Energy levels of 92Nb and 94Tc from 92Zr, 94Mo(p,nγ)92Nb, 94Tc

The γ-decays of the energy levels of ⁹²Nb and ⁹⁴Tc were measured following the ⁹²Zr(p, n)⁹²Nb and ⁹⁴Mo(p, n)⁹⁴Tc reactions. For ⁹²Nb the γ-ray thresholds were measured and a detailed excitation function was obtained near the ⁹³Zr ground-state isobaric analog resonance. Also, γ-γ coincidence measurements with two Ge(Li) detectors were made for both ⁹²Nb and ⁹⁴Tc. The coincidence measurements determined ten energy levels in ⁹²Nb and two levels in ⁹⁴Tc. A comparison with particle transfer reactions is used to elucidate the structure of ⁹²Nb levels observed here but not seen in the transfer reactions.     

Absence of proton weak coupling states in93Nb

The reactions⁹³Nb(p, p)⁹³Nb,⁹³Nb(p, n)⁹³Mo, and⁹³Nb(p, p′) through isobaric analog resonances in⁹⁴Mo have been used to study the structure of low-lying⁹³Nb states, specifically the degree to which these states can be described as single protons coupled to the core states of⁹²Zr. Excitation curves were obtained from 5.8 to 7.6 MeV incident proton energy, for eight states in⁹³Nb. Results of this work are compared to results from Coulomb excitation studies,⁹³Nb(n, n′γ),⁹³Nb(d,p),⁹²Zr(³He,d),⁹⁴Mo(d,³He) and other related work.     

Gamow-Teller strength and giant resonances in 90Zr(n,p) at 198 MeV

The ⁹⁰Zr(n,p)⁹⁰Y reaction has been studied at a bombarding energy of 198 MeV in a search of GT⁺ strength. Up to 10 MeV excitation in ⁹⁰Y, we obtain a firm upper limit of S_β⁺ ⩽ 3.6. The majority of the strength at low excitations is identified as spin dipole.     

The excitation functions and Isomer ratios for neutron-induced reactions on Mo and Zr

Absolute cross sections of the reactions ⁹²Mo(n, 2n)^91m,91gMo, ⁹²Mo(n, p)⁹²Nb and ⁹²Mo(n, α)^{89m, 89g}Zr, relative cross sections of the reaction ⁹⁰Zr(n, 2n) ^89mZr and isomer ratios of the ⁹⁰Zr(n, 2n) reaction have been measured in the neutron energy range 13–15 MeV. The results for the (n, 2n) reactions are in good agreement with those of the previous studies. The present results for the (n, p) and (n, α) reactions are in disagreement with the previous works. The experimental data are analysed by the statistical model to determine the level-density parameter a, the moment of inertia ? and the strength of the γ-ray transition S_l in order to simultaneously reproduce the experimental data on the excitation function and the isomer ratio in the (n, 2n) reaction. The γ-ray competition, the yrast level and the experimental information on the excited levels of the residual nucleus in the (n, 2n) reaction are taken into account. The obtained values of a, ? and S_l are consistent with those deduced from other types of nuclear data.     

High-spin states in 90Nb

The high-spin states in ⁹⁰Nb have been studied by in-beam γ-ray spectroscopy using the 35 MeV ⁸⁹Y(α, 3nγ)⁹⁰Nb and 33 MeV ⁹⁰Zr(³He, p2nγ)⁹⁰Nb reactions. A new isomeric state with half-life 0.44±0.02 σs and J^π = 11^? has been located in this nucleus. The level scheme derived from these measurements is compared with shell-model calculations.     

The level scheme of91Nb from the reaction90Zr (p, γ)91Nb

The level scheme of⁹¹Nb has been investigated with the reaction⁹⁰Zr(p, γ)⁹¹Nb. Proton energies between 3.0 and 7.2 MeV were used. The γ spectra were taken with Ge(Li) detectors. Primary γ transitions to 36 excited states of⁹¹Nb up to 3.8 MeV excitation energy and many secondary γ transitions from the decay of those states were observed, leading to an extension of the known level scheme. The proton binding energy for⁹¹Nb was determined as (5167± 5) keV. Eleven γ transitions in⁹⁰Zr, part of them new, from the competing reaction⁹⁰Zr(p, p′ γ) were also observed.     

Excitation functions for the production of90Nb and88Y by irradiation of zirconium with deuterons

Excitation functions for reactions induced by deuterons on natural zirconium and⁹⁰Zr targets have been measured radiochemically with the stacked foil technique between ≈9 and 27 MeV. The observed reactions are Zr(d,xn)⁹⁰ Nb, Zr(d,axn)⁸⁸Y,⁹⁰Zr(d, 2n)⁹⁰Nb and⁹⁰Zr(d, α)⁸⁸Y. The excitation functions for the reactions⁹¹Zr(d, 3n)⁹⁰Nb and⁹¹Zr(d, αn)⁸⁸Y have been deduced from the results mentioned above. Calculations of the excitation function for the (d, 2n) reaction have been performed with two different treatments, each one taking into account two competitive mechanisms. The compound-statistical model plus Hittmair's stripping theory accounts quite well for the (d, 2n) cross sections observed. However, the agreement obtained with Peaslee-Otozai's theory is excellent and the set of parameters used more reasonable. It has been assumed that the stripping mechanism can have no contribution to the (d, α) reaction. Accordingly, calculations for this reaction have been done using the compound-statistical model and the entire absorption process followed by the evaporation of an α particle. No good agreement is obtained with either theory.     

Study of energy levels of 92Nb observed in the 92Zr(p,n) reaction

The energy levels of ⁹²Nb were observed in the ⁹²Zr(p, n) reaction by use of the neutron time-of-flight technique. Limits on the spins and parities of the ⁹²Nb levels were placed by observing the enhancement in the relative yield of the final states at an isobaric analog resonance and by the measurement of the relative yield off resonance. The measured enhancements and off-resonance yields are compared with values calculated using the statistical compoundnucleus model. Comparison of results from the two methods is used to obtain spin-parity assignments for nine energy levels lying between 976 and 1739 keV in ⁹²Nb.     

Two-proton pickup in the zirconium region using the (6Li, 8B) reaction

Spectra at θ_lab = 8° have been measured for the ^{92, 94, 96, 98, 100}Mo(⁶Li, ⁸B)^{90, 92, 94, 96, 98}Zr and ^{90, 92, 94}Zr(⁶Li, ⁸B)^{88, 90, 92}Sr two-proton pickup reactions at 90 MeV. The strongest transitions to levels in the Zr isotopes are the 0⁺ → 0⁺ transitions to the g.s. and first excited 0⁺ states. The salient feature of the five Zr spectra is the large increase in 0⁺₂ transition strength relative to that of the g.s. which is observed in the two heaviest isotopes, ^{96, 98}Zr. In ⁹⁰Zr, the 0⁺₂ transition is weak whereas it is the dominant transition in ⁹⁸Zr with twice the g.s. strength. These large variations in relative cross section, which are reproduced by DWBA calculations, are attributed to changing g.s. proton configurations in Zr. In the Sr spectra, which are distinctly different from the Zrspectra, the strongest transitions to excited states are to 2⁺ rather than 0⁺ states. The (⁶Li, ⁸B) reaction seems to be adequately described as a one-step cluster transfer of a T = 1, S = 0 proton pair. The suitability of this reaction for measurements of two-proton pickup is discussed.     

Radiative strength functions for dipole transitions in 90Zr

Partial cross sections for the (p, γ) reaction on the ⁸⁹Y nucleus that were measured previously at proton energies between 2.17 and 5.00 MeV and which were averaged over resonances were used to determine the absolute values and the energy distribution of the strength of dipole transitions from compound-nucleus states to low-lying levels of the ⁹⁰Zr nucleus. The data obtained in this way were compared with the predictions of various models.     

Damping of the giant spin-flip dipole and spin-flip quadrupole charge exchange modes in 90Zr

The damping of the giant spin-flip dipole (L=1, S=1, T=1, J^π=0⁻, 1⁻, 2⁻) and spin-flip quadrupole (L=2, S=1, T=1, J^π=1⁺, 2⁺, 3⁺) charge exchange resonances in ⁹⁰Zr is calculated in a microscopic nuclear structure model which includes both one-particle-one-hole (1p1h) and two-particle-two-hole (2p2h) configurations in a systematic way. It is shown that the coupling to 2p2h configurations gives rise to a strongly energy dependent spreading width which shifts a large fraction of transition strength to high excitation energies. The implications for the analysis of intermediate energy ⁹⁰Zr(p,n) data is discussed.     

Missing Gamow-Teller strength in mass 42

The reaction ⁴²Ca(p,n)⁴²Sc at E_p = 160 MeV is used to measure the Gamow-Teller (GT) strength function. Normalization of the dominant peak in the (p,n) spectrum to B(GT) determined from the analogous transition in the beta decay of ⁴²Ti renders the strength function absolute, and the total measured stregth is about half of the minimum value required for a T = 1 nucleus. Shell model calculations are presented which reproduce the shape of the strength function, but overpredict the absolute measured strength by about a factor of two. Evidently the missing strength has been moved out of the region of nucleon particle-hole excitations, and quenching, due possibly to Δ₃₃ coupling, is indicated. Symmetry implications of an observed strong suppression of the T_> component of the GT strength are discussed.     

Energy dependence of one- and two-particle transfer reactions in the system16O+90Zr

The energy dependence of the one- and two-particle transfer reactions⁹⁰Zr(¹⁶O,¹⁷O)⁸⁹Zr,⁹⁰Zr(¹⁶O,¹⁵N)⁹¹Nb and⁹⁰Zr(¹⁶O,¹⁴C)⁹²Mo was studied at bombarding energies of 80MeV, 138.2 MeV and 194.4 MeV. A comparison with one-step DWBA calculations shows good agreement for the one-particle transfers over the whole energy range. For the two-proton transfer reaction (¹⁶O,¹⁴C) the discrepancies between experiment and theory are large with an exponential decrease towards higher energies. Current theories are unable to describe this behavior.     

Statistical model calculations for medium and strong absorption in theA=90 region

Calculations of elastic, inelastic and total neutron cross sections as well as of (p, n) reaction cross sections in theA=90 mass region have been performed, using the statistical model of Hofmann, Richert, Tepel and Weidenmüller. The optical model parameters were obtained by Finckh et al. from the neutron decay of IAR in the⁹¹Zr(p, n)⁹¹ Nb reaction. The calculations are in very good agreement with the published experimental data. The optical model parameters proved to be satisfactory up to aboutE _n=8.0 MeV. The present work might be relevant to the evaluation of neutron data for reactor technology.     

Investigation of pairing vibration states in 88Sr and 90Zr via the (d, 6Li) reaction and search for α-vibration states

With a 45 MeV deuteron beam the ⁹⁴Mo(d, ⁶Li)⁹⁰Zr and ⁹²Zr(d, ⁶Li)⁸⁸Sr reactions have been studied to search for α-vibration states in ⁹⁰Zr and in ⁸⁸Sr. No evidence is found for such a state in both nuclei with a strength > 5 % than that of the ground state. Strongly excited 0⁺ states in ⁸⁸Sr and ⁹⁰Zr are those also seen in two-nucleon stripping and pickup reactions.     

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.     

Origin of quenching of Gamow-Teller strength in54Fe

The recent charge exchange (n,p) and (p,n) reaction data on⁵⁴Fe is used to determine the importance of Δ-isobar excitations and RPA-type correlations in the quenching of GT strength.     

Scattering and transfer reactions for 6, 7Li + 90, 91Zr

Elastic scattering data have been measured for the ⁷Li + ⁹⁰Zr, ⁶Li + ⁹⁰Zr, and ⁶Li + ⁹¹Zr systems at E(Li) = 34 MeV. Inelastic scattering data for the ⁷Li + ⁹⁰Zr and ⁶Li + ⁹⁰Zr systems were also measured for the 2⁺(2.18 MeV) and 3^?(2.75 MeV) states in ⁹⁰Zr and the $\text{1}\text{2}{}^{\mathrm{?}}\text{(0.48}\text{MeV}\text{)}$ state in ⁷Li. Optical model analyses of the elastic scattering data and DWBA analyses for the states in ⁹⁰Zr were performed. The deduced deformation lengths for the 2⁺ state agreed with those extracted in other studies but the deformation length for the 3^? state was smaller. The ⁹⁰Zr(⁷Li, ⁶Li)⁹¹Zr angular distributions were measured for the 1.21 and 2.03 MeV states and the 2.19 MeV doublet in ⁹¹Zr. Also, ⁹⁰Zr(⁷Li, ⁶He)⁹¹Nb angular distributions were measured for the ground states, 0.10, 3.41 and 4.82 MeV states in ⁹¹Nb. The transitions well matched in angular momentum were described by finite-range DWBA calculations, while other transitions displayed the same phase problems seen with heavier ions. The extracted spectroscopic information was consistent with the results of other reaction studies. At the present energy, it was not possible to determine whether the l = 1 phase problem that occurs for heavy-ion single-nucleon transfer reactions on 2s-1d shell nuclei occurs in ⁹¹Zr also.     

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.