Kenton J. Moody

New cold and ultra hot binary and cold ternary spontaneous fission modes for 252Cf and new band structures with gammasphere

Abstract Prompt γ-γ-γ and x-γ-γ coincidence studies following the spontaneous fission of 252Cf were carried out first at Oak Ridge then at Gammasphere first with 36 and later with 72 Ge and two x-ray detectors and a long x-ray-γ coincidence experiment at Idaho. Many new cases of correlated pairs in cold (zero neutron emission) binary fission are observed and for the first time the heavier correlated pairs are identified in ternary fission where the third partners are α, 6He (or α2n), 10Be and, tentatively, 14C. Theoretical calculations were carried out of the relative yields of cold binary and ternary fission. There is reasonable agreement between the relative theoretical and experimental yields. New level structures and isotopes include new octupole deformations, identical bands and other structures. Stable octupole deformation is now observed in N=86, 140Xe, 142Ba and 144Ba and 143Ba, 148Ba and to higher spin in 144Ba. The 142–146Ba data provide the first evidence for the predicted disappearance of stable octupole deformation at high spins from band crossings. Identical yrast bands are observed with widely different neutron and proton numbers in 98,100Sr, 108,110Ru, 112,116Pd, 144,146Ba, 152,154,156Nd, 156Sm, 160Gd and a new type of shifted identical bands in 156,158,160Sm as well as the first identical octupole bands in 142,144Ba. Other new level structures are found. Yields and neutron multiplicities were measured directly for Srue5f8Nd, Zrue5f8Ce, Moue5f8Ba, Ruue5f8Xe, and Pdue5f8Te correlated pairs. A new ultra hot fission mode was discovered going via only 108Moue5f8144Ba, 107Moue5f8145Ba, and/or 106Moue5f8146Ba pairs where one or more of 44,145,146Ba are hyperdeformed at scission with 3:1 axis ratio.

Attempts to produce superheavy elements by fusion of 48Ca with 248Cm in the bombarding energy range of 4.5–5.2 MeV/u

Abstract A search for superheavy elements which are expected to occur around the predicted nuclear shell closures at atomic number 114 and neutron number 184 was made in bombardments of 248 Cm with 48 Ca ions. We have carried out this search at energies close to the Coulomb barrier to keep the excitation energy of the compound nucleus Z = 116, A = 296 as low as possible. The experiments were performed at the accelerators SUPERHILAC (Lawrence Berkeley Laboratory (LBL)) and UNILAC (GSI) and used a variety of improved physical and chemical techniques for the isolation and detection of superheavy elements to increase the sensitivity relative to earlier experiments. The small-angle separator system (SASSY) at LBL and the separator for heavy-ion reaction products (SHIP) at GSI were used for shortlived nuclides, and several radiochemical techniques were applied for longer half-lives. Although a broad range of half-lives, 10 −6 to 10 8 s, and excitation energies, 16 to 40 MeV, has been examined, no evidence for the formation of superheavy elements with cross sections greater than 10 −34 to 10 −35 cm 2 was found.

Actinide yields from the reactions of 40Ca and 48Ca with 248Cm

Abstract Radiochemical techniques were applied to determine the actinide yields for nuclides with mass numbers below and above that of the target from the reaction 48 Ca + 248 Cm at energies near the interaction barrier. This work was initiated in the hope that the very neutron-rich projectile 48 Ca might give access to new neutron-rich isotopes of target-like actinide elements by nucleon transfer reactions. However, for trans-target elements, the use of 48 Ca did not result in increased production rates for such isotopes compared with those obtained in previous investigations. In contrast, rather neutronrich isotopes of below-target elements were observed at a remarkably high cross-section level. This observation is interpreted as a massive mass drift towards symmetry for this system. These investigations have been extended to the system 40 Ca + 248 Cm to explore the effect of the very different neutron-to-proton N Z ratios of 40 Ca and 48 Ca on the production of heavy target-like products.

Continuous separation and identification of neutronrich neptunium isotopes from heavy-ion reactions by means of the centrifuge system “sisak”

Abstract Neutron-rich neptunium isotopes formed in direct transfer reactions between 136 Xe projectiles and targets of 244 Pu have been separated from other reaction products with the on-line liquid-liquid extraction system SISAK. A four-detector delay method was used to determine the half-lives of the previously unknown isotopes 243 Np and 244 Np. From the decay of the 287.4 keV γ-line a half-life of 1.8 ± 0.3 min was determined for 243 Np. From the decay curves of the γ-lines at 162.7 and 216.7 keV, which fit into the known level scheme of 244 Pu, a half-life of 2.5 ± 0.3 min results for 244 Np.

Procedures for a fast separation of berkelium from complex mixtures of reaction products

Two procedures are described for fast separations of berkelium from complex mixtures of reaction products arising from heavy ion reactions, such as18O+248Cm. The first procedure uses a combination of several extraction steps with a final separation on a cation exchanger, the second procedure starts with an anion exchange column which is followed by multiple extractions in different media. The elements separated in the different steps were analyzed and overall decontamination factors are given.

Heavy isotope production by multinucleon transfer reactions with 254Es

Abstract Fast automated on-line and quasi-on-line radiochemical techniques are applied to search for new isotopes, to measure their decay characteristics and to study the cross sections of the heaviest most neutron-rich actinide isotopes in reactions of 16,18O and 22Ne projectiles with 254Es as a target. The measured yields for isotopes up to lawrencium-260 are three or more orders of magnitude higher than in any other reaction used so far. A comparison with data for similar transfers from 248Cm targets is made. Transfer cross sections are extrapolated for the production of unknown, neutron-rich isotopes of elements 101 through 105, and the unique potential of 254Es as a target to make these exotic nuclei accessible is demonstrated.

The decay of 251Bk

Abstract The decay of 251 Bk has been studied by radiochemical techniques. The half-life of 55.6 ± 1.1 min agrees with the literature. Relative intensities of γ-rays, including previously unknown γ-rays at 129.9 keV and 163.8 keV, have been determined. The estimated absolute intensity of the γ-ray at 177.7 keV is (5.2 ± 0.6)%. The maximum β-energy has been measured to be 915 ± 10 keV and was used to determine the mass of 251 Bk. A decay scheme based on these measurements is given.

Deducing the 237U(n,f) cross-section using the Surrogate Ratio Method

The authors have deduced the {sup 237}U(n,f) cross-section over an equivalent neutron energy range of 0 to 20 MeV using the Surrogate Ratio method. A 55 MeV {sup 4}He{sup 2+} beam from the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory was used to induce fission in the following reactions {sup 238}U({alpha},{alpha}f) and {sup 236}U({alpha},{alpha}f). The {sup 238}U reaction was a surrogate for {sup 237}U(n,f) and the {sup 236}U reaction was used as a surrogate for {sup 235}U(n,f). The energies of the scattered alpha particles were detected in a fully depleted segmented silicon telescope array (STARS) over an angle range of 35{sup o} to 60{sup o} with respect to the beam axis. The fission fragments were detected in a third independent silicon detector located at backward angles between 106{sup o} to 131{sup o}.

Isomer ratio measurements for the reaction 29Si(18O, p2n)44m, 44gSc

Abstract Isomer ratios for the reaction 29 Si( 18 O, p2n) 44g Sc, 44g Sc have been deduced from activity measurements for projectile energies between 30 and 99 MeV. Statistical model calculations show that the isomer ratio dependence on projectile energy up to about 80 MeV can be adequately described by assuming a fixed ratio of quadrupole to dipole γ-ray strengths. Such a ratio of E2/E1 strengths agrees with corresponding values deduced from the literature. The values of the γ-ray strength ratios needed to fit the experimental isomer ratios are extremely sensitive to the relative amounts of quadrupole γ-ray admixture and to the presence of discrete levels other than those which conform to the yrast line.

Analytical Chemistry of Plutonium

In 1940, shortly after the discovery of fission, McMillan and Abelson studied the recoil range of fission products induced by neutrons incident on a thin uranium foil (McMillan, 1939; McMillan and Abelson, 1940). While fission products were mostly ejected from the foil, two activities were significantly retained, one with a half-life of 23 min and the other with a half-life of 2.3 days.