K. Wisshak

Neutron Capture in Low-Mass Asymptotic Giant Branch Stars: Cross Sections and Abundance Signatures

Recently improved information on the stellar (n, γ) cross sections of neutron magic nuclei at N = 82, and in particular of 142Nd, turn out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low-mass asymptotic giant branch (AGB) stars. The origin of this phenomenon is identified as lying in the high neutron exposures at low neutron density obtained between thermal pulses when 13C burns radiatively in a narrow layer of a few 10-4 M☉. This effect is studied in some detail, and the influence of the currently available nuclear physics data is discussed with respect to specific further questions. In this context, particular attention is paid to a consistent description of s-process branchings in the region of the rare earth elements. It is shown that, in certain cases, the nuclear data are sufficiently accurate that the resulting abundance uncertainties can be completely attributed to stellar modeling. Thus, the s-process becomes important for testing the role of different stellar masses and metallicities as well as for constraining the assumptions used in describing the low neutron density provided by the 13C source.

Pulse shape analysis of liquid scintillators for neutron studies

The acquisition of signals from liquid scintillators with Flash ADC of high sampling rate ð 1G S=sÞ has been investigated. The possibility to record the signal waveform is of great advantage in studies with g’s and neutrons in a high count-rate environment, as it allows to easily identify and separate pile-up events. The shapes of pulses produced by g-rays and neutrons have been studied for two different liquid scintillators, NE213 and C6D6: A 1-parameter fitting procedure is proposed, which allows to extract information on the particle type and energy. The performance of this method in terms of energy resolution and n=g discrimination is analyzed, together with the capability to identify and resolve pile-up events. r 2002 Elsevier Science B.V. All rights reserved.

The Karlsruhe 4π barium fluoride detector

A new experimental approach has been implemented for accurate measurements of neutron capture cross sections in the energy range from 5 to 200 keV. The Karlsruhe 4π barium fluoride detector consists of 42 crystals shaped as hexagonal and pentagonal truncated pyramids forming a spherical shell with 10 cm inner radius and 15 cm thickness. All crystals are supplied with reflector and photomultiplier, thus representing independent gamma-ray detectors. The energy resolution of the 4π detector is 14% at 662 keV and 7% at 2.5 MeV gamma-ray energy, the overall time resolution is 500 ps and the peak efficiency 90% at 1 MeV. The detector allows to register capture cascades with 95% probability above a threshold energy of 2.5 MeV. Neutrons are produced via the 7Li(p, n)7Be reaction using the pulsed proton beam of a Van de Graaff accelerator. A collimated neutron beam is passing through the detector and hits the sample in the centre. The energy of captured neutrons is determined via time of flight, the primary flight path being 77 cm. The combination of short primary flight path, a 10 cm inner radius of the spherical BaF2 shell, and the low capture cross section of barium allows to discriminate background due to capture of sample-scattered neutrons in the scintillator by time of flight, leaving part of the neutron energy range completely undisturbed. This feature, together with the high efficiency and good energy resolution for capture gamma-rays, allows to separate the various background components reliably enough, that the capture cross section ratio of two isotopes can be determined with an accuracy of ≤ 1.0%. The detector will be used for nuclear astrophysics to investigate the origin of the heavy elements in the slow neutron capture process.

A detector for (n,γ) cross-section measurements at a spallation neutron source

Abstract The quest for improved neutron capture cross-sections for advanced reactor concepts, transmutation of radioactive wastes as well as for astrophysical scenarios of neutron capture nucleosynthesis has motivated new experimental efforts based on modern techniques. Recent measurements in the keV region have shown that a 4 π BaF 2 detector represents an accurate and versatile instrument for such studies. The present work deals with the potential of such a 4 π BaF 2 detector in combination with spallation neutron sources, which offer large neutron fluxes over a wide energy range. Detailed Monte Carlo simulations with the GEANT package have been performed to investigate the critical backgrounds at a spallation facility, to optimize the detector design, and to discuss alternative solutions.

An optimized C6D6 detector for studies of resonance-dominated (n, γ) cross-sections

Hydrogen-free scintillators are indispensable for determining the small, resonance-dominated neutron capture cross-sections of light and neutron magic nuclei, data which are needed for advanced reactor concepts, for transmutation of radioactive wastes as well as for astrophysical scenarios of neutron capture nucleosynthesis. A critical comparison of the existing detector concepts by means of detailed GEANT simulations revealed large differences in neutron sensitivity. Based on these simulations, an optimized detector was developed and successfully tested. Compared to a commercial detector, the neutron sensitivity of this solution is more than an order of magnitude lower, thus allowing even extremely small capture/scattering ratios to be measured reliably.

Prototype crystals for the Karlsruhe 4π barium fluoride detector

Abstract The energy and time resolution of large BaF 2 crystals has been improved significantly by the use of a PTFE reflector and a modified voltage divider chain. We found that an energy resolution of 8.7% (662 keV) and a time resolution of 325 ps ( 60 Co, 300 keV threshold) can be obtained simultaneously from a cylindrical test crystal of 1 l volume. The corresponding results as obtained from prototype crystals for the Karlsruhe 4 π detector are only slightly worse (9.6% and 400 ps). These crystals are truncated hexagonal and pentagonal pyramids shaped from cylindrical single crystals of 14 cm diameter and 15 cm thickness.

Fast neutron capture cross sections and related gamma-ray spectra of niobium-93, rhodium-103, and tantalum-181

The capture cross sections of /sup 93/-Nb, /sup 103/Rh, and /sup 181/Ta were measured in the 10- to 70-keV neutron energy range, using /sup 197/Au as a standard. Hauser-Feshbach calculations were performed that yielded not only the neutron cross sections of the isotopes considered up to 4-Mev neutron energy but also partial capture cross sections and capture gamma-ray spectra. For these calculations a consistent set of input parameters was determined from available experimental information or from model-guided systematics. The influence of these parameters on the results is discussed. Refs.

128Xe and 130Xe: Testing He-Shell Burning in Asymptotic Giant Branch Stars

The s-process branching at 128I has been investigated on the basis of new, precise, experimental (n, γ) cross sections for the s-only isotopes 128Xe and 130Xe. This branching is unique, since it is essentially determined by the temperature- and density-sensitive stellar decay rates of 128I and only marginally affected by the specific stellar neutron flux. For this reason it represents an important test for He-shell burning in asymptotic giant branch stars. The description of the branching by means of the complex stellar scenario reveals a significant sensitivity to the timescales for convection during He shell flashes, thus providing constraints for this phenomenon. The s-process ratio 128Xe/130Xe deduced from stellar models allows for a 9% ± 3% p-process contribution to solar 128Xe, in agreement with the Xe-S component found in meteoritic presolar SiC grains.

176Lu/176Hf : A sensitive test of s-process temperature and neutron density in AGB stars

The s-process branching at A ¼ 176 has been analyzed on the basis of significantly improved experimental cross sections.Thisworkreportsonactivationmeasurementsofthepartial(n ;� )crosssectionof 176Lufeedingtheisomeric state in 176 Lu. In total, six irradiations were performed at the Karlsruhe 3.7 MV pulsed Van de Graaff accelerator, and the induced activities were measured with HPGe clover detectors. In combination with previous data, partial cross sections of 3185 � 156 and 1153 � 30 mbarn were deduced at kT ¼ 5:1 and 25 keV, respectively. With these results and a recent time-of-flight measurement of the total stellar (n ;� ) cross section, the isomeric ratio was found to be constant in the relevant thermal energy range of the main s-process component. Based on these new data, a comprehensive analysis of the branching at 176 Lu was carried out for testing the temperature and neutron density conditions duringHeshellflashesinthermallypulsinglow-massasymptoticgiantbranchstars.Itwasfoundthatthelong-standing problem of the mother/daughter ratio of the two s-only isotopes 176 Lu and 176 Hf could be solved, if the temperaturedependent � -decay half-life of 176 Lu was considered with sufficient resolution over the temperature profile of the

Neutron capture cross section of 232Th

Abstract The neutron capture cross section of 232Th has been measured in the energy range from 5 to 225 keV at the Karlsruhe 3.7-MV Van de Graaff accelerator relative to the gold standard. Neutrons were produced via the 7Li(p,n) 7Be reaction by bombarding metallic Li targets with a pulsed proton beam, and capture events were registered with the Karlsruhe 4π barium fluoride detector. The main difficulty in this experiment is the detection of true capture events characterized by a comparably low binding energy of 4.78 MeV in the presence of the high-energy gamma background (up to 3.96 MeV) associated with the decay chain of the natural thorium sample. With the high efficiency and the good energy resolution of the 4π detector, the sum energy peak of the capture cascades could be reliably separated from the background over the full range of the neutron spectrum, yielding cross-section uncertainties of; 2% above 20 keV and of 4% at 5 keV. The clear identification of the various background components represents a significant improvement compared to existing data for which sometimes high accuracy was claimed, but which were found to be severely discrepant. A comparison to the evaluated files shows reasonable agreement in the energy range above 15 keV, but also severe discrepancies of up to 40% at lower neutron energies.