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New Target Methodology: Polymer-Assisted Deposition and Its Applications on Gas-Phase Nuclear Chemistry with Rutherfordium

  • Author(s): Garcia, Mitch Andre
  • Advisor(s): Nitsche, Heino
  • et al.

This work focuses on three research topics that were distinctly different but broadly centered around developing new methods to perform transactinide gas-phase chemistry. First, the application of a new materials science methodology, Polymer-Assisted Deposition (PAD), to target manufacturing is described. Second, the construction of a new experimental apparatus to conduct gas-phase chemistry and the design of circuits and electronics for the measurement of alpha-decay energy is discussed. Third, a new nuclear reaction for the production of 261Rf, an ideal candidate for gas-phase chemistry due to its relatively-long half life (t1/2 = 69 s), is detailed.

The PAD method was used to make metal oxides of hafnium, europium, and thulium on silicon backings that were found to be homogenous and highly uniform metal oxide films. The films, after a single application of the PAD method, ranged in thickness from 30 to 320 nm. The reapplication of the PAD method on already existing metal oxide films (six times) using the 10% by weight hafnium (IV) solution resulted in equally homogeneous and uniform films with a total thickness of 600 nm.

The irradiation of targets made by the PAD method on silicon nitride backings (thickness of 1000 nm, 344 ug/cm2) were irradiated with an 40Ar beam at a laboratory frame energy of 210 MeV (50 particle nA). The root mean squared (RMS) roughness prior to irradiation was 1.1 nm for a 250 nm (220 ug/cm2) Tm2O3 target, and a RMS roughness of 2.0 nm after irradiation was measured by Atomic Force Microscopy (AFM). These experiments demonstrated the resilience of the targets to heavy-ion irradiation and will prove useful when next generation ion sources come online with subsequently larger beam intensities.

The design, construction and testing of the alpha-wheel apparatus for gas-phase chemistry reactions was completed. Testing of the device was done with 205Fr, produced from the nuclear reaction of 169Tm(40Ar, 4n)205Fr. The francium atoms were quickly transported to the alpha-wheel apparatus, and a total of 164 decays were detected. They centered at an energy of 6.8 MeV which corresponded to the known 7.01 MeV alpha particle from 205Fr; discrepancies in the energy are due to energy-loss from the helium atmosphere in the apparatus.

The first nuclear synthesis of Rutherfordium-261 using uranium targets (238U) were conducted with the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. The nuclear reaction consisted of irradiating the uranium targets with a 26Mg beam. The production of 261Rf went through a 3n exit channel, a channel not normally seen in hot-fusion reactions, and had a cross section of 28 pb.

In summary, this work shows that Polymer-Assisted Deposition is a good method to produce homogeneous, uniform, and crack-free metal oxide targets for nuclear science applications. It also showed that the construction of an alpha-wheel apparatus to measure the alpha-decay of radionuclides is reliable. Finally, it has been shown that the production of 261Rf is possible with uranium targets at a sufficient cross-section to make a gas-phase chemistry reaction with this isotope feasible.

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