Rudolf Siemssen

Binary l-matched reactions in 14N + 159Tb collisions

Abstract Various channels from the 14 N + 159 Tb reaction at 140 MeV were studied using γ-particle coincidences combined with γ-ray multiplicity measurements. A wide class of binary reactions ranging from typical incomplete fusion processes to few-nucleon capture reactions was selected and investigated. Energy spectra, absolute cross sections and average γ-ray multiplicities were measured for a large number of reaction channels. From a detailed kinematical analysis it was found that all the reactions quite precisely satisfy the “recoil formula” of Siemens et al. for hard-grazing collisions. It is attempted to describe this wealth of reaction data on the common footing of the sum-rule model that combines the idea of partial statistical equilibrium with a generalized concept of critical angular momentum. The experimental results seem to confirm the basic assumptions of the model.

Incomplete fusion in 12C+160Gd collisions☆

Abstract The mechanism of the production of fast α-particles in 12 C induced reactions was studied over a wide range of bombarding energies (7.5–16.7 MeV/ A ) by measuring α-γ coincidences. Absolute cross sections for 160 Gd ( 12 C , αx n ) 168−x Er and 160 Gd ( 12 C , 2αx n ) 164−x Dy reactions, as well as inclusive α-particle production cross sections, have been determined. Depending on the bombarding energy, a fraction of 0.2–0.4 of the singles α-particles can be explained as resulting from incomplete fusion reactions ( 12 C, α) and ( 12 C, 2α) which correspond to a capture of “ 8 Be” and “ 4 He”, respectively. The remaining fragments of the projectile have, on the average, the beam-velocity energies and their angular distributions are forward peaked. Distributions of side-feeding to the yrast bands in the target-residue nuclei indicate that low partial waves are strongly hindered in the incomplete fusion reactions. The energy dependence of the cross sections for ( 12 C, α) and ( 12 C, 2α) incomplete fusion as well as for ( 12 C, 3α) projectile breakup is given. A generalized concept of critical angular momentum which explains a competition between complete fusion, incomplete fusion and breakup reactions is proposed.

The 116Sn(d, t)115Sn reaction and the investigation of deeply bound hole states in the odd-A tin isotopes

Abstract Single-neutron pick-up via the (d, t) reaction was studied at E d = 50 MeV on all stable, even- A tin isotopes with an emphasis on the pick-up from the first major shell below the valence shell. The deeply bound hole states thus observed exhibit themselves as a gross structure phenomenon with a systematic variation of its width as a function of neutron number. The most extensive study was done for the 116 Sn(d, t) 115 Sn reaction for which angular distributions were obtained not only for the deeply bound hole states but also for the pick-up of valence nucleons, and for the excitation of the isobaric analogue states. A DWBA analysis yielded the full sum-rule strength for the pick-up of the valence nucleons. It located in the gross structure about 45% of the combined 1g 9 2 , 2p 1 2 , 2p 3 2 and 1f 5 2 strength. Additional strength may be found in the continuous “background” on which the gross structure peaks are riding. A systematic study of the gross structure shapes and cross sections in all odd- A tin isotopes is presented. The observed variation in the spreading of the 1g 9 2 hole strength as a function of neutron number is compared with recent calculations.

Production of pi(0) and eta mesons in carbon-induced relativistic heavy-ion collisions

The production of π0 and η mesons has been investigated in the system 12C+12C at 0.8A GeV, 1.0A GeV, and 2.0A GeV using the TAPS photon detector. The production cross sections and transverse-momentum distributions measured around midrapidity extend the existing systematics of neutral-meson production to light systems and the highest energy available at the GSI accelerator facility. At all beam energies scaling of the differential production cross section with the transverse mass of the emitted meson is observed. In addition π0 production has been studied in the mass-asymmetric systems 12C+197Au and 197Au+12C at 0.8A GeV.

Semi-microscopic description of three-nucleon transfer and the Sn(p, α)In reaction☆

Abstract Differential cross sections have been measured in the angular range from 10° to ≈ 60° for the 112, 116, 118, 124Sn(p, α)109, 113, 115, 121In reaction at an incident energy of Ep = 22 MeV. The 124Sn(p, α)121In reaction also was studied at Ep = 30 MeV. From the observed differences between the (p, α) and one-proton pickup data it is inferred that coherence effects play an important role arising from additional contributions to the reaction amplitude in which the picked-up neutrons do not form a zero-coupled pair. A semi-microscopic model has been developed to calculate relative transition strengths assuming a one-step pickup process for the (p, α) reaction. Detailed calculations were carried out for the 118Sn(p, α)115In reaction by employing weak-coupling wave functions for 115In. The calculations account well for the observed differences between the one-proton pickup and the (p, α) data and correctly reproduce the relative intensities for the 0 g 9 2 , 1 p 1 2 and 1 p 3 2 proton-hole states and the |0 g 9 2 −1 ⊗2 + 〉 weak-coupling multiplet. In addition to the reaction mechanism study new sctroscopic information was obtained for some of the odd-A indium isotopes.

Unusually strong excitation of the 1.59 MeV 0+ state in the 100Mo(d,6Li)96Zr reaction

In a study of the 94,96,98,100Mo(d, 6Li)90,92,94,96Zr reactions at Ed = 45 MeV an unusually strong excitation of the 1.59 MeV 0+ state in 96Zr is observed. The variation in the 0+ strength from one zirconium isotope to the other can be qualitatively accounted for by the changing proton configuration with neutron number.

Sequential ejectile decays and uncorrelated breakup processes in the 14N + 159Tb reaction

From a study of particle-particle correlations, involving position-sensitive detector telescopes, conclusive evidence for sequential ejectile decay was found, and the relative importance of sequential decay and of breakup processes, which were observed to be uncorrelated in angle, could be determined.

Bremsstrahlung photons as a probe of hot nuclei

Abstract Aside from the dominant production of hard photons in first-chance p-n collisions, a significant hard-photon production in a later stage of heavy-ion reactions is predicted by the BUU theory. These thermal hard photons are emitted from a nearly thermalized source and still originate from bremsstrahlung produced in individual p-n collisions. The calculations predict that the production of the thermal hard photons is strongly correlated with the incompressibility of nuclear matter. Tentative experimental evidence for their production is found in the hard-photon energy spectra measured in the systems 86 Kr + nat Ni at 60.0 A MeV, 181 Ta+ 197 Au at 39.5 MeV and 208 Pb+ 197 Au at 29.5 A MeV.

Astrophysical rate of O-15(alpha,gamma)Ne-19 via the (p, t) reaction in inverse kinematics

A recoil coincidence technique has been applied to measure the alpha-decay branching ratios of near-threshold states in Ne-19. Populating these states using the (p,t) reaction in inverse kinematics, we detected the recoils and their decay products with 100% geometric efficiency using a magnetic spectrometer. Combining our branching ratio measurements with independent determinations of the radiative widths of these states, we calculate the astrophysical rate of O-15(alpha,gamma)Ne-19. Using this reaction rate, we perform hydrodynamic calculations of nova outbursts and conclude that no significant breakout from the hot CNO cycles into the rp process occurs in novae via O-15(alpha,gamma)Ne-19.

PARTICLE-PARTICLE CORRELATION STUDIES FOR THE SYSTEM N-14+TB-159 AT 10 AND 12 MEV/A

Abstract Energy and angular correlations of p, n and α-particles in coincidence with projectile-like fragments have been studied for the reaction 14 N + 159 Tb at 10 and 12 MeV/ A . The measurements involved in-plane, and at one energy also out-of-plane, correlations. Position-sensitive detectors were employed to investigate sequential ejectile decays. The correlations indicate the presence of at least two processes: (a) formation in a binary process of an excited fragment that decays sequentially by light particle emission and (b) emission of a fast light particle at an early stage of the interaction. The second is characteristic of an “uncorrelated” process that can be described by a factorisation prescription. For the heavier fragments, the first process is associated with the quasielastic component while the second component corresponds to inelastic events.