Application of the method of “molecular plating” to prepare actinide targets suitable for accelerator bombardment is presented. Two example applications involving 229Th and 254Es are discussed along with the merits and liabilities of the method.

During the past few years we have made measurements of (n, p) and (n, α) cross sections on several radioactive nuclei of importance to nuclear astrophysics. The measurements were made at the Manuel Lujan, Jr. Neutron Scattering Center (LANSCE) from thermal neutron energy to approximately 100 keV. Successful measurements have been completed on the radioisotopes 7Be, 22Na and 36Cl while preliminary data have been taken on targets of 54Mn and 55Fe. Similar measurements have also been made on the stable isotopes 14N, 17O and 35Cl. We are currently assembling a 4π barium fluoride (BaF2) detector which will allow us to expand our program to (n, γ) measurements. The (n, γ) (and in some cases future (n, p)) measurements will require targets with higher specific activity and greater chemical purity than we have so far been able to use. We discuss the fabrication techniques used for the samples produced so far, the requirements the future (n, γ) targets must meet and our current plans for producing them, and the physics motivations for the measurements.

Targets of iron, tungsten, carbon, and calcium or areal densities 2.3–5.8 g/cm2 were fabricated to high precision for a fixed-target experiment performed in 1987 at Fermilab to measure relative Drell-Yan cross sections. The experiment used 800-GeV protons at an intensity of 2 × 1012 protons per 23-s spill. Areal densities were determined to an accuracy of approximately 1 part in 104. The calcium targets were vacuum-encapsulated in stainless steel by electron-beam welding.

Targets are made at Los Alamos for experiments at the Ion Beam Facility (Van de Graaff), the Medium Energy Physics Facility (LAMPF), and for experiments conducted at many other accelerators in the US and Europe. Thin, isotopic targets are made by sputtering and evaporation. Versatile, large-scale facilities exists for ceramics and plastics fabrication, electroplating, powder metallurgy, fabrication by pressing, casting and rolling, chemical and physical vapor deposition, and sputtering. Special developments include ultra-precision machining, cryogenic targets, and shaped-foil targets.

For electron-scattering experiments we have prepared osmium targets of 20 to 40 mg/cm2 sandwiched in graphite by centrifuging and hot-pressing in argon. They are strong and withstand beam-induced temperatures of at least 500°C.

The empirically determined conditions for making strong, self-supporting foils, 100–300 μg/cm2, of nickel and platinum by electron-gun vacuum evaporation include substrate temperatures of 250–350°C. X-ray diffraction studies have been made to determine what changes in crystal structure with increasing temperature are correlated with foil strength.

Measurements with a semiconductor detector system and a two-dimensional analyzer have yielded information on the mass distributions and the details of the kinetic energy reiease from a series of charged particle-induced fission reactions. The fissioning compound nuclei range from thallium for which the mass distributions are symmetric, to plutonium for which the fission is predominantly asymmetric. In an intermediate region, the charged particle-induced fission of Ra/sup 226/ yields comparable contributions of symmetric and asymmetric fission. The results are quantitatively consistent with a two-mode hypothesis for the fission process and indicate that within each mode the distance between the charge centers of the two fragments at the scission point is approximately the same for all mass divisions. The results show a lower total kinetic energy release from symmetric fission than from asymmetric fission, indicating that the distance between the charge centers at the scission point is about 10% greater for the symmetric mode than for the asymmetric mode. (auth)

A back-to-back semiconductor counter system was used to study the energetics of 10.8 and 13.6-Mev deuteroninduced fission of Ra/sup 226/ and 25-Mev He/sup 3/-induced fission of Au/sup 197/ and Bi/sup 209/. Evidence for two distinct modes of Ra/sup 226/ fission with the total kinetic energy release arising from Coulomb fragment repu1sion was found. It is suggested that the fission process at moderate excitation energies is different from that at low energies. (L.N.N.)

An investigation has been made of the relative amounts of scintillation light emitted from mixtures of the noble gases during the 0.5 μsec interval following the stopping of an α-particle. A bellows pump and a hot uranium furnace in a closed circulation system maintained gas purity. A diphenyl stilbene coating on the photomultiplier tube and the walls of the scintillation volume acted as a wavelength shifter for the primary ultraviolet radiation. Data on the efficiency of various proportions of binary combinations of xenon, krypton, argon, neon, and helium are presented. They show a characteristic large drop in the light for mixtures containing a small proportion of the heavier gas in a major fraction of the ligher. These results may be qualitatively interpreted in terms of noble gas molecular ion formation. The large light output of the 10% Xe-90% He mixture might be useful in constructing a neutron spectrometer of high efficiency using the He3(n,p)H3 reaction.