R.J. Dougan

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.

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.

Spontaneous fission properties of 252,254No and 256,258[104] and the disappearance of the outer fission barrier

Abstract We have measured the mass and total kinetic energy distributions from the spontaneous fission of 252No, 254No, 256[104] and 258[104]. The results, in combination with earlier measurements for 256No, 258No and 262No, show a sharp transition from asymmetrical mass division in 256No to symmetrical division for 258No and 262No. However, all isotopes of element 104, including 260[104], appear to yield broadly symmetrical mass distributions. The total kinetic energies around 200 MeV for the element 104 isotopes indicate that they fission by the low-energy mode of bimodal fission. Recent calculations of static potential energy surfaces including higher-order asymmetric deformations suggest that the outer fission barrier is well below the ground state in energy, which accounts for the observance of the symmetric mass division.

Intermediate structure in the fission cross sections of the even curium isotopes

Abstract The neutron-induced fission cross sections of Cm-244, Cm-246, and Cm-248 which were measured with the RINS lead spectrometer system at RPI have been analyzed for intermediate structure. Gross structure is observed in Cm-244 near 200, 400, 800, and 2000 eV and in Cm-246 near 350, 800, and 3300 eV. For Cm-248 a very broad type of structure is observed near 10 keV. Assuming double-barrier fission, the Cm-244 and Cm-246 structure is consistent with the outer potential well lying 2 to 3 MeV above the inner potential well. The broad structure observed in Cm-248 fission implies significantly different barrier parameters than for Cm-244 or Cm-246.

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.

Fission Neutron Multiplicity for the 242 m Am( n,f ) Reaction

The fission neutron multiplicity, ν, of 242mAm(n,f) was measured relative to that of 235U(n,f) using the neutron time-of-flight facility at the Lawrence Livermore National Laboratory 100-MeV elect...

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.