Beyond the N = 50 shell closure: High-spin excitations of 87Kr and ground-state spin of 87Br

The ⁸⁷Kr nucleus has been produced as fission fragment in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85MeV bombarding energy and studied with the Euroball IV array. High-spin states of this neutron-rich isotope have been identified for the first time. Its level scheme has been obtained up to 6.3MeV excitation energy and spin I ∼ 23/2ℏ. Its structure is interpreted by analogy with those of the heavier isotones. The proposed configurations involve both proton and neutron excitations from several sub-shells located close to the Fermi levels, particularly νd_5/2, πp_3/2f_5/2 and πg_9/2. Moreover, a revised spin value of 5/2^- for the ⁸⁷Br ground state is proposed.     

Measurement and analysis of alpha particle induced reactions on gold

Stacked foil activation technique and Ge(Li) gamma ray spectroscopy have been used for the measurement of excitation functions of¹⁹⁷Au(α,xn) (x=1−3),¹⁹⁷Au(α,2pn) and¹⁹⁷Au(α,αn) reactions up to 50 MeV. The experimental cross-sections were compared with the predictions of pre-equilibrium hybrid model, as well as with the more recent index model. A general agreement was found in all reactions using initial exciton numbern ₀=4(4p0h) except for¹⁹⁷Au(α,n) reaction, where index model gives fairly good agreement withn ₀=5(5p0h).     

Beta-gamma-gamma directional correlation in103Rh

β-γ-γ directional correlation studies for the cascades (i)β-rays ofE _max=0.12 MeV,γ-rays of 557 keV andγ-rays of 53 keV and (ii)β-rays ofE _max=0.21 MeV,γ-rays of 444 keV andγ-rays of 53 keV have been made. The triple correlation functionsW(θ) were obtained to beW(θ)=1+(−0.153±0.031)P ₂(cosθ)+(0.004±0.035)P ₄(cosθ) forβ-rays ofE _max 0.12 MeV→557→53 keV cascade andW(θ)=1+(0.163±0.042)P ₂(cosθ)+(−0.035±0.058)P ₄(cosθ) forβ rays ofE _max=0.21 MeV→444 keV→53 keV cascade. Spins and parities of the 650, 537 and 93 keV levels of¹⁰³Rh are deduced by triple angular correlation and the internal conversion coefficient studies. Multipolarities of the transitions are also determined.     

Intermediate width structures in the reaction50Ti(p,n)50V

The total (p, n) reaction cross section for⁵⁰Ti has been measured from threshold upto about 4.9 MeV proton energy, with fine energy resolution. The excitation function averaged over about 100 keV energy interval exhibits “intermediate width structures”. Detailed statistical analysis of the data has revealed the presence of two kinds of structures having average widths which are characteristic of the compound nuclear and intermediate reaction mechanisms. The level spacings of these intermediate structures are consistent with Izumo's partial equilibrium model.     

A first experimental approach to the 15O + α elastic scattering

The ¹⁵O(α,α)¹⁵O elastic scattering is investigated using a ¹⁵O radioactive beam and a He gas cell limited by Mylar windows. The width of a ¹⁹Ne state at an excitation energy of 5.35MeV is measured as Γ_α = 3.2±1.6keV, in agreement with charge symmetry estimate.     

Search for narrow dibaryon resonances in neutral pion photoproduction from the deuteron

The reaction γd↦π⁰ X has been measured with TAPS at MAMI in the energy range E _γ = 140-300 MeV. Using the Glasgow tagging spectrometer a photon energy resolution of 0.8 MeV was achieved. The energy excitation functions of integral and differential total cross-sections show no structures of statistical significance > 2σ. Upper limits for the production of narrow isoscalar or isovector dibaryons with masses m? 2100 MeV/c² were deduced. They are in the range 2-5 μb averaged over the 0.8 MeV energy resolution. Received: 25 October 2000 / Accepted: 24 November 2000     

Level structure of 99Rh at high spins

High spins states in ⁹⁹Rh were populated via the ⁶⁶Zn(³⁷Cl, 2p2n)⁹⁹Rh reaction at an incident beam energy of 130 MeV. Seventeen new transitions have been observed in the present study and the level scheme has now been extended up to a spin of J∼ 25ħ and an excitation energy of about E _x∼ 10 MeV. The observation of a positive parity E2 cascade based on the 9/2⁺ isomeric level is suggestive of collective behaviour in this nucleus up to high spins. Spherical shell model (within restricted model space) and Cranked shell model calculations were performed to obtain an insight into the observed level structure. The new collective band observed up to a spin of J∼ 25ħ is suggested to be based on (πg _9/2 ³) ⊗ (νg _7/2 ²) quasi-particle excitations. Received: 12 July 1999 / Revised version: 14 September 1999     

Surface vibrations and the nature of the adsorption state: C2H2 on Pt(111)

Molecular vibrations of C₂H₂ and C₂D₂ adsorbed on Pt(111) at 140 K and ∼300K have been measured by high resolution electron energy loss spectroscopy. The comparison of C₂H₂ and C₂D₂ spectra allows an unambiguous assignment of the observed losses to the excitation of C−H bending, C−H stretching, and C−C stretching modes of nondissociatively adsorbed acetylene. From the relative intensities of losses the hybridisation state is determined to be nearsp ². The C−C stretching frequency indicates a C−C bond order of ∼1.8.     

Total (p,n) reaction cross section study on51V over the incident energy range 1·56 to 5·53 MeV

The total (p, n) reaction cross section for⁵¹V has been measured as a function of proton energy in the energy range 1·56 to 5·53 MeV with thick and thin targets. The fluctuations in the fine resolution excitation functions were analysed, to extract 〈Γ〉, the coherence width. The thick target excitation function suitably averaged over appropriate energy intervals has been compared with the optical model, Hauser-Feshbach and Hauser-Feshbach-Moldauer calculations. The strong isobaric analog resonance atE _p ～ 2·340 has been shape analysed to extract the proton width Γ _p , the spreading withW and the spectroscopic factor.     

High-spin excitations of 84 35Br49 and 85 35Br50: Mapping the proton sub-shells towards 78Ni

The ^{84, 85}Br nuclei have been produced as fission fragments in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85MeV bombarding energy and studied with the Euroball IV array. The high-spin states of the odd-odd ⁸⁴Br nucleus have been identified for the first time, the observed structure being built on the known 6^- isomeric state. High-spin states of ⁸⁵Br have been observed up to 5.4MeV excitation energy and spin I ∼ 21/2. From angular correlation analysis, spin values have been assigned to most of the ⁸⁵Br excited states up to 4MeV. None of these excited states has been found to exhibit a delayed decay having T _1/2 > 10ns. All the observed states in ^{84, 85}Br can be described by various proton excitations involving at least the two sub-shells ( πf _5/2 and πp _3/2) located just above the Z = 28 shell closure.     

Rotational bands and signature inversion phenomena in πh9/2⊗νi13/2 and πi13/2⊗νi13/2 structures in odd-odd 176Ir

The ^109,111,113Rh nuclei have been produced as fission fragments in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85 MeV. Their level schemes have been built from gamma-rays detected using the Euroball IV array. High-spin states of the neutron-rich ^111,113Rh nuclei have been identified for the first time. Several rotational bands with the odd proton occupying the πg _9/2, πp _1/2 and π(g _7/2/d _5/2) sub-shells have been observed. A band of low-energy transitions has been identified at excitation energy around 2 MeV in ^109,111Rh, which can be interpreted in terms of three-quasiparticle excitation, πg _9/2νh _11/2νg _7/2/d _5/2. In addition another structure built on states located at low excitation energy (608 keV in ¹¹¹Rh, 570 keV in ¹¹³Rh) points out that, as already observed in the lighter isotopes ^107,109Rh, triaxial deformation plays a role in the neutron-rich Rh isotopes well beyond the mid-shell. Received: 15 July 2002 / Accepted: 9 October 2002 / Published online: 3 December 2002 RID="a" ID="a"e-mail: porquet@csnsm.in2p3.fr RID="b" ID="b"Present address: CSNSM IN2P3-CNRS and Université Paris-Sud 91405 Orsay, France. RID="c" ID="c"Present address: CEA/Saclay, DSM/DAPNIA/SPhN, 91191 Gif-sur-Yvette Cedex, France. Communicated by D. Schwalm     

On the unusual properties of the 282 keV state in <Superscript>135</Superscript>Sb

Recently the first excited state in ¹³⁵Sb has been observed at the unexpectedly low excitation energy of only 282keV and interpreted as mainly d _5/2 proton coupled to the ¹³⁴Sn core. Based on theoretical considerations it was suggested that its low excitation energy is related to a relative shift of the proton d _5/2 and g _7/2 orbits induced by the neutron excess. We have measured the lifetime of the 282keV state by the advanced time-delayed βγγ(t) method. The measured half-life, T _1/2 = 6.1(4)ns, yields exceptionally low limits of B(M1;5/2₁ ⁺→7/2₁ ⁺)≤3.0×10^-4 μ ² _N and B(E2;5/2₁ ⁺→7/2₁ ⁺)≤54e ² fm ⁴. These strongly hindered M1 and slow E2 transition rates are similar to those for the transition de-populating the first excited state at 405keV in ²¹¹Bi. Results of shell model calculations with realistic interactions are presented. The M1 decay rate was found to be extremely sensistive both to the wave function and to the M1 effective operator.     

A study of pre-equilibrium emission of neutrons in <Superscript>93</Superscript>Nb(α, xn) reactions

With a view to study the pre-equilibrium emission mechanism in α-induced reactions the excitation functions for ⁹³Nb(α, n)^96m Tc, ⁹³Nb(α, n)⁹⁶Tc, ⁹³Nb(α, 2n)^95m Tc, ⁹³Nb(α, 2n)^95g Tc and ⁹³Nb(α, 3n)⁹⁴Tc reactions have been measured in the energy range threshold to ≈ 10MeV/nucleon using the activation technique. The measured excitation functions have also been compared with theoretical predictions based on the semi-classical code, which takes into account compound nucleus as well as pre-equilibrium emission. The analysis of the data indicates significant contribution from pre-equilibrium emission at these energies particularly in the high-energy tail portion of EFs. The effect of the variation of the parameters used in the code has been studied. The isomeric cross-section ratios have also been measured. It has been observed that the pre-equilibrium fraction increases rapidly with the increase in α-particle bombarding energy.     

Thermonuclear reaction rates from (p,n) reactions

Thermonuclear reaction rates for the temperature range 1≤T ₉≤5 have been extracted from experimentally measured (p, n) cross sections for⁴⁵Sc⁵⁰Ti,⁵¹V,⁵⁴Cr,⁵⁵Mn and⁵⁹Co nuclides below 5 MeV bombarding energy. These reaction rates are important in the build-up of medium and heavy nuclides in the stellar evolution process and nucleosynthesis. To enhance the usefulness of these reaction rates in astrophysical calculations, they have been fitted to an analytic function of temperature, valid throughout the temperature range considered here.     

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     

High-spin structure of the neutron-rich 107,109 45Rh isotopes: the role of triaxiality

The ^107,109Rh nuclei have been produced as fission fragments following the fusion reaction ²⁸Si +¹⁷⁶Yb at 145 MeV bombarding energy and studied with the Eurogam2 array. In both nuclei three new rotational bands with the odd proton occupying the πg_9/2, πp_1/2 and π(g_7/2/d_5/2) sub-shells have been observed. In ¹⁰⁷Rh, two other bands involving strong M1 transitions have been identified at excitation energy larger than 2 MeV. They can be interpreted in terms of three quasiparticle excitations. In addition new structures consisting of four transitions, built on states located at low excitation energy (680 keV in ¹⁰⁷Rh and 642 keV in ¹⁰⁹Rh), point out the importance of triaxial deformation in these two isotopes. Received: 1 September 1999     

Resonances inη-light nucleus systems

We locate resonances inη-light nucleus elastic scattering using the time delay method. We solve few-body equations within the finite rank approximation in order to calculate the t-matrices and hence the time delay for theη-³He andη-⁴He systems. We find a resonance very close to the threshold inη-³He elastic scattering, at about 0.5 MeV above threshold with a width of ∼2 MeV. The calculations also hint at the presence of sub-threshold states in both the cases     

Levels in74As from the74Ge(p,nγe)74As reaction

The low-lying levels in⁷⁴As have been studied by means ofγ-ray and internal conversion electron spectroscopy following the⁷⁴Ge(p,n)⁷⁴As reaction. New levels at 372.7, 532.8, 632.1, 731.6, 752.7, 758.3, 801.6, 902.9 and 1128.5 keV, not observed in earlier studies, have been established.J ^π assignments have been made to several low-lying levels. An earlier ambiguity regarding the identification of an isomeric level has been clarified. The half-life of a level at 271.4 keV has been measured to be 1.0±0.1 nsec; in addition, limits on half-lives of levels at 182.7, 277.5 and 425.4 keV have been assigned. The level structure is discussed on the basis of available nuclear models.     

Measurement and analysis of excitation functions for alpha induced reactions on iodine and cesium

The excitation functions for the reactions¹²⁷I(α, 2n)¹²⁹Cs,¹²⁷I(α, 4n)¹²⁷Cs,¹³³Cs(α, 2n)¹³⁵La and¹³³Cs(α, 4n)¹³³La have been measured up to ≈50 MeVα-particle energy using the stacked foil activation technique. Measured excitation functions are compared with pre-equilibrium geometry dependent hybrid model calculations. It has been found that theoretical calculations using an initial exciton numbern ₀=4 (2p+2n+0h) give good agreement with experimental excitation functions.