Transition probabilities and the structure of some negative-parity states in126,130Xe and132Ba

The lifetimes of several negative-parity states in^126,130Xe and¹³²Ba have been determined by means of the generalized centroid-shift method. The reactions^124,128Te(α,2n) and¹²²Sn(¹³C, 3n) have been used. Following results were obtained:T _1/2(2758 keV)=1.3±0.2 ns in¹²⁶Xe,T _1/2(2060 keV)=0.20±0.10 ns,T _1/2(2104 keV)=0.50±0.10 ns,T _1/2(2376 keV)=0.30±0.10 ns andT _1/2(2973 keV)=4.6±0.4 ns in¹³⁰Xe as well asT _1/2(2120 keV)=0.40 _?0.10 ^+0.20 ns in¹³²Ba. A systematics of the B(E2; 7 _? ¹ ?5 ₁ ^? ) values in theN=76 nuclei is presented. Electric dipole and quadrupole transition rates are discussed in terms of octupole and quadrupole collectivity. The structure of the 5 ₁ ^? and 7 ₁ ^? states is considered. Nuclear reactions:^124,128Te(α, 2 _n ),E=26 MeV,¹²²Sn (¹³C, 3n),E=53 MeV; measuredE _γ I _γ , γ-r.f. DeducedT _1/2, B(σL) in^126,130Xe and¹³²Ba. Ge detectors. Generalized centroid-shift analysis.     

Transition probabilities and the structure of some negative-parity states in 126,130Xe and 132Ba

The lifetimes of several negative-parity states in ^126,130Xe and ¹³²Ba have been determined by means of the generalized centroid-shift method. The reactions ^124,128Te(α, 2n) and ¹²²Sn(¹³C, 3n) have been used. Following results were obtained: T_1/2(2758 keV) = 1.3 ± 0.2 ns in ¹²⁶Xe, T_1/2 (2060 keV) = 0.20 ± 0.10 ns, T_1/2 (2104 keV) = 0.50 ± 0.10 ns, T_1/2 (2376 keV) = 0.30 ± 0.10 ns and T_1/2 (2973 keV) = 4.6 ± 0.4 ns in ¹³⁰Xe as well as T _1/2(2120 keV) = 0.40 _?0.10 ^+0.20 ns in ¹³²Ba. A systematics of the B(E2; 7 ₁ ^? ? 5 ₁ ^? ) values in the N = 76 nuclei is presented. Electric dipole and quadrupole transition rates are discussed in terms of octupole and quadrupole collectivity. The structure of the 5 ₁ ^? and 7 ₁ ^? states is considered. Nuclear reactions: ^124,128Te(α, 2n), E = 26 MeV, ¹²²Sn (¹³C, 3n), E = 53 MeV; measured E_{gg}, I_{gg}, γ-r.f. Deduced T_1/2, B(σL) in ^126,130Xe and ¹³²Ba. Ge detectors. Generalized centroid-shift analysis.     

Decay studies of131Ba

Bombarding a⁸⁵Rb target by 27 MeVα-particles the following half-lives of excited states in different nuclei have been determined by means of the generalized controid shift method: in⁸⁷Y,T _1/2(2827.1 keV)=0.75±0.10 ns,T _1/2(2675.9 keV)=0.25±0.10 ns,T _1/2(3595.3 keV)=0.5±0.2 ns andT _1/2(2986.9, 3094.4 keV)<0.1 ns; in⁸⁸Y,T _1/2(232.1 keV)=0.8±0.1 ns and in⁸⁵Rb,T _1/2(151.2 keV)=0.6±0.1 ns. In⁸⁷Y, the members of the doublet (vg ₉ ^2/?2 )₈₊ (πp_1/2) are identified. E2 transition rates 21/2⁺? 17/2⁺ in odd A≈ 90 nuclei are shown to represent essentially core transitions. Evidence for the existence of seniority-five configurations is found.     

Core vibration of99Tc

The gyromagnetic ratios and the half-lifes of the 141 keV and 181 keV states of⁹⁹Tc have been remeasured. The results,g(141 keV)=+1.280(44);g(181 keV)=+1.446(20) andT _1/2(141 keV)=0.205(4)ns;T _1/2(181 keV)=3.44(3) ns are in only fair agreement with prior published data but more precise. They confirm that both states are members of the ground state core vibration multiplet. The hyperfine field of TcFe has been determined asB _hf ^10K (TcFe)=30.42 (30)T;B _hf ^290K (TcFe)=29.47 (29)T     

Decay of 0 2 + states in88,92,94Zr andE0 systematics of Zr isotopes

Branching ratios ofE0 andE2 transitions depopulating the 0 ₂ ⁺ states of^{88, 92, 94}Zr have been determined using conversion-electron and γ-ray spectroscopy. Two different lifetime-measurement methods were applied in the remeasurement of the half-lives of the 0 ₂ ⁺ states in^{92, 94}Zr, yielding consistent results ofT _1/2(0 ₂ ⁺ ,⁹²Zr)=85(15) ps andT _1/2(0 ₂ ⁺ ,⁹⁴Zr)=280(40) ps. The monopole strengths extracted areρ ²(⁹²Zr)=8.4(17) ×10^?3 andρ ²(⁹⁴Zr)=11.9(20)×10^?3. The proton configurations of the 0 ₂ ⁺ states are discussed in view of these values and proton transfer data.     

Study of excited states in49V with the48Ti (p, γ)49V reaction

In the⁴⁸Ti(p, γ)⁴⁹V reaction gamma decays of thirteen resonances betweenE _p =960 and 1570 keV are investigated. Level energies within ±0.5–±2.0 keV andQ-value 6756.8±1.5 keV are obtained. Branching ratios for the resonance states and strongly populated bound states are given. Gamma-ray angular distribution measurements yield the followingJ(keV) assignments of⁴⁹V bound and resonance states:J(1140)=5/2,J ^π(2235)=5/2^(?),J(2264)=(3/2),J(2308)=3/2,J(3912)=3/2,J(8105,Ep=1374keV)=(1/2) andJ ^π(8289,E _p =1564keV)=3/2^(?). Multipolarity mixing ratios for all measured primary and secondary gamma rays are tabulated. Dopplershift attenuation measurements yield the mean lifetimes τ _m (keV) of the following bound states in⁴⁹V:τ _m (748)=(200± ₁₀₀ ⁴⁰⁰ )fs, τ _m (1140)=(250± ₁₀₀ ⁵⁰⁰ )fs, τ _m (1155)>400 fs, τ _m (1515)=(45± ₂₀ ³⁰ )fs, τ _m (1644)=(55± ₂₀ ³⁰ )fs, τ _m (1661)=(25±5)fs, τ _m (1994)>400 fs, τ _m (2235)=(30± ₁₅ ³⁰ )fs, τ _m (2264)=(45± ₁₅ ³⁰ )fs and τ _m (2308)=(20±10)fs.     

Gyromagnetic ratios of excited states in186W

The gyromagnetic ratios of the 4 ₁ ⁺ , 6 ₁ ⁺ , and 2 ₂ ⁺ states in¹⁸⁶W were measured relative to that of the 2 ₁ ⁺ level by means of the transient field implantation perturbedγ-ray angular distribution technique. The nuclei in the states of interest were Coulomb excited using a beam of 220-MeV⁶³Cu projectiles and recoiled swiftly through a thin, polarized Fe foil. The present measurements yielded ratiosg(4 ₁ ⁺ )/g(2 ₁ ⁺ )=1.04±0.07,g(6 ₁ ⁺ )/g(2 ₁ ⁺ )=1.03 ±0.20 andg(2 ₂ ⁺ )/g(2 ₁ ⁺ )=0.63±0.13. The sizable disparity between the measuredg-factors of the ground- and excited-band is examined within the context of the interacting boson approximation model.     

Absolute transition probabilities from the 53.2 keV level in105Ag

The halflife of the 9/2 ₁ ⁺ , 53.2 keV state in¹⁰⁵Ag has been measured by the delayed-coincidences technique to 2.33±0.08 ns. The reduced transition probabilities between the 9/2 ₁ ⁺ and 7/2 ₁ ⁺ states have been deduced and compared with the theoretical predictions within the cluster-vibration coupling model (Alaga model).     

Measurement of theg-factor of the 4 1 + -state of202Pb

Theg-factor of the 4 ₁ ⁺ -state of²⁰²Pb was investigated by the IPAC-technique. Inspite of the long half-life, which was measured as:T _1/2(4 ₁ ⁺ -state)=1.97(2)ns and the strong applied external magnetic field of 95.0 kG no rotation of the 787 keV-(422keV)-961 keVγ-γ angular correlation could be observed. Theg-factor must therefore be very small. A computer fit gave the limits:g(4 ₁ ⁺ -state)=+0.002(4). This smallg-factor can be understood by the assumption of an accidental cancellation of a positive contribution by the main [f _5/2 ² ]₄-configuration and several negative contributions by admixtures of all other possible two neutron configurations.     

Untersuchung des Zerfalls von134Cs durch γ-γ-Winkelkorrelationsmessungen

γ-γ-angular correlation measurements with Ge(Li) detectors and NaJ(Tl) detectors provided theE2/M1 mixing ratios of the following gamma transitions: 3 ₁ ⁺ →2 ₂ ⁺ (475.3 keV), 2 ₂ ⁺ →2 ₁ ⁺ (563.3 keV), 4 ₂ ⁺ →4 ₁ ⁺ (569.4 keV), 3 ₁ ⁺ →2 ₁ ⁺ (1,039 keV). The angular correlation measurements were only consistent with spin 3 of the 1,643 keV level. The half life of the 1,401 keV level was determined by delayed coincidence techniques to beT _1/2 (1,401 keV)≦30 ps.     

Pion photoproduction on197Au to ground and isomeric states in197Hg

The lifetime of the 331.3 keV 0 ₂ ⁺ state in¹⁰⁰Zr has been measured at the gas-filled recoil separator for fission products JOSEF. By observing the delayed coincidences between theβ-particles populating the level and theE0 conversion electrons from its decay into the ground state, a half-life of 3.37±0.30 ns has been obtained. From the measured lifetime and the relative intensities of the 0 ₂ ⁺ →0 ₁ ⁺ and 0 ₂ ⁺ →2 ₁ ⁺ transitions, values of 0.493±0.015 for theE0 strength parameterρ, and of 16 single particle units forB(E2,2 ₁ ⁺ → 0 ₂ ⁺ ) have been deduced. The enhanced nature of theE0 transitions suggests mixing of the 0 ₁ ⁺ and 0 ₂ ⁺ states which may be estimated by comparing the experimentalB(E2) values for the 2 ₁ ⁺ →0 ₁ ⁺ and 2 ₁ ⁺ →0 ₂ ⁺ transitions with the predictions of the asymmetric VMI model.     

Gyromagnetic ratios of 4+ and 6+ states in156Gd and158Gd

The followingg-factors have been derived from time integral measurements of γ-γ angular correlations in the static magnetic hyperfine field of magnetized gadolinium metal probes:¹⁵⁶Gd:g(4 ₁ ⁺ )=+0.310(19)g(6 ₁ ⁺ )=+0.25(21)g(4 ₃ ⁺ , 1511 keV)=+0.809(27)¹⁵⁸Gd:g(4 ₁ ⁺ )=+0.409(15). The 5.35d ¹⁵⁶Tb sources were produced by the reaction¹⁵⁶Gd(d, 2n)¹⁵⁶Tb in our cyclotron. A carrier-free 150y ¹⁵⁸Tb source was obtained from ISOLDE/CERN. In comparison with the precisely knowng-factors of the 2 ₁ ⁺ states,g(2 ₁ ^su+ ,¹⁵⁶Gd) =+0.386(4) andg(2 ₁ ⁺ ,¹⁵⁸Gd)=0.381(4), we observe a large reduction for the¹⁵⁶Gd 4 ₁ ⁺ state whereasg increases slightly for¹⁵⁸Gd. The half-life of the 4 ₁ ⁺ state of¹⁵⁸Gd was remeasured as¹⁵⁸Gd:T _1/2(4 ₁ ⁺ )=148(2) ps. A measurement of the rotation in the 4 ₃ ⁺ state of¹⁵⁶Gd in external magnetic fields of various strengths up toB _ext=9.5 T did not confirm the anomalous dependence of the magnetic hyperfine field in gadolinium metal on the external field, which has been reported by Persson et al. [29].     

New isomers in83Br,85Rb,85Kr and86Kr

Using methods of in-beam spectroscopy in connection with 35 MeV⁷Li bombardment of⁸²Se targets four new isomers have been identified. They are observed in⁸³Br at 3070 keV (T_1/2=0.6±0.2 μs), in⁸⁵Rb at 2826 keV (T_1/2=12.5±0.6 ns), in⁸⁵Kr at 1991 keV (T_1/2=1.2+ _0.4 ^1.0 μs) and in⁸⁶Kr at 2250 keV (T_1/2=3.1±0.6 ns). Spin and parity 19/2^? have been assigned to the new isomer in⁸⁵Rb that is interpreted as the coupling of a p_3/2 proton to the g ₉ ^2/?2 two-neutron excitation of the N=48 neutron system. Using the TDPAD method an upper limit for the g-factor of ¦g¦≦0.17 has been estimated for this isomer. The same two-neutron excitation is also believed to be responsible for the new isomer in⁸³Br. The new isomers in⁸⁵Kr and⁸⁶Kr are interpreted as configurations containing the two-proton excitation (f ₅ ^2/?1 , p ₃ ^2/?1 ) where in the isomeric 17/2⁺ level of⁸⁵Kr an additional g_9/2 neutron hole is involved.     

Microscopic foundation of the interacting boson model in thes d-shell nuclei

Starting from an isospin invariant shell-model hamiltonian, we describe a method for deriving microscopically the IBM-hamiltonian appropriate to lights d-shell nuclei. The key ingredients of our approach are:a) the Belyaev-Zelevinsky-Marshalek (BZM) bosonization procedure;b) two successive unitary transformations that extract the “maximally decoupled” collective bosons with angular momentaJ=0(s _ππ ⁺ ,s _νν ⁺ ,s _πν ⁺ ) andJ =2(d _ππ ⁺ ,d _νν ⁺ ,d _πν ⁺ (T=0),d _πν ⁺ (T=1)). The method is applied to obtain the low-energy spectra and the electron scattering form factors for the 0 ₁ ⁺ →2 ₁ ⁺ transitions in²⁰Ne and²⁴Mg. Good agreement with the exact shell-model results is achieved. The inclusion of proton-neutron bosons (s _πν ⁺ ,d _πν ⁺ (T=1),d _πν ⁺ (T=0)), as well as the renormalization of boson parameters due to the non-collective degrees of freedom, are shown to play a crucial role.     

Winkelkorrelationsmessungen an88Sr

The γγ ande _? ^KL γ directional correlations have been measured for the (898–1836) keV cascade in⁸⁸Sr following the decay of⁸⁸Y. The observed correlation coefficients areA ₂₂(γγ)=?0.0784±0.0042 andA ₂₂(e _? ^KL γ)=0.0102±0.0046. The γγ experiment is consistent with a mixing ratio δ(γ)=0.009±0.005 for the 898 keV transition. Using the result of thee _? ^KL γ experiment the most probable value of the ratio of the penetration matrix element to the normal γ-ray matrix element was determined to be η=0.03±0.30 showing a normal conversion process. This penetration parameter agrees with a hindrance factorH _W(E1)=2.1 · 10^?3 for theE1 transition.     

The penetration parameter λ of the anomalousM1-conversion of the 482 keV transition in181Ta

Byγ- e _K ^? ande _K ^? -γ TDPAC measurements with the 133 keV–482 keV cascade of¹⁸¹Ta in a polycrystalline beryllium matrix theA ₂(e _K ^? )-coefficient of the 482 keV transition was determined as: A₂(482e _K ^? )=?0.095±0.015. The theoretical value is 6.0 times larger. A remeasurement of the E2/M1 mixing ratio of the same transition by integralγ- γ angular correlation measurements with a system of three 20 ccm Ge(Li)-detectors gave the result:δ(482)=4.96±0.25. Theα _K -coefficient was redetermined as:α _K (482)=0.0246±0.0018. From these data the penetration parameter:λ=157±11 is derived and compared with previously published values and theoretical estimates.     

Evidence for a 3.6 minute isomeric 12− state of the πh 11 2/−1 vi 13 2/−1 configuration in206Tl and effective two-particle interactions

Using the²⁰⁴Hg(α, pn)-reaction andα-particles of energies 39–55 MeV, we have found an isomeric 3.6 min 12^? state in²⁰⁶Tl at 2,642.9 keV which has the two-hole configurationπh ₁₁ ^2/?1 vi ₁₃ ^2/?1 The 12^? state decays mainly by anE5 transition of energy 1,021.4 keV to a 7⁺ state at 1,621.5 keV whose main configuration isπs ₁ ^2/?1 vi ₁₃ ^2/?1 There is, in addition, evidence for a weak 565 keVM 4 branch to an 8⁺ state at 2,078 keV whose main configuration should beπh ₁₁ ^2/?1 vf ₅ ^2/?1 . The 7⁺ state decays by a stretched cascade ofγ-rays to states of the following values ofJ ^π and excitation energy: 5 _? ⁺ , 1,405.4 keV; 4^?, 952.1 keV; 2^?, 265.8 keV and 0^?, 0 keV. The main configurations of these states areπh ₁₁ ^2/?1 vp ₁ ^2/?1 ,πd ₃ ^2/?1 vf ₅ ^2/?1 ,πd ₃ ^2/?1 vp ₁ ^2/?1 andπs ₁ ^2/?1 vp ₁ ^2/?1 respectively. From the nuclear masses of²⁰⁸Pb,²⁰⁷Pb,²⁰⁷Tl, and²⁰⁶Tl and the experimental excitation energies it is possible to obtain the proton hole-neutron hole interaction in²⁰⁶Tl. This interaction is compared with the calculations of Kuo and Herling and the discrepancies are discussed. The 12^?→8⁺ M4 transition rate is reduced because of destructive interference between the protonh _11/2→d _3/2 and the neutroni _13/2→f _5/2 contributions. The magnitude of the reduction is accurately reproduced by the wave functions of Kuo and Herling. The 12^?→7⁺ E5 transition rate is about twice as large as the single-holeπh ₁₁ ^2/?1 πs ₁ ^2/?1 transition rate. This deviation is fully explained by the configuration admixtures in the 7⁺ state, given by Kuo and Herling.     

Spin des 921 keV-Zustandes in99Tc

The angular correlation of the⁹⁹Tc 739-181 keV gamma-gamma cascade has been measured in a liquid molybdenium source. The result was:A ₂=0.0930 ± 0.0028,A ₄=?0.0095 ± 0.0042. The spin of the 921 keV level was determined to 3/2⁺. The mixing ratio of the 739 keV gamma-transition was measured to δ (E2/M1)=5.9 _?0.3 ^+0.4 .     

E1,ΔK=0−Übergänge in deformierten Kernen ungerader Massenzahl

The half lives of the 5/2 5/2^?[523] level at 25.7 keV in ₆₆ ¹⁶¹ Dy and the 5/2 5/2⁺[642] level in ₆₈ ¹⁶³ Er, found in this work to lie at 69.2 keV, have been determined by the delayed coincidence method to be T_1/2=27.8± 1.5 nsec, and T_1/2=8.8± 0.5 nsec, respectively. The following hindrance factors relative to the single particle Weisskopf estimate (F _W and the Nilsson estimate (F_N were obtained: ₆₆ ¹⁶¹ Dy 5/2 5/2^?[523] → 5/2 5/2⁺[642]:F_W=(6.6± 1.3) × 10³, F_N=0.48± 0.10 ₆₈ ¹⁶³ Er 5/2 5/2⁺[642] → 5/2 5/2^?[523]:F_W=(2.4± 0.5)× 10⁴, F_N=1.8 ± 0.4 A systematic difference between transitions in odd proton nuclei and odd neutron nuclei was found: E1, ΔK=0 transitions in odd neutron nuclei have hindrance factors F_N from 2.9 to 0.16, this means, these transitions are in agreement with the predictions of the Nilsson model within a factor of 10. For transitions in odd proton nuclei one has hindrance factors F_N from 75 to 9.9 × 10^?4. It is shown that a small difference between the deformation of the initial and the final state changes the transition probability of both, proton and neutron transitions, considerably.