arXiv subjects

A. R. Junghans

Publications and source records attributed to A. R. Junghans.

30 records · Page 2Linked to original sources

arXiv2007

Nuclear-fission studies with relativistic secondary beams: analysis of fission channels

Nuclear fission of several neutron-deficient actinides and pre-actinides from excitation energies around 11 MeV was studied at GSI Darmstadt by use of relativistic secondary beams. The characteristics of multimodal fission of nuclei around 226Th are systematically investigated and interpreted as the superposition of three fission channels. Properties of these fission channels have been determined for 15 systems. A global view on the properties of fission channels including previous results is presented. The positions of the asymmetric fission channels are found to be constant in element number over the whole range of systems investigated.

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arXiv2007

Photodisintegration studies on p-nuclei: The case of Mo and Sm isotopes

In explosive stellar environments like supernovae, the temperatures are high enough for the production of heavy neutron-deficient nuclei, the socalled p-nuclei. Up to now, the knowledge of the reaction rates of p-nuclei is based on theoretical parameterizations using statistical model calculations. At the bremsstrahlung facility of the superconducting electron accelerator ELBE of FZ Dresden-Rossendorf, we aim to measure the photodisintegration rates of heavy nuclei experimentally. Photoactivation measurements on the astrophysically relevant p-nuclei 92Mo and 144Sm have been performed with bremsstrahlung end-point energies from 10.0 to 16.5 MeV. First experiments on the short-lived decays following the reaction 144Sm(gamma,n) are carried out using a pneumatic delivery system for rapid transport of activated samples. The activation yields are compared with calculations using cross sections from recent Hauser-Feshbach models.

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arXiv2007

Photon strength distributions in stable even-even molybdenum isotopes

Electromagnetic dipole-strength distributions up to the particle separation energies are studied for the stable even-even nuclides $^{92,94,96,98,100}$Mo in photon scattering experiments at the superconducting electron accelerator ELBE of the Forschungszentrum Dresden-Rossendorf. The influence of inelastic transitions to low-lying excited states has been corrected by a simulation of $γ$ cascades using a statistical model. After corrections for branching ratios of ground-state transitions, the photon-scattering cross-sections smoothly connect to data obtained from $(γ,n)$-reactions. With the newly determined electromagnetic dipole response of nuclei well below the particle separation energies the parametrisation of the isovector giant-dipole resonance is done with improved precision.

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arXiv2005

Photodissociation of p-process nuclei studied by bremsstrahlung induced activation

A research program has been started to study experimentally the near-threshold photodissociation of nuclides in the chain of cosmic heavy element production with bremsstrahlung from the ELBE accelerator. An important prerequisite for such studies is good knowledge of the bremsstrahlung distribution which was determined by measuring the photodissociation of the deuteron and by comparison with model calculations. First data were obtained for the astrophysically important target nucleus 92-Mo by observing the radioactive decay of the nuclides produced by bremsstrahlung irradiation at end-point energies between 11.8 MeV and 14.0 MeV. The results are compared to recent statistical model calculations.

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arXiv2004

New signatures on dissipation from fission induced by relativistic heavy-ion collisions

Fissile nuclei with small shape distortion relative to the ground-state deformation and with low angular momentum were produced in peripheral heavy-ion collisions. Under the conditions of small shape distortions and low angular momentum, the theoretical description of the fission process can be considerably simplified, and the relevant information on dissipation can be better extracted than in conventional experiments based on fusion-fission reactions. In addition, this experimental approach induces very high excitation energies, a condition necessary to observe transient effects. The experimental data were taken at GSI using a set-up especially conceived for fission studies in inverse kinematics. This set-up allowed determining three observables whose sensitivity to dissipation was investigated for the first time: the total fission cross sections of 238U at 1 A GeV as a function of the target mass, and, for the reaction of 238U at 1 A GeV on a (CH2)n target, the partial fission cross sections and the partial charge distributions of the fission fragments. The comparison of the new experimental data with a reaction code adapted to the conditions of the reactions investigated leads to clear conclusions on the strength of dissipation at small deformation where the existing results are rather contradictory.

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arXiv2004

A critical analysis of the modelling of dissipation in fission

The time-dependent flux over the fission barrier of an excited nucleus under the influence of dissipation is investigated. Characteristic features of the evolution of the amplitude of the probability distribution and the velocity profile at the fission barrier are derived. Analytical results are compared to numerical Langevin calculations and used to develop a new analytical approximation to the solution of the Fokker-Planck equation for the time-dependent fission-decay width. This approximation is shown to be more realistic than previously proposed descriptions, which were widely used in the past.

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arXiv2004

Manifestation of transient effects in fission induced by relativistic heavy-ion collisions

We examine the manifestation of transient effects in fission by analysing experimental data where fission is induced by peripheral heavy-ion collisions at relativistic energies. Available total nuclear fission cross sections of 238U at 1 A GeV on gold and uranium targets are compared with a nuclear-reaction code, where transient effects in fission are modelled using different approximations to the numerical time-dependent fission-decay width: a new analytical description based on the solution of the Fokker-Planck equation and two widely used but less realistic descriptions, a step function and an exponential-like function. The experimental data are only reproduced when transient effects are considered. The deduced value of the dissipation strength depends strongly on the approximation applied for the time-dependent fission-decay width and is estimated to be of the order of 2x10**21 s**(-1). A careful analysis sheds severe doubts on the use of the exponential-like in-growth function largely used in the past. Finally, we discuss which should be the characteristics of experimental observables to be most sensitive to transient effects in fission

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arXiv2004

Transient effects in fission evidenced from new experimental signatures

A new experimental approach is introduced to investigate the relaxation of the nuclear deformation degrees of freedom. Highly excited fissioning systems with compact shapes and low angular momenta are produced in peripheral relativistic heavy-ion collisions. Both fission fragments are identified in atomic number. Fission cross sections and fission-fragment element distributions are determined as a function of the fissioning element. From the comparison of these new observables with a nuclear-reaction code a value for the transient time is deduced.

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arXiv2003

Comment on "Electromagnetic dissociation of ^8B and the astrophysical S-factor for ^7Be(p,gamma)^8B"

Recently, Davids and Typel recommended a "low" value of S_{17}(0) based on fits to published direct and Coulomb dissociation data, in which they excluded the precise result of Junghans et al. We show that their statistical analysis is incorrect, due to a substantial underestimate of the experimental uncertainties, and leads to conclusions that are not supported by a proper analysis.

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arXiv2003

Precise measurement of the ^7Be(p,gamma)^8B S-factor

We present new measurements of the ^7Be(p,gamma)^8B cross section from \bar{E}_{cm} = 116 to 2460 keV, that incorporate several improvements over our previously published experiment, also discussed here. Our new measurements lead to S_{17}(0) = 22.1 +- 0.6(expt) +- 0.6(theor) eV b based on data from \bar{E}_{cm} = 116 to 362 keV, where the central value is based on the theory of Descouvemont and Baye. The theoretical error estimate is based on the fit of 12 different theories to our low energy data. We compare our results to other S_{17}(0) values extracted from both direct (^7Be(p,gamma)^8B) and indirect (Coulomb dissociation and heavy-ion reaction) measurements, and show that the results of these 3 types of experiments are not mutually compatible. We recommend a "best" value, S_{17}(0) = 21.4 +- 0.5(expt) +- 0.6(theor) eV b, based on the mean of all modern direct measurements below the 1^+ resonance. We also present S-factors at 20 keV which is near the center of the Gamow window: the result of our measurements is S_{17}(20) = 21.4 +- 0.6(expt) +- 0.6(theor) eV b, and the recommended value is S_{17}(20) = 20.7 +- 0.5(expt) +- 0.6(theor) eV b.

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arXiv2002

Electromagnetic-induced fission of 238U projectile fragments, a test case for the production of spherical super-heavy nuclei

Isotopic series of 58 neutron-deficient secondary projectiles (205,206At, 205-209Rn, 208-212,217,218Fr, 211-223Ra, 215-226Ac, 221-229Th, 226-231Pa, 231-234U) were produced by projectile fragmentation using a 1 A GeV 238U beam. Cross sections of fission induced by nuclear and electromagnetic interactions in a secondary lead target were measured. They were found to vary smoothly as a function of proton and neutron number of the fissioning system, also for nuclei with large ground-state shell effects near the 126-neutron shell. No stabilization against fission was observed for these nuclei at low excitation energies. Consequences for the expectations on the production cross sections of super-heavy nuclei are discussed.

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arXiv2001

7Be(p,gamma)8B astrophysical S-factor from precision cross section measurements

We measured the 7Be(p,gamma)8B cross section from E_cm = 186 to 1200 keV, with a statistical-plus-systematic precision per point of better than +- 5%. All important systematic errors were measured including 8B backscattering losses. We obtain S_17(0) = 22.3 +- 0.7(expt) +- 0.5(theor) eV-b from our data at E_cm <= 300 keV and the theory of Descouvemont and Baye.

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