Radioactive targets for neutron-induced cross section measurements

Measurements using radioactive targets are important for the determination of key reaction path ways associated with the synthesis of the elements in nuclear astrophysics (sprocess), advanced fuel cycle initiative (transmutation of radioactive waste), and stockpile stewardship. High precision capture cross-section measurements are needed to interpret observations, predict elemental or isotopical ratios, and unobserved abundances. There are two new detector systems that are presently being commissioned at Los Alamos National Laboratory for very precise measurements of (n,{gamma}) and (n,f) cross-sections using small quantities of radioactive samples. DANCE (Detector for Advanced Neutron-Capture Experiments), a 4 {pi} gamma array made up of 160 BaF{sub 2} detectors, is designed to measure neutron capture cross-sections of unstable nuclei in the low-energy range (thermal to {approx}500 keV). The high granularity and high detection efficiency of DANCE, combined with the high TOF-neutron flux available at the Lujan Center provides a versatile tool for measuring many important cross section data using radioactive and isotopically enriched targets of about 1 milligram. Another powerful instrument is the Lead-slowing down spectrometer (LSDS), which will enable the measurement of neutron-induced fission cross-section of U-235m and other short-lived actinides in a energy range from 1-200 keV with sample sizes down to 10 nanograms. Due to the short half-life of the U-235m isomer (T{sub 1/2} = 26 minutes), the samples must be rapidly and repeatedly extracted from its {sup 239}Pu parent. Since {sup 239}Pu is itself highly fissile, the separation must not only be rapid, but must also be of very high purity (the Pu must be removed from the U with a decontamination factor >10{sup 12}). Once extracted and purified, the {sup 235m}U isomer would be electrodeposited on solar cells as a fission detector and placed within the LSDS for direct (n,f) cross section measurements. The production of radioactive targets of a few milligrams will be described as well as the containment for safe handling of these targets at the Lujan Center at LANSCE. To avoid any contamination, the targets are electrochemically fixed onto thin Ti foils and two foils are placed back to back to contain the radioactive material within. This target sandwich is placed in a cylinder made of aluminum with thin translucent windows made of Kapton. Actinides targets, such as {sup 234,235,236,238}U, {sup 237}Np, and {sup 239}Pu are prepared by electrodeposition or molecular plating techniques. Target thicknesses of 1-2 mg/cm{sup 2} with sizes of 1 cm{sup 2} or more have been made. Other targets will be fabricated from separation of irradiated isotopically enriched targets, such as {sup 155}Eu from {sup 154}Sm,{sup 171}Tm from {sup 170}Er, and {sup 147}Pm from {sup 146}Nd, which has been irradiated in the high flux reactor at ILL, Grenoble. A radioactive sample isotope separator (RSIS) is in the process of being commissioned for the preparation of other radioactive targets. A brief summary of these experiments and the radioactive target preparation technique will be given.

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