R. Jahn

Search for η-mesic nuclei in a recoil-free transfer reaction

We have studied the reaction p+{sup 27}Al {yields} {sup 3}He+p+{pi}{sup -}+X at recoil-free kinematics. An {eta} meson possibly produced in this reaction would be thus almost at rest in the laboratory system and could therefore be bound with high probability, if nuclear {eta} states exist. The decay of such a state through the N*(1535) resonance would lead to a proton-{pi}{sup -} pair emitted in opposite directions. For these conditions we find some indication of such a bound state. An upper limit of {approx_equal}0.5 nb is found.

High resolution study of the Λp final state interaction in the reaction p+p→K++(Λp)

The reaction pp ! K + + (�p) was measured at Tp = 1.953 GeV and � = 0 ◦ with a high missing mass resolution in order to study thep final state interaction. The large final state enhancement near thep threshold can be described using the stan- dard Jost-function approach. The singlet and triplet scattering lengths and effective ranges are deduced by fitting simultaneously thep invariant mass spectrum and the total cross section data of the freep scattering.

Systematics of nn states with high spin: A study of the (α, 2He) reaction on fp shell nuclei☆

Abstract At 57 MeV bombarding energy the (α, 2 He) reaction has been investigated on targets of 54,56 Fe, 58,60,62,64 Ni, 64,66 Zn, and 70 Ge. Selective excitation of the 2n configurations ( f 5 2 g 9 2 ) 7− , ( g 9 2 ) 8 + 2 , and ( g 9 2 2 d 5 2 ) 6+ was observed in all final nuclei. A linear A and T dependence of the binding energies of these states was observed. This systematic behaviour is well described by the Bansal-French model. The values obtained for the strength of the isoscalar and the isovector parts of the particle-hole interaction are consistent with the average of the values describing the corresponding single-particle states.

Cross section and tensor analysing power of the dd→ηα reaction near threshold

The angular distributions of the unpolarised differential cross section and tensor analysing power A xx A x x of the d → d → α η reaction have been measured at an excess energy of 16.6 MeV. The ambiguities in the partial-wave description of these data are made explicit by using the invariant amplitude decomposition. This allows the magnitude of the s -wave amplitude to be extracted and compared with results published at lower energies. In this way, firmer bounds could be obtained on the scattering length of the ηα system. The results do not, however, unambiguously prove the existence of a quasi-bound ηα state.

12C + p → 13N(T = 32) resonance at 14.23 MeV

Abstract The excitation functions of the elastic scattering 12 C(p, p) 12 C and the reactions 12 C(p, p′) 12 C (4.44 MeV) and 12 C(p, α) 9 B g.s. have been measured at θ = 175° in the region of the first T = 3 2 state in 13 N with high energy resolution. A straightforward resonance analysis of the elastic scattering data yields Γ = 1.10 ± 0.09 keV, Γ p = 210 ± 11 eV and Γ p Γ = 0.191 ± 0.017 . The results are compared with shell-model predictions. The additionally measured angular distributions of the elastic scattering 12 C(p, p) 12 C above and below the resonance are compared with optical model calculations. The combination of the present results for Γ p and Γ p / Γ with previous results for Γ γ / Γ p and Γ γ Γ p / Γ yields Γ γ = 27.0 ± 2.2 eV as a new determination of the 13 N (T = 3 2 , 15.07 MeV → T = 1 2 , 0 MeV ) γ-decay width. The comparison with the analogous γ-decay in 13 C is discussed with respect to the charge symmetry of corresponding ΔT = 1 γ-transitions in mirror nuclei.

Stretched proton-neutron configurations in fp-shell nuclei *

Abstract The (α, d) reaction has been investigated at bombarding energies around 55 MeV on 40,42,44,48 Ca, 50,52,54 Cr, 54,56,58 Fe, 58,60,62,64 Ni, 64,66 Zn and 70,72 Ge targets. The angular distributions of prominent transitions were analyzed with the distorted-wave Born approximation and shown to correspond to unnatural-parity stretched proton-neutron configurations. Strong transitions of ( g 9 2 ) 2 9 + and ( p 3 2 , g 9 2 ) 6 − character were identified in most final nuclei. Additionally, dominant transitions with ( f 7 2 , g 9 2 ) 8 − structure were observed for target nuclei with an open proton ( f 7 2 shell. These stretched high-spin states could be consistently described by an average normalization constant N = 225 ± 85 indicating their relatively pure two-nucleon structure. In the lower-mass Sc and Mn isotopes, configurations built on g 9 2 states are increasingly damped because of the strong fragmentation of the underlying single-particle strength.

Quasi-elastic single-nucleon transfer in 48Ti + 42,44Ca collisions well above the Coulomb barrier

Abstract Single-nucleon transfer reactions in quasi-elastic collisions between 48 Ti and 42,44 Ca have been studied at incident energies per nucleon E / A = 8– l 5 MeV. The measured energy spectra display structures, characteristic of the incident energy and the reaction channel. The data have been analyzed in a fully microscopic direct one-step transfer model where the spectra are interpreted as the transition of all possible states in the valence shell of the donor nucleus to bound and unbound states in the recipient. Realistic single-particle (hole) strength distributions are obtained from a quasiparticle core coupling model. It is based on the coupling of a quasiparticle to the core-excited spectrum of the acceptor (donor) which results from a large-scale two-quasiparticle RPA calculation. The present approach includes the contributions from ground-state pairing correlations and higher radial components in the particle response functions. General features of the data such as angular distributions, shape, and absolute magnitude as a function of bombarding energy are described fairly well for all four reaction channels. With increasing incident energy a growing importance of transfer to higher shells of the acceptor nucleus is demonstrated. The present model, however, does not account for additional strength observed in the spectra between 5 and 10 MeV excitation energy. As a possible explanation, the role of higher order effects is discussed.

The 16O + p → 17F (T = 32) resonances at Ep = 11.26,12.71 and 14.57 MeV

Abstract Excitation functions for 16 O+p reactions have been measured with high energy resolution in the region of the first, second and seventh T = 3 2 resonances in 17 F at extreme backward angles. The observed resonance shapes have been analyzed with a single-level resonance formula taking the off-resonance spin-flip amplitude into account. The resonance parameters of the 17 F first T = 3 2 state studied with special emphasis are E x = 11193.3 ± 2.3 keV, Γ = 200 ± 40 eV and Γ p 0 = 19 ± 3 eV. This result and other results are compared with previous studies and theoretical predictions. The comparison with data of the mirror nucleus 17 O is discussed with respect to the observed charge asymmetry of the isospin-forbidden particle decay widths.

STATUS OF THE EDDA EXPERIMENT AT COSY

The EDDA experiment is designed to study p + p excitation functions with high energy resolution and narrow step size in the kinetic energy range from 250 MeV to 2500 MeV at the Cooler Synchrotron COSY. Measurements during the accelertion phase in conjunction with internal targets will allow to achieve a fast and precise energy variation. Prototypes of the detector elements and the fiber target have been extensively tested with proton and electron beams; the detector performance and trigger efficiency have been studied in Monte Carlo simulations. In this contribution, results concerning detector design, prototype studies, Monte Carlo simulations and the anticipated detector resolutions will be reported.

Experimental study of the pd -> He-3 K+K- and pd -> He-3 phi reactions close to threshold

Two--kaon production in proton--deuteron collisions has been studied at three energies close to threshold using a calibrated magnetic spectrograph to measure the final