Mass measurements of neutron-deficient nuclides close to A = 80 with a Penning trap

The masses of ^{80, 81, 82, 83}Y, ^{83, 84, 85, 86, 88}Zr and ^{85, 86, 87, 88}Nb have been measured with a typical precision of 7keV by using the Penning trap setup at IGISOL. The mass of ⁸⁴Zr has been measured for the first time. These precise mass measurements have improved S_p and Q_EC values for astrophysically important nuclides.     

g-Factor Measurements of Rotational States in 84,86Zr

The g-factors of the positive parity yrast rotational states in ^84,86Zr have been precisely measured using a TMF-IMPAD method. The measured g factors indicate the proton alignment followed by neutron alignment in the g _9/2 high-j shell for ⁸⁴Zr and the neutron alignment followed by proton alignment for ⁸⁶Zr. The present results also reveal the structure transition at N=46 for Zr isotopes. This revised version was published online in September 2006 with corrections to the Cover Date.     

The decays of the Tz = 32 β-delayed proton precursors 83Zr, 87Mo and 91mRu

The decays of the nuclides ⁸³Zr,⁸⁷Mo and ^91mRu have been studied with singles proton counting as well as with pX and pγ coincidences. The half-lives for the three nuclides were measured to be 37.8 ± 1.1 s, 13.3 ± 0.4 s and 7.6 ± 0.8 s, respectively. The proton branch in the decay of ⁸³Zr that populates the first excited state in ⁸²Sr was measured to be (6 ± 4) % of the total delayed proton decay. The corresponding branch in the decay of ⁸⁷Mo, populating the first excited state in ⁸⁶Zr was measured to be (15 ± 8) %. Restrictions to the possible spin and parity assignments of ⁸³Zr and ⁸⁷Mo were deduced from comparisons between the experimental excited-state branches, and those predicted from a statistical model of delayed particle decay. Fits to the endpoints of the delayed proton spectra from ⁸³Zr and ⁸⁷Mo yielded values of the energy available for delayed proton decay (Q_e.c.?B_p) of 2.75 ± 0.10 MeV and 3.7 ± 0.3 MeV, respectively. Comparison between calculated and experimental proton spectra was used as an argument that the source of ⁹¹Ru activity is actually a $\text{1}\text{2}{}^{\mathrm{?}}$ isomer.     

Heavy-ion in-beam studies of the nucleus87Nb

High spin states in the transitional nucleus⁸⁷Nb up to 14 MeV excitation have been established for the first time via the reactions⁴⁰Ca(⁵⁰Cr, 3p)⁸⁷Nb and⁵⁸Ni (³²S, 3p)⁸⁷Nb. The⁸⁷Nbγ-radiations have been identified throughγ-ray spectra taken in coincidence with the evaporation residues detected in the Daresbury recoil separator or with multiple proton emission. Gamma-gamma coincidences, DCO ratios,γ-ray angular distributions and lifetimes have been measured. A total of some 100 transitions have been placed into a level scheme comprising of sixty states. The one-quasiparticle (1qp) bands of either parity and several other band-like structures have been identified, some containing alignedg _9/2 nucleons. Moderately enhancedE2 in-band transitions of 13–48 W.u. as well as several weakE2 yrast transitions connecting bands with different quasiparticle numbers have been found. Similarities with respect to theN=46 isotones⁸³Rb,⁸⁴Sr,⁸⁵Y,⁸⁶Zr and⁸⁸Mo are discussed.     

Cross Section Measurements for (n,2n),(n,p)and (n,α) Reactions on Strontium Isotopes at the Neutron Energy About 14MeV

Cross-sections for ⁸⁴Sr(n, 2n)⁸³Sr, ⁸⁶Sr(n, 2n)^{85mSr,86Sr(n, 2n)85Sr,88Sr(n, 2n)87mSr,84Sr(n, p)84Rb,86Sr(n, p)86Rb, 88Sr(n, p)88Rb and 88Sr(n,α)85mKr reactions were measured at neutron energies from 13.5 to 14.6MeV using activation technique and γ-ray spectrometry. The neutron flux was determined using the monitor reaction 93Nb(n, 2n)92mNb and the neutron energies were measured by the method of cross section ratios for $^{90}$Zr(n, 2n)$^{89m+g}$Zr to $^{93}$Nb(n, 2n)$^{92m}$Nb reactions. The results of present work were compared with data published previously.}     

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     

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.     

Neutron inelastic scattering to octupole states in single-closed-shell nuclei

Differential cross sections for the excitation of the first octupole-vibrational state in the closed- neutron-shell nuclides ⁸⁸Sr and ⁹⁰Zr and in the closed-proton shell-nuclei ^{116, 118, 120, 124}Sn by 11 MeV neutrons are presented. The distorted-wave Born approximation is used to obtain deformation lengths, σ(3^?), for each state. Results are compared with earlier measurements of inelastic proton scattering to the same states. Although limited resolution in the neutron time- of-flight spectrometer complicates the interpretation of the Sn data, the overall conclusion that σ_nn′(3^?) ≈ σ_pp′(3^?) is supported by all of the measurements.     

Isomers of astrophysical interest in neutron-deficient nuclei at masses A = 81, 85 and 86

Decay properties of neutron-deficient exotic nuclei close to A=80 have been investigated at the IGISOL facility. The studied nuclei, ⁸¹Y, ⁸¹Sr, ^81mKr, ⁸⁵Nb, ⁸⁵Zr, ⁸⁶Mo and ⁸⁶Nb, were produced by a ³²S beam from the Jyv?skyl? isochronous cyclotron on ⁵⁴Fe and ^natNi targets. The internal conversion coefficient for a 190.5 keV isomeric transition in ^81mKr has been measured and the internal transition rate has been determined. The internal transition rate has been used to estimate a neutrino capture rate on ⁸¹Br, which yields a log ft of 5.13±0.09 for the reaction ⁸¹Br( ν, e ^-)^81mKr. A new isomer with a half-life of 3.3±0.9 s has been observed in ⁸⁵Nb. The existence of an earlier reported isomer with a half-life of 56 s in ⁸⁶Nb has not been confirmed.     

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.     

A study of87Zr by (α,x n) reactions

The reactions⁸⁴Sr(α,n)⁸⁷Zr and⁸⁶Sr(α, 3n)⁸⁷Zr were used to populate excited states in⁸⁷Zr. The de-excitation of these states was studied by in-beamγ-ray spectroscopy. A number of new high-spin states were observed. Lifetime measurements were also performed. The results are discussed within the frame-works of the shell model and the particle-core coupling model.     

Isomere Wirkungsquerschnittsverhältnisse beim thermischen Neutroneneinfang im Bereich der 2p 1/2 und 1g 9/2 Schalenmodellzustände

Thermal capture cross sections and isomeric cross section ratios in thermal neutron capture were determined for even-even nuclei in the region of the 2p _1/2-1g _9/2 neutron shell. Capture cross sections for formation of Zn 69 ^m , Zn 71 ^m , Zn 71, Ge 71, Ge 75 ^m , Ge 77 ^m , Se 77 ^m , Se 79 ^m , Se 83 ^m , Se 83, Sr 85 ^m and Sr 87 ^m were measured by the activation method. From these data and additional measurements of cross section ratios the isomeric ratios for thermal capture in Zn 68, Zn 70, Ge 70, Ge 74, Ge 76, Se 76, Se 78, Se 80, Se 82 and Sr 84 were determined. In addition epithermal capture isomer ratios were determined for Zn 68, Ge 74, Ge 76, Se 80 and Sr 84. The isomer ratios are compared with calculations based on the statistical model ofHuizenga andVandenbosch. With the exception of Se 80 the measured ratios can be explained theoretically with reasonable values of the spin-cutoff-factorσ and gamma-ray multiplicityN, however there seems to be unexplained difference in eitherσ orN between the two groups of nuclei leading to either 1/2–7/2 or 1/2–9/2 isomers.     

New neutron-deficient isotopes 83Nb and 85Nb

New neutron-deficient isotopes of ^83,85Nb were identified with the aid of results of the in-beam spectroscopic studies of ^83,85Zr. The activities were generated by the ⁵⁸Ni(²⁸Si, p2n)⁸³Nb, ⁶⁰Ni(²⁸Si, p2n)⁸⁵Nb and ⁵⁸Ni(³²S, αp)⁸⁵Nb, reactions, and the radiations were measured by means of βγ-coincidence. Then the half-lives of ⁸³Nb and ⁸⁵Nb were found to be 4.1 ± 0.3 and 20.9 ± 0.7 sec, and the Q_EC values were determined to be 7.5 ± 0.3 and 6.0 ± 0.2 MeV for ⁸³Nb and ⁸⁵Nb, respectively, which are noticeably lower than predicted values from mass formulae.     

Proton alignment in 82Sr investigated by g-factor measurements

The proton alignment in 82Sr has been investigated by the g-factor measurements of the ground state rotational band levels up to spin I = 8+. The g-factors were measured by a transient-magnetic-field ion implantation perturbed angular distribution method. The obtained g-factors increase with the increasing of spin along the band and clearly show the g9/2 proton alignment that starts at I = 6+.     

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.     

States in86Sr,84Sr and87Y populated in the (p,t) reaction

The (p,t) reactions on⁸⁸Sr,⁸⁴Sr and⁸⁹Y have been carried out at a bombarding energy of 49.5 MeV. Some low-lying excitations in⁸⁶Sr and⁸⁴Sr associated with configurations of 1g _9/2 and 2p _1/2 neutrons have been identified. Levels populated to 4 MeV of excitation in the strontium nuclei and to 2.5 MeV in⁸⁷Y have been studied in the present experiment. In the case of⁸⁷Y most of the levels populated are of a complex nature.     

Level schemes of100Y and100Zr from the beta-decay of mass separated100Sr

The beta-decay of¹⁰⁰Sr and the low-spin isomer in the daughter nucleus¹⁰⁰Y were investigated from sources of¹⁰⁰Sr produced from thermal neutron fission in a high-temperature ion source of the online-separator facility OSTIS installed at the Institut Laue-Langevin (ILL), Grenoble. Gamma-single, gamma-multispectra and gamma-gamma-coincidence measurements were performed resulting in the first level scheme of¹⁰⁰Y as well as a considerably extended scheme of¹⁰⁰Zr. The schemes are placed in level systematics and implications for the appearance of shape coexistence atN=60 are discussed.     

High-spin states in89Nb,88Zr and88Nb with a shell-model description of theN=48 andN=47 nuclei

The reactions⁸⁹Y(α,4nγ),⁸⁹Y(α,p4nγ) and⁸⁹Y(α,5nγ) were used to populate high-spin states in, respectively,⁸⁹Nb,⁸⁸Zr and the previously unstudied nucleus⁸⁸Nb. These states were deduced via in-beam gamma-ray spectroscopy. The results of a shell-model study ofN=48⁸⁸Zr,⁹⁰Mo,⁸⁷Y,⁸⁹Nb andN=47⁸⁷Zr,⁸⁸Nb nuclei are compared to experiment.