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].     

Structure of high spin states in83Y

We report on the first in-beam study of high spin levels in⁸³Y which were established up to 9.0 MeV excitation energy and probable spin of 41/2⁺ resp. 27/2^? by means of the reaction⁵⁸Ni(²⁸Si, 3p). Ten lifetimes and six lifetime limits in the 10^?12–10^?9 s range were determined in two recoil distance Doppler shift experiments. The positive parity yrast states form ag _9/2 Coriolis decoupled band with partial alignment, near-rigid rotor moment of inertia and deformationβ ₂=0.29. The negative parity yrast band has very similar collective properties; it shows a pronounced band crossing at rotational frequency ?ω≈0.40 MeV which we associate with twoquasiparticleg _9/2 proton alignment. At 2.56 MeV excitation, a second ΔI=1 band starting with 17/2^? was found. On the basis of the similarity to⁸⁵Y and the very weakE2 decay of this state, we suggest that this band has (3qp) configuration with an alignedg _9/2 neutron pair. The lifetimes of the lowest 2⁺ and 4⁺ state in⁸⁴Zr populated in the reaction⁵⁸Ni (²⁸Si, 2p) were determined to be τ(2)=17.8(11) ps resp. τ(4)=3.5(4) ps.     

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.     

The β-decay of98Y and the level scheme of98Zr

Study of theβ-decay of⁹⁸Y and of the level scheme of⁹⁸Zr was undertaken by the use of the two recoil fission product separators JOSEF and LOHENGRIN. Twoβ-decay modes have been detected for⁹⁸Y, the half-lives of which are 0.65±0.05 s and 2.0±0.2 s. The spins and parities of the two states which undergoβ-decay are proposed to be 1⁺ and 4^?, respectively, with the configurations (π1g _9/2)(ν1g _7/2) and (π2p _1/2 (ν1g _7/2). The measuredQ _β -value is 7300±400 keV for the 0.65 s decay. The level scheme of⁹⁸Zr was deduced fromγ,γ-coincidence data. Absoluteγ-intensities were obtained from two independent measurements. The half-life of the first excited state of⁹⁸Zr at 853.0 keV with spin and parity 0⁺ was determined through delayedγ, e^?-measurements to be 63 ±7ns. No indication of the coexistence of two different nuclear shapes of⁹⁸Zr was found.     

Core polarization and 5/2+ states in91Nb

The energies and spectroscopic factors ofJ ^π=5/2⁺ states of nucleus⁹¹Nb excited via a reaction transferring a proton to the 2d _5/2 orbit of⁹⁰Zr target state have been calculated. Effective two-body interaction used has been extracted from the experimentally observed two-body energies of (1g ₉ ^2/?1 (n) 2d _5/2(n)), (1g ₉ ^2/?1 (n) 1g _9/2(p)) and (1g _9/2(p)-2d _9/2(n)) multiplets in⁹⁰Zr,⁹⁰Nb and⁹²Nb nuclei respectively. Most of the calculated energies and the strengths ofJ ^π=5/2⁺ levels have reasonably good counterparts in the experimental spectrum, however the calculation shows about 17% strength lying at 6.8 MeV, without having a confirmed counterpart in the observed level scheme. The reduced transition strengthsB(M1) forM l transitions from 5/2^? T>(11/2) state to the various components of 5/2⁺ T<(=9/2) state have also been reported; but the corresponding experimental values are not available. The main feature of the reduced transition strengths is that theM1 transition to the state at 3.69 MeV is inhibited whereas that to the state at 6.79 MeV is enhanced, the relevant core-configuration, interfering destructively in the former case and constructively in the latter.     

Fragmented 2d 5/2, 1g 7/2 and 1g 9/2 deep proton-hole states of207TI

The three proton-hole states ?2d_5/2, 1g_7/2 and 1g_9/2 are found to be fragmented as a result of coupling of these states with the 3^?, 5^?, 2⁺, 4⁺ and 6⁺ collective states of²⁰⁸Pb. The excited states in²⁰⁸Pb (t,α) reaction can be quantitatively explained in terms of altered 2d_5/2, 1g_7/2 and 1g_9/2 states with the hole-core coupling model.     

Theg factor of the lowest 7+ state in100Rh and 6− state in104Rh

Theg-factor of the 7⁺ state in¹⁰⁰Rh withT _1/2=140(5) ns and of the 6^? state in¹⁰⁴Rh withT _1/2=47(3)ns has been measured by the time-differential perturbedγ-ray angular distribution method (TDPAD). The obtained values are +0.67(2) and +0.33(1) respectively. The two nuclei were populated with reactions induced by⁷Li on^96,100Mo at a bombarding energy of 30 MeV. Shell model calculations using effective single particle moments show that the 7⁺ state in¹⁰⁰Rh has mostly a πg _9/2 ? vd _5/2 configuration, while the main component of the 6^? state in¹⁰⁴Rh is the πg _9/2 ? vh _11/2.     

Study of the excited 0+ states through conversion electron spectroscopy of neutron rich Zr and Mo fission products

A windowless Si(Li) electron detector has been used in conjunction with the gas filled fission product separator JOSEF to study totally converted transitions in Zr and Mo isotopes. The existence of twoβ-decay modes in⁹⁶Y,⁹⁸Y and¹⁰⁰Nb is confirmed. The half-lives of theβ-decaying levels feeding O ₂ ⁺ states in⁹⁶Zr and⁹⁸Zr are 6.0 ± 0.4 sec and 0.60±0.05 sec respectively. A search for other first excited O ₂ ⁺ states shows that no such states are apparent in¹⁰⁰Zr and¹⁰⁴Mo.     

Nuclearg-factors of theJ π=21/2+isomer in112In and theJ π=6+ 112Sn

Theg-factor of theJ ^π=21/2⁺ isomeric state in¹¹¹In (T _1/2=13.3 ns) and of theJ ^π=6⁺ isomeric state in¹¹²Sn (T _1/2=13.7 ns) were measured using the spin rotation method. The result obtained for theJ ^π=21/2⁺ level in¹¹¹In,g=+0.47 (2), indicates that this state has an almost pure ((πg _9/2)^?1 νg _7/2 νd _5/2) shell model configuration. The experimental valueg=+0.04 (3) for theJ ^π=6⁺ isomer in¹¹²Sn agrees with the theoretical value calculated within the frame of the BCS model.     

New investigation of83Zr and85Zr beta decay

The decay of on-line mass separated⁸³Zr and⁸⁵Zr sources has been studied by singles and coincident X-ray and-ray and conversion electron measurements. The previous level scheme of⁸³Y is confirmed and 5 new levels have been added up to 2.74 MeV excitation. A decay scheme of^85gZr with 40 levels up to 2.66 MeV in⁸⁵Y is deduced. The experimental observation below 2 MeV excitation in⁸⁵Y agrees qualitatively with the predictions of the interacting boson-fermion model.     

Theβ-decay of100Y: Discovery of a very low lying 0+ state in100Zr

Theβ-decay of¹⁰⁰Y has been investigated at the gas filled recoil separator JOSEF by means ofγ-ray and conversion electron spectroscopy. Twoβ-decay modes, of half lives 0.94±0.03 s and 0.55±0.15 s, have been observed. Using Xγ-, Xce-, γγ- andγce-coincidences the level scheme of¹⁰⁰Zr has been constructed. An excited 0 ₂ ⁺ state at 331.3 keV has been discovered in this nucleus. Theβ-decay from the 0.55 s decay mode in¹⁰⁰Y shows a strong preference for the ground state compared to the 0 ₂ ⁺ state in¹⁰⁰Zr. The structure of¹⁰⁰Zr and the nature of the 0 ₂ ⁺ level is discussed in the light of the present results.     

Properties of the 13/2+ isomeric decay in201Hg

The isomeric decay of theT _1/2=94 us isomeric state in²⁰¹Hg was studied in beam via the reaction¹⁹⁸Pt(α, n). Gamma-ray and conversion-electron spectra were recorded. From the conversion coefficients, the multipolarities of the two depopulating transitions were determined to beM 2 + 10%E 3 andE 2, respectively. These assignments give the depopulating transitions to be 13/2⁺ → 9/2^? and 9/2^? → 5/2^?, and these thus substantiate the previous suggestion that the isomeric state hasJ ^π=13/2⁺. For the 13/2⁺ → 9/2^? M 2 transition a detailed study ofE 3 andM 4 admixed amplitudes was done. The microscopic properties of the 13/2⁺ and 9/2^? states are discussed, and a comparison to the corresponding ones in^199,203,205Hg is made.     

Two-neutrino double β decay of96Zr to excited 2+ state of96Mo

The two-neutrino double beta decay of⁹⁶Zr isotope for 0⁺ → 2⁺ transition has been studied in the PHFB model. In our earlier work, the reliability of the intrinsic wave functions of⁹⁶Zr and⁹⁶Mo isotopes has been established by obtaining an overall agreement between a number of theoretically calculated spectroscopic properties as well as half-lives of 2vββ decay for 0⁺ → 0⁺ transition and the available experimental data. In the present work, the half-life of 2vββ decay for 0⁺ ar 2⁺ transition T ₁ ^2/2v (0⁺ →²⁺) has been calculated using the same set of intrinsic wave functions.     

Lifetime measurements in99Sr and100Zr

The lifetime of the first excited state in⁹⁹Sr has been determined by the delayed γ-γ coincidence method. The value of t_/2=0.58(9) ns corresponds to a deformation parameter ß=0.38(4). The deduced z> value confirms the previously proposed ν[411]3/2 Nilsson assignment for the⁹⁹Sr ground state. In addition, lifetime values for the 2⁺ (t_1/2=0.40(8) ns) and 0⁺ (t_1/2=5.36(23) ns) states in¹⁰⁰Zr are obtained, which deviate from older measurements and indicate a larger deformation of ß=0.39(4).     

Decay of three-particle high spin states in85Y

High spin states in⁸⁵Y have been excited in the reaction⁷²Ge(¹⁶O,p2n) at 48–60 MeV beam energy. From measurements ofnγ andγγ coincidences, excitation functions and angular distributions, the high spin spectrum of⁸⁵Y has been established up to 5.4 MeV excitation energy and spinI≦29/2. Lifetimes or limits of lifetimes have been determined for 14 levels via the recoil distance and Doppler shift attenuation method. Theg _9/2 decoupled proton band is found to backbend atI ^π=17/2⁺ as also seen in the reduction of the 17/2⁺→13/2⁺ and 21/2⁺→17/2⁺ E2 transition strengths. The lowest (3qp) positive parity band has predominantly aν ² g _9/2×πg _9/2 structure. The observed negative parity yrast states in the spin range 15/2^?≦I≦29/2^? are most probably of (g _9/2)² ×(f _5/2,p _1/2) (3qp) nature as suggested by a comparison of the⁸⁴Sr and⁸⁵Y level structures and electromagnetic transition strengths.     

Enhanced octupole collectivity at double subshell closure in the96Zr region

A value of 84 ± 44 ps has been established for the half-life of the first 3^? state of⁹⁶Zr, using the centroid shift method. It is shown that the corresponding large electric octupole transition strength is in accordance with systematics, suggesting largest octupole collectivity when both the 1g_9/2 Proton and 1h_11/2 neutron orbitals are nearly empty due to the Z=40, N=56 double subshell closure.     

FastM1 transitions in85Rb

Excited states in⁸⁵Rb have been studied via the⁸²Se(⁷Li, 4n) reaction by using in-beamγ-ray spectroscopy. On the basis of new experimental data on coincidence relations, angular distributions and linear polarizations of theγ-rays the yrast sequence has been extended up to a 33/2⁺ level at 7.1 MeV excitation energy. Using the DSA method mean lifetimes have been determined for 9 levels. Above the 21/2⁺ yrast state 5 fast M1 transitions withB (M 1) ≧0.3 W.u. have been identified. They are interpreted as transitions between recoupled states of the configuration πg_9/2ν(g_9/2) ₈₊ ^?2 or πg_9/2(f ₅ ² /?1p ₃ ^2?1 )₄₊ν(g_9/2) ₈₊ ^?2 .     

The147Sm(n,α)144Nd Reaction on 2 keV neutrons

Through (p,2n) in-beam γ and e^? measurements the excitation of the 235 μs 9⁺ isomer in¹⁴⁶Eu has been determined to be 666.3 keV. High spin members of the πd ₅ ^2/?1 νf_7/2 and πg ₇ ^2/?1 νf_7/2 particle hole multiplets populated in the isomeric decay were located.     

TDPADg-factor measurement of the 21/2+ state in89zr

Theg-factor of the 21/2⁺ state in⁸⁹Zr has been determined to be 0.89(4). The TDPAD method has been applied in the reaction⁸⁵Rb(⁷Li, 3n)⁸⁹Zr at 28 MeV. The obtained value agrees with shell model predictions.     

Magnetic moment of the 96.7 keV 9/2+ level in79Rb

Theg-factor of the 9/2⁺ 96.7 keV excited state with a half-lifeT _1/2=18.6(5) ns in⁷⁹Rb populated in the⁷⁹Br(³He, 3n) reaction has been measured by the TDPAD method. The experimentalg-factorg _exp=+1.117(16) has been analysed in the frame of particle-rotor models and gave evidence for a large deformation of this state.