J. Konijn

An improved parametrization of the optical potential for pionic atoms

We extended the parametrization of the phenomenological optical potential for pionic atoms by allowing for an isospin-dependence of the absorption parameters. Least squares fits to the 140 selected pionic atom data are performed. The new parameter set indicates a significant isospin dependence and is able to provide a simultaneous description of shifts and widths for the previously problematic deeply bound 3d-orbits in elements such as 181Ta, natRe, natPt, 197Au, 208Pb and 209Bi.

The level structure of 156Gd studied by means of the (α, 2nγ) reaction

Abstract A complete set of conventional γ-ray spectroscopic techniques has been applied to investigate the level structure of 156 Gd. A total of twenty-five new levels has been established; unambiguous spin assignments could be given for twelve of them on the basis of angular distributions and conversion electron measurements. The proposed level scheme contains 49 levels, which can be ordered in seven rotational bands. The ground-state band was excited up to J π = 14 + , the β-band up to 10 + , the γ-band up to (11 + ), the second K π = 0 + band tentatively up to (10 + ), the K π = 4 + band up to (8 + ). Two negative-parity bands, one with even spins and one with odd spins, were excited to J π = (12 − ) and (13 − ). An isomeric state was established with T 1 2 = 1.3 μs, J π = 7 − , E x = 2137.7 keV . The properties of the K π = 4 + band and the isomeric state can be well explained by two-quasiparticle configurations. The negative-parity bands are interpreted as aligned octupole bands. Positive and negative-parity bands have been calculated in terms of the IBA model. Good agreement with the experimental results is obtained.

An in-beam Ge(Li)-NaI(Tl) Compton suppression spectrometer

Abstract A Compton suppression spectrometer used for in-beam gamma-ray spectroscopy is described. The system consists of a large NaI(Tl)-crystal (O 25.4 cm, 25.4 cm long) and a large-volume closed-end Ge(Li) detector. The overall suppression factor is about 11, with an optimum of 14.5 at low energies. Examples are given of the use at proton- and π − -beams.

The four quasi-particle 178Hf isomeric state, its excitation energy and multipolarities of deexciting transitions

Abstract The decay properties of the 31 y 178Hf isomeric state have been investigated by means of a Ge(Li)-NaI(Tl) Compton suppression device and a Si(Li) detector, as well as with Ge(Li) and Ge(Li), intrinsic Ge and Si(Li) coincidence arrangements. From the E3 character of the unobserved isomeric transition (11.7 ≦ EIT ≦ 16.7 keV), deduced from L-subshell and M/L ratios, Iπ, K of the isomeric level was determined as 16+, 16. The {p[404] ↓ + p1514] ↑ + n[514] ↓ + n[624]↑}16+ + four quasi-particle configuration was assigned to this level at 2447.5 ± 2.5 keV. As proposed earlier the isomeric E3 transition decays to the 13− level of the band built upon the Iπ = 8− isomer. The |gK−gR| values derived from E 2 M 1 ratios of interband transitions show that the 8− isomer contains about 36 % of the p[404] ↓ + p[514] ↑ configuration and about 64 % of the n[624] ↑ + n[514] ↓ configuration.

The 12C(e, e′p) and 12C(e, e′pp) reactions in the Δ-resonance region

Abstract Coincidence cross sections for the 12 C(e, e′pp) and 12 C(e, e′p) reactions have been measured in the Δ-resonance region. The 12 C(e, e′pp) reaction has been measured at three different angular settings of the proton detectors to investigate the angular correlation between the emitted protons. The data, which have a low statistical accuracy, are compared with a calculation based on the direct-knockout mechanism, which includes one- and two-body currents. NN-correlations are accounted for via a correlation function, Δ-excitation via a two-body current and final-state interactions of the emitted protons via an optical potential. The 12 C(e, e′p) data, taken at large proton-emission angles, cover the region of high missing-energy values (145–275 MeV). The data are compared to results of a calculation taking into account meson exchange, intermediate Δ-excitation and pion production. This comparison shows that there is ample evidence for processes leading to multi-nucleon knockout.

Anomalous strong interaction shifts and widths of the 3d state in pionic Pt and Au

Abstract The poinic 4f → 3d X-ray transitions in Pt and Au have been observed. The strong interaction monopole shifts ϵ o and widths Γ o of the 4f and 3dd levels have been deduced. For the pionic 4f levels standard optical potentials predict the experimental values quite well, whereas the deeper bound 3d states have shifts and widths that are smaller by a factor of about two than the theoretical predictions.

A study of the strong interaction effects on pionic 3d and 4f levels in 181Ta, natRe, natPt, 197Au, 208Pb, 209Bi and 237Np

Abstract The pionic X-ray spectra of 181 Ta, nat Re, nat Pt, 197 Au, 208 Pb, 209 Bi and 237 Np have been investigated to obtain data for a systematical study of the pionic 3d and 4f levels. In order to reduce background the experimental method has been improved by using an array of Compton suppression BGO-shields and neutron time-of-flight discrimination. The widths of the pionic 3d levels are observed to be narrower by a factor 1.5 than theoretical predictions obtained with standard optical potentials. A deviation is also found for the strong interaction quadrupole shift, ϵ 2 , for both the 4f and 3d orbits. A possible explanation for the smaller width, Γ 0 (3 d ), can be found by extending the parametrization of the optical potential with absorption terms.

Two-nucleon knock-out investigated with the semi-exclusive 12C(e, e′p) reaction

Abstract The 12C(e, e′p) reaction has been studied in the dip region, at an energy transfer ω = 212 MeV and a three-momentum transfer |q| = 270 MeV/c. The data cover a missing energy range 0–180 MeV and include missing momenta up to 750 MeV/c. The cross sections, measured at various angles between the detected proton and q, show a clear signature for two-nucleon knock-out. At large angles, the measured strength can mainly be attributed to the coupling of the virtual photon to pion exchange currents and intermediate Δ-excitation. The data at small angles cannot be explained by these mechanisms alone. It is suggested that additional strength originates from the emission of a correlated nucleon pair.

Strong interaction effects in pionic 208Pb

Abstract The X-ray spectrum of pionic 208 Pb has been measured. The deduced values of the strong interaction monopole shift with respect to the point Coulomb energy are ϵ 0 (4f) = 1.49±0.02 keV and ϵ 0 (3d) = 19.4±1.2 keV. The observed s strong interaction absorption widths of this spherical nucleus are found to be Γ 0 (4f) = 1.25±0.02 keV and Γ 0 (3d) = 47.0±3.6 keV. Important in the analysis of the pionic 4f level is the intensity balance for this level, giving an additional check on the measured absorption width of the pionic 4f level. For the more peripheral 4f state the measured strong interaction shifts and widths are well explained by standard optical model calculations. The values for the 3d state, however, are not in agreement with these calculations.

Crossing of negative parity bands in even-even nuclei around A≈150

Level energies of negative parity yrast bands (NPB) have been studied with a recently proposed, sensitive method. The irregular behaviour of the NPBs in theN=88 nuclei around spin 9–11 and the smooth behaviour in156Dy (up toI=13) and in238U (up to I=19) support calculations by Vogel, where NPBs are described as aligned octupole bands up to a critical spin where intersection with two-quasiparticle bands takes place. Considering the NPB levels (I≦13) in156Dy as members of an aligned octupole band, we obtain a remarkably good reproduction of their decay properties and energies using the VMI model.