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.     

Simultaneousg 9/2 proton and neutron alignments in76Kr

Yrast states with spins as high as 22 ? have been identified in⁷⁶Kr with the reactions⁵⁸Ni(²⁴Mg,α2p) at 110 MeV and⁴⁰Ca (⁴⁰Ca, 4p) at 155 MeV. Simultaneous band crossings due to g_9/2 proton and neutron quasiparticles have been observed in the frequency range 0.60≦?ω≦0.75 MeV. The moment of inertia increases rapidly at the crossing frequency. The systematics of proton and neutron alignments in the Kr isotopes are discussed.     

Level structure of proton-richN=83 Nuclei150Ho and152Tm

The yrast levels of theN=83 odd-odd nuclei¹⁵⁰Ho and¹⁵²Tm have been studied byγ-ray spectroscopy following reactions induced by 240–250 MeV⁶⁰Ni ions. Isomers have been identified in¹⁵⁰Ho at 1.096, 2.625 and ～8.0 MeV and in¹⁵²Tm at 2.555 and ～ 6.3 MeV, and the level schemes of both nuclei have been established up to 2.6 MeV. The level spectra are interpreted using the shell model in terms of the coupling of anf _7/2,h _9/2 or i_13/2 valence neutron to the yrast excitations of the neighboring N=82 nuclei.     

Decay schemes and total decay energies of89Br and90Br

Spectroscopical investigations of⁸⁹Br and⁹⁰Br have been performed, utilizing the formation of the molecular ion AlBr⁺ in the ion-source of an isotope separator on-line a reactor. Detailed level schemes have been constructed for⁸⁹Kr and⁹⁰Kr, and theβ-strength function of⁸⁹Br and⁹⁰Br below the neutron binding energy is presented. The total decay energy of^89,90Br, as well as of⁸⁹Kr, have been measured by means ofβ γ-coincidences, and the population of excited states in⁸⁹Kr via⁹⁰Br delayed neutrons has been deduced.     

Structure of the Triplet of low-lying states in101Mo

The properties of the triplet of low-lying states in¹⁰¹Mo have been studied through spectroscopy of theγ radiation following thermal neutron capture in¹⁰⁰Mo and β^? decay of¹⁰¹Nb and through a measurement of the proton angular distributions in the¹⁰⁰Mo(d,p) reaction with 14 MeV deuteron energy. The half-lives of the 13.5 keV state and the 57.0 keV 5/2⁺ state have been measured as 226(7) and 133(7)ns, respectively. These values and the quadrupole/dipole mixing ratios of the 13.5 keV and 43.5 keV transitions yield spin and parity 3/2⁺ for the 13.5 keV level. The E2 components in the 13.5 (3/2⁺ →1/2⁺) and 43.5 keV (5/2⁺→3/2⁺) transitions are ≦ 8·10^?4 and 54(9)%, respectively. The possibility of an additional state near to the 57.0 keV level is discussed. IBFM/PTQM calculations, taking into consideration the transitional character of the¹⁰⁰Mo boson core, account for the electromagnetic-transition and transfer-reaction pattern of the triplet of states.     

Three-nucleon yrast states in145Sm

In-beamγ-ray and conversion electron measurements with (α, xn) reactions have established the¹⁴⁵Sm highspin states up toI ^π=25/2⁺ at 3.5 MeV excitation. A shell model analysis using empirical two- and one-body energies from neighbouring nuclei classifies the low-lying odd-parity levels as 3-quasiparticle states formed by the¹⁴⁴Sm two-proton-hole excitations and thef _7/2 valence neutron. The higher-lying positive-parity states involve particle-hole core excitations with one proton inh _11/2.     

Levels in 74Se, 78, 80Kr and 84Sr and systematics of quasi-γ bands

Quasi-band structures in ⁷⁴Se, ^{78, 80}Kr and ⁸⁴Sr have been investigated using (p, 2nγ) reactions. The members of the quasi-γ band have been observed up to 5⁺ in ⁷⁴Se, ^{78, 80}Kr and up to 3⁺ in⁸⁴Sr. The analyses of the energy systematics of the quasi-γ bands studied in this mass region as well as in other regions make clear the evolution of the quasi-γ band to the γ-band in well-deformed nuclei. In addition to these positive-parity bands, negative-parity levels were observed in ⁷⁴Se, ⁸⁰Kr and ⁸⁴Sr.     

Rotational bands in77Kr

Collective bands in⁷⁷Kr have been studied with the help ofγ singles spectra,γγ coincidences,γ ray angular distributions and recoil-distance life time experiments measured during the irradiation of⁶³Cu and⁶⁴Ni targets with¹⁶O ions. Positive and negative parity bands with the following level energies, spins, and parities have been found: g.s. 5/2⁺; 150.1 keV 7/2⁺; 279.1 9/2⁺; 784.8 11/2⁺; 1003.7 13/2⁺; 1660.5 15/2⁺; 1919.1 (17/2⁺); 66.5 keV (3/2)^?; 245.3 (5/2)^?; 499.6 (7/2)^?; 799.5 (9/2)^?; 1177.3 (11/2)^?; 1569.9 (13/2)^?; 2063.2 (15/2)^?. A mean life time of 170±17 nsec for the 66.5 keV state was measured withγγt coincidences and a mean life time of 48±13 psec for the 245.3 keV state was obtained with a recoildistance Doppler shift measurement.     

Collectivity and the role of two-proton and two-neutron excitations in 82Kr

Excited states in ⁸²Kr have been studied in the reaction ⁸⁰Se(α, 2n) by using in-beam γ-ray spectroscopy. Measurements of γγ coincidences, excitation functions, angular distributions and the linear polarization of the γ-rays have been carried out. All together, 38 levels have been identified up to $\text{I = 12}\text{h}\text{?}$ and excitation energies up to 6 MeV. For 23 of these levels the mean lifetime could be determined by Doppler shift and the pulsed-beam γ-timing methods. Most of the levels could be grouped into collective bands according to measured B(E2) values. Two sequences, I^π = 8⁺, 10⁺, 12⁺, are interpreted as rotation-aligned 2q.p. bands built up on $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ protons and neutrons, respectively. Interaction matrix elements between these bands were deduced as V ? 10 keV. Additional states, I^π = (6⁺), 7⁺, 8⁺, indicate coexistence of rotation-aligned and deformation-aligned 2q.p. excitations. An odd-parity ΔI = 1 band is ascribed to proton excitations of the type $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{?}\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$. Several fast M1 transitions with B(M1) = 0.1–1 W.u. have been observe states of equal spin. They are interpreted as being due to configuration mixing.     

In-beam study of 80Kr; Quasiparticle excitations in nuclei around mass 80

The excited states in ⁸⁰Kr have been studied in the reactions ⁷⁷Se(α, n), ⁷⁸Se(α, 2n), ⁸⁰Se(α, 4n) and ⁶⁵Cu(¹⁸O, p2n) by using in-beam γ-ray spectroscopy. In addition to γγ-coincidences, excitation functions and angular distributions, linear polarization of γ-rays and conversion electrons were also measured. All together, 32 levels have been identified up to spin 14 at an excitation energy of 6.7 MeV in ⁸⁰Kr. For 21 of these levels the mean lifetime could be determined by Doppler shift methods and by the pulsed-beam γ-timing method. The B(E2) values of 30–60 W.u., derived for many transitions, indicate strong collectivity and the existence of several band structures is suggested. Above 2.5 MeV 2-quasiparticle (qp) excitations become important. The excitation energies of ⁸⁰Kr and its neighbours ^{77, 78, 79}Kr, ⁷⁷Br and ⁸¹Rb have been analysed in terms of the cranked shell model. In ^78,80Kr two-proton excitations have been found to be responsible for the observed band crossing. Quasiparticle excitations strongly influence the pairing and stabilize the deformation. The anomalies in the negative-parity bands of ⁸¹Rb and ⁷⁷Br are interpreted as a crossing of a 3qp and a 1qp band and the relatively low frequency of the crossing point is ascribed to the blocking effect.     

Beyond the N = 50 shell closure: High-spin excitations of 87Kr and ground-state spin of 87Br

The ⁸⁷Kr nucleus has been produced as fission fragment in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85MeV bombarding energy and studied with the Euroball IV array. High-spin states of this neutron-rich isotope have been identified for the first time. Its level scheme has been obtained up to 6.3MeV excitation energy and spin I ∼ 23/2ℏ. Its structure is interpreted by analogy with those of the heavier isotones. The proposed configurations involve both proton and neutron excitations from several sub-shells located close to the Fermi levels, particularly νd_5/2, πp_3/2f_5/2 and πg_9/2. Moreover, a revised spin value of 5/2^- for the ⁸⁷Br ground state is proposed.     

High-spin levels in the odd-odd nucleus142Eu

High-spin states up to I=22? have been observed for the first time in the odd-odd nucleus¹⁴²Eu above the 8^? β⁺ -decaying isomer. A new isomer of mean life τ_m=9.0± 0.6 ns and J^π=8⁺ has been identified. The low energy part of the level scheme is interpreted in terms of single-particle excitations in the 2d_5/2 and h_11/2 orbitals and of the N=80 core excitations.     

Structure of the neutron-rich nucleus 37 94 rb57

A level scheme has been established for ₃₇ ⁹⁴ Rb₅₇ based on studies at the fission-product separator JOSEF ofγγ coincidences following theβ- decay of⁹⁴Kr. For the 190.7 keV level a half-life of 2.5(8) ns has been determined through the centroid-shift method. The majorβ-decay branch of⁹⁴Kr proceeds with log ft=3.7 into the 988.4 keV state in⁹⁴Rb. This transition must be of the 0⁺→1⁺ GT type involving the transitionνg_7/2→πg_9/2. IBFFM calculations have been performed for⁹⁴Rb, starting from the parametrization used in the previous IBM investigations for the isotope⁹³Rb and the isotone⁹⁶Y. A theoretical interpretation of the low-lying levels including propositions for the spins is given in a comparison to the available data.     

Anomalous shift in crossing frequency for the proton 1/2[541] band in175Ta

High-spin states in¹⁷⁵Ta have been populated in the¹⁶⁰Gd (¹⁹F,4n)¹⁷⁵Ta reaction with beams provided by the HI-13 tandem accelerator at the Institute of Atomic Energy in Beijing. A level scheme was constructed from γ-γ coincidence experiments. Seven decay sequences built on 5/2⁺[402], 1/2^?[541], 7/2⁺[404] and 9/2^? [514] proton Nilsson configuration have been extended to higher spin. An anomalously large delay of the neutron AB crossing built on the h_9/2 proton Nilsson State 1/2^?[541] is discussed.     

Multipolarity determination fromL x-ray spectra in the decays of227Th,243Am,and249Cf

A new method to determine the multipolarity of a converted γ-transition from itsL x-ray spectrum has been extended by a coincidence technique and applied in the complex decays of²²⁷Th,²⁴³Am, and²⁴⁹Cf. TheL x-ray spectra of four low energy transitions have been measured in coincidence with cascading high energy γ-rays and theirM1-E2 mixing ratios obtained by comparison with calculatedL x-ray line intensities. The following results were obtained: for the 3/2⁺→l/2⁺ ground-state transition in²²³Ra, an admixture of (74±12)%E2, for the 7/2⁺→5/2⁺ ground-state transition in²³⁹Np, less than 30%E2, for the 9/2⁺→7/2⁺ ground-state transition in²⁴⁵Cm, (63±10)%E2, and for the 11/2^?→ 9/2^? transition in the third rotational band in²⁴⁵Cm, (32±10)%E2.     

The46Ti(d,τ)45Sc reaction

The⁴⁶Ti(d, τ)⁴⁵Sc reaction has been investigated at an incident energy of 52 MeV. Angular distributions have been taken for 14 τ-groups corresponding to excitation energies below 4 MeV in⁴⁵Sc. Spectroscopic factors were extracted through DWBA calculations. Spins and parities of 5/2⁺ are proposed for states at 1.30, 1.80, 2.91, 3.48, and 3.72 MeV. The hole state spectrum of⁴⁵Sc closely resembles that of⁴⁷Sc. A strong fractionation of the 1d _3/2 strength was not observed in contrast to other recent work. The data provide evidence for (1f _7/2 · 1f _5/2 ¹) components in the ground state wave function of⁴⁶Ti, which demonstrates that such configurations are not restricted to neutron excitations.     

In-beam study of74Br and a first cascade in75Kr

An in-beam study of⁷⁴Br has been carried out using γ-ray spectroscopy in connection with the reactions (³He,p2in) and (d,2n) on⁷⁴Se targets. Separate level schemes on top of the two β-decaying isomers of⁷⁴Br are proposed. Two cascades connecting highspin states are assigned to populate theI ^π=4⁽⁺⁾ isomer. In this level scheme, the excited states at 72.1, 188.3 and 472.0 keV are found to have half-lives of 13.3, 0.7 and 0.7 ns, respectively. The low-spin isomer of⁷⁴Br is tentatively assigned asI ^π=(0^?). This state is populated by two other cascades. For the low-spin levels two-particle configurations of proton and neutron Nilsson orbitals are proposed. The high-spin states are tentatively interpreted as arising from the configurations (πg _9/2,νg _9/2) and (πg _3/2,νg _9/2). As a by-product, a cascade of four transitions has been observed during the³He bombardment of⁷⁴Se and tentatively assigned to negative-parity states in⁷⁵Kr.     

Description of even-even Krypton isotopes by the Interacting Boson Approximation

Energy spectra andB(E2) values of the even-even Kr isotopes^74–82Kr have been analyzed in terms of the IBA-2 model of Otsuka, Arima and Iachello [6, 7]. We can reproduce the variation of collectivity along the isotope chain by means of a consistent set of 5 parameters, essentially only one of them has been varied with neutron number. The low lying 0 ₂ ⁺ states have been found to be strongly influenced by shell effects. Predictions are given for the not yet observed gamma bands of^74,76Kr.     

Low-lying four-quasiparticle state in82Rb

The g-factor of the 6⁺ 191.3 keV excited state with a half-life T_1/2=12.3(6) ns in⁸²Rb populated in the⁷⁹Br(⁴He,n) and⁸¹Rb(³He,2n) reactions has been measured by the TDPAD method. On the basis of the experimental g-factor g _exp =+0.670(8) the four-quasiparticle configuration [πg9/2?ν(g9/2) ₇ ^2/3 ]₆₊ involving an anomalous neutron coupling has been assigned to this state.     

On theβ − decay of the 2s isomer in98Y and the existence of an intruder band in98Zr

The γ radiation from the ß^? decay of the high-spin isomer in⁹⁸Y has been studied at the separator JOSEF. A main goal was to get insight into the spins of levels in⁹⁸Zr which have been interpreted as members of an intruder band based on the 0₂ ⁺ state at 854 keV. The measuredγ-γ angular-correlations lead to the assignment of spins of 4 and 3 ? to the 2491 and 1844 keV levels, respectively, and contradict their interpretation as the 6⁺ and 4⁺ states of the proposed band. For the 2.0s isomer of⁹⁸Y, the ß{?}-decay properties suggest I=5 with a major configuration [πg_9/2,νs_1/2(g² _7/2)₀].