Search Results (70 total)

The structure of ¹⁹⁴Ir is investigated via (n,γ),(n,e^-),(d,p), and (d→,α) spectroscopy. The use of different methods leads to an almost complete level scheme up to high excitation energies including γ-decay and spin-parity assignments. A reanalysis of the formerly published (n,γ) data was triggered by our new (d,p) and (d→,α) transfer reactions. The experimental level scheme is compared to predictions using extended supersymmetry. Herein, the classification of states was done according to quantum numbers, excitation energies, and (d→,α) transfer strengths. A one-to-one correspondence in excitation energies was obtained for the 23 lowest lying theoretical states with similar structures for the experimental and calculated level schemes. The two-nucleon transfer strengths show remarkable agreement. A Nilsson classification is discussed as well.

The HERMES experiment has measured the transverse polarization of Λ and Λ̅ hyperons produced inclusively in quasireal photoproduction at a positron beam energy of 27.6 GeV. The transverse polarization P_n^Λ of the Λ hyperon is found to be positive while the observed Λ̅ polarization is compatible with zero. The values averaged over the kinematic acceptance of HERMES are P_n^Λ=0.078±0.006(stat)±0.012(syst) and P_n^Λ̅ =-0.025±0.015(stat)±0.018(syst) for Λ and Λ̅ , respectively. The dependences of P_n^Λ and P_n^Λ̅ on the fraction ζ of the beam’s light-cone momentum carried by the hyperon and on the hyperon’s transverse momentum p_T were investigated. The measured Λ polarization rises linearly with p_T and exhibits a different behavior for low and high values of ζ, which approximately correspond to the backward and forward regions in the center-of-mass frame of the γ^*N reaction.

Proton threshold states in ²³Mg are important for the astrophysically relevant proton capture reaction ²²Na(p,γ)²³Mg. In the indirect determination of the resonance strength of the lowest states, which were not accessible by direct methods, some of the spin-parity-assignments remained experimentally uncertain. We have investigated these states with shell model, Coulomb displacement, and Thomas-Ehrman shift calculations. From the comparison of calculated and observed properties, we relate the lowest relevant resonance state at E_x=7643 keV to an excited 3/2⁺ state in accordance with a recent experimental determination by Jenkins From this we deduce significantly improved values for the ²²Na(p,γ)²³Mg reaction rate at stellar temperatures below T₉=0.1 K.

With a resolution of 3 keV, the two lowest 0^- states in ²⁰⁸Pb are identified by measurements of the reaction ²⁰⁷Pb(d,p) with the München Q3D magnetic spectrograph in the region E_x=5.2-5.7 MeV where the average level spacing is 6 keV. Precise relative spectroscopic factors are determined. Matrix elements of the residual interaction among one-particle one-hole configurations in a two-level scheme are derived for the two lowest 0^- states in ²⁰⁸Pb. The off-diagonal mixing strength is determined as 110±10  (experimental)±15  (systematic) keV. Measurements of the reaction ²⁰⁸Pb(p,p^') via isobaric analog resonances in ²⁰⁹Bi support the structure information obtained.

The ¹¹²Sn(p,t)¹¹⁰Sn reaction was studied in a high-resolution experiment at an incident proton energy of 26 MeV. Angular distributions for 27 transitions to levels of ¹¹⁰Sn up to an excitation energy of ~4.3 MeV were measured. A distorted-wave Born approximation (DWBA) analysis of experimental angular distributions using conventional Woods-Saxon potentials were done, allowing either the confirmation of previous spin and parity values or the assignment of new spin and parity to a large number of ¹¹⁰Sn states. A shell-model study was performed using an effective interaction derived from the CD-Bonn nucleon-nucleon potential. The energy spectra are calculated and compared with experiment, whereas the theoretical two-nucleon spectroscopic amplitudes, evaluated in a truncated seniority space, are used in the microscopic DWBA calculation of some cross-section angular distributions.

The nature of 0⁺ excitations, especially in transitional and deformed nuclei, has attracted new attention. Following a recent experiment studying ¹⁵⁸Gd, we investigated a large group of nuclei in the rare-earth region with the (p,t) pickup reaction using the Q3D magnetic spectrograph at the University of Munich MP tandem accelerator laboratory. Outgoing tritons were recorded at various lab angles, and their angular distributions are compared to those calculated using the distorted-wave Born approximation. Using the unique shape of the L=0 angular distribution, more than double the number of 0⁺ states than were previously known are identified. The distribution of 0⁺ energies and cross sections is discussed in terms of collective and noncollective degrees of freedom, and the density of low-lying 0⁺ states is discussed as a corroboration of a characteristic feature of phase transition regions. The degree of level mixing, as extracted from Brody distribution fits to the energy spacings of adjacent 0⁺ levels, is also explored.

Inelastic proton scattering via isobaric analog resonances allows derivation of rather complete information about neutron particle-hole states. We applied this method to the doubly-magic nucleus ²⁰⁸Pb by measuring angular distributions of the reaction ²⁰⁸Pb(p,p^') on top of the isobaric analog resonances in ²⁰⁹Bi with the Q3D magnetic spectrograph at München. We identify the six states of the i_11/2f_5/2 multiplet and the four states of the i_11/2p_3/2 multiplet in the energy range 4.6  MeV<E_x<5.3 MeV. Firm spin assignments for the 10 states are given, most of them new. Additional measurements of the reaction ²⁰⁷Pb(d,p) confirm the assignments. A new state at E_x=5239 keV has a dominant proton particle-hole structure with spin 4^-.

Excited states in the deformed nucleus ¹⁶⁸Er have been studied with high-energy resolution, in the (p,t) reaction, with the Munich Q3D spectrograph. A number of 25 excited 0⁺ states (four tentative) and 63  2⁺ states have been assigned up to 4.0 MeV excitation energy. This unusually rich characterization of the 0⁺ and 2⁺ states in a deformed nucleus, close to a complete level scheme, offers a unique opportunity to check, in detail, models of nuclear structure that incorporate many excitation modes. A comparison of the experimental data is made with two such models: the quasiparticle-phonon model (QPM), and the projected shell model (PSM). The PSM wave functions appear to contain fewer correlations than those of the QPM and than required by the data.

The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6  GeV/c positron beam of DESY HERA. The use of a tensor-polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry A_zz^d and the tensor structure function b₁^d for average values of the Bjorken variable 0.01<⟨x⟩<0.45 and of the negative of the squared four-momentum transfer 0.5  GeV²<⟨Q²⟩<5  GeV². The quantities A_zz^d and b₁^d are found to be nonzero. The rise of b₁^d for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.

The ¹²³Sb(p→,α)¹²⁰Sn reaction has been studied in a high resolution measurement at incident energy of 24 MeV using a polarized proton source and a Q3D spectrometer. The differential cross sections and asymmetries for transitions to levels of ¹²⁰Sn homologous to the lowest energy states of ¹¹⁹In have been measured and interpreted in terms of the experimental differential cross sections and asymmetries of the parent ¹¹⁹In states, on the basis of the weak-coupling model. Several (17) states of the ¹²⁰Sn residual nucleus, not previously reported in the literature, have been detected and spins and parities have been proposed for seven of them, while parities have been proposed for five other previously known levels. In order to analyze the transitions populating the homologous levels, for both differential cross sections and asymmetries, microscopic DWBA calculations have been performed using spectroscopic amplitudes obtained from a shell-model study.

Polarized deep-inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double-spin asymmetries of cross sections. Inclusive and semi-inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a reanalysis of previously published data. Inclusive and semi-inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti-up, anti-down, and strange sea quarks were computed from these asymmetries in a “leading order” QCD analysis. The polarization of the up-quark is positive and that of the down-quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data.

The well deformed atomic nucleus ¹⁵⁹Gd was investigated by means of radiative neutron capture and single neutron transfer reactions. Nearly 70 secondary γ rays from the (n,γ) reaction studied with high-resultion bent-crystal spectrometers at Grenoble are assigned to ¹⁵⁹Gd. About 200 levels with spin up to 11 / 2 are observed in this nucleus below 2.3  MeV in (d,p) and (d,t) reactions investigated at the Tandem Van de Graaff accelerator in Garching using unpolarized (18  MeV) and polarized (22  MeV) deuteron beams, respectively. The proposed level scheme for this nucleus is arranged into 21 rotational bands. Experimentally observed levels are interpreted using predictions obtained within the quasiparticle-phonon model and the quasiparticle-rotor approach for a well deformed nucleus.

To provide information for comparison with predictions from a dynamical supersymmetry, the odd-odd nucleus ¹⁹⁶Au was studied via transfer reactions. With a polarized deuteron beam we measured (d⃗,t) and (d⃗,α), and with unpolarized beams we measured (p,d), (³He,d), and (α,d) transfer reactions. From the high-resolution ¹⁹⁷Au(p,d)¹⁹⁶Au spectrum, a rather complete set of excitation energies was obtained. Quantum numbers and spectroscopic factors were obtained from angular distributions of single-neutron transfer in ¹⁹⁷Au(d⃗,t)¹⁹⁶Au, single-proton transfer in ¹⁹⁵Pt(³He,d)¹⁹⁶Au, and two-nucleon transfer ¹⁹⁸Hg(d⃗,α)¹⁹⁶Au. We obtain firm J^π assignments for 21 out of the 27 states with negative parity observed up to 490  keV excitation energy, by combining our data with that taken using γγ and conversion electron spectroscopy. The number of states and the firm or restricted assignments are in agreement with the predictions from the dynamical U_ν(6∕12)⊗U_π(6∕4) supersymmetric scheme. Including our (α,d) data, we can deduce spectroscopic factors for four different transfer channels. When model predictions of spectroscopic factors become available, these data will provide a further critical test as to what extent this symmetry is realized in nature.

By means of the (p,t) reaction we study the excitation spectra of 0⁺ states in the deformed nuclei ²²⁸Th, ²³⁰Th, and ²³²U, using the Q3D magnetic spectrograph facility at the Munich tandem accelerator. At small reaction angles the 0⁺ transfer angular distributions have steeply rising cross sections which allow us to identify these states in otherwise very complicated and dense spectra. For each of these nuclei we resolve typically about ten excited states with safe 0⁺ assignments. The studied excitation energies range up to 2.5, 2.7, and 2.3  MeV, respectively, and the summed transfer strengths add to more than 60% of the ground state strength. As in a recent study of ¹⁵⁸Gd we compare with interacting boson approximation (IBA) calculations in the spdf boson space. This highly schematic collective model description, including octupole collectivity, but neglecting other relevant degrees of freedom, gives numbers of excited 0⁺ states in these actinide nuclei that are rather close to the observed ones.

The ¹¹⁶Sn(p,t)¹¹⁴Sn reaction has been studied in a high-resolution experiment at an incident proton energy of 26  MeV. Angular distributions for 61 transitions to levels of ¹¹⁴Sn up to an excitation energy of ∼4.1  MeV have been measured. A distorted-wave Born approximation analysis of experimental angular distributions using conventional Woods-Saxon potentials has been done, allowing either the confirmation of previous spin and parity values or the assignment of new spin and parity to a large number of ¹¹⁴Sn states. A shell-model study of ¹¹⁴Sn has been performed using a realistic effective interaction derived from the CD–Bonn nucleon-nucleon potential. The model space has been truncated to states with seniority up to 4. Comparison between the experimental and calculated energy spectra for both positive- and negative-parity states shows a quite satisfactory agreement.

The ²⁴Na nucleus was investigated via (d⃗,p) and (n,γ) reactions. Within the range of excitation energy from 0 to 6.3  MeV 75 levels were observed. The angular distribution of partial cross sections and vector analyzing powers for 70 levels were determined within the framework of distorted-wave Born approximation. Of 240 γ transitions assigned to ²⁴Na, 234 were placed in the decay scheme. Based on the extensive (n,γ) data the neutron separation energy was deduced to be 6959.44±0.05  keV. Using spectroscopic information from our (d⃗,p) measurement we investigated the role of the direct capture mechanism in the ²³Na(n,γ)²⁴Na reaction.

Double-spin asymmetries of semiinclusive cross sections for the production of identified pions and kaons have been measured in deep inelastic scattering of polarized positrons on a polarized deuterium target. Five helicity distributions including those for three sea quark flavors were extracted from these data together with reanalyzed previous data for identified pions from a hydrogen target. These distributions are consistent with zero for all three sea flavors. A recently predicted flavor asymmetry in the polarization of the light quark sea appears to be disfavored by the data.

The transfer reaction ⁹⁴Zr(d,p)⁹⁵Zr has been studied to check the existence of a low-lying first excited level at 23  keV in ⁹⁵Zr. This level—if it exists—might have significant influence on the branching of the astrophysical s process at the branching nucleus ⁹⁵Zr. Our experimental spectra, measured with very good energy resolution, do not show any evidence for the existence of such a low-lying level. This is in agreement with experimental results from several reactions and contradicts the only claim for evidence of such a low-lying level which was also derived from ⁹⁴Zr(d,p)⁹⁵Zr data, however with moderate resolution, but still the best of the previous experiments.

Spin-dependent lepton-nucleon scattering data have been used to investigate the validity of the concept of quark-hadron duality for the spin asymmetry A₁. Longitudinally polarized positrons were scattered off a longitudinally polarized hydrogen target for values of Q² between 1.2 and 12  GeV² and values of W² between 1 and 4  GeV². The average double-spin asymmetry in the nucleon resonance region is found to agree with that measured in deep-inelastic scattering at the same values of the Bjorken scaling variable x. This finding implies that the description of A₁ in terms of quark degrees of freedom is valid also in the nucleon resonance region for values of Q² above 1.6  GeV².

Exclusive coherent and incoherent electroproduction of the ρ⁰ meson from ¹H and ¹⁴N targets has been studied at the HERMES experiment as a function of coherence length (l_c), corresponding to the lifetime of hadronic fluctuations of the virtual photon, and squared four-momentum of the virtual photon (-Q²). The ratio of ¹⁴N to ¹H cross sections per nucleon, called nuclear transparency, was found to increase (decrease) with increasing l_c for coherent (incoherent) ρ⁰ electroproduction. For fixed l_c, a rise of nuclear transparency with Q² is observed for both coherent and incoherent ρ⁰ production, which is in agreement with theoretical calculations of color transparency.

A new high-precision (p,t) study of the ¹⁵⁸Gd nucleus was carried out with the Q3D spectrometer at the University of Munich. The result is the observation for the first time of a deformed nucleus with 13 excited 0⁺ states below an excitation energy of approximately 3.1 MeV. Seven of these 0⁺ states are observed for the first time and an additional three are new confirmations of previous tentative assignments. This abundance of 0⁺ states provides significant new information on these poorly understood excitations. ¹⁵⁸Gd can now be viewed as a unique laboratory for further investigations on the nature of 0⁺ excitations in nuclei.

Excited states in ¹⁸⁰Ta, populated in the ¹⁸⁰Hf(p,n) and (d,2n) reactions, were investigated by in-beam γ-ray and conversion-electron spectroscopy. In addition to the excited two-quasiparticle bands with K^π=0^- and 1^- known from previous work, we have identified six other low-lying two-quasiparticle bands with K<~4. At intermediate K, 12 previously unkown levels were identified that could, however, not be associated reliably with rotational bands. These levels might be of significance in connection with the as yet unresolved problem of the stellar production mechanism for the 9^- isomer of ¹⁸⁰Ta. At higher K we observe only levels known from the previous in-beam γ-spectroscopic studies with heavier projectiles, but suggest for some levels a different ordering into rotational bands based on band-mixing considerations. We also report a deuteron spectrum measured in the ¹⁸¹Ta(p,d)¹⁸⁰Ta reaction with high resolution and statistics. This spectrum served for configuration assignments and yields an energy of 78±1 keV for the 9^- isomer in ¹⁸⁰Ta.

Spin transfer in deep-inelastic Λ electroproduction has been studied with the HERMES detector using the 27.6 GeV polarized positron beam in the DESY HERA storage ring. For an average fractional energy transfer 〈z〉=0.45, the longitudinal spin transfer from the virtual photon to the Λ has been extracted. The spin transfer along the Λ momentum direction is found to be 0.11±0.17(stat)±0.03(syst); similar values are found for other possible choices for the longitudinal spin direction of the Λ. This result is the most precise value obtained to date from deep-inelastic scattering with charged lepton beams, and is sensitive to polarized up quark fragmentation to hyperon states. The experimental result is found to be in general agreement with various models of the Λ spin content, and is consistent with the assumption of helicity conservation in the fragmentation process.

The beam-spin asymmetry in hard electroproduction of photons has been measured. The data have been accumulated by the HERMES experiment at DESY using the HERA 27.6 GeV longitudinally polarized positron beam and an unpolarized hydrogen-gas target. The asymmetry in the azimuthal distribution of the produced photons in the angle φ relative to the lepton scattering plane was determined with respect to the helicity state of the incoming positron beam. The beam-spin analyzing power in the sinφ moment was measured to be -0.23±0.04(stat)±0.03(syst) in the missing-mass range below 1.7 GeV. The observed asymmetry is attributed to the interference of the Bethe-Heitler and deeply virtual Compton scattering processes.