P. A. Ellison

Measurement of the 240Am production cross section via proton irradiation of 242Pu

Abstract A new nuclear reaction for the production of 240Am was experimentally investigated. Targets of 150 – 500 μg/cm2 242Pu on 2 μm Ti were produced through molecular deposition. Five irradiations, in which 242Pu, natTi, and natNi targets were jointly activated with protons from the Lawrence Berkeley National Laboratory 88-Inch Cyclotron produced 240Am, 48V, and 57Ni, respectively. The radioactive decay of these nuclides was monitored using high-purity Ge gamma ray detectors in the weeks following irradiation. A maximum 242Pu(p, 3n)240Am nuclear reaction cross section was measured to be 45 ± 13 mb with 23 MeV protons. While this value is lower than theoretical predictions, it is high enough to be the most viable nuclear reaction for the large-scale production of 240Am.

Excitation Function for the 74Se(18O,p3n) Reaction

Abstract The 74Se( 18O,p3n)88gNb excitation function was measured and a maximum cross section of 495±5 mb was observed at and 18O energy of 74.0 MeV. Experimental cross sections were compared to theoretical calculations using the computer code ALICE-91 and the values were found to be in good agreement. The half-life of 88gNb was determined to be around 14.56±0.11 min.

Nuclear spectroscopy of the heaviest elements: studies of 254No, 257Rf, and 261Sg

Recently it has become possible to perform detailed spectroscopy on nuclei beyond Z = 100 with the aim of understanding the underlying single-particle structure of superheavy elements. A number of such experiments have been performed at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory using the Berkeley Gas-filled Separator (BGS), coupled with delayed γ-ray and electron-decay spectroscopy. Experiments have been performed on 254No (Z = 102), 257Rf (Z = 104), and 261Sg (Z = 106). The results provide new information on the properties of transactinide nuclei, which is important for testing models of the heaviest elements.

Electromagnetic decays of excited states in {sup 261}Sg (Z=106) and {sup 257}Rf (Z=104)

An isomeric one-quasineutron state, likely based on the [725]11/2{sup -} Nilsson level, was identified in {sup 261}Sg by its decay via internal conversion electrons. The state has an excitation energy of approx =200 keV and a half-life of 9.0{sub -1.5}{sup +2.0} mus. {sup 261}Sg has the highest Z and A of any nucleus in which the electromagnetic decay of an isomeric state was observed to date. A separate experiment was performed on the alpha daughter nucleus of {sup 261}Sg, namely {sup 257}Rf. Spectroscopy of delayed gamma rays and converted electrons from {sup 257}Rf resulted in the identification of a K isomer at an excitation energy of approx =1125 keV with a half-life of 134.9 +- 7.7 mus. The spin of the isomeric state is tentatively assigned I=21/2,23/2 and the state likely decays to a rotational band built on the [725]11/2{sup -} Nilsson level via a DELTAK=5 or 6 transition. The present results provide new information on excited states in the transactinide region, which is important for testing models of the heaviest elements.

Extraction of niobium and tantalum isotopes using organophosphorus compounds - Part I - Extraction of 'carrier-free' metal concentrations from HCl solutions

Abstract The extraction of niobium (Nb) and tantalum (Ta) from hydrochloric acid media by bis(2-ethylhexyl) hydrogen phosphate (HDEHP) and bis(2-ethylhexyl) hydrogen phosphite (BEHP) was studied. The goal of the experiments is to find a system that demonstrates selectivity between the members of group five of the Periodic Table and is also suitable for the study of dubnium (Db, Z=105). Experiments were performed at the trace level (10-16 M Nb or Ta) using hydrochloric acid with concentrations ranging from 1−11 M and short-lived isotopes of Nb and Ta produced in nuclear reactions. When HDEHP was used as the extractant, the Nb extraction yield decreased with increasing acid concentrations above 6 M, while the amount of Ta extracted remained over 75% for all acid concentrations studied. Tantalum was found to be extracted by BEHP at acid concentrations above 6 M, while niobium was not significantly extracted. The data obtained are used as the basis to discuss the speciation of Nb and Ta under the conditions studied and to evaluate possible extraction mechanisms.

Independent Verification of Element 114 Production in the {sup 48}Ca+{sup 242}Pu Reaction

Independent verification of the production of element 114 in the reaction of 244-MeV {sup 48}Ca with {sup 242}Pu is presented. Two chains of time- and position-correlated decays have been assigned to {sup 286}114 and {sup 287}114. The observed decay modes, half-lives, and decay energies agree with published results. The measured cross sections at a center-of-target energy of 244 MeV for the {sup 242}Pu({sup 48}Ca,3-4n){sup 287,286}114 reactions were 1.4{sub -1.2}{sup +3.2} pb each, which are lower than the reported values.

High-K multi-quasiparticle states and rotational bands in {sub 103}{sup 255}Lr

Two isomeric states have been identified in 255 Lr. The decay of the isomers populates rotational structures. Comparison with macroscopic-microscopic calculations suggests that the lowest observed sequence is built upon the [624]9/2+ Nilsson state. However, microscopic cranked relativistic Hartree-Bogoliubov (CRHB) calculations do not reproduce the moment of inertia within typical accuracy. This is a clear challenge to theories describing the heaviest elements.

New isotope {sup 263}Hs

A new isotope of Hs was produced in the reaction {sup 208}Pb({sup 56}Fe,n){sup 263}Hs at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory. Six genetically correlated nuclear decay chains have been observed and assigned to the new isotope {sup 263}Hs. The measured cross section was 21{sub -8.4}{sup +13} pb at 276.4 MeV lab frame center-of-target beam energy. {sup 263}Hs decays with a half-life of 0.74{sub -0.21}{sup +0.48} ms by {alpha}-decay and the measured {alpha}-particle energies are 10.57 {+-} 0.06, 10.72 {+-} 0.06, and 10.89 {+-} 0.06 MeV. The experimental cross section is compared to a theoretical prediction based on the Fusion by Diffusion model [W. J. SwiaPtecki et al., Phys. Rev. C 71, 014602 (2005)].

Comparison of reactions for the production of {sup 258,257}Db: {sup 208}Pb({sup 51}V,xn) and {sup 209}Bi({sup 50}Ti,xn)

Excitation functions for the 1n and 2n exit channels of the 208Pb(51V,xn)259-xDb reaction were measured. A maximum cross section of the 1n exit channel of 2070+1100/-760 pb was measured at an excitation energy of 16.0 +- 1.8 MeV. For the 2n exit channel, a maximum cross section of 1660+450/-370 pb was measured at 22.0 +- 1.8 MeV excitation energy. The 1n excitation function for the 209Bi(50Ti,n)258Db reaction was remeasured, resulting in a cross section of 5480+1750/-1370 pb at an excitation energy of 16.0 +- 1.6 MeV, in agreement with previous values [F. P. Hebberger, et al., Eur. Phys. J. A 12, 57 (2001)]. Differences in cross section maxima are discussed in terms of the fusion probability below the barrier.