Charles W Alexander

Evaluation of neutron inelastic scattering for radioisotope production

Abstract A systematic study of the production of 117mSn, 119mSn and 195mPt in the hydraulic tube facility (HT) of the ORNL High Flux Isotope Reactor (HFIR) has demonstrated that in all three cases the yields from neutron inelastic scattering ( A Z[n,n′] Am Z) were higher than yields obtained from neutron capture reactions ( ( A −1) Z[n,γ] Am Z) . The corresponding fission-averaged cross-sections were 222 ± 16, 168 ± 12 and 287 ± 20 mb, respectively. The relative gains in the specific activity were 1.4 for 195mPt, 3.3 for 117mSn and 4.4 for 119mSn. The larger gain from 119mSn could be attributed to the relatively lower excitation energy (89.5 keV) of this metastable nucleus. As a part of these studies, the thermal neutron capture cross-sections and resonance integrals for the above reactions and the cross-sections for the [n,2n] reactions leading to the above nuclides were also evaluated.

Neutron flux characterization of a peripheral target position in the High Flux Isotope Reactor.

The High Flux Isotope Reactor at the Oak Ridge National Laboratory provides the highest steady-state thermal neutron flux in the western world for a wide range of experiments and for isotope production. The highest available fluxes are located in a flux trap region created inside the nested fuel elements. The experimentally determined thermal and the empirically obtained epithermal flux values along the vertical axis of the peripheral target position were fit to cosine curves, with the thermal flux ranging from 1.1 x 10(15)ns(-1)cm(-2) at outer positions to 1.5 x 10(15)ns(-1)cm(-2) at the center. The corresponding epithermal flux ranged from 3.5 x 10(13) to 7.5 x 10(13)ns(-1)cm(-2), respectively. The fast neutron flux (En > or = 0.32 MeV in two positions and En > or = 1.5 MeV in two other positions) was approximately 6 x 10(14)ns(-1)cm(-2), corresponding to a fast to thermal ratio of approximately 0.4.

Neutron absorption cross section of sup 236 U

Uranium-236 neutron absorption is measured as a function of neutron time of flight from 20 eV to 1 MeV. The neutron flux is monitored with a {sup 6}Li glass scintillator. Average cross sections from 3 keV to 1 MeV are derived. Estimated uncertainties are < 5% below 600 keV and increase to 9.5% at 1 MeV. From 20-eV to 4.2-keV neutron energy, 293 resonance peaks are parameterized.

Radioisotope Micropower System Using Thermophotovoltaic Energy Conversion

Recent advances in thermophotovoltaic (TPV) energy conversion efficiency have increased interest in investigating the use of TPV in a wider spectrum of applications. This paper discusses a micropower system under development that utilizes a radioisotope heat source, microelectromechanical (MEM) thermal insulation, 3‐D tungsten photonic crystal (PC) emitter, and thermophotovoltaic modules for the energy conversion. The 3‐D tungsten photonic crystal (PC) emitter is a selective emitter that is designed to match the bandgap of the TPV module. Compared to bulk tungsten, the PC has increased emittance in the convertible wavelength range and decreased emittance in the nonconvertible wavelength range, which results in a greater than 10% improvement in conversion efficiency.

Production and decay of the heaviest odd-Z nuclei in the 249Bk + 48Ca reaction

The reaction of 249Bk with 48Ca has been investigated with an aim of synthesizing and studying the decay properties of isotopes of the new element 117. The experiments were performed at five projectile energies (in two runs, in 2009-2010 and 2012) and with a total beam dose of 48Ca ions of about 9x1019 The experiments yielded data on a-decay characteristics and excitation functions of the produced nuclei that establish these to be 293117 and 294117 – the products of the 4n- and 3n-evaporation channels, respectively. In total, we have observed 20 decay chains of Z=117 nuclides. The cross sections were measured to be 1.1 pb for the 3n and 2.4 pb for the 4n-reaction channel. The new 289115 events, populated by α decay of 117, demonstrate the same decay properties as those observed for 115 produced in the 243Am(48Ca,2n) reaction thus providing cross-bombardment evidence. In addition, a single decay of 294118 was observed from the reaction with 249Cf – a result of the in-growth of 249Cf in the 249Bk target. The observed decay chain of 294118 is in good agreement with decay properties obtained in 2002-2005 in the experiments with the reaction 249Cf(48Ca,3n)294118. The energies and half-lives of the odd-Z isotopes observed in the 117 decay chains together with the results obtained for lower-Z superheavy nuclei demonstrate enhancement of nuclear stability with increasing neutron number towards the predicted new magic number N=184.

Sensitivity Studies and Experimental Evaluation for Optimizing Transcurium Isotope Production

Abstract This work applies to recent initiatives at the Radiochemical Engineering Development Center at Oak Ridge National Laboratory to optimize the production of transcurium isotopes in the High Flux Isotope Reactor in such a way as to prolong the use of high-quality heavy curium feedstock. By studying the sensitivity of fission and transmutation reaction rates to the neutron flux energy spectrum, a flux filtering methodology is explored for increasing the fraction of (n,γ) reactions per neutron absorption. Filter materials that preferentially absorb neutrons at energies considered detrimental to optimal transcurium production are identified, and transmutation rates are examined with high-energy resolution. Experimental capsules are irradiated employing filter materials, and the resulting fission and activation products are studied to validate the filtering methodology. Improvement is seen in the production efficiency of heavier curium isotopes in 244Cm and 245Cm targets and potentially in the production of 252Cf from mixed californium targets. Further analysis is recommended to evaluate longer-duration irradiations more representative of typical transcurium production.

Comparison of Cf-252 thin-film sources prepared by evaporation or self-transfer

Thin-film sources containing Cf-252 were prepared by two techniques—evaporation and self-transfer—to determine whether sources prepared by simple evaporation work as well as sources prepared by self-transfer for alpha particle studies. The sources were analyzed by alpha and gamma spectroscopy. Results indicate that self-transfer sources exhibit less alpha energy straggling and alpha energy loss than evaporative sources. Fission fragments may also self-transfer, and sources made by self-transfer may need time to decay before reaching radioactive equilibrium.