Study of analogue states in52Cr through proton capture by51V

γ-Ray yield function has been studied for the proton capture by vanadium in the proton energy range 720–1300 keV. Isobaric analogues of low lying states in⁵²V have been identified. At two resonances theγ-decay andγ-ray angular distributions have been obtained and the branching ratios and the multipole mixing ratios have been deduced. The analogue-antianalogue M1 transition in⁵²Cr is found to be strongly hindered as in other f_7/2 nuclei. TheQ-value obtained for this reaction is (10500±2.8) keV and the Coulomb displacement energy is (8.06±0.01) MeV. An upper limit of 2 meV has been obtained for theα- decay strength of the 11.395 MeV state in⁵²Cr.     

Structure of28Si from the spectroscopy of high-spin states

A search for high-spin states in²⁸Si has been performed byn?y coincidence measurements in the²⁵Mg(α,nγy) reaction atE _α=14 and 15.5 MeV. Spin-parity assignments of the observed levels were obtained fromn?γ angular correlation and lifetime measurements atE _α=14.5 MeV. Theγ-decay of the 9,164 keV level was investigated separately with the²⁷Al(p, γ) reaction at theE _p=2,160 and 2,312 keV resonances. Rotational bands withK ^π=3^? (comprising levels atE _x=6,879, 8,413, 10,188 and 12,204 keV),K ^π =5^? (comprising levels atE _x=9,702, 11,577 and 13,741 keV) andK ^π=0⁺ (comprising levels atE _x=6,691, 7,381, 9,164 and 11,509 keV) were observed. The finding of the latter band supports the idea of coexisting oblate and prolate shapes in²⁸Si. A level at 14,643 keV excitation energy has the properties of theI ^π=8⁺ member of the ground state band. There are additional positive-parity high-spin states which do not fit into rotational bands. All types of positive-parity states are well accounted for by shell model calculations.     

The structure of28Si above 10 MeV excitation energy II: Assignments of quantum numbers

New assignments of quantum numbers have been obtained for more than 50 levels in²⁸Si between 8953 and 15915 keV excitation energy. They are based on the measurement of -ray angular distributions or anisotropies on 29 resonances of the²⁷Al(p, ) reaction and on the²⁴Mg(a, ) resonance atE =3355 keV. A total of 15 high-spin states withI=5–7 has been obtained and the spectrum ofT=1 states has been identified up to 15915 keV excitation energy and a maximum spinI=6.     

Narrow α + <Superscript>28</Superscript>Si elastic-scattering states at high excitation in <Superscript>32</Superscript>S

The excitation function and angular distributions of elastic α-particle scattering on ²⁸Si have been measured in the laboratory energy range 6-28 MeV using a backscattering technique on a thick target, yielding a continuous energy distribution. More than 200 narrow states are observed, with widths in the range ∼ 30-100 keV at excitation energies E ^* = 13-32 MeV. Angular distributions at backward angles were measured, and angular momentum values of more than 83 states have been deduced. The analysis gives spin-parities J ^π, α-partial widths Γ_α and reduced widths of the narrow high-lying resonant states in ³²S. The experimentally observed states display both the negative- and the positive-parity rotational-like sequences with seemingly no parity splitting, a finding which is at variance with most potential-model predictions. The deduced effective moment of inertia indicates a more extended structure than the ground-state configuration. The observed strength of each ℓ-value is analyzed in terms of an underlying split doorway state of Lorentzian form, which yields an interpretation as fragmented rotational α + ²⁸Si states. Received: 26 June 2000 / Accepted: 16 September 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: kkallman@abo.fi RID="b" ID="b"Present address: Swedish Polytechnic, FIN-65200 Vasa, Finland. Communicated by D. Guerreau     

Isospin-forbidden β-decay of 28Mg

We have measured the branching ratio for the isospin-forbidden Fermi β-decay of ²⁸Mg to the 0.972 MeV 0⁺, T = 1 level in ²⁸Al. This decay occurs through an admixture of the 0⁺, t = 2 analog state at 5.992 MeV into the anti-analog level at 0.972 MeV. A charge-dependent matrix element of 17.0_?5.8^+4.3 keV is deduced from the observed (0.21 ± 0.12)% branch. Comparisons are made with matrix elements deduced in other nuclei. An analysis based on a simple shell model with fourfold degenerate Orbitals indicates the importance of the two-body Coulomb interaction in isospin mixing in nuclei with more than one valence proton.     

Isospin dependence of the nuclear level width in the compound nucleus 30P

The excitation functions of ²⁹Si(p, α₀)²⁶Al, ²⁹Si(p, α₁)²⁶Al and ²⁹Si(p, α₂)²⁶Al were measured with high beam energy resolution in order to determine the isospin dependence of the nuclear level width of the composite nucleus, ³⁰P, at an average excitation energy of 19.6 MeV. From an auto-correlation analysis of these excitation functions, the level widths of the T_< = 0 and T_> = 1 isospin states are determined as 81 ± 17 keV and 104 ± 35 keV, respectively. An analysis including isospin mixing is also performed. With the aid of the statistical theory of nuclear reactions, the coherence energies are used to deduce the relative densities of two isospin states. The predictions of the Fermi gas model of level densities including isospin are in good agreement with the obtained results.     

Photodisintegration of 28Si to excited residual states

Integrated cross sections up to 28 MeV have been measured to excited residual states in ²¹Si, ²⁷Al and ²⁴Mg, following the photodisintegration of ²⁸Si. From comparisons with spectroscopic factors of nucleon pickup reactions on ²⁸Si, leading to the same residual states, it is concluded that the photonucleon emission process is predominantly single particle in nature. Estimates of isospin mixing in the ²⁸Si giant dipole resonance are also given.     

Identification of the 109Xe and 105Te α-decay chain

The α-decay chain ¹⁰⁹Xe → ¹⁰⁵Te → ¹⁰¹Sn was identified at the Holifield Radioactive Ion Beam Facility. Advances in digital electronics have made possible the identification of both alpha emitters in the same experiment despite the disparate half-lives of 13 ±2 ms and 620 ±70 ns for ¹⁰⁹Xe and ¹⁰⁵Te, respectively. Two α-decay transitions were observed from ¹⁰⁹Xe with Q_α values of 4067 ±10 and 4217 ±8 keV. One transition between the ground states of ¹⁰⁵Te and ¹⁰¹Sn was observed with a Q_α value of 4889 ±6 keV. Using the measured half-lives, branching ratios, and Q_α values the reduced α-decay widths, δ², were determined. Comparison of the δ² value for ¹⁰⁵Te with ²¹³Po indicates a “superallowed" character in the α emission of ¹⁰⁵Te.     

The structure of28Si above 10 MeV excitation energy III: Level scheme and shell model interpretation

²⁸Si level scheme up to 14.5 MeV excitation energy is reevaluated using information from two preceding papers. It consists of approximately 250 levels which are almost completely characterized according to the quantum numbersI, π, T of the levels. The properties of positive-parity states are compared to the predictions of shell model calculations within the completes-d basis space using the unifieds-d shell Hamiltonian. A spectrum of 48 experimentalT=1 states between 9.3 and 16 MeV is reproduced with a rms deviation of only 150 keV. A calculation of radiative widths and γ-decay modes which uses free-nucleong-factors yields excellent agreement with experiment and confirms that quenching of M1 transitions is only marginal in²⁸Si. The detailed shell model analysis of theT=0 spectrum is extended to the limiting energy whereT=1 wave function admixtures, not contained in the theory, become important experimentally. This happens at 6–8 MeV above the yrast state, depending on the spin value. Altogether it appears that a spectrum of 171 levels below 14.5 MeV, which have positive or unassigned parity, is almost completely accounted for by the model. Apparent intruder states from outside thes-d shell space are observed atE _x =10 945 keV (I ^π=4⁺) and 12 860 keV (I ^π=6⁺) and are interpreted as members of aK ^π=0⁺ rotational band.     

Derp 3/2-Analogzustand in51V

In an investigation ofT=7/2 analogue states in⁵¹V the⁵⁰Ti(p, γ) excitation curve has been measured for proton bombarding energies 1280–1480 keV and 2340–2660 keV. From the (p, γ) resonances 29 new virtual levels in the region of 9316–9510 keV excitation energy in⁵¹V were determined. The strong resonance atE _p=1 371 keV has been identified as the isobaric analogue state of the⁵¹Ti ground state by determining spin and parity of this resonance to be 3/2^?. There is no evidence for a strong analogue resonance in⁵¹V corresponding to the 1.16 MeV _{p 1/2} state in⁵¹Ti. The γ-decay of the _{p 3/2} analogue state has been studied by measuring branching ratios and angular distributions of primary γ-transitions with a Ge(Li) detector.M1E2 mixing ratios have been determined for these transitions. The total width of the resonance for γ-decay is found to be Γ_γ=1.6±0.4 eV. New bound levels in⁵¹V have been introduced at 3576, 4651 and 4661 keV excitation energy. TheJ ^π values of the 3085, 4770, and 4863 keV states are determined to be 5/2^?, 5/2^?, 3/2^?, respectively. The analogue-antianalogueM1 transition strength is found to be considerably reduced compared to the situation ins-d shell nuclei.     

Nuclear spins, magnetic moments and α-decay spectroscopy of long-lived isomeric states in 185Pb

Alpha-decay properties of the neutron-deficient isotope ¹⁸⁵Pb were studied at the PSB-ISOLDE (CERN) on-line mass separator using the resonance ionisation laser ion source (RILIS). The nuclei of interest were produced in a 1.4 GeV proton-induced spallation reaction of a uranium graphite target. In contrast to previous studies, two α-decaying isomeric states were identified in ¹⁸⁵Pb. The relative production of the isomers, monitored by their α-counting rates, could be significantly changed when a narrow-bandwidth laser at the RILIS setup was used to scan through the atomic hyperfine structure. Based on the atomic hyperfine structure measurements, along with the systematics for heavier odd-mass lead isotopes, the spin and the parity of these states were interpreted as 3/2^- and 13/2⁺ and their nuclear magnetic moments were deduced. The α-decay energy and half-life value for the I ^π = 13/2⁺ isomer are E _α = 6408(5) keV, T _1/2 = 4.3(2) s, respectively; while for the I ^π = 3/2^- isomer ( T _1/2 = 6.3(4) s) two α-decays with E _α1 = 6288(5) keV, I _α1 = 56(2)% and E _α2 = 6486(5) keV, I _α2 = 44(2)% were observed. By observing prompt α-γ coincidences new information on the low-lying states in the daughter isotope ¹⁸¹Hg was obtained. Received: 7 February 2002 / Accepted: 19 February 2002     

Search for low-energy resonances in25Mg(p,γ)26Al

The reaction²⁵Mg(p,γ)²⁶Al has been investigated atE _p (lab)=80-350keV. The existence of the resonance atE _p =317keV has been verified and new resonances have been found atE _p =198, 255 and 304 keV. TheE _p = 304keV resonance represents a new compound state in²⁶A1. Information on branching ratios,Ωγ values, total widths andJ ^π assignments for the observed resonances as well as upper limits on themy strengths for the expected resonances atE _p =97 and 134 keV are given. The astrophysical aspects of the data are discussed in the light of the renewed interest in the precise location of²⁶A1 nucleosynthesis.     

The S-D mixing and dielectron widths of higher charmonium 1−− states

The dielectron widths of ψ(4040), ψ(4160), and ψ(4415), and their ratios are shown to be in good agreement with experiment, if in all cases the S-D mixing with a large mixing angle θ ≅ 34° is taken. Arguments are presented why continuum states give small contributions to the wave functions at the origin. We find that the Y (4360) resonance, considered as a pure 3 ³ D ₁ state, would have very small dielectron width, Γ _ee (Y (4360)) = 0.060 keV. On the contrary, for large mixing between the 4 ³ S ₁ and 3 ³ D ₁ states with the mixing angle θ = 34.8°, Γ _ee (ψ(4415)) = 0.57 keV coincides with the experimental number, while a second physical resonance, probably Y (4360), has also a rather large Γ _ee (Y (∼4400)) = 0.61 keV. For the higher Y (4660) resonance, considered as a pure 5 ³ S ₁ state, we predict the dielectron width Γ _ee (Y (4660)) = 0.70 keV, but it becomes significantly smaller, namely 0.31 keV, if the mixing angle between the 5 ³ S ₁ and 4 ³ D ₁ states has the characteristic value θ = 34°. The mass and dielectron width of the 6 ³ S ₁ charmonium state are calculated. The text was submitted by the authors in English.     

T=1 states in26Al

The resonance reaction²⁵Mg(p, γ)²⁶Al in the energy rangeE _p =300–400 keV was used to populate high-lying bound states in the self-conjugate nucleus²⁶Al. The existence of three resonances atE _p =304, 316 and 388 keV was verified, the spins of the ones at 316 and 388 keV were found to be 3^? and 2⁺, respectively. The spin of the bound level at 4,547 keV level was fixed to beJ ^π=2⁺. Several strong isovector Ml transitions were observed, which led toT=1 assignments for the levels atE _x =4,191, 4,547, 4,599, 5,141 and 5,542 keV. The results for excitation energies and values ofJ ^π,T together with previous experimental and theoretical data on²⁶Al and the neighboring |T _z |=1 nuclei²⁶Mg and²⁶Si are discussed in the framework of the isospin model.     

High resolution gamma-ray spectroscopy of the27Al(p,γ)28Si resonance reaction

High resolution gamma-ray spectra have been measured from the²⁷Al (p,γ)²⁸Si reaction for the resonances atE _p=2·482, 2·511 and 2·735 MeV at ϑ _pγ=0°, 30°, 55° and 90° using a Ge (Li) gamma spectrometer. From the spectra and the angular distributions the properties of the resonance states have been obtained. These states are the isobaric analogues of the levels at 4·69, 4·75 and 4·93MeV levels respectively in the parent nucleus A²⁸l.     

Radioactive decay of 217Pa

The radioactive decay of ²¹⁷Pa was investigated by means of α-γ-spectroscopy. Fine structure in the ground-state α-decay was established. Ambiguities in the fine structure of the α-decay of the previously known isomeric state could be clarified by α-γ-coincidence measurements. A previously unknown α-transition of E _α = (8306 ± 5) keV was detected and identified by means of delayed α-α- and α-γ-γ-coincidence measurements. A second isomeric state decaying by α-emission was not observed. The quality of the previously reported data of the α-decay fine structure of ²¹⁷Th was improved. Received: 29 April 2002 / Accepted: 17 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: f.p.hessberger@gsi.de Communicated by J. ?yst?     

Isospin impurity of 64Ga ground state

It is well known that the observation of a nonzero Fermi matrix element for a β-transition between states that differ in isospin can provide information about the relative isospin purity of the states involved. We have determined the log ft value for the 0⁺ → 0⁺ (ΔT ≠ 0) β-transition in ${}^{64}\text{Ga}\text{→}{}^{64}\text{Zn}$ decay as 6.516 ± 0.020. From this log ft value, we have deduced |M_F| = (43.4 ± 1.1) × 10^?3, |α| = (21.7 ± 0.6) × 10^?3 and |〈V_CD〉| = 41.7 ± 1.1 keV, where M_F,α² and 〈V_CD〉 represent the Fermi matrix element, isospin impurity and Coulomb matrix element, respectively. The Coulomb matrix element of 41.7 keV found for ⁶⁴Ga is one of the largest known from β-decay experiments. The experimental procedure involved a careful measurement of the intensity of the annihilation radiation relative to that of the other γ-rays from ⁶⁴Ga decay. As a by-product, we have obtained an improved ⁶⁴Ga decay scheme. We have also summarized the existing information on the isospin impurities of nuclear states as deduced from β-decay experiments.     

Stellar reaction rate of26Mg(p, γ)27Al

The reaction²⁶Mg(p, γ)²⁷Al has been investigated atE _p (lab)=80–355 keV. The existence of the resonances atE _p =292 and 338 keV has been verified and new resonances were found atE _p =227 and 275 keV. Information on branching and mixing ratios,ωγ values, total widths, andJ ^π assignments for the observed resonances is given. Upper limits on theωγ strengths for expected resonances in the energy range covered are presented. The astrophysical aspects of the data are discussed in the light of the²⁶Mg(²⁶Al) isotopic anomaly.     

Gamma-rays from the24Mg(p,γ)25Al reaction for proton energiesE p=800 keV toE p=1,830 keV

The yield curve and resonance strengths of resonances in the²⁴Mg(p, γ)²⁵Al reaction for the proton energy rangeE _p=800 keV toE _p=1,830 keV, have been determined and the existence of a resonance atE _p=1.184 MeV confirmed. Single energy spectra as well as decay schemes and branching ratios are given for these resonances. An upper level of 2.0×10^?15 sec for the lifetime of the 3.079 MeV level in²⁵Al results from a Doppler shift attenuation measurement carried out on the 2.624 MeV gamma line originating in the 3.070 MeV→0.455 MeV transition in²⁵Al.