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.     

Electric quadrupole and magnetic dipole moments of the first excited 2+ states of186Os and188Os

The spectroscopic quadrupole moments and the magnetic dipole moments of the lowest 2⁺ states in¹⁸⁶Os (137 keV) and¹⁸⁸Os (155 keV) have been determined by Mößbauer transmission experiments. The electric quadrupole momentsQ ₂₊(Os 186)=? (1.80±0.22) b andQ ₂₊(Os 188)=?(1.81±0.24) b as well as their ratioQ ₂₊(Os 188)/Q ₂₊(Os 186)=1.00±0.07 within the limits of error agree withB(E2) data, if a comparison on the basis of the rotational model is made. For the g-factors and their ratio g₂₊(Os 186)=0.281±0.008, g₂₊(Os 188)=0.305±0.015 andg ₂₊(Os 188)/g ₂₊(Os 186)=1.08±0.05 was obtained. All results are compared with recent model calculations.     

High-spin states in the N = 84 nucleus 152Er

High-spin states in ¹⁵²Er have been populated through the ¹⁴⁴Sm(¹²C, 4n)¹⁵²Er reaction. Excitation functions, angular distributions, prompt and delayed coincidences against the beam burst were measured. Three isomeric states at 2183 keV (τ = 1.8 ns), 4519 keV (τ = 1.2 ns) and 4915 keV (τ = 6.6 ns) have been observed. The decay scheme is developed up to 7 MeV and is discussed and compared with that of ¹⁵⁰Dy.     

Observation of the weakly deformedvi 13 2/2 band in188Hg

High-Spin states in¹⁸⁸Hg have been investigated using the¹⁶⁸Er(²⁴Mg,4n) reaction. The rotation aligned vi ₁₃ ^2/2 band, which intersects the weakly oblate ground state band atI≈10, is observed up toI ^π =20⁺ with an isomeric 12⁺ level with T_1/2=134(15)ns. The coexisting strongly deformed band, built on the 825 keV 0⁺ level, is observed up to the 14⁺ state. The systematics of the vi ₁₃ ^2/2 rotation aligned bands is discussed.     

Levels of188Os populated by thermal neutron capture

Direct and coincidence gamma spectroscopy has been applied to the reaction¹⁸⁷Os(n, γ)¹⁸⁸Os. A neutron binding energy of 7,989.3±0.3 keV is deduced and a level scheme is established. The experimental level energies andB(E2) ratios are compared to calculations in the framework of the Interacting Boson Model.     

Microsecond isomers in 187Tl and 188Pb

Lifetime measurements of states in nuclei with A=187 and 188 have been performed, using reactions between ¹⁵⁵Gd and ³⁶Ar and following the transport of evaporation residues to the focal plane of a gas-filled recoil separator. In a separate experiment using the ¹⁵⁹Tb(³²S, 4n) reaction the γ-decay of isomeric levels in ¹⁸⁷Tl has been studied using delayed γ-γ coincidence measurements. From observation of their subsequent γ decay, the mean lifetimes were measured to be 1000 ± 55 ns and 1600 ± 100 ns. Although it was not possible to characterize the isomers completely, they are proposed as candidates for one-proton, two-neutron excitations. In the course of this study, the decay of an isomer in ¹⁸⁸Pb was also observed and its lifetime was measured as 1150 ± 30 ns, and limits of 20–600 ns were placed on the meanlife of an isomer conjectured in ¹⁸⁷Pb. Received: 20 August 1999     

Mössbauer studies and nuclear quadrupole moments of186, 188, 189, 190 Os

Mössbauer transmission experiments with the 137, 155, 69 and 187 keV gamma rays of^{186, 188, 189, 190}Os, respectively, yielded the electric quadrupole splitting of these gamma resonance lines in OsO₂ and OsP₂. From the results the following ratios of quadrupole moments were derived:Q ₂ ⁺(¹⁸⁶Os, 137 keV):Q ₂ ⁺(¹⁸⁸Os, 155keV):Q ₂ ⁺(¹⁹⁰Os, 187 keV):Q _3/2 ^?(¹⁸⁹Os, g.s.)=(+1.100±0.020): 1.0:(+0.863±0.051): (?0.586±0.011) andQ _5/2 ^?(¹⁸⁹Os, 69 keV)/Q _3/2 ^?(¹⁸⁹Os, g.s.)=?0.735 ±0.012. The ratios for^{180, 188, 190}Os are, within their limits of error, in agreement with the expectation of the rotational model, indicating that the pairing-plusquadrupole model calculations of Kumar and Baranger predict too rapid a transition form rotational to vibrational nuclei. Applying this argument to¹⁸⁶Os in particular and using the measured ratios, one obtains a set of values for the quadrupole moments themselves, namelyQ ₂ ⁺ (¹⁸⁶Os)=?(1.50 ± 0.10)b,Q ₂ ⁺(¹⁸⁸Os)=?(1.36± 0.09) b,Q ₂ ⁺ (¹⁹⁰Os)=?(1.18 ± 0.08) b,Q _3/2 ^?(¹⁸⁹Os)=+ (0.80 ± 0.06) b, andQ _5/2 ^? (¹⁸⁹Os)=? (0.59 ± 0.05) b. For the electric field gradient at Os nuclei in Re metal a value ofV _zz=?(3.3 ± 0.6) · 10¹⁷ V/cm² was found. A measurement with a magnetically split source yielded δ=+ 0.685 ± 0.025 for theE2/M1 mixing parameter of the 69 keV transition of¹⁸⁹Os,g _5/2 ^?/g _3/2 ^?=0.895 ± 0.006 for the ratio of theg-factors of the 69 keV state and the groundstate, andH _i=?(1135 ± 20) kOe for the hyperfine field at Os nuclei in an iron matrix.     

The mass yield distribution of fragments studied with improved antisymmetrized molecular dynamics

The radiative decays of 4.7 d^119m Te and 16 h^119g Te have been re-investigated. Compared to previous investigations, almost twice more transitions have been observed. Revised decay schemes are proposed in which most of the observed transitions are attributed. The half lives of the levels at 270.45 keV (g 7/2), 644.01 keV (s 1/2), 1212.69 keV (9/2⁺) and 1366.15 keV (h 11/2) have been measured by means of the delayed coincidence method and found to be 35±10ps, <10ps, <10 ps and 112±15 ps respectively. The states are discussed in the framework of current models.     

Der Zerfall der Cäsiumisotope Cs129 und Cs130

Cs¹²⁹(T_1/2=32 h) and Cs¹³⁰(T_1/2=30 min) were produced by J¹²⁷(α,2n)Cs¹²⁹and J¹²⁷(α,n)Cs¹³⁰reactions, respectively. The γ-ray-spectra from the decays of Cs¹²⁹ and Cs¹³⁰ have been investigated using Li-drifted Ge-counters and scintillation spectrometers. In the decay of Cs¹²⁹15 γ-rays have been observed having the following energies and relative intensities: 95 keV (1.6), 177 keV (1.1), 267 keV (1.7), 278 keV (4.8), 317 keV (7.6), 371 keV (100), 410 keV (69), 511 keV (≦ 0.1), 548 keV (10.5), 587 keV (1.7), 623 keV (0.12), 660 keV (0.07), 864 keV (0.09), 907 keV (0.67) and 948 keV (0.2). The electron capture branching ratios to the ground state and excited states of Xe¹²⁹were determined; logft values for the various decay branches were derived from these ratios. In the decay of Cs¹³⁰twoγ-transitions with the following energies and intensities relative to the β⁺ emission have been found: 535 keV (11) and 585 keV (1.3). A new level at 1122 keV in Xe¹³⁰is proposed.     

Alpha decay of neutron-deficient isotopes with 78 ⪕ Z ⩽ 83 including the new isotopes 183, 184Pb and 188Bi

Using on-line mass separation of evaporation residues from heavy-ion-induced fusion reactions, three new isotopes were identified by their alpha decay, E_α given in keV: ¹⁸³Pb, 6715(20), ¹⁸⁴Pb, 6632(10) and ¹⁸⁸Bi, 6820(20). Half-lives were determined for ¹⁸⁴Pb, 0.55(6)s, ¹⁸⁵Pb, 4.1(3)s and ¹⁸³Tl, 0.060(15) s. The alpha branching ratios were re-measured for ^182–184Hg and ^{178, 179}Pt, the results are discussed within the framework of alpha width systematics.     

The γ-decay of two-particle states in 210Bi

The γ-decay of excited states in ²¹⁰Bi up to 3330 keV excitation energy has been studied using the ²⁰⁹Bi(d, pγ)²¹⁰Bi reaction. From the observed decay scheme, tentative J^π values are made for 25 states. The excitation energies, (d, p) strengths and decay branching ratios of these states are compared with those calculated using the two-particle shell-model wave functions of Kim and Rasmussen and Kuo and Herling.     

Magnetic moment of the 188 kev excited state in125I

Theg-factor of the 188 keV 3/2⁺ state in¹²⁵I has been determined with the perturbed angular correlation method to be g=0.70 (5). This result is close to the value reported earlier for the corresponding level in¹²⁵I.     

The decay of 20 min179Re

The decay of 20 min¹⁷⁹Re has been studied by means of Ge(Li) and Si(Li) detectors and a magneticβ-ray spectrometer. From the measured positon endpoint aQ-value of 2690±50 keV has been deduced. The decay scheme given is supported by coincidence relations. Fast beta transitions (logft≈5.1) to levels at 720keV and 1680 keV in¹⁷⁹W can be explained by a three quasi-particle character for these states. Nilsson assignments to these and other levels are discussed.     

Der radioaktive Zerfall des Fe61

Fe⁶¹(T _1/2=5.95 min) was produced irradiating Ni⁶⁴ and natural nickelfoils with fast neutrons in the reaction Ni⁶⁴(n, α)Fe⁶¹; Fe was chemical seperated. The gamma and beta spectrum were investigated with Ge(Li)-counters, NaJ(Tl) and plastic scintillation spectrometers. Fe⁶¹ emits five gamma rays having the energies and intensities per Fe⁶¹ decay: 121 keV (0.064), 177 keV (0.027), 298 keV (0.293), 1025 keV (0.461), 1202 keV (0.539). Threeγγ-cascades could be identified: 121–177 keV, 121–1202 keV, 298–1025 keV. Inγ β-coincidence measurements the maximal beta energy was determined:E _βmax=2.86 MeV. A decay sceme Fe⁶¹-Co⁶¹ was constructed: Fe⁶¹, spin and parityJ ^π=3/2^?, decays by means of allowed beta decay to three excited states of Co⁶¹: 1323 keV (1/2^?), 1202 keV (3/2^?), 1025 keV (5/2^?). The energies and intensities per Fe⁶¹ decay of the three beta decay modes are: 2.56 MeV (0.367), 2.68 MeV (0.495), 2.86 MeV (0.138). — A 88 keV gamma ray (T_1/2=33 sec) was observed coming from the Co⁶³ decay.     

Study of the excited states of119Sn by the decay of119g+m In

The decay of^119g+m In to the excited states of¹¹⁹Sn was investigated.¹¹⁹In was produced by the¹²⁰Sn(γ,p)¹¹⁹In reaction on an enriched SnO₂ target. The isomeric transition of 311.25 keV to the¹¹⁹In ground state was observed. In the beta decay of^119m In excited states at 23.9, 920.5, 921.4, 1089.0, 1187.9 and 1249.6keV in¹¹⁹Sn withJ ^π values of 3/2⁺, 3/2⁺, 5/2⁺, 5/2⁺, 3/2⁺ and 1/2⁺ respectively, are fed. In the decay of the¹¹⁹In ground state only the 7/2⁺ level in¹¹⁹Sn at 787.0 keV is fed.     

Investigation of dual decay of70Ga (21 min)

The weakβ ^?-branchings in⁷⁰Ga decay leading to excited states in⁷⁰Ge at 1039 keV, and 1215 keV were measured byβ ^?-ray and γ-ray detection in 4π geometry to be 0.36 ±0.02%, and 0.32±0.01%, respectively. The weak electron capture decay of⁷⁰Ga was confirmed and its intensity measured to be 0.41±0.05%. The energy of the γ-rays was remeasured to be 176.17±0.02 keV, and 1039.20±0.08 keV.     

Elektroneneinfang beim Zerfall des Ti45

Electron capture to the 718 keV, 1238 keV, 1408 keV and 1665 keV excited states of Sc⁴⁵ have been observed in the decay of Ti⁴⁵. The branching ratios were determined to be (0,4±0,04)% (0,05±0,02)%, (0,27±0,03)% and (0,08±0,01)% per decay of Ti⁴⁵ respectively.     

Levels of 188os populated in the 189Os(d,t)188Os reaction and in the decay of 41h 188Ir

The 0 level structure of ¹⁸⁸Os has been investigated by the ¹⁸⁹Os(d, t)¹⁸⁸Os reaction using a broad range magnetic spectrograph, and the properties of the ¹⁸⁸Os levels populated in the decay of ¹⁸⁸Ir have been re-examined. The (d, t) results yield new information about the location of two-neutron excitations in ¹⁸⁸Os involving the $\text{3}\text{2}$[512] orbital. Since the ¹⁸⁸Os ground state contains admixtures of both $\text{K =}\text{3}\text{2}\text{and}\text{K =}\text{1}\text{2}$ character, cross-section formulae for single-neutron transfer from a target state which is not pure in K are considered, and it is found that rather small $\text{K =}\text{1}\text{2}$ admixtures in the ¹⁸⁹Os ground state give rise to striking interference effects, which are manifested in the experimental (d, t) cross sections into the members of the ¹⁸⁸Os ground state band. The consequences of the mixed character of the target state on the (d, t) population of members of the K^π = 2⁺ γ-vibration and of higher-lying two-quasiparticle bands are also discussed.     

Search for correlated two-photon emission from e+(82Sr)+Th interactions

A previously observed electron line at INS at 330.3±0.4 keV in e⁺+Th interactions has been implied by these authors to originate from the decay of an unknown neutral object X⁰. QED predicts a two-photon decay mode of X⁰ leading to a correlated two-photon coincidence peak at 841.3±0.4 keV. To search for such a peak γγ-coincidence experiments were performed. From an observed peak-like structure in the spectrum at 841.7±0.3 keV and an associated cross section of 10.6±3.1 (stat) ± 6.9 (syst) μb an upper limit for the cross section of 15.2±1.5 (syst) μb for the production and 2γ decay of the X⁰ was deduced with a statistical confidence level of 95%.     

The α-decay rate of 188Pb

As part of our investigation of the reported anomalous α-decay rates of even-even lead nuclei with A ≦ 192 we have measured the α-decay branching ratio of ¹⁸⁸Pb. The isotope was produced in the ¹⁸⁰W(¹⁶O, 8n) reactions, and its decay properties were investigated with the use of the Oak Ridge on-line isotope separator facility. Gamma-ray and α-particle spectra were measured with the detectors placed in calibrated geometries. From these measurements the ¹⁸⁸Pb α-decay branching ratio was determined to be (22 ± 7)%, a value about seven times greater than that available in the literature. This result, together with our recent data for ¹⁹⁰Pb and ¹⁹²Pb, is discussed within the context of α-decay-rate systematics in the lead region.