Alpha-decay widths of neutron-deficient francium and astatine isotopes

The use of mass-separated samples of neutron-deficient francium isotopes, produced at the ISOLDE on-line mass-separator facility at the CERN 600 MeV synchro-cyclotron, has permitted precise determinations through genetic relationships of /(EC+β⁺) branching ratios for a range of light francium and astatine isotopes. The experiments verify previous mass assignments and provide improved values for the half-lives. The systematics of -widths and of β-strength functions is surveyed.     

Alpha-decay characteristics of neutron-deficient 190Po, 189Bi and 186Pb isotopes

The neutron-deficient ¹⁹⁰Po and ¹⁸⁹Bi isotopes have been produced in the 2n respectively p2n channels of the ⁴⁸Ti +¹⁴⁴ Sm reaction. Cross sections for different reaction channels are given. For ¹⁹⁰Po, improved values of α-energy E_α = 7550(15) keV and half-life T_1/2 = 1.9 _?0.4 ^+0.6 ms were measured. By using correlated α-decay chains an α-branching ratio of 0.50(25) was deduced for ¹⁸⁶Pb. The reduced α-width systematic is extended for both of these isotopes, confirming a persistence of the saturation towards the lighter Po isotopes and the absence of any anomalies for the neutron-deficient Pb isotopes. For ¹⁸⁹Bi improved values of E_α = 7300(15) keV and T_1/2 = 4.8(5) ms were measured.     

Measurement of isotope shift and hyperfine splitting of190, 191, 193, 197Pb isotopes by collinear laser spectroscopy

We report here on the measurement of isotope shift and hyperfine splitting of^{190, 191, 193, 197}Pb for the 723 nm atomic optical transition. Detailed analysis of the optical data has been done by combining them with the available muonic and electronicx-ray isotope shift data. The magnetic dipole moments and the electric quadrupole moments of the odd isotopes have been extracted from the hyperfine coupling constants of the atomic states involved in the optical transition used.     

Nuclearg-factors of the first excited states in Ir191 and Ir193

Mössbauer experiments were performed to determine the nuclearg-factor of the 82 keV 1/2⁺ state in Ir¹⁹¹ (g=+1.083±0.009) and the 73 keV 1/2⁺ state in Ir¹⁹³ (g=+0.9400±0.0019). TheE2/M1 mixing parameters of the corresponding (1/2⁺) (E2/M1) (3/2⁺) transitions were found to be ¦δ¦ (Ir¹⁹¹, 82 keV)=0.80±0.06 andδ(Ir¹⁹³, 73 keV)=?0.558±0.005.     

The low-energy level structure and the electromagnetic properties of191Ir and193Ir

TheM 1 andE 2 reduced transition probabilities in¹⁹¹Ir and¹⁹⁹Ir have been calculated on the weak particle-core coupling model. The static moments of the ground state and the first three low-lying excited states have been computed. The theoretical values are in good agreement with the experimental results, in particular, the existing discrepancy between the theoretical and the experimental value for the magnetic moment of the first excited 5/2⁺ state, in both these nuclei, has been removed. The predicted sign of the quadrupole moment of the 2⁺ state in the neighbouring even-even nuclei is consistent with the results reported for Platinum nuclei. The level structure of both these nuclei has been calculated assuming the particle-core interaction to be dipole-dipole and quadrupole-quadrupole type.     

Alpha-decay fine structures of U isotopes and systematics for isotopic chains of Po and Rn

A model of the cluster radioactivity of even-even nuclei is presented. In this model, the zero-point vibrations in the charge-asymmetry coordinate determine the cluster formation (spectroscopic factor), while the tunneling in the coordinate of the relative separation of the centers of mass of the cluster and the daughter nucleus determines the penetrability of the barrier of the nucleus-nucleus potential. The quality of the model is demonstrated for describing cluster decay half-lives and the fine structure in alpha decays of even-even U isotopes. The model is applied to describe the alpha decays in the isotopic chains of Po, Rn, and U. The correspondence of the calculated half-lives to the Geiger-Nuttall law is discussed.     

Alpha decay studies of new neutron-deficient francium isotopes and their daughters

Neutron-deficient francium isotopes were produced by proton-induced spallation reactions and investigated by alpha spectroscopy following on-line mass separation. The new isotopes^{201, 202}Fr were identified and their half-lives measured. An improved value for the half-life of²⁰³Fr was obtained. Alpha branching ratios were determined for^{198m, g, 199}At. The derivation of the mass excess of²⁰²Fr and the systematics of alpha energies and reduced widths are discussed.     

Isotope shifts of neutron-deficient gold isotopes with 193≧A≧190

The isotope shift between¹⁹⁷Au (stable) and the radioactive Au isotopes¹⁹⁰Au,¹⁹¹Au,¹⁹²Au and¹⁹³Au were determined by resonance fluorescence spectroscopy in the 6s ² S _1/2–6p ² P _1/2.λ=267.6nm line. The unstable Au isotopes were produced at the ISOLDE mass separator at CERN. The nuclei were investigated semi on-line in a resonance vessel, heated to 1,400°C. The results areδν ^190,197=?11.12(39) GHz,δν ^191,197=?9.67(12) GHz,δν ^192,197=?8.32(15) GHz,δν ^193,197=?6.29(11) GHz, corresponding to a change of the mean-square charge radius byδ〈r ²〉^190,197 =0.261(12) fm²,δ〈r ²〉^191,197=0.227(5) fm²,δ〈r ²〉^192,197=0.195(5) fm²,δ〈r ²〉^193,197 =0.148(4) fm².     

Hyperfine structure investigations in the4F9/2 atomic ground state of191Ir and193Ir

The5d ⁷6s^{2 4}F_9/2 atomic ground state of¹⁹¹Ir and¹⁹³Ir has been studied using the atomic-beam magnetic-resonance method. The results are:¹⁹³Ir:g _J(⁴F_9/2)=1.29694 (3)¹⁹¹Ir:Δv(⁴F_9/2; F=6?F=5)=659.26496 (12) MHzΔv( ⁴F_9/2; F=5?F=4)=189.44002 (09) MHzΔv( ⁴F_9/2; F=3?F=4)=84.05040 (80) MHzA=57.52148 (04) MHzB=471.20425 (57) MHzC=?0.020 (30) kHz¹⁹³Ir:Δv( ⁴F_9/2; F=6?F=5)=660.09043 (12) MHzΔv( ⁴F_9/2; F=5?F=4)=224.47848 (13) MHzΔv( ⁴F_9/2; F=?F=4)=33.53453 (89) MHzA=62.65556 (05) MHzB=426.23546 (64) MHzC=0.020 (30) kHz Using the magnetic dipole moments known by NMR-technique [1] we obtained for the electric quadrupole moments as calculated from the hyperfine interaction constantsA andB:Q(¹⁹¹Ir)=0.78 (20) barns,Q(¹⁹³Ir)=0.70 (18) barns (uncorrected for core polarization effects). A calculation of the hyperfine anomaly yields:₁₉₁ Δ ₁₉₃=?0.00023 (10). The ratio of theB factors which should be the same as for the quadrupole moments turned out to be:B(¹⁹¹Ir)/B(¹⁹³Ir)=Q(¹⁹¹Ir)/Q(¹⁹³Ir)=1.105502(3).     

Alpha-decay of neutron-deficient 200Fr and heavier neighbours

A new measurement of the ²⁰⁰Fr -decay half-life, with improved accuracy compared to previous measurements, has been carried out at the Isolde-CERN on-line mass separator. A half-life of 49(4) ms has been obtained, which is substantially different from earlier literature values. For the ¹⁹⁶At daughter decay, a half-life of 389(54) ms and an branching ratio b = 94(5)% were measured. In addition, the half-lives of ^{201, 203, 205}Fr and ^197,199At are reported.     

Alpha-decay study of188Pb and180,182Hg

Theα decay of mass-separated¹⁸⁸Pb and^180,182Hg has been studied at the GSI on-line mass separator. Alpha singles spectra as well asα-X-t andα-e-t coincidence events were collected. Fine structure in theα decay of¹⁸⁸Pb was measured for the first time, feeding the 2₁ ⁺ and 0₂ ⁺ states in¹⁸⁴Hg. Theα decay of^180,182Hg feeds low-lying states in^176?178Pt. One of these states could be identified as a 0⁺ state. The deexcitation pattern could be established. As a by-product an alpha-decay scheme of¹⁸⁰Au towards levels in¹⁷⁶Ir is given.     

Structure studies of neutron-rich beryllium and carbon isotopes

The structure of neutron-rich beryllium isotopes has been investigated using different heavy-ion induced transfer reactions. A strong sensitivity to the chosen core nucleus is found for the cores of ⁹Be and ¹⁰Be, respectively. With a ⁹Be core, molecular rotational bands up to high excitation energies are observed due to the pronounced 2α-cluster structure, whereas only few states at low excitation energies are populated with a ¹⁰Be core. For ¹¹Be a detailed investigation has been performed for the three states at 3.41 MeV, 3.89 MeV and 3.96 MeV, which resulted in the most probable spin-parity assignments 3/2⁺, 5/2^? and 3/2^?, respectively. Furthermore we have studied particlehole states of ¹⁶C using the ¹³C(¹²C,⁹C)¹⁶C reaction and found 14 previously unknown states.     

五种新同位素的发现仅是开始

尽管某些同位素可以存活数小时、数周甚至数千年,但另一些同位素却以极快的速度闪现,以至于科学家无法确认它们的存在。天体物理学家认为,这些稀有的同位素粒子尽管寿命短暂,却在恒星的演化和宇宙重元素的形成中起着重要的作用。最近,密歇根州立大学稀有同位素束设施(FRIB)的研究人员已经制造并鉴别了五种从未见过的稀有同位素,这些同位素都包含大量的中子。

     

Alpha decay of the new isotopes 188,189Po

New neutron-deficient isotopes ^188,189Po have been produced in the complete fusion reaction of ⁵²Cr ions with a ¹⁴²Nd target at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of α-γ and α-conversion electron coincidence measurements and of α-α position and time correlations. In ¹⁸⁹Po a ground state to ground state α decay with E_α1= 7540(20) keV, T_1/2= 5(1) ms and two fine structure α-decays at E_α2= 7264(15) keV and E_α3= 7316(15) keV have been observed. In ¹⁸⁸Po (T_1/2= 400⁺²⁰⁰ ₋₁₅₀μs) a ground state to ground state α decay at E_α= 7915(25) keV and a fine structure α decay at E_α= 7350(40) keV have been found. Improved data on the α-decay of ¹⁸⁹Bi were obtained. Received: 15 October 1999     

The spin(6) scheme and electromagnetic transitions in 191Ir and 193Ir

The Coulomb excitation of natural Ir and Pt targets has been measured with protons and α-particles of energies between 5.0 and 6.0 MeV. Accurate γ-ray angular distributions were taken to determine the relative yields of observed γ-rays. Using ¹⁹⁴Pt as standard the reduced electric quadrupole transition probabilities to the ground states from Coulomb excited levels in the Ir and Pt isotopes were determined. Other B(E2) and B(M1) values were determined from the decay schemes obtained and from mixing ratios deduced from γ-ray angular distributions or from previously measured internal conversion coefficients. The detailed sets are compared with calcula- tions based on the Spin(6) scheme, which predicts several symmetries in ¹⁹¹Ir and ¹⁹³Ir. The possible existence of wider supersymmetry multiplets in this region of nuclei is examined on the basis of comparison of electromagnetic properties of Ir nuclei with those of Pt and Os isotopes.     

Alpha decay studies of neutron-deficient radium isotopes

Neutron-deficient isotopes of radium have been produced in the reactions ³⁵Cl + ¹⁷⁵Lu, ⁴⁰Ar + ^170,171Yb, and ³⁶Ar + ^170,171Yb and separated in-flight with a gas-filled recoil separator. The activities have been identified on the basis of correlated alpha decay chains. The alpha particle energy E_α and the half-life T_1/2 of an isomeric state of the new isotope, ^203mRa, have been determined to be (7615±20) keV and (33 _?10 ⁺²² ), respectively. An assignment of another decay with E_α = (7577±20) keV and T_1/2 = (1.1 _?0.5 ^+5.0 ) ms to ^203g Ra is made on the basis of one observed three-decay chain. Tentative evidence for the production of the new isotope ²⁰²Ra is also given. The values E_α = (7860±60) keV and T_1/2 = (0.7 _?0.3 ^+3.3 ) ms were measured. Improved decay data have been obtained for ²⁰⁴Ra. The results are E_α = (7484±10) keV and T_1/2 = (59 _?9 ⁺¹² ). Earlier findings concerning the decay of two states in ²⁰⁵Ra and an isomer in ²⁰⁷Ra have been confirmed. Approximate values for cross sections have been determined for ²⁰²Ra, produced using the ³⁶Ar + ¹⁷⁰Yb reaction and for ²⁰³Ra, ²⁰⁴Ra, and ²⁰⁵Ra using the ³⁵Cl + ¹⁷⁵Lu reaction. The results are 2 nb (²⁰²Ra), 4 nb (²⁰³Ra), 40 nb (²⁰⁴Ra), and 800 nb (²⁰⁵Ra).     

Alpha decay studies of neutron-deficient radium isotopes

Neutron-deficient isotopes of radium have been produced in the reactions³⁵Cl+¹⁷⁵Lu,⁴⁰Ar+^170,171Yb, and³⁶Ar+^170,171Yb and separated in-flight with a gas-filled recoil separator. The activities have been identified on the basis of correlated alpha decay chains. The alpha particle energy E_α and the half-life T_1/2 of an isomeric state of the new isotope,^203m Ra, have been determined to be (7615±20) keV and (33 _?10 ⁺²² ) ms, respectively. An assignment of another decay with E_α = (7577±20) keV and T_1/2=(1.1 _?0.5 ^+5.0 ) ms to^203g Ra is made on the basis of one observed three-decay chain. Tentative evidence for the production of the new isotope²⁰²Ra is also given. The values E_α = (7860±60) keV and T_1/2=(0.7 _?0.3 ^+3.3 ) ms were measured. Improved decay data have been obtained for²⁰⁴Ra. The results are E_α = (7484±10) keV and T_1/2=(59 _?9 ⁺¹² ) ms. Earlier findings concerning the decay of two states in²⁰⁵Ra and an isomer in²⁰⁷Ra have been confirmed. Approximate values for cross sections have been determined for²⁰²Ra, produced using the³⁶Ar+¹⁷⁰Yb reaction and for²⁰³Ra,²⁰⁴Ra, and²⁰⁵Ra using the³⁵Cl+¹⁷⁵Lu reaction. The results are 2 nb (²⁰²Ra), 4 nb (²⁰³Ra), 40 nb (²⁰⁴Ra), and 800 nb (²⁰⁵Ra).