decreasing center-of-mass energy. This finding calls for the theoretical treatment of a subtle electromagnetic effect presently not incorporated in rigorous three-nucleon scattering calculations, before it is justified to invoke charge-dependent three-nucleon forces and/or other new physics.

Energy Dependence of Fission Product Yields from {sup 235}U, {sup 238}U and {sup 239}Pu for Incident Neutron Energies Between 0.5 and 14.8 MeV

Abstract Fission Product Yields (FPY) have historically been one of the most observable features of the fission process. They are known to have strong variations that are dependent on the fissioning species, the excitation energy, and the angular momentum of the compound system. However, consistent and systematic studies of the variation of these FPY with energy have proved challenging. This is caused primarily by the nature of the experiments that have traditionally relied on radiochemical procedures to isolate specific fission products. Although radiochemical procedures exist that can isolate all products, each element presents specific challenges and introduces varying degrees of systematic errors that can make inter-comparison of FPY uncertain. Although of high importance in fields such as nuclear forensics and Stockpile Stewardship, accurate information about the energy dependence of neutron induced FPY are sparse, due primarily to the lack of suitable monoenergetic neutron sources. There is a clear need for improved data, and to address this issue, a collaboration was formed between Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL) and the Triangle Universities Nuclear Laboratory (TUNL) to measure the energy dependence of FPY for 235 U, 238 U and 239 Pu. The measurements have been performed at TUNL, using a 10 MV Tandem Van de Graaff accelerator to produce monoenergetic neutrons at energies between 0.6 MeV to 14.8 MeV through a variety of reactions. The measurements have utilized a dual-fission chamber, with thin (10-100 μ g/cm2) reference foils of similar material to a thick (100-400 mg) activation target held in the center between the chambers. This method allows for the accurate determination of the number of fissions that occurred in the thick target without requiring knowledge of the fission cross section or neutron fluence on target. Following activation, the thick target was removed from the dual-fission chamber and gamma-ray counted using shielded HPGe detectors for a period of 1-2 months to determine the yield of various fission products. To the extent possible all irradiation and counting procedures were kept the same to minimize sources of systematic errors. FPY have been determined at incident neutron energies of 0.6, 1.4, 2.4, 3.5, 4.6, 5.5, 8.9 and 14.8 MeV.

Simultaneous measurement of (n,γ) and (n,fission) cross sections with the DANCE 4π BaF2 array

Neutron capture cross section measurements on many of the actinides are complicated by low‐energy neutron‐induced fission, which competes with neutron capture to varying degrees depending on the nuclide of interest. Measurements of neutron capture on 235U using the Detector for Advanced Neutron Capture Experiments (DANCE) have shown that we can partially resolve capture from fission events based on total photon calorimetry (i.e. total γ‐ray energy and γ‐ray multiplicity per event). The addition of a fission‐tagging detector to the DANCE array will greatly improve our ability to separate these two competing processes so that improved neutron capture and (n,γ)/(n,fission) cross section ratio measurements can be obtained. The addition of a fission‐tagging detector to the DANCE array will also provide a means to study several important issues associated with neutron‐induced fission, including (n,fission) cross sections as a function of incident neutron energy, and total energy and multiplicity of prompt fission photons. We have focused on two detector designs with complementary capabilities, a parallel‐plate avalanche counter and an array of solar cells.