R.J. Dupzyk

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.

Lifetime of the long-lived isomer of 236Np from α-, β- and electron-capture decay measurements

Abstract The half-life of long-lived 236Np, due to α, β and electron-capture decay, was found to be 1.55 × 105 yr. Of all decays, 88% populate excited states in 236U and 12% populate levels in 236Pu. Lifetimes measured by growth of the ground states of 236U and 236Pu agree with values from corresponding γ de-excitations in these daughter nuclei. Therefore, nearly all the electron-capture decays populate the 6+ level of the ground-state band in 236U. Similarily, essentially all the β− decay populates an analogous 6+ level in 236Pu, which de-excites through a previously unreported transition of 158.3 keV. If a very weak γ-ray at 894 keV can be ascribed to a level in 232U populated by β decay of 232Pa, its existence establishes a 0.2% α-branching decay in 236Np.

Alpha decay of 255Md, 256Md and 257Md

Abstract The α-decay of 255Md,256Md and257Md has been studied following production of these isotopes in a series of helium-ion bombardments of an einsteinium target. The mendelevium was collected as the atoms recoiled from the target during bombardment. Following chemical purification the mendelevium was separated into various mass fractions in an electromagnetic isotope separator. The α-particle emission of each fraction was examined with the following results: 255 Md ,T 1 2 = 27±3 min , α/total = 0.100±0.014, Eα = 7.340±0.005 MeV ; 256 Md , T 1 2 = 77±5 min , α/total = 0.099±0.005, Eα = 7.16±0.015 MeV (16±2 %), Eα = 7.23±0.01 MeV (63 ±4%), Eα = 7.33±0.03 MeV (4±1 %), Eα = 7.46±0.03 MeV (5±1 %), Eα = 7.49±0.02 MeV (6±1 %), Eα = 7.67±0.03 MeV (2±1 %), Eα = 7.72±0.02 MeV (4±1 %) ; 257 Md , T 1 2 = 300±20 min , α/total MeV . The observed α-decay rates are compared with calculated values derived from α-decay theory. Lower limits have been determined for the partial spontaneous fission half-lives:255Md > 12 d,256Md > 2 d, 257Md > 6 d.

Nuclear decay properties of 256Fm

Abstract The nuclide 256 Fm was produced in 4 He ion bombardment of a 3μg 253 Es target. With samples where the fermium fraction was isolated chemically, the mass 256 was enriched in an electromagnetic isotope separator. The nuclear decay properties of 256 Fm have been measured as follows: The half-life is 157±2 min. The alpha branching ratio is ζ /( ζ +SF) = 0.081±0.003, and the alpha-particle energy of the most abundant group is 6.925±0.005 MeV. The measured alpha-decay rate shows satisfactory agreement with a calculated value derived from theory. The cross section for the reaction 255 Fm(n, γ) 256 Fm with well-modulated reactor neutrons has been measured and found to be 30±4 b.

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.

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.