The 198Au βhalf-life in the metal Au⋆

For the β^--decay of ¹⁹⁸Au in a Au metallic environment the half-life was observed to be longer by 0.4±0.7% at room temperature (T = 293K) and by 4.0±0.7% when the metal was cooled to T = 12K, both compared to the literature value of T _1/2 = 2.6943±0.0008d.     

Alpha-decay half-life of <Superscript>221</Superscript>Fr in different environments

A possible dependence of the alpha decay process on the solid-state environment of the decaying nucleus has been investigated by measuring the alpha-decay half-life of ²²¹Fr at room temperature in different materials ( Au, W, Si and polyethylene). The change in half-life between the insulator Si and the metals Au/ W is found to be 0.30(17)% / 0.42(21)%, respectivly. Furthermore an improved value of 4.79(2)min was found for the absolute half-life of ²²¹Fr.     

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.     

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     

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     

Super-allowed α decay above doubly-magic <Superscript>100</Superscript>Sn and properties of <Superscript>104</Superscript>Te = <Superscript>100</Superscript>Sn ⊗ α

α-decay half-lives for ^{104, 105, 106}Te and ^{108, 109, 110}Xe close above the doubly-magic ¹⁰⁰Sn are calculated from systematic double-folding potentials. The derived α preformation factors are compared to results for ^{212, 213, 214}Po and ^{216, 217, 218}Rn above the doubly-magic ²⁰⁸Pb. α-decay energies of Q _α = 5.42±0.07MeV and 4.65±0.15MeV are predicted for ¹⁰⁴Te and ¹⁰⁸Xe; the corresponding half-lives are T _1/2 ≈ 5ns for ¹⁰⁴Te and of the order of 60μs for ¹⁰⁸Xe. Additionally, the properties of rotational bands in ¹⁰⁴Te are analyzed, and the first excited 2⁺ state in ¹⁰⁴Te is predicted at E _x = 650±40keV; it decays preferentially by γ emission with a reduced transition strength of 10 Weisskopf units to the ground state of ¹⁰⁴Te and with a minor branch by α emission to the ground state of ¹⁰⁰Sn.     

(EC + β<Superscript>+</Superscript>) decays of the 11/2<Superscript>-</Superscript> isomer and 1/2<Superscript>+</Superscript> ground state of <Superscript>143</Superscript>Dy

The 1/2⁺ ground state and a 11/2^- isomer of very neutron-deficient isotope ¹⁴³Dy were produced by irradiation of an enriched target of ¹⁰⁶Cd with ⁴⁰Ca and studied by using a helium-jet fast tape-transport system in combination with proton-γ, X-γ and γ-γ coincidence measurements. A simple ( EC + β⁺) decay scheme of ^143mDy with a half-life of 3.0(3) s and a tentative ( EC + β⁺) decay scheme of ^143gDy with a half-life of 5.6(10) s are proposed. As a by-product, the 347- and 545-keV γ transitions in ¹³⁸Sm following the β-delayed proton emission of ¹³⁹Gd decay and the 323-keV γ transition in ¹³⁹Eu following the β-delayed proton emission of ¹⁴⁰Tb decay could be observed for the first time. Received: 20 August 2002 / Accepted: 28 October 2002 / Published online: 11 February 2003 RID="a" ID="a"e-mail: xsw@ns.lzb.ac.cn Communicated by D. Schwalm     

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.     

First observation of medium-spin excitations in the <Superscript>138</Superscript>Cs nucleus

The Penning trap mass spectrometer JYFLTRAP, coupled to the Ion Guide Isotope Separator On-Line (IGISOL) facility at Jyv?skyl?, was employed to measure the atomic masses of neutron-rich ^70-73Ni and ^{73, 75}Cu isotopes with a typical accuracy less than 5keV. The mass of ⁷³Ni was measured for the first time. Comparisons with the previous data are discussed. Two-neutron separation energies show a weak subshell closure at ⁶⁸ ₂₈Ni₄₀ . A well established proton shell gap is observed at Z = 28 .     

Stable collective triaxial rotation in <Superscript>77</Superscript>Kr

Based on the γ -spectroscopic analysis of G.N. Sylvan et al., configuration-dependent cranked Nilsson-Strutinsky calculations have been performed. They describe the nucleus ⁷⁷Kr globally in good agreement with the experimental findings and the expectations derived from single-particle orbitals in a deformed potential. Within these calculations, the unambiguous assignment of a configuration with three protons and five neutrons in the g _9/2 shell to the high-spin Yrast states of negative parity indicates that the nucleus ⁷⁷Kr has stable triaxial deformation and rotates about the intermediate axis. This not only disproves the recent interpretation of an occupation of the d _5/2 orbital being responsible for the strong deformation in this band, but also represents one of the first examples in this mass region for such a rotation.     

High-precision measurement of the 14O half-life using a mass-separated radioactive beam

A high-precision measurement of the ¹⁴O half-life has been performed using a mass-separated radioactive beam in combination with a germanium detector set-up. This is the first ¹⁴O half-life measurement with a contamination-free source. The final result of 70.560 ± 0.049 seconds is in agreement with the generally adopted mean value. Received: 5 April 2001 / Accepted: 6 August 2001     

(EC + β+) decay of 133Sm and 1.1 s isomer of 133Pr

The very neutron-deficient isotope ¹³³Sm was produced by irradiation of an enriched target of ⁹⁶Ru with ⁴⁰Ca and studied by using a helium-jet fast tape transport system in combination with X-γ and γ-γ coincidence measurements. A simple ( EC + β⁺) decay scheme of ¹³³Sm was proposed for the first time. As a by-product ¹³³Pr was produced simultaneously because of the ^98-102Ru content of the ruthenium target. The half-life of a 11/2^- low-lying isomeric state in ¹³³Pr was measured to be 1.1±0.2 s. Received: 22 March 2001 / Accepted: 18 July 2001     

Multiple particle break-up study of low excited states in <Superscript>9</Superscript>Be: The ghost peak in the <Superscript>8</Superscript>Be excitation energy spectrum visited

The aim of this work is to find out the origin of the anomalous resonance in ⁸Be seen in the reactions through excited states in ⁹Be. We have populated the ⁹Be excited states by β-decay of ⁹Li. Energy and direction of the two α particles has been detected and the neutron spectra reconstructed. In our work we identified the “anomalous resonance" in ⁸Be observed in several reaction studies as coming from the decay of the 2.43 and 2.78 MeV states in ⁹Be. This anomalous resonance appears when the two detected α particles are assumed to form a resonance in ⁸Be. We argued that the main decaying channels for these two levels in ⁹Be do not involve ⁸Be.     

Nonmesonic decay of the Λ-hyperon in hypernuclei produced by p + Au collisions

The β-delayed proton precursor ¹⁴⁹Yb was synthesized in the reaction ¹¹²Sn(⁴⁰Ca, 3n) and identified by using a proton-gamma coincidence measurements in combination with a helium-jet fast tape transport system. Its β-delayed proton spectrum was observed. The half-life of ¹⁴⁹Yb was determined to be 0.7±0.2 s. The 253, 101, and 365 keV γ transitions in ¹⁴⁷Dy, following β-delayed proton emission of ¹⁴⁸Ho decay, were reported for the first time. Received: 14 March 2001 / Accepted: 26 September 2001     

Gamow-Teller beta decay of 105Sn

The β-decay of ¹⁰⁵Sn was re-investigated at the ISOL facility of GSI Darmstadt by using a total absorption spectrometer. The experimental results include the half-life and β-delayed proton branching ratio of 32.7(5)s and 1.1(4) ^. 10^-4, respectively, and the contribution of electron capture to the ¹⁰⁵Sn decay of 0.420(35). The Gamow-Teller strength distribution peaking at 3.6MeV was measured, yielding a summed Gamow-Teller strength of 3.0(4). The latter data are discussed in comparison with shell-model predictions based on an empirical interaction.     

In-beam γ-ray spectroscopy of <Superscript>190</Superscript>Po: First observation of a low-lying prolate band in Po isotopes

Gamma rays from excited states of ¹⁹⁰Po have been observed using the Jurosphere Ge-detector array coupled to the RITU gas-filled separator. They were associated with a collective band which from spin 4 onwards resembles the prolate rotational bands known in the isotones ¹⁸⁸Pb and ¹⁸⁶Hg. This indicates that in ¹⁹⁰Po the prolate configuration becomes yrast above I = 2. The experimental results are interpreted in a two-band mixing calculation and are in agreement with α-decay data and potential energy surface calculations. Received: 10 January 2003 / Accepted: 19 February 2003 / Published online: 7 May 2003     

Change of the 7Be electron capture half-life in metallic environments

For the electron capture of ⁷Be in the metallic environments Pd and In the ⁷Be half-life was observed to increase by 0.9±0.2 and 0.7±0.2%, respectively, while in the insulator Li₂O it was unchanged within experimental error (all samples cooled to T = 12K). The observations are consistent with the predictions of the Debye plasma model applied to the quasi-free electrons in the metals.     

Is the weak interaction constant really constant?

A comparison is made of the probability of the process of two neutrino double-beta decay for ⁸²Se and ⁹⁶Zr in direct (counter) and geochemical experiments. The experimental data for ¹³⁰Te are also analyzed. It is shown that the probability is systematically lower in geochemical experiments, which characterize the probability of decay a few billions years ago. In addition geochemical measurements on young minerals give lower values of T (¹³⁰Te) as compared to measurements on old minerals. It is proposed that this could be due to a change in the weak interaction constant with time. The possibilities of new precise measurements to be performed with the aid of counters and geochemical experiments are discussed. A new geochemical experiment with ¹⁰⁰Mo is proposed. Received: 24 February 2000 / Accepted: 4 March 2000     

Decay study of neutron-rich zirconium isotopes employing a Penning trap as a spectroscopy tool

A new technique to produce isobarically pure ion beams for decay spectroscopy by using a gas-filled Penning trap was commissioned at the ion guide isotope separator on-line facility, IGISOL. β-decays of neutron-rich ¹⁰⁰Zr, ¹⁰²Zr and ¹⁰⁴Zr isotopes were studied with this technique. In addition, the values of ^100,102,104Zr β-decays were determined from the direct mass measurements of zirconium and niobium isotopes performed with a high-precision Penning trap. The mass of ¹⁰⁴Nb was directly measured for the first time and the obtained mass excess value for the longer-living (1⁺) state is -71823±10 keV. For the ground states of ¹⁰⁰Nb and ¹⁰²Nb the obtained mass excess values were -79802±20 keV and -76309±10 keV, respectively. The observed distribution of the β strength supports a prolate deformation assignment for ^100,102,104Zr isotopes.     

Nuclear deformation and the two-neutrino double- β decay in <Superscript>124, 126</Superscript>Xe , <Superscript>128, 130</Superscript>Te , <Superscript>130, 132</Superscript>Ba and <Superscript>150</Superscript>Nd isotopes

The two-neutrino double-beta decay of ^{124, 126}Xe , ^{128, 130}Te , ^{130, 132}Ba and ¹⁵⁰Nd isotopes is studied in the Projected Hartree-Fock-Bogoliubov (PHFB) model. Theoretical 2ν β^-β^- half-lives of ^{128, 130}Te , and ¹⁵⁰Nd isotopes, and 2ν β⁺β⁺ , 2ν β⁺ EC and 2ν ECEC for ^{124, 126}Xe and ^{130, 132}Ba nuclei are presented. Calculated quadrupolar transition probabilities B(E2 : 0⁺ → 2⁺) , static quadrupole moments and g -factors in the parent and daughter nuclei reproduce the experimental information, validating the reliability of the model wave functions. The anticorrelation between nuclear deformation and the nuclear transition matrix element M _2ν is confirmed.