Study of the yrast band in the doubly odd156Tm

The yrast band in the doubly odd¹⁵⁶Tm nucleus was studied through¹⁴⁴Sm(¹⁹F,2p5n)¹⁵⁶Tm reaction at beam energy of 105MeV. Several high-spin states of¹⁵⁶Tm were identified and the highest spin of the band with configuration7/2^–[523] v1/2⁺[660] could be built up to spin 25. The level structure shows the onset of a non- or weak collectivity which generally appears at neutron number of 87 in neutron-deficient rare-earth nuclei.This work was supported in part by the Fok Ying Tung Education Foundation and the Natural Science Fund of China.     

Analogous negative parity spectra of125Xe and127Xe

The low spin states of the nucleus¹²⁷Xe have been investigated with the (, 2n) reaction by means of in-beam--spectroscopy, using theosiris-cube-spectrometer at the FNTandem accelerator facility of the University of Cologne. In the negative parity system, four excited states for each of the spins 11/2^–.15/2^– and two 5/2^–.9/2^– — states could be established at excitation energies up to 1MeV above the Yrast line. With this new data, we are able to compare the non-Yrast states and their decay properties to the corresponding states of¹²⁵Xe, which had been studied with similar methods in Cologne. The almost one-to-one correspondence between the states of these neighboring neutron-odd nuclei is a clear indication that their excitations at low spin remain collective well above the Yrast line.     

Structure of negative parity yrast bands in odd mass <Superscript>125–131</Superscript>Ce nuclei

The negative parity yrast bands of neutron-deficient ^125–131Ce nuclei are studied by using the projected shell model approach. Energy levels, transition energies and B(M1)/B(E2) ratios are calculated and compared with the available experimental data. The calculations reproduce the band-head spins of negative parity yrast bands and indicate the multi-quasiparticle structure for these bands.     

The\bar pp \to e^ + e^ - reaction at rest and the hierarchy of the annihilation ranges

A new procedure is devised to evaluate the proton form-factor at the ¯pp threshold from experimental data on ¯ppe ⁺ e ^– annihilation at rest. Comparison of the ¯ppe ⁺ e ^– and¯pp ^+– processes reveals that annihilation ranges for various channels are ranked in line with quark dynamics.This work has been funded in part by the German Federal Minister for Research and Technology (BMFT) under the contract number 06HD756     

Proton alignment in the 21/2+ yrast state of89Mo

The magnetic moment of the isomeric 2584 keV 21/2⁺ yrast state in⁸⁹Mo was measured via the time differential perturbed angular distribution method after recoil implantation (IMPAD) into a polarized Ni target. A g-factor of +0.79(4) was extracted, indicating a (g _9/2)² v(g _9/2)^–1 structure of this state. The result is placed in a systematic survey of g-factors in this mass region as well as compared with the prediction of shell model calculations.Communicated by: D. Schwalm     

Excitations in208Pb nearE x=5683 keV

A search for the recently proposed two-phonon octupole vibrational (2-POV) 4⁺ or 6⁺ state in²⁰⁸Pb at an excitation energy of 5683 keV has been performed using the²⁰⁷Pb(d,p),²⁰⁸Pb (p,p) and (,) reactions at high energy resolution. No evidence for a two-phonon excitation at this energy is found.This work was supported in part by the DFG under contract nr. II C4-Gr 894/2-1 and by the DFG-Graduiertenkolleg Struktur der Hadronen und Kerne under contract nr. Mu 705/3-1.     

Characterization of the superdeformed band in189Hg

New measurements of the pionic double charge exchange on⁵⁶Fe exhibit a pronounced energy dependence of this reaction at low energies in accordance with similar observations on other nuclei. We show that this peculiar energy dependence finds its natural explanation by a resonance in theNN subsystem withJ ^p =0^–,T=0 and a mass of 2.065 GeV.This work has been funded by the German Federal Minister for Research and Technology (BMFT) under the contract numbers 06 TÜ 656 and 06 KA 266 and by the DFG (Mu 704/3, Graduiertenkolleg)     

Optical gain on the 476.5 nm Argon-ion laser line in a gas-target excited by a heavy ion beam

Optical gain on the 476.5 nm Ar II 4p–4s ion laser transition has been observed in argon-gas excited by 2.5 ns pulses of 90 MeV³²S ions with a repetition rate of 4883 Hz. The energy per pulse was 23 J. The projectiles were stopped in the target at pressures between 5 and 20 kPa. Gain was determined from a measured transient increase of the intensity of a 476.5 nm probe laser beam sent along the ion beam axis and back reflected by an aluminum foil. The maximum gain observed was (0.4±0.1)×10^–3 at a target-gas pressure of 5 kPa. Control experiments using krypton as target-gas were performed and yielded a null result. The optical gain observed in argon is consistent with the result from an analysis of spectroscopic studies of rare-gas targets excited by heavy ion beams.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday. This work has been funded by the German Federal Ministry for Research and Technology (BMFT) under contract No. 06 TM 310 I, Gesellschaft für Schwerionenforschung (GSI), Darmstadt, and the Tandem accelerator laboratory, Munich     

Influence of particle-number projection on potential energy surfaces of fissioning nuclei

The structure of high-spin states in the transitional nuclei^191,193Hg is described in the framework of the interacting boson fermion model with one broken pair. The results of model calculation reproduce in detail the structure of yrast states up toJ=49/2⁺ ?, including the first backbending, and the two lowest three-neutron negative parity bands.     

Subthreshold antiproton andK − production in heavy ion collisions

Subthreshold ¯p andK ^– and energetic ^– production was studied in Ne + NaF, Cu, Sn and Bi, and in Ni + Ni collisions with incident energies between 1.6 and 2 GeV/u. The measured cross sections indicate a dominant contribution of baryonic resonances. This is also consistent with a generalized scaling behaviour of the cross sections with the energy available in the collision and the energy necessary to produce particles as observed with Ne induced reactions. Deviations from scaling especially pronounced in the Ni-Ni system will be discussed in terms of absorption effects. The flat slope of the excitation function for ¯p production can be related to a reduced production threshold caused by a reduction of the antiproton mass in the dense and heated medium by about 100—150 MeV/c². A similar in-medium mass reduction is also indicated forK ^– mesons. An increased ¯p reabsorption probability for the heavier systems is concluded from the comparison of the ¯p yields in Ne + NaF, Ne + Sn and Ni + Ni collisions.Dedicated to Professor Klaus Dietrich on the occasion of his 60th birthday.     

Solving the Gross-Neveu model with relativistic many-body methods

The Gross-Neveu model provides a unique opportunity to apply relativistic many-body techniques (Dirac-Hartree approximation, RPA) in a context where all calculations can be done analytically and — in the largeN limit — yield the exact results. The physical fermion as well as multifermion (baryon) and fermion-antifermion (meson) bound states are discussed in this spirit, with special emphasis on the role of the Dirac sea.Supported by the Bundesministerium für Forschung und Technologie     

Positive parity low spin states of odd-mass tellurium isotopes

In this work, we analyse the positive parity of states of odd-mass nucleus within the framework of interacting boson fermion model. The result of an IBFM-1 multilevel calculation with the lg_9/2, 2d_5/2, 2d_3/2, 3s_1/2 and one level, 1h_11/2 with negative parity, single particle orbits is reported for the positive parity states of the odd mass nucleus ^123-125Te. Also, an IBM-1 calculation is presented for the low-lying states in the even-even ^124-126Te core nucleus. The energy levels and B (E2) transition probabilities were calculated and compared with the experimental data. It was found that the calculated positive parity low spin state energy spectra of the odd-mass ^123-125Te isotopes agree quite well with the experimental data.     

Investigation of the isospin-forbidden reaction6Li(6Li,d 2)10 B 0+,1

Angular distributions of the isospin-forbidden reaction⁶Li(⁶Li, d₂)¹⁰ B _0+,1 were measured at six beam energies in the range from 3 to 8 MeV. The contribution of two-step transfer mechanism to the reaction under consideration was found to be negligible. Hauser-Feshbach model cross sections, obtained with parameters derived from a previous analysis of isospin-allowed reactions in the⁶Li +⁶Li system are consistent with the data, provided a 2% reduction factor reflecting isospin mixing of compound nucleus levels. From this factor an isospin-breaking matrix element of H _c ^{2 1/2}13 keV was deduced. The presence of one broad or several resonances with (J ^P ,T)=(1^–, 1) or (2^–,1) concentrated at an excitation energy of about 30.3 MeV in¹²C seems to be responsible for the observed shape of angular distributions and their energy dependence.This work was supported by Deutsche Forschungsgesellschaft under contact No. Fr575/2-1,2.     

GRID lifetime measurements in59, 61, 63Ni following thermal neutron capture

The GRID-method has been used to measure the lifetimes of ten excited states in^{59, 61, 63}Ni following thermal neutron capture in Ni targets of natural isotopic composition. Four of the lifetimes have been determined for the first time, the other six lifetimes can be compared with the results of conventional DSA-measurements following charged particle induced reactions. Cascade feeding effects have been included in the analysis. Level energies and electromagnetic properties of negative parity states in⁵⁹Ni have been compared with the results of shell model calculations in 3p0h and 4p 1h model spaces. Statistical model estimates of the lifetimes as function of excitation energy and spin are also given.Work partially supported by Deutsches BMFT under contract 06GOE141     

A new determination of the γπ → ππ anomalous amplitude via πe<Superscript>-</Superscript> → π<Superscript>-</Superscript>e<Superscript>-</Superscript>π<Superscript>0</Superscript> data

We discuss the reaction π^-e^- → π^-e^-π⁰ with the purpose of obtaining information on the γπ → ππ anomalous amplitude _3π. We compare a full calculation at (p⁶) in chiral perturbation theory and various phenomenological predictions with the existing data of Amendolia et al. By integrating our theory results using Monte Carlo techniques, we obtain σ = 2.05 nb at (p⁶) and σ = 2.17 nb after including the dominant electromagnetic correction. Both results are in good agreement with the experimental cross-section of σ = (2.11±0.47) nb. On the basis of the ChPT results one would extract from the experimental cross-section as amplitudes _3π^(0)extr = (9.9±1.1) GeV^-3 and _3π^(0)extr = (9.6±1.1) GeV^-3, respectively, which have to be compared with the low-energy theorem _3π = e/(4π²F_π³) = 9.72GeV^-3. We emphasize the need for new data to allow for a comparison of experimental and theoretical distributions and to obtain _3π with smaller uncertainty.     

First observation of excited states in the <Superscript>111</Superscript>Tc nucleus --A new region of deformation at 40 ⩽ Z ⩽ 46, N ≥ 68?

The ¹¹¹Tc nucleus, populated in the spontaneous fission of ²⁴⁸Cm, was studied by means of prompt -ray spectroscopy using the EUROGAM2 array. Excited states in ¹¹¹Tc were observed for the first time. Systematics of energy levels in odd-A Tc isotopes, obtained in our study of ¹⁰⁷Tc and ¹⁰⁹Tc provide a reliable spin and parity assignment I = 5/2⁺ to the head of the new band in ¹¹¹Tc, interpreted as the 5/2⁺[422] orbital originating from the proton g_9/2 shell. This level is most likely the ground state. Therefore, the (9/2⁺,7/2⁺) spin-parity assignment to the ground state of ¹¹¹Tc, reported previously, is unlikely. Properties of the yrast band in ¹¹¹Tc suggest prolate deformation of this band. There are hints that the deformation of ¹¹¹Tc is larger than that of ¹⁰⁹Tc, possibly due to admixtures of oblate-deformed configurations, which lower their excitation energy with increasing neutron number.     

Complete spectroscopy of51,52V via the50,51V(n, γ) reactions

The gamma-ray spectra emitted after thermal neutron capture in highly enriched⁵⁰V and^natV targets have been studied using in-pile targets at the ILL high flux reactor and pair and germanium detectors. The neutron binding energies in⁵¹V and⁵²V were determined to beB _n (⁵¹V)=11051.11(17) keV andB _n (⁵²V)=7311.22(26) keV. The thermal neutron capture cross-section in⁵⁰V was measured to be 21 _–2 ⁺⁴ b. From 724 lines attributed to⁵¹V, 330 transitions, comprising 90% of the-ray flux, were placed into the level scheme. Fifty-nine primary dipole transitions from the 11/2⁺ or 13/2⁺ capture states in⁵¹V were established from which the E1 strength distribution was deduced. The energy scaling of these primary transitions was found to follow the E1-giant dipole resonance dominance. Many new levels were established; a number of states proposed in previous (d,p) and (n,) work were confirmed from their primary population and decaying secondary radiations. The density of levels in the high spin (9/2I15/2) region was parametrized with the Fermi gas model. The spin distributions of⁵¹V were analyzed and a spin cut-off parameter =2.8 (3) was deduced. — A nearly complete level scheme of⁵²V up to 3.5 MeV excitation and similar results concerning the level density and the primary -ray spectrum were obtained in the⁵¹V (n, )⁵²V study.Supported by Deutsches BMFT under contract 06GÖ141     

Defects in pulsed laser and thermal processed ion implanted silicon

The evolution of the nature and concentration of the defects produced by 100 or 300 keV As ions at fluences 1 to 4×10^–12 cm^–2 inn-type, Fz Silicon doped with 10¹⁵ to 10¹⁶ cm^–3 has been studied as function of thermal treatments (in the range 500°–900 °C) and of the energy density (in the range 0.3–0.6 J cm^–2) of a light pulse from a ruby laser (15 ns, 0.69 m). Deep-level transient spectroscopy (DLTS) combined with capacitance — voltage (C-V) measurements were used to get the characteristics (energy level, crosssection for the capture of majority carriers) of the defects and theirs profiles. The difficulties encountered in the analysis of the results, due to the large compensation of free carriers in the implanted region and to the abrupt defect and free carrier profiles, are discussed in detail and the corrections to apply on the C-V characteristics and the DLTS spectra are described. The defects resulting from the two types of treatments are found to be essentially the same. Only, for laser energies higher than 0.5 J cm^–2, the laser treatment appears to introduced new defects (atE0.32 eV) which should result from a quenching process. The fact that a laser energy smaller than the threshold energy for melting and recrystallization is able to anneal, at least partially, the defects produced by the implantation, demonstrates that the annealing process induced by the laser pulse is not a purely thermal process but is enhanced by a mechanism involving ionization.     

Wide- and narrow-band saturated fluorescence measurements of hydroxyl concentration in premixed flames from 1 bar to 10 bar

We have studied the use of wide-band detection in conjunction with saturation of a rovibronic transition of OH within itsA ^{2 +}–X ²(0,0) band. For wide-band detection, in which fluorescence is detected from the entire excited rotational manifold, the fluorescence yield is sensitive to collisions in two ways. First, it is sensitive to the ratio of rate coefficients describing rotational energy transfer and electronic quenching; this ratio determines the number of neighboring rotational levels that are populated during the laser pulse. Second, the fluorescence yield can vary with the total collisional rate coefficient; only after a sufficient number of collisions, corresponding to 2.5 ns in an atmospheric flame, does the rotational manifold reach steady state. We also compare measurements employing wide-band (detecting theR ₁ andR ₂ branches) and narrow-band (detecting a single transition) saturated fluorescence of OH. Over a wide range of conditions — obtained by varying the equivalence ratio, temperature, N₂ dilution, and pressure — the wide- and narrow-band fluorescence techniques compare well. Given this good agreement, wide-band saturated fluorescence could be especially useful for analyzing atmospheric flames with XeCl-excimer lasers; one can potentially obtain 2—D images of OH which have a high signal-to-noise ratio and a reduced sensitivity to laser irradiance and quenching.