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K. -H. Schmidt

Publications and source records attributed to K. -H. Schmidt.

At least 37 records · Page 2Linked to original sources
arXiv2010

Extending the north-east limit of the chart of nuclides

The existence of nuclei with exotic combinations of protons and neutrons provides fundamental information on the forces acting between nucleons. The maximum number of neutrons a given number of protons can bind, neutron drip line1, is only known for the lightest chemical elements, up to oxygen. For heavier elements, the larger its atomic number, the farther from this limit is the most neutron-rich known isotope. The properties of heavy neutron-rich nuclei also have a direct impact on understanding the observed abundances of chemical elements heavier than iron in our Universe. Above half of the abundances of these elements are thought to be produced in rapid-neutron capture reactions, r-process, taking place in violent stellar scenarios2 where heavy neutron-rich nuclei, far beyond the ones known up today, are produced. Here we present a major step forward in the production of heavy neutron-rich nuclei: the discovery of 73 new neutron-rich isotopes of chemical elements between tantalum (Z=72) and actinium (Z=89). This result proves that cold-fragmentation reactions3 at relativistic energies are governed by large fluctuations in isospin and energy dissipation making possible the massive production of heavy neutron-rich nuclei, paving then the way for the full understanding of the origin of the heavier elements in our Universe. It is expected that further studies providing ground and structural properties of the nuclei presented here will reveal further details on the nuclear shell evolution along Z=82 and N=126, but also on the understanding of the stellar nucleosyntheis r-process around the waiting point at A~190 defining the speed of the matter flow towards heavier fissioning nuclei.

nucl-ex↗
arXiv2008

R-process nucleosynthesis calculations with complete nuclear physics input

The r-process constitutes one of the major challenges in nuclear astrophysics. Its astrophysical site has not yet been identified but there is observational evidence suggesting that at least two possible sites should contribute to the solar system abundance of r-process elements and that the r-process responsible for the production of elements heavier than Z=56 operates quite robustly producing always the same relative abundances. From the nuclear-physics point of view the r-process requires the knowledge of a large number of reaction rates involving exotic nuclei. These include neutron capture rates, beta-decays and fission rates, the latter for the heavier nuclei produced in the r-process. We have developed for the first time a complete database of reaction rates that in addition to neutron-capture rates and beta-decay half-lives includes all possible reactions that can induce fission (neutron-capture, beta-decay and spontaneous fission) and the corresponding fission yields. In addition, we have implemented these reaction rates in a fully implicit reaction network. We have performed r-process calculations for the neutrino-driven wind scenario to explore whether or not fission can contribute to provide a robust r-process pattern.

astro-ph↗
arXiv2008

Production of medium-mass neutron-rich nuclei in reactions induced by 136Xe projectiles at 1 A GeV on a beryllium target

Production cross sections of medium-mass neutron-rich nuclei obtained in the fragmentation of 136Xe projectiles at 1 A GeV have been measured with the FRagment Separator (FRS) at GSI. 125Pd was identified for the first time. The measured cross sections are compared to 238U fission yields and model calculations in order to determine the optimum reaction mechanism to extend the limits of the chart of the nuclides around the r-process waiting point at N=82.

nucl-ex↗
arXiv2008

Experimental investigation of the residues produced in the 136Xe+Pb and 124Xe+Pb fragmentation reactions at 1 A GeV

This paper presents the nuclide cross sections and the longitudinal velocity distributions of residues produced in the reactions of 136Xe and 124Xe at 1 A GeV in a lead target, measured at the high-resolution magnetic spectrometer, the Fragment Separator (FRS) of GSI. The data cover a broad range of isotopes of the elements between Z = 3 and Z = 56 for 136Xe and between Z = 5 and Z = 55 for 124Xe, reaching down to cross sections of a few microbarns. The velocity distributions exhibit a Gaussian shape for masses above A = 20, while more complex behaviour is observed for lighter masses. The isotopic distributions for both reactions preserve a memory on the projectile N/Z ratio over the whole residue mass range.

nucl-ex↗
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.

nucl-ex↗
arXiv2007

First access to beta half-lives approaching the r-process path near N=126

Heavy neutron-rich nuclei close to N=126 were produced by fragmentation of a 1 A GeV 208Pb beam at the FRS at GSI. The beta-decay half-lives of 8 nuclides have been determined. The comparison of the data with model calculations including an approach based on the self-consistent ground-state description and continuum QRPA considering the Gamow-Teller and first-forbidden decays provide a first indication on the importance of first-forbidden transitions around A=195. The measured data indicate that the matter flow in the r-process to heavier fissioning nuclei is faster than previously expected.

nucl-ex↗
arXiv2007

A new analysis method to determine beta-decay half-lives in experiments with complex background

This paper reports the first application of a new technique to measure the beta-decay half -lives of exotic nuclei in complex background conditions. Since standard tools were not adapted to extract the relevant information, a new analysis method was developed. The time distribution of background events is established by recording time correlations in backward time. The beta half lives of the nuclides and the detection efficiency of the set-up are determined simultaneously from a least-squares fit of the ratio of the time-correlation spectra recorded in forward and in backward time, using numerical functions. The necessary numerical functions are calculated in a Monte-Carlo code using the known operation parameters of the experiment and different values for the two free parameters, half-life and detection efficiency, as input parameters.

nucl-ex↗
arXiv2007

Measurement of the complete nuclide production and kinetic energies of the system 136Xe + hydrogen at 1 GeV per nucleon

We present an extensive overview of production cross sections and kinetic energies for the complete set of nuclides formed in the spallation of 136Xe by protons at the incident energy of 1 GeV per nucleon. The measurement was performed in inverse kinematics at the FRagment Separator (GSI, Darmstadt). Slightly below the Businaro-Gallone point, 136Xe is the stable nuclide with the largest neutron excess. The kinematic data and cross sections collected in this work for the full nuclide production are a general benchmark for modelling the spallation process in a neutron-rich nuclear system, where fission is characterised by predominantly mass-asymmetric splits.

nucl-ex↗
arXiv2007

First experiment on fission transients in highly fissile spherical nuclei produced by fragmentation of radioactive beams

We report on a novel experimental approach for studying the dissipative spreading of collective motion in a meta-stable nuclear system, using, for the first time, highly fissile nuclei with spherical shape. This was achieved by fragmentation of 45 radioactive heavy-ion beams at GSI, Darmstadt. The use of inverse kinematics and a dedicated experimental set-up allowed for the identification in atomic number of both fission fragments. From the width of their charge distributions, a transient time of (3.3 +/- 0.7) 10-21 s is deduced for initially spherical nuclei.

nucl-ex↗
arXiv2006

Spallation Residues in the Reaction 56Fe + p at 0.3, 0.5, 0.75, 1.0 and 1.5 A GeV

The spallation residues produced in the bombardment of 56}Fe at 1.5, 1.0, 0.75, 0.5 and 0.3 A GeV on a liquid-hydrogen target have been measured using the reverse kinematics technique and the Fragment Separator at GSI (Darmstadt). This technique has permitted the full identification in charge and mass of all isotopes produced with cross-sections larger than 10^{-2} mb down to Z=8. Their individual production cross-sections and recoil velocities at the five energies are presented. Production cross-sections are compared to previously existing data and to empirical parametric formulas, often used in cosmic-ray astrophysics. The experimental data are also extensively compared to different combinations of intra-nuclear cascade and de-excitation models. It is shown that the yields of the lightest isotopes cannot be accounted for by standard evaporation models. The GEMINI model, which includes an asymmetric fission decay mode, gives an overall good agreement with the data. These experimental data can be directly used for the estimation of composition modifications and damages in materials containing iron in spallation sources. They are also useful for improving high precision cosmic-ray measurements.

nucl-ex↗
arXiv2006

Nucleosynthesis in neutrino heated matter: The vp-process and the r-process

This manuscript reviews recent progress in our understanding of the nucleosynthesis of medium and heavy elements in supernovae. Recent hydrodynamical models of core-collapse supernovae show that a large amount of proton rich matter is ejected under strong neutrino fluxes. This matter constitutes the site of the vp-process where antineutrino absorption reactions catalyze the nucleosynthesis of nuclei with A > 64. Supernovae are also associated with the r-process responsible for the synthesis of the heaviest elements in nature. Fission during the r-process can play a major role in determining the final abundance patter and in explaining the almost universal features seen in metal-poor r-process-rich stars.

astro-ph↗
arXiv2006

Systematic comparison of ISOLDE-SC yields with calculated in-target production rates

Recently, a series of dedicated inverse-kinematics experiments performed at GSI, Darmstadt, has brought an important progress in our understanding of proton and heavy-ion induced reactions at relativistic energies. The nuclear reaction code ABRABLA that has been developed and benchmarked against the results of these experiments has been used to calculate nuclide production cross sections at different energies and with different targets and beams. These calculations are used to estimate nuclide production rates by protons in thick targets, taking into account the energy loss and the attenuation of the proton beam in the target, as well as the low-energy fission induced by the secondary neutrons. The results are compared to the yields of isotopes of various elements obtained from different targets at CERN-ISOLDE with 600 MeV protons, and the overall extraction efficiencies are deduced. The dependence of these extraction efficiencies on the nuclide half-life is found to follow a simple pattern in many different cases. A simple function is proposed to parameterize this behavior in a way that quantifies the essential properties of the extraction efficiency for the element and the target - ion-source system in question.

nucl-ex↗
arXiv2005

Evaporation residues produced in spallation of 208Pb by protons at 500A MeV

The production cross sections of fragmentation-evaporation residues in the reaction Pb+p at 500A MeV have been measured using the inverse-kinematics method and the FRS spectrometer (GSI). Fragments were identified in nuclear charge using ionisation chambers. The mass identification was performed event-by-event using the B-rho - TOF - Delta-E technique. Although partially-unresolved ionic charge states induced an ambiguity on the mass of some heavy fragments, production rates could be obtained with a high accuracy by systematically accounting for the polluting ionic charge states. The contribution of multiple reactions in the target was subtracted using a new, partly self-consistent code. The isobaric distributions are found to have a shape very close to the one observed in experiments at higher energy. Kinematic properties of the fragments were also measured. The total and the isotopic cross sections, including charge-pickup cross sections, are in good agreement with previous measurements. The data are discussed in the light of previous spallation measurements, especially on lead at 1 GeV.

nucl-ex↗
arXiv2005

Transient and quasi-stationary dissipative effects in the fission flux across the barrier in 1 A GeV 238U on deuterium reactions

Isotopic cross sections of all projectile residues with Z above 23 produced in collisions induced by 238U at 1 A GeV on deuterium have been measured. The isotopic distributions reflect the role of evaporation and fission in the formation process of these nuclei. The comparison of the measured cross sections with Monte-Carlo de-excitation codes including an analytical description of the dynamics of fission shows the sensitivity of the data to nuclear dissipation. Moreover, the large excitation-energy range covered in this experiment together with the high accuracy of the measured cross sections allowed to clearly separate and quantify the role of transient and quasi-stationary dissipative effects in the fission-decay width.

nucl-ex↗
arXiv2005

Light Nuclides Produced in the Proton-Induced Spallation of 238U at 1 GeV

The production of light and intermediate-mass nuclides formed in the reaction 1H+238U at 1 GeV was measured at the Fragment Separator (FRS) at GSI, Darmstadt. The experiment was performed in inverse kinematics, shooting a 1 A GeV 238U beam on a thin liquid-hydrogen target. 254 isotopes of all elements in the range from Z=7 to Z=37 were unambiguously identified, and the velocity distributions of the produced nuclides were determined with high precision. The results show that the nuclides are produced in a very asymmetric binary decay of heavy nuclei originating from the spallation of uranium. All the features of the produced nuclides merge with the characteristics of the fission products as their mass increases.

nucl-ex↗
arXiv2004

Recoil spectrometers for heavy-ion identification and secondary-beam production: pushing the low-energy limit

The feasibility of low-energy fragmentation experiments using a magnetic spectrometer is discussed. The main challenge is the multiplicity of the ionic charge states, which can hamper the identification in both Z and A of the fragments. Three topics are covered. First, a specific set-up for ionization chambers, based on a very large gas thickness, is presented. Its satisfactory performances are discussed in light of the observations during a 500A MeV Pb+p experiment performed at the FRS (GSI). As a second topic, the possibility to use a thick layer of matter (a degrader) as a passive measurement device to identify the nuclear charge and the ionic charge state of fragments is discussed. This method, successfully used for Z identification in experiments such as Pb+p at 1A GeV, fails to measure the charge states at 500A MeV for the same system. It is shown that surface defects of the degrader are probably responsible for this failure. The third topic is the description of new analysis techniques developed in order to account for and subtract the contribution of polluting charge states in the spectrometer, thus making possible a clean estimation of the production cross sections of all fragments. The combination of those new experimental and analysis techniques made the 500A MeV spallation experiment a success.

nucl-ex↗
arXiv2004

High-resolution velocity measurements on fully identified light nuclides produced in 56Fe + hydrogen and 56Fe + titanium systems

New experimental results on the kinematics and the residue production are obtained for the interactions of 56Fe projectiles with protons and (nat)Ti target nuclei, respectively, at theincident energy of 1 A GeV. The titanium-induced reaction serves as a reference case for multifragmentation. Already in the proton-induced reaction, the characteristics of the isotopic cross sections and the shapes of the velocity spectra of light residues indicate that high thermal energy is deposited in the system during the collision. In the 56Fe+p system the high excitation seems to favour the onset of fast break-up decays dominated by very asymmetric partitions of the disassembling system. This configuration leads to the simultaneous formation of one or more light fragments together with one heavy residue.

nucl-ex↗