The half-life of <Superscript>198</Superscript>Au : High-precision measurement shows no temperature dependence

We have measured the half-life of the β -decay of ¹⁹⁸Au in a metallic environment, both at low temperature (19K), and also at room temperature. We find the half-lives at both temperatures to be the same within 0.04%, a finding that contradicts a recent report of a 3.6±1.0 % difference in the ¹⁹⁸Au half-life when measured at essentially the same two temperatures. Our results for the half-life, 2.6949±0.0009 d at room temperature and 2.6953±0.0008 d at 19K, also agree well with previous precision room temperature measurements.     

Assignment of levels in <Superscript>208</Superscript>Fr and 10<Superscript>-</Superscript> isomers in the odd-odd isotones <Superscript>206</Superscript>At and <Superscript>208</Superscript>Fr

Excited states in ²⁰⁸Fr have been identified using the ¹⁹⁷Au(¹⁶O, 5n)²⁰⁸Fr reaction and a variety of time-correlated g \gamma -ray and conversion electron spectroscopic techniques. Transitions above and below a t \tau = 623(16) ns 10^- isomer are placed in the level scheme. This isomer is analogous to that observed in the odd-odd isotone ²⁰⁶At for which additional spectroscopic information is also obtained, including a precise lifetime of t \tau = 1173(30) ns. The g \gamma -rays assigned to ²⁰⁸Fr are the same as the main transitions erroneously assigned to ²⁰⁹Fr in previous work.     

Investigations into the alpha-decay of <Superscript>195</Superscript>At

The low-energy nuclear structure and decay properties of the neutron-deficient isotopes ¹⁹⁵At and ¹⁹¹Bi have been studied. ¹⁹⁵At was produced in the reaction ¹⁴²Nd(⁵⁶Fe,p2n)¹⁹⁵At and ¹⁹¹Bi as the daughter activity of ¹⁹⁵At. The activities were implanted in a position-sensitive silicon detector after being separated from the primary beam by a gas-filled recoil separator. The 1/2⁺ intruder state was determined to be the ground state in ¹⁹⁵At with an alpha-decay energy of E _α = 6953(3) keV and a half-life T _1/2 = 328(20) ms. Another state with an alpha-decay energy E _α = 7075(4) keV and a half-life T _1/2 = 147(5) ms was found to decay to a 148.7(5) keV excited state in ¹⁹¹Bi for which a spin and parity of 7/2^- were deduced. Consequently, the same 7/2^- character was assigned to the initial state at 32(7) keV in ¹⁹⁵At on the basis of unhindered alpha-decay. The 9/2^- state, being the ground state in heavier odd-mass astatine isotopes, was not observed. Received: 16 September 2002 / Accepted: 19 December 2002 / Published online: 25 March 2003 RID="a" ID="a"e-mail: heikki.kettunen@phys.jyu.fi RID="b" ID="b"Present address: Laboratory of Radiochemistry, P.O. Box 55, FIN-00014, University of Helsinki, Finland. RID="c" ID="c"Present address: Radiation and Nuclear Safety Authority, P.O. Box 14, FIN-00881 Helsinki, Finland. Communicated by W. Henning     

On the possibility of accelerating α decay: To the problem of disposing of radioactive nuclear waste

A number of experimental tests of the recent information on the possibility of accelerating α decay by implanting α-emitting isotopes into metals with their further cooling to a low temperature were performed using the nuclear orientation technique. The measurement data on half-lives for ²⁵³Es in Fe at 4 K and 50 mK, ²²⁴Rn, ²²⁵Ra, and ²²⁷Ac in Fe at temperatures down to 25 mK, and ²²¹Fr in nonmetal (Si) and metal (Au) at 4 K and ∼20 mK are reviewed. In all cases, no change in half-life was found within an accuracy of 1%.     

Mass distribution studies in the <Superscript>19</Superscript>F + <Superscript>197</Superscript>Au reaction

Mass distribution and evaporation residue measurements have been carried out in the reaction ¹⁹F + ¹⁹⁷Au using the recoil catcher technique followed by off-line γ-ray spectrometry. The random neck rupture model (RNRM) has been used to compute the variance of the mass distribution ( σ²_A) and the average kinetic energy ( ˉ) of the fission fragments for the present system. The results of model calculations have been found to be in good agreement with the experimental observations. Measured evaporation residue cross-sections have been compared with the statistical model calculations.     

Inelastic excitation of <Superscript>187</Superscript>Re

Excited states in ¹⁸⁷Re populated by inelastic scattering of a 500 MeV ⁸²Se beam have been studied by means of in-beam γ-ray spectroscopy. Levels built on the 5/2⁺[402] and 9/2^-[514] one-quasiparticle states were measured up to I ^π = (21/2⁺) and (21/2^-), respectively. In addition, several new levels including an isomer at 1682 keV with a half-life of 114(23) ns have been found. Quasiparticle configurations of the levels and the transition rates are discussed. Received: 12 December 2002 / Accepted: 13 March 2003 / Published online: 13 May 2003     

Source velocity at relativistic beams of <Superscript>4</Superscript>He

The source velocities (β = ν/c) extracted from rapidity plots of the fragment invariant probability distribution in terms of the longitudinal versus transversal velocity components has been studied for ⁴He + Au collisions at 4 and 14.6 GeV. It was found transition from broad range source velocities distribution in case of ⁴He(4 GeV) + Au to fixed source velocity in case of ⁴He(14.6 GeV) + Au.     

Near-barrier neutron transfer in reactions <Superscript>6</Superscript>He + <Superscript>45</Superscript>Sc, <Superscript>64</Superscript>Zn,and <Superscript>197</Superscript>Au

Experimental cross sections of formation of isotopes ⁴⁶Sc (in reaction ⁶He + ⁴⁵Sc), ^196,198Au (in reaction ⁶He + ¹⁹⁷Au), and ⁶⁵Zn (in reaction ⁶He + ⁶⁴Zn) are analyzed. The time-dependent Schrödinger equation for the outer neutrons of ⁶He and ¹⁹⁷Au nuclei is solved numerically to calculate the probability of neutron transfer and transfer cross sections. In reaction ⁶He + ¹⁹⁷Au, the contribution of fusion and subsequent evaporation to experimental data can be neglected, while the corresponding contributions to reactions ⁶He + ⁴⁵Sc and ⁶He + ⁶⁴Zn are considerable. Fusion–evaporation is taken into account using the computational code of the NRV knowledge base. The results of calculations are in satisfactory agreement with the experimental data.     

Investigating <Superscript>64</Superscript>Zn<Superscript>+</Superscript> ion-doped silicon under conditions of hot implantation

The results from visualizing the structure and identifying the composition of surface and the nearsurface layers of CZ n-Si (100) implanted by ⁶⁴Zn⁺ ions with dose of 5 × 10¹⁶ cm^–2 and energy of 50 keV under conditions of a substrate heated to 350°C are presented. It is found that there is no Si amorphization after Zn implantation, and only one layer 200 nm thick forms and is damaged because of radiation-induced defects. Zn nanoparticles 10–100 nm in size are found on a sample’s surface and in its near-surface layer. Computer analysis and mapping of the elemental and phase composition of FIB crater walls and the surface show that the main elements (54%) in the sample near-surface layer are Si, O, and Zn. The presence of ZnO phase is recorded to a depth of 20 nm in the sample.     

Magneto-optical trap and mass-separator system for the ultra-sensitive detection of <Superscript>135</Superscript>Cs and <Superscript>137</Superscript>Cs

We report the first magneto-optical trapping of radioactive ¹³⁵Cs and ¹³⁷Cs and a promising means for detecting these isotopes at ultra-sensitive levels by coupling a magneto-optical trap (MOT) to a mass separator. A sample containing both isotopes was placed in the source of a mass separator, ionized, mass-separated, and implanted in a Zr foil within the trapping cell. After implantation, atoms were released from the foil by inductive heating and then captured in a MOT that used large-diameter beams and a dry-film-coated cell to achieve high trapping efficiency. Trapped-atom numbers in the case of either isotope ranged from 10⁴ to 10⁷, as determined from the MOT fluorescence signal. Over this trapped-atom range, the MOT fluorescence signal was found to increase linearly with the number of atoms implanted in the foil and without isotopic bias to within 4%. In principle, this method can then provide a measurement of the ¹³⁷Cs/¹³⁵Cs ratio accurate to within 4% through the direct ratio of MOT fluorescence signals. The fluorescence signal from stable ¹³³Cs, when implanted and released from the foil, was suppressed relative to MOT signals by more than seven orders of magnitude when the system was tuned to trap ¹³⁵Cs or ¹³⁷Cs. When combined with the isotopic selectivity of ≥10⁵ for the mass separator, the overall suppression of ¹³³Cs is expected to exceed 10¹². At present our system delivers atoms from sample to MOT with an efficiency of 0.5%, has a trapped-atom detection limit of 4000 atoms, and achieves a sample-detection sensitivity of one million atoms. Received: 23 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +1-505/667-0440, E-mail: mdd@lanl.gov     

Distribution of <Superscript>28</Superscript>Si, <Superscript>29</Superscript>Si,and <Superscript>30</Superscript>Si isotopes under plastic deformation in subsurface layers of Si: B crystals

The redistribution of ²⁸Si, ²⁹Si, and ³⁰Si isotopes in subsurface layers of Si: B single crystals after their plastic deformation has been revealed. It has been found that the distribution profile of ²⁸Si and ²⁹Si isotopes becomes smoother after deformation, whereas the ³⁰Si isotope distribution remains unchanged. A change in the subsurface profile of the ²⁹SiO oxide is observed, which indicates the migration of the ²⁹Si isotope in the composition of oxygen complexes during plastic deformation.     

Different formulations of <Superscript>3</Superscript>He and <Superscript>3</Superscript>H photodisintegration

Different momentum space Faddeev-like equations and their solutions for the radiative pd-capture and the three-nucleon photodisintegration of ³He are presented. Applications are based on the AV18 nucleon-nucleon and the Urbana IX three-nucleon forces. Meson exchange currents are included using the Siegert theorem. A very good agreement has been found in all cases indicating the reliability of the used numerical methods. Predictions for cross-sections and polarization observables in the pd-capture and the complete three-nucleon breakup of ³He at different incoming-deuteron/photon energies are presented.     

On the unusual properties of the 282 keV state in <Superscript>135</Superscript>Sb

Recently the first excited state in ¹³⁵Sb has been observed at the unexpectedly low excitation energy of only 282keV and interpreted as mainly d _5/2 proton coupled to the ¹³⁴Sn core. Based on theoretical considerations it was suggested that its low excitation energy is related to a relative shift of the proton d _5/2 and g _7/2 orbits induced by the neutron excess. We have measured the lifetime of the 282keV state by the advanced time-delayed βγγ(t) method. The measured half-life, T _1/2 = 6.1(4)ns, yields exceptionally low limits of B(M1;5/2₁ ⁺→7/2₁ ⁺)≤3.0×10^-4 μ ² _N and B(E2;5/2₁ ⁺→7/2₁ ⁺)≤54e ² fm ⁴. These strongly hindered M1 and slow E2 transition rates are similar to those for the transition de-populating the first excited state at 405keV in ²¹¹Bi. Results of shell model calculations with realistic interactions are presented. The M1 decay rate was found to be extremely sensistive both to the wave function and to the M1 effective operator.     

Nuclear quadrupole moments of 5/2<Superscript>-</Superscript> and 9/2<Superscript>-</Superscript> states in <Superscript>169</Superscript>Ta

The nuclear spectroscopic quadrupole moments for the πh_9/25/2^-, 1/2^-[541] and the πh_11/29/2^-, 9/2^-[514] isomeric states in ¹⁶⁹Ta have been measured employing the time differential perturbed angular-distribution technique following the nuclear reaction ¹⁵⁹Tb(¹⁶O, 6nγ)¹⁶⁹Ta at beam energy 104 MeV. The ratio of the intrinsic quadrupole moments has been derived as 1.87(13) from the measured quadrupole precession frequencies of the corresponding states. The model-independent analysis of the equilibrium deformation indicates strong prolate- and oblate-driving nature of the 1/2^-[541] and 9/2^-[514] orbitals in ^169,171Ta isotopes, respectively.     

<Superscript>29</Superscript>Si, <Superscript>27</Superscript>Al, <Superscript>1</Superscript>H and <Superscript>23</Superscript>Na MAS NMR Study of the Bonding Character in Aluminosilicate Inorganic Polymers

²⁹Si, ²⁷Al, ¹H and ²³Na solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to relate nominal composition, bonding character and compressive strength properties in aluminosilicate inorganic polymers (AIPs). The ²⁹Si chemical shift varies systematically with Si-to-Al ratio, indicating that the immediate structural environment of Si is altering with nominal composition. Fast ¹H MAS and ²⁹Si T _SiH/T _1ρ relaxation measurements demonstrated that occluded pore H₂O mobility within the disordered cavities is slow in comparison with H₂O mobility characteristics observed within the ordered channel structures of zeolites. The ²⁷Al MAS NMR data show that the Al coordination remains predominantly 4-coordinate. In comparison with the ²⁹Si MAS data, the corresponding ²⁷Al MAS line shapes are relatively narrow, suggesting that the AlO₄ tetrahedral geometry is largely unperturbed and the dominant source of structural disorder is propagated by large distributions of Si–O bond angles and bond lengths. Corresponding ²³Na MAS and multiple-quantum MAS NMR data indicate that Na speciation is dominated by distributions of hydration states; however, more highly resolved ²³Na resonances observed in some preparations supported the existence of short-range order. New structural elements are proposed to account for the existence of these Na resonances and an improved model for the structure of AIPs has also been proposed. Authors' address: John V. Hanna, NMR Facility, Institute of Materials and Engineering Science, Lucas Heights Research Laboratories, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia     

The beta-decay scheme of <Superscript>232</Superscript>Fr and the K = 0 ground-state band in <Superscript>232</Superscript>Ra

The beta-decay of ²³²Fr to excited states in ²³²Ra has been studied using gamma-gamma coincidence detection combined with the isotope separator on-line technique at the ISOLDE PSB facility at CERN. Earlier findings are confirmed and three new gamma lines are reported. In addition to the beta-decay characteristics of ²³²Fr, the K = 0 ground-state band in ²³²Ra is identified. A yield survey of neutron-rich Fr isotopes, important also for the EURISOL project, is incorporated.Received: 20 April 2004, Revised: 12 May 2004, Published online: 13 July 2004PACS: 21.10.-k Properties of nuclei; nuclear energy levels - 23.20.-g Electromagnetic transitions - 28.60. + s Isotope separation and enrichment - 29.25.Rm Sources of radioactive nuclei     

Structure of low-lying <Superscript>12</Superscript>C resonances

The hyperspherical adiabatic expansion is combined with complex scaling and used to calculate low-lying nuclear resonances of ¹²C in the 3α model. We use Ali-Bodmer potentials and compare results for other potentials α-α with similar ⁸Be properties. A three-body potential is used to adjust the ¹²C resonance positions to desired values extending the applicability of the method to many-body systems decaying into three α-particles. For natural choices of three-body potentials we find 14 resonances below the proton separation threshold, i.e. two 0⁺, three 2⁺, two 4⁺, one of each of 1^±, 2^-, 3^±, 4^-, and 6⁺. The partial-wave decomposition of each resonance is calculated as a function of the hyperradius. Strong variation is found from small to large distance. The connection to previous experimental and theoretical results is discussed and agreements as well as disagreements are emphasized.     

Double-beta decay of <Superscript>100</Superscript>Mo to 0<Superscript>+</Superscript> excited states in <Superscript>100</Superscript>Ru

A brief review on the 2νββ decay of ¹⁰⁰Mo to the 0⁺ excited state in ¹⁰⁰Ru is performed. A weighted-average half-life value for the decay has been obtained, T_1/2 = (6.8 ± 1.2) × 10²⁰ yr. The corresponding average value for the nuclear matrix element was extracted, 0.095 ± 0.009.     

Search for superheavy hydrogen isotopes <Superscript>6</Superscript>H and <Superscript>7</Superscript>H in stopped π-absorption reactions

Experimental search for superheavy hydrogen isotopes ⁶H and ⁷H was performed in the stopped pion absorption on ⁹Be and ¹¹B nuclei. The structures in the missing-mass spectra were observed in the reaction channels ⁹Be(π^-, pd )X and ¹¹B(π^-, p⁴He)X . Four states of ⁶H were proposed. Evidences for ⁷H formation were obtained in the reaction channels ⁹Be(π^-, pp)X and ¹¹B(π^-, p³He)X .