Double beta decay of 96Zr

After 10357 h of running the NEMO-2 tracking detector with an isotopically enriched zirconium source (0.084 mol yr of ⁹⁶Zr), a ββ2ν decay half-life of T_1/2=(2.1^+0.8(stat)_−0.4(stat)±0.2(syst))·10¹⁹ y was measured. Limits with a 90% C.L. on the ⁹⁶Zr half-lives of 1.0·10²¹ y for ββ0ν decay to the ground state, 3.9·10²⁰ y to the 2⁺ excited state and 3.5·10²⁰ y for ββ0νχ⁰ decay with a Majoron (χ⁰) were obtained. The data also provide direct limits at the 90% C.L. for the ⁹⁴Zr half-lives. These limits are 1.1·10¹⁷ y for ββ2ν decay to the ground state, 1.9·10¹⁹ y for ββ0ν decay to the ground state and 2.3·10¹⁸ y for ββ0νχ⁰ decay to ground state.     

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.     

Theoretical study of fusion reactions 32S + 94,96Zr and 40Ca + 94,96Zr and quadrupole deformation of 94Zr

The dynamic coupling effects on fusion cross sections for reactions~(32)S + ~(94,96)Zr and ~(40)Ca + ~(94,96)Zr are studied with the universal fusion function formalism and an empirical coupled channel(ECC) model. An examination of the reduced fusion functions shows that the total effect of couplings to inelastic excitations and neutron transfer channels on fusion in ~(32)S +~(94)Zr(~(40)Ca +~(94)Zr) is almost the same as that in ~(32)S +~(96)Zr(~(40)Ca +~(96)Zr). The enhancements of the fusion cross section at sub-barrier energies due to inelastic channel coupling and neutron transfer channel coupling are evaluated separately by using the ECC model. The results show that effect of couplings to inelastic excitations channels in the reactions with94 Zr as target should be similar as that in the reactions with ~(96) Zr as target. This implies that the quadrupole deformation parameters β_2of ~(94)Zr and~(96) Zr should be similar to each other.However, β_2 's predicted from the finite-range droplet model, which are used in the ECC model, are quite different. Experiments on~(48) Ca +~(94)Zr or~(36) S +~(94)Zr are suggested to solve the puzzling issue concerning β_2for~(94)Zr.     

Role of thermal and photobeta decays in processes of nucleosynthesis of problematic p nuclei of 113In, 115Sn, 92,94Mo, and 96,98Ru in massive stars

The temperature dependence of the rates of ¹¹³Cd →¹¹³In, ¹¹⁵In →¹¹⁵Sn, ⁹²Zr→⁹²Nb →⁹²Mo, ⁹⁴Zr→⁹⁴Nb →⁹⁴Mo, ⁹⁶Mo →⁹⁶Tc→⁹⁶Ru, and ⁹⁸Mo→⁹⁸Tc→⁹⁸Ru thermal beta transitions was studied at temperatures of massive-star matter in the range of 1 × 10⁸–6 × 10⁹ K. These decays are the possible channel of synthesis of the p nuclei of ¹¹³In, ¹¹⁵Sn, ⁹²,⁹⁴Mo, and ⁹⁶,⁹⁸Ru. The abundances of these nuclei present a challenge for models that study the explosivemechanism of synthesis. The contribution of photobeta decay to the synthesis of the aforementioned p nuclei was estimated. It was shown that the channel of thermal beta decay for ¹¹³In, ¹¹⁵Sn, ⁹⁴Mo, and ⁹⁸Ru p nuclei and the channel of photobeta decay for the ⁹⁶Mo p nucleus could be efficient at the high-temperature quasiequilibrium stage of massive-star evolution.     

The Zr(d,n)Nb and Zr(d,n)Nb reactions

The (d, n) reaction on ⁹⁰Zr and ⁹⁶Zr has been studied at 12 MeV deuteron bombarding energy using the neutron time-of-flight technique with an overall neutron time resolution of 1.9 ns. Angular distributions of neutron groups leading to states in ⁹¹Nb and ⁹⁷Nb were measured in the angular range between 15° and 60°. The measured cross sections were analyzed in the framework of the distorted-wave theory of stripping reactions to deduce l-values and proton spectroscopic factors of states in the residual nuclei. The results are compared with the corresponding data available from (³He, d) studies. The fractionation of the single-particle proton states and their centroid energies are determined.     

The[g 9 2/2 ]8+ state in96Zr

An 8⁺ state has been identified at 4390 keV in the doubly semi-magic nucleus⁹⁶Zr through the study of angular correlations among the γ radiation which is emitted in the β^? decay of the 10 s isomer of⁹⁶Y. This state probably has the configuration πg ₉ ^2/2 and is populated through a GT decay from the 8⁺ member of the [πg_9/2?νg_7/2] multiplet in⁹⁶Y. The deexcitation indicates that it belongs to a band based on the 0⁺ state in⁹⁶Zr.     

Studies of multi-nucleon transfer reactions in <Superscript>90</Superscript>Zr(<Superscript>18</Superscript>O,X) and <Superscript>90</Superscript>Zr(<Superscript>16</Superscript>O,X)

Multi-nucleon transfer reactions in ¹⁸O + ⁹⁰Zr and ¹⁶O + ⁹⁰Zr have been studied at an incident energy of 90 MeV. The energy spectra and angular distributions are measured. The data have been analyzed to obtain cross-section dependence on the number of nucleons transferred and on the ground-state Q-values. In the ⁹⁰Zr(¹⁸O, X); X = ¹⁶O, ^17,16,15,14N, ^14,13,12C, ^12,11,10B, ^10,9,7Be and ^7,6Li reactions, 2n and 2n-correlated transfer cross-sections are observed to be enhanced as compared to the ¹⁶O + ⁹⁰Zr reaction. A detailed comparison in the multi-particle stripping and elastic-scattering cross-section between these two systems are made in order to investigate the possible influence of the two valence neutrons in ¹⁸O nucleus. Diffractional model DWBA calculations, based on the direct surface transfer model, have been performed to understand the reaction mechanism of multi-nucleon transfer to continuum.Received: 12 September 2002, Revised: 24 September 2003, Published online: 27 January 2004PACS: 25.70.Hi Transfer reactions - 25.70.Bc Elastic and quasielastic scattering     

Lifetime of the first excited state in Zr

The reaction ⁹⁶Zr(p, p′ e) was used to measure the lifetime of the 0₂⁺(1.59 MeV) state in ⁹⁶Zr through the observation of delayed coincidences between a pulsed proton beam and the E0 internal conversion electrons. The half-life is 3.80 ± 1.5 ns. This value is compared to a simple shell model calculation.     

Transfer reactions and sub-barrier fusion in 40Ca + 90, 96Zr

The two systems ⁴⁰Ca + ^90,96Zr have been studied by measuring nucleon transfer reactions at two energies near the Coulomb barrier, thus complementing the available sub-barrier fusion cross-sections. Angular distributions for various transfer channels have been determined. Significantly larger neutron transfer cross-sections are found for the target ⁹⁶Zr that exhibits the larger enhancement in the sub-barrier fusion cross-sections. All data have been analyzed with a new model for heavy-ion collisions that calculates simultaneously transfer cross-sections, fusion excitation functions and barrier distributions. The model gives a good account of both transfer and fusion data. Received: 2 May 2002 / Accepted: 4 June 2002 / Published online: 26 November 2002 RID="a" ID="a"e-mail: montagnoli@pd.infn.it, Fax +39049 8277102, Tel. +39049 8277117. RID="b" ID="b"On leave from the China Institute for Atomic Energy, 102413 Beijing, China. Communicated by C. Signorini     

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.     

Investigation of double-beta decay with the NEMO-3 detector

The double-beta-decay experiment NEMO-3 has been taking data since February 2003. The aim of this experiment is to search for neutrinoless (0 νββ) decay and investigate two neutrino doublebeta decay in seven different isotopically enriched samples (¹⁰⁰Mo, ⁸²Se, ⁴⁸Ca, ⁹⁶Zr, ¹¹⁶Cd, ¹³⁰Te, and ¹⁵⁰Nd). After analysis of the data corresponding to 3.75 yr, no evidence for 0 νββ decay in the 100Mo and 82Se samples was found. The half-life limits at the 90% C.L. are 1.1 × 10²⁴ and 3.6 × 10²³ yr, respectively. Additionally for 0 νββ decay the following limits at the 90% C.L. were obtained, >1.3 × 10²² yr for ⁴⁸Ca, >9.2 × 10²¹ yr for ⁹⁶Zr, and >1.8 × 10²² yr for 150Nd. The 2 νββ decay half-life values were precisely measured for all investigated isotopes.     

Nuclear moments around the Z=40 shell closure:91mY,95Zr and97Nb

Magnetic moments around the Z=40 shell closure have been established using nuclear magnetic resonance on oriented nuclei (NMR/ON) in iron. From the resonance frequencies we established |μ(⁹¹Y;9/2⁺)|=5.96(6)μ_N, |μ(⁹⁵Zr;5/2⁺)|= 1.103(23)μ_N, | μ(⁹⁷Nb;9/2⁺)| = 6.153(5)μ_N. From the electric quadrupole alignement of⁹⁵Zr+⁹⁵Nb in a Zr single crystal Q(⁹⁵Zr)=+0.29(5)b and Q(⁹⁵Nb)<0 have been derived. The results obtained are discussed using the Nilsson deformed single particle model. It is shown that for certain deformation regions, a measurement of the magnetic moment can give information on the nuclear quadrupole deformation. the NICOLE-ISOLDE Collaboration, CERN     

Decay of97Zr to nuclear states of97Nb

The gamma rays emitted in disintegration of⁹⁷Zr have been investigated by Ge(Li) detectors, operated singly and in coincidence. In all, quanta emitted in thirty-five nuclear transitions have been observed. The gamma ray energies and relative intensities have been measured. Beta spectra have been studied with Ge(Li) detector and anthracene counter in coincidence. Nine beta spectra have been identified as to end point and intensity, either by direct measurement or by inference from the gamma ray data. The results of all measurements have been combined to provide a decay scheme for⁹⁷Zr. Nuclear excited states in⁹⁷Nb are indicated at energies of 743, 1,148, 1,251, 1,276, 1,434, 1,548, 1,652, 1,750, 1,765, 1,852, 1,958, 2,056, 2,106, and 2,247 keV. Possible spin-parity assignments have been suggested for some of these states.     

Der Zerfall des Co60m und Zr90m

Co^60m was produced by the reaction Co⁵⁹ (n,γ) Co^60m ; its decay has been investigated by scintillation spectrometers. The half-period has been remeasured yielding a value ofT _1/2=(10·35 ±0·20) min. The isomeric decay mainly leads to the Co⁶⁰ ground state by a (58·6±0·6) keV transition. TheK-conversion coefficientα _K= 41±3 and the total conversion coefficientα=47±3 indicate the transition to beM3 and the spin of Co^60m to be 2⁺. Further Co^60m decays by beta transitions to Ni⁶⁰. The beta component (8·6±1·2)·10^?3%, log ft=7·32 leading to a (2·16±0·02) MeV level has been measured for the first time. A second component (0·24±0·03)%, log ft=7·23 leads to the 1·33 MeV level. The 2·16 MeV level decays to the Ni⁶⁰ ground state mainly by a 0·83–1·33 MeV cascade, in a few cases by a direct transition. The spin of the 2·16 MeV level is 2⁺. Zr^90m was produced by the reaction Zr⁹⁰ (n,n′) Zr^90m . The half-period was measured yielding a value ofT _1/2=(0·83±0·03) sec. Zr^90m decays in the following way: (77±5) % by a direct (2·31±0·02) MeV —E 5 transition to the Zr ground state and (23±5)% by a (132·5±1·5) keV-(2·18±0·03) MeV cascade detected for the first time in the isomeric decay. TheK-conversion coefficient of the 132·5 keV transition isα _k=(2·2±0·3) indicating this transition to beE3 and the spin of the 2·18 MeV level to be 2⁺. The Zr^90m -decay sheme is in accordance with the level sequence of Zr⁹⁰ as measured in the Nb⁹⁰ decay.     

Investigation of neutron deficient Zr and Nb nuclei with heavy ion induced compound reactions

³²S and¹²C induced compound reactions on Fe, Ni and Se targets have been used to produce neutron deficient nuclei in the mass region 84≦A ≦87. In-beamγ-ray spectroscopy consisting of the measurement of excitation functions,γγ coincidences andγ-ray angular distributions has been performed. The following level energies and spin-parity values have been deduced:⁸⁴Zr: 540 keV, 2⁺; 1262.8, (4⁺); 2137.2, (6⁺); 3089.2⁸⁶Zr: 751.9, 2⁺; 1666.6, (4⁺); 2670.0, (6⁺); 2705.5, (5); 3271.3; 3298.5, (8⁺); 3423.5, (7); 3531.5, (10⁺); 3645.9; 4326.1; 4417.3 Half-lives and energies ofγ-rays from the residual activities have been measured. The existence of three new isotopes could be established by their radioactive decay:⁸⁴Nb(12±3 s)→⁸⁴Zr⁸⁶Nb(80±12 s)→⁸⁶Zr⁸⁷Mo(14.6±1.5 s)→⁸⁷Nb→⁸⁷Zr     

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.     

Search for double- \beta decays of 96Ru and 104Ru by ultra-low background HPGe \gamma spectrometry

A search for double- $ \beta$ processes in ⁹⁶Ru and ¹⁰⁴Ru was realized in the underground Gran Sasso National Laboratories of the INFN (Italy) with the help of ultra-low background HPGe $ \gamma$ spectrometry. After 158h of data taking with 473g ruthenium sample, new improved limits on double-beta processes in ⁹⁶Ru have been established on the level of 10¹⁸-10¹⁹yr. For the first time the double-beta decay of ¹⁰⁴Ru to the 2⁺ ₁ excited level of ¹⁰⁴Pd has been restricted as T _1/2 > 3.5×10¹⁹ yr.     

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.     

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.