E.K. Hulet

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.

DETERMINATION OF THE No(II)--No(III) POTENTIAL FROM TRACER EXPERIMENTS.

Abstract The value of the standard oxidation potential of the No(II)No(III) couple in aqueous solution was estimated from the average behavior observed during repetitive series of experiments with 50–100 atoms per experiment. The distinction between No(II) and No(III) was made on a few atoms at a time basis by multiple solvent extractions with di(2-ethylhexyl)orthophosphoric acid (HDEHP) using the column elution technique of extraction chromatography. By comparing the extraction of nobelium from dilute acid solutions containing different oxidants with the extraction behavior of tracer quantities of radioactive elements of known standard oxidation potentials, the standard potential of the half-reaction No(II)No(III)+ e − was estimated to be −1·4 to −1·5 V.

Studies of californium(II) and (III) iodides

Abstract The compounds CfI2 and CfI3 have been prepared in microgram quantities using high-vacuum techniques. These compounds were characterized by both X-ray powder diffraction and optical spectroscopy. Californium diiodide is prepared by H2 reduction of CfI3 at elevated temperatures and exhibits two crystalline modifications: the CdCl2-type rhombohedral structure with a0 = 7.434 ± 0.011 A and α = 35.83 ± 0.07°, and the CdI2-type hexagonal structure with a0 = 4.557 ± 0.004 A and c0 = 6.992 ± 0.006 A. Optical spectra taken of the samples reveal absorption bands in the wavelength region 300–1100 nm consistent with the presence of Cf(II). Californium triiodide exhibits the rhombohedral BiI3-type structure with a0 = 8.205 ± 0.004 A and α = 55.08 ± 0.01°. This structure is stable up t0 700°C; linear coefficients of thermal expansion were obtained for the temperature range 20–700°C. Optical spectra taken of CfI3 showed absorption-peak positions consistent with those expected by extrapolation from known CfCl3 and CfBr3 peak positions.

Decay properties of heavy mendelevium isotopes

Abstract We investigated the nuclear properties of several mendelevium isotopes produced in the reactions of heavy ions with 245Esg. We measured the alpha particles, spontaneous fissions (SF), and photons emitted by counting samples resulting from chemical and/or mass separations. The 256Md half-life is (78.1 ± 1.8) min; it decays primarily by electron capture (EC), but also by alpha emission (11 ± 3)% of the time. The 256Md ground state has J ⩽ 2 (probably Jπ = 1−), and a mass excess of (87.611 ± 0.053) MeV. The 257Md half-life is (5.523 ± 0.050) h; it decays primarily by EC, but also by alpha emission (15.2 ± 2.6)% of the time and by SF less than 1% of the time. The 256Md mass excess is (88.989 ± 0.003) MeV. The 258Mdg (Jπ = 8−) half-life is (51.50 ± 0.29) d. It decays by alpha emission; the sum of SF, EC, and β− decay branches is less than 3 × 10−3 %. The alpha decay of 258Mdg populates 254Esm (0.60 ± 0.08)% of the time. The 256Mdg mass excess is (91.691 ± 0.007) MeV. The half-life of J π = 1 − 258 Md m is (57.0 ± 0.9) min. It decays by EC; the branch for decay by alpha emission is less than 1.2%. The sum of SF and β− decay branches is less than 30%. The half-life of 259Md is (1.60 ± 0.06) h; it decays primarily by SF. The alpha decay branch of 259Md is less than 1.3%. From the 259Md half-life we calculated a SF hindrance factor associated with the 7 2 − [514] proton configuration of 3.6 × 106. From our data we proposed partial level schemes for 252Es, 253Es, and 254Es.

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 + .

Observation of enhanced nuclear stability near the 162 neutron shell

In bombardments of {sup 248}Cm with {sup 22}Ne the authors discovered two new isotopes, {sup 265}106 and {sup 266}106, by establishing genetic links between {alpha} decays of the 106 nuclides and SF or {alpha} decays of the daughter (grand-daughter) nuclides. For {sup 266}106 they measured E{sub {alpha}}=8.62{+-}0.06 MeV followed by the SF decay of {sup 262}104 for which they measured a half-life value of 1.2{sup +1.0}{sub {minus}0.5} s. For {sup 265}106 they measured E{sub {alpha}}=8.82{+-}0.06 MeV. They estimated {alpha} half-lives of 10-30 s for {sup 266}106 and 2-30 s for {sup 265}106 with SF branches of {approximately}50% or less. The decay properties of {sup 266}106 indicate a large enhancement in the SF stability of this N=160 nuclide and confirm the existence of the predicted neutron-deformed shell N=162.

Some nuclear properties of fermium-257☆

Abstract A total half-life of 100·5±0·2 days and an alpha-to-spontaneous-fission decay ratio of 475±10 have been measured for 257 Fm by counting in ionization chambers. Thermal-neutron fission cross sections for 257 Fm (2950±160 b) and its daughter, 253 Cf (1300±240 b), have been measured in a thermal-neutron column using mica as a fission-fragment track recorder.

Two new isotopes with N = 157: 256Es and 255Cf

Abstract We produced the new nuclides 256 Es and 255 Cf by reactor-neutron irradiations of 255 Es and 254 Cf. The β-decay half lives of 256 Es and 255 Cf are 25.4±2.4 (2δ) min and 85±18 (2δ) min, respectively. We measured effective cross sections of 55±10 b for the 255 Es( n , γ) 256 Es reaction and 4.5±1 barns for the 254 Cf( n , γ) 255 Cf reaction.

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.

The discovery of 260Md and the decay properties of 258Fm, 258m,gMd and 259Md☆

Abstract We have discovered a new neutron-rich isotope, 260Md, from 18O and 22Ne bombardments of 254Es. We observed a spontaneous-fission (SF) activity with a half-life of 32 days in electromagnetically separated fractions with mass number 260 from these bombardments and we measured the mass and kinetic energy distributions of this SF activity. The mass distribution was symmetric with the principal energy peak at a total kinetic energy (TKE) of 234 MeV, similar to previous observations for heavy fermium isotopes. Surprisingly, we also observed a smaller symmetric component with a TKE of 195 MeV. We interpret these two peaks in the TKE distribution as arising from two types of fission in the same nucleus, or bimodal fission. The observed fission activity may be either from the SF decay of 260Md or from 260Fm which would arise from electron-capture (EC) decay of 260Md. We have eliminated the possible β− decay of 260Md by measuring β−-SF time correlations for the decay of 260Md and we plan to determine whether 260Md decays by EC by measuring time correlations between fermium X-rays and SF events. We also measured various properties of the heavy fermium and mendelevium isotopes and obtained 1. (1) more accurate cross-sections for the neutron-rich mendelevium isotopes which we use to predict the production rates of yet undiscovered nuclides, 2. (2) improved half-Me measurements for 258m,gMd and 259Md, 3. (3) confirmation of the EC decay of 258mMd by measurement of the fermium X-rays preceding the SF decay of 258Fm and 4. (4) very substantially improved mass and TKE distributions for the SF decay of 258Fm and 259Md.