J.H. Landrum

Chloride complexation of element 104

Abstract We have investigated the chloride complexation of element 104 and compared it to that of the actinides and Hf. The 65- s α-emitting isotope 261 104 was produced via the bombardment of 248 Cm with 18 O ions, and the recoil products were transported to a fast computer-controlled apparatus for performing chemical experiments. This system allowed the experiments to be repeated many times to compensate for the very small number or 261 104 atoms produced in each bombardment. The formation of anionic-chloride complexes with element 104 was compared to those with Hf, Cm, and Fm by testing their relative absorption onto a column containing a quaternary amine. The results show that in 12 M HCl solutions the chloride complexation of element 104 is clearly stronger than that of the trivalent actinides and is quite similar to that of Hf, which is expected to be its homolog in the periodic table.

Radiopolarography of mendelevium in aqueous solutions

Abstract The possible existence of a monovalent ion of mendelevium has been examined further by radiopolarography. Experiments were conducted in several aqueous media with 241 Am, 249 Cf, 254 Es, 255,256 Fm and 256 Md tracers. Half-wave amalgamation potentials of Md in tetramethylammonium perchlorate and in LiCl were identical within the experimental errors at −1.755 ± 0.005 V vs the saturated calomel electrode, in good agreement with an earlier study by radiocoulometry in an ammonium acetate medium. No shift in the half-wave potential of Md was observed due to NH 4 + or Cl − ions; hence, these ions do not act as complexing agents for Md ions present during electrochemical reductions. The shift in the half-wave potential observed in the presence of citrate as a complexing agent was characteristic of the reduction process Md 2+ → Md 0 (Hg). In noncomplexing media, the slope of the logarithmically transformed wave of Md was ∼ 30 mV, consistent only with a reversible, two-electron reduction process. All of our results provide evidence for electrochemical reduction of the Md 2+ ion only and disprove the existence of an the intermediate Md + ion in aqueous solution with properties like those of Cs + , Ag + or Cu + .

A new isotope of curium and its decay properties: 251Cm☆

Abstract The nuclide 251Cm ( t 1 2 = 16.8 ± 0.2 min ) was produced by neutron capture from 250Cm with an approximate cross section of 80 barns. In the decay of 251Cm, we measured the energies and intensities of 12 γ-rays from which we constructed a level scheme for 251Bk and assigned configurations for six single-particle states in 251Bk and for the ground state of 251Cm ( 1 2 ) + . The levels of 251Bk fed by 251Cm β-decay are compared with those of 249Bk fed by 249Cm β-decay. We also compare our measured Qβ− value of 1.42 MeV for 251Cm with previous closed-cycle estimates.

Non-observance of monovalent Md☆

Abstract Cocrystallization experiments were reported in 1972 that indicated a monovalent state existed for Md. We recently repeated some of these experiments and performed a series of new ones in which we attempted to prepare Md(I) by reduction with Sm2+ in an ethanolic or fused KCl medium. The oxidation state of Md was determined by the amount of 256Md tracer (≥ 105 atoms) carried on precipitates of SmF 2 SmCl 2 , SmCl2, RbCl and Rb2PtCl6. Comparisons of the coprecipitation behavior of Md with the behavior of tracer amounts of the elements Es, Fm, Eu, Sr, Y and Cs showed that Md consistently follows the behavior of Fm2+, Eu2+ and Sr2+, rather than the behavior of Cs+. Therefore, we conclude that Md cannot be reduced to a monovalent oxidation state with Sm2+ and that the earlier claim for Md(I) is unsubstantiated.

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.

Discovery of a 7. 6-hour high-spin isomer of einsteinium-256

A 7.6-hour, beta-emitting isomer of /sup 256/Es has been produced via the (t,p) reaction by bombarding /sup 254g/Es with 16-MeV tritons. No evidence for an alpha branch was found. A number of gamma rays were observed, on the basis of which a partial decay scheme is proposed. It is concluded that the isomer has spin 7 or 8.

Attempt to produce element 120 in the {sup 244}Pu+{sup 58}Fe reaction

An experiment aimed at the synthesis of isotopes of element 120 has been performed using the {sup 244}Pu({sup 58}Fe,xn){sup 302-x} 120 reaction. No decay chains consistent with fusion-evaporation reaction products were observed during an irradiation with a beam dose of 7.1 x 10{sup 18} 330-MeV {sup 58}Fe projectiles. The sensitivity of the experiment corresponds to a cross section of 0.4 pb for the detection of one decay.

Synthesis, Decay Properties, and Identification of Superheavy Nuclei Produced in 48Ca‐induced Reactions

Thirty‐four new nuclides with Z = 104–116, 118 and N = 161–177 have been synthesized in the complete‐fusion reactions of 238U, 237Np, 242,244Pu, 243Am, 245,248Cm, and 249Cf targets with 48Ca beams. The masses of evaporation residues were identified through measurements of the excitation functions of the xn‐evaporation channels and from cross bombardments. The decay properties of the new nuclei agree with those of previously known heavy nuclei and with predictions from different theoretical models. A discussion of self‐consistent interpretations of all observed decay chains originating from the parent isotopes 282,283112, 282113, 286–289114, 287,288115, 290–293116, and 294118 is presented. Decay energies and lifetimes of the neutron‐rich superheavy nuclei as well as their production cross sections indicate a considerable increase in the stability of nuclei with the approach to the theoretically predicted nuclear shells with N = 184 and Z = 114.

Measurement of the prompt fission neutron multiplicity from the 245Cm(n, f) and 242mAm(n, f) reactions☆

Abstract The prompt fission neutron multiplicity, v p , of 245 Cm was measured relative to that of 235 U using the neutron time-of-flight facility at the Lawrence Livermore National Laboratory (LLNL) 100 MeV electron linac. Incident neutron energies ranged from 0.05 eV to 14 MeV. In addition, a monoenergetic measurement of v p for 245 Cm relative to 235 U was made at E n = 14.1 MeV using the LLNL Insulated Core Transformer (ICT) Neutron Source. Fission fragments were detected using a hemispherical ionization chamber containing ∼ 200 μg of 245 Cm. A separate fission chamber contained 8.3 mg of 235 U. Fission neutrons were detected in liquid scintillators using pulse-shape discrimination to separate γ-rays from neutrons. The measurements reported here for 245 Cm are in agreement with each other and indicate an overall rate of increase in v p with incident neutron energy which departs significantly from systematic trends. Interpretations are presented which suggest that these results are a consequence of changing fragment kinetic energies. Results from a concurrent measurement of v p for 242m Am at E n = 14.1 MeV are also included and agree with previously reported data.