R. Reifarth

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.

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.

Neutron reactions in astrophysics

The quest for the origin of matter in the Universe had been the subject of philosophical and theological debates over the history of mankind, but quantitative answers could be found only by the scientific achievements of the last century. A first important step on this way was the development of spectral analysis by Kirchhoff and Bunsen in the middle of the 19th century, which provided first insight in the chemical composition of the sun and the stars. The energy source of the stars and the related processes of nucleosynthesis, however, could be revealed only with the discoveries of nuclear physics. A final break-through came eventually with the compilation of elemental and isotopic abundances in the solar system, which reflect the various nucleosynthetic processes in detail. This review focuses on the mass region above iron, where the formation of the elements is dominated by neutron capture, mainly in the slow (s) and rapid (r) processes. Following a brief historic account and a sketch of the relevant astrophysical models, emphasis is put on the nuclear physics input, where status and perspectives of experimental approaches are presented in some detail, complemented by the indispensable role of theory.

Stellar Neutron Capture on Promethium: Implications for the s-Process Neutron Density

The unstable isotope 147Pm represents an important branch point in the s-process reaction path. This paper reports on the successful determination of the stellar (n, γ) cross section via the activation technique. The experiment was difficult because the relatively short 147Pm half-life of 2.62 yr enforced the sample mass to be restricted to 28 ng or 1014 atoms only. By means of a modular, high-efficiency Ge Clover array the low induced activity could be identified in spite of considerable backgrounds from various impurities. Both partial cross sections feeding the 5.37 day ground state and the 41.3 day isomer in 148Pm were determined independently, yielding a total (n, γ) cross section of 709 ± 100 mbarn at a thermal energy of kT = 30 keV. The (n, γ) cross sections of the additional branch point isotopes 147Nd and 148Pm as well as the effect of thermally excited states were obtained by detailed statistical model calculations. The present results allowed considerably refined analyses of the s-process branchings at A = 147/148, which are probing the neutron density in the He-burning shell of low-mass asymptotic giant branch stars.

Neutron capture cross section of unstable 63Ni: implications for stellar nucleosynthesis.

The 63Ni(n,γ) cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from   kT=5-100  keV with uncertainties around 20%. Stellar model calculations for a 25M⊙ star show that the new data have a significant effect on the s-process production of 63Cu, 64Ni, and 64Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

Acquisition-analysis system for the DANCE (detector for advanced neutron capture experiments) BaF/sub 2/ gamma-ray calorimeter

The DANCE detector is a segmented 4/spl pi/ gamma-ray calorimeter for measuring (n, /spl gamma/) and (n,fission) cross-sections of stable and long-lived radioactive isotopes. DANCE uses waveform digitization to acquire the basic gamma-ray data, which maximizes the information available for event reconstruction, but has necessitated the development of several techniques for handling the resulting high data rates. This paper describes the basic experimental requirements for acquisition and analysis and how we have satisfied these requirements primarily by extending existing acquisition and analysis frameworks.

The Stellar Neutron-Capture Rate of 34S: The Origin of 36S Challenged

The (n, γ) cross section of 34S has been measured by means of the activation technique using a quasi-stellar neutron spectrum corresponding to a thermal energy of kT = 25 keV. In spite of the very small cross section of this neutron-magic nucleus and a number of additional experimental complications, the stellar value of σv/vT = 226 ± 10 μb could be reliably determined. Because of this small cross section, 34S acts as a bottleneck for the reaction flow to 36S. Consequently, the suggested interpretation of the 36S abundance as a neutron monitor for the s-process in massive stars must be ruled out. This implies, however, that the origin of 36S remains an open problem.

Measurement of the 60Fe(n, gamma)61Fe Cross Section at Stellar Temperatures.

Observations of galactic gamma-ray activity have challenged the current understanding of nucleosynthesis in massive stars. Recent measurements of (60)Fe abundances relative to ;{26}Al;{g} have underscored the need for accurate nuclear information concerning the stellar production of (60)Fe. In light of this motivation, a first measurement of the stellar (60)Fe(n, gamma)(61)Fe cross section, the predominant destruction mechanism of (60)Fe, has been performed by activation at the Karlsruhe Van de Graaff accelerator. Results show a Maxwellian averaged cross section at kT = 25 keV of 9.9 +/-_{1.4(stat)};{2.8(syst)}mbarn, a significant reduction in uncertainty with respect to existing theoretical discrepancies. This result will serve to significantly constrain models of (60)Fe nucleosynthesis in massive stars.

PINO-a tool for simulating neutron spectra resulting from the 7Li(p,n) reaction

The 7 Li(p,n) reaction in combination with a 3.7 MV Van de Graaff accelerator was routinely used at FZK to perform activation as well as time-of-flight measurements with neutrons in the keV-region. Planned new setups with much higher proton currents like SARAF and FRANZ and the availability of liquid-lithium target technology will trigger a renaissance of this method. A detailed understanding of the neutron spectrum is not only important during the planning phase of an experiment, but also during for the analysis of activation experiments. Therefore, the Monte-Carlo based program PINO (Protons In Neutrons Out) was developed, which allows the simulation of neutron spectra considering the geometry of the setup and the proton-energy distribution.