Neutron capture cross sections of 69Ga and 71Ga at 25 keV and e peak = 90 keV

This project was supported by EFNUDAT, ERINDA, the EuroGENESIS project MASCHE, HIC for FAIR and BMBF (05P15RFFN1).We measured the neutron capture cross sections of 69Ga and 71Ga for a quasi-stellar spectrum at kBT = 25 keV and a spectrum with a peak energy at 90 keV by the activation technique at the Joint Research Centre (JRC) in Geel, Belgium. Protons were provided by an electrostatic Van de Graaff accelerator to produce neutrons via the reaction 7Li(p,n). The produced activity was measured via the γ emission of the product nuclei by high-purity germanium detectors. We present preliminary results.publishersversionpublishe

First Results of the 140^{140}Ce(n,γ)141^{141}Ce Cross-Section Measurement at n_TOF

An accurate measurement of the $^{140}$Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the $^{140}$Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in $^{140}$Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the $^{140}$Ce Maxwellian-averaged cross-section

First Results of the 140^{140}Ce(n,γ)141^{141}Ce Cross-Section Measurement at n_TOF

An accurate measurement of the $^{140}$Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the $^{140}$Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in $^{140}$Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the $^{140}$Ce Maxwellian-averaged cross-section

Measurement of the 244^{244}Cm and 246^{246}Cm Neutron-Induced Cross Sections at the n_TOF Facility

The neutron capture reactions of the $^{244}$Cm and $^{246}$Cm isotopes open the path for the formation of heavier Cm isotopes and of heavier elements such as Bk and Cf in a nuclear reactor. In addition, both isotopes belong to the minor actinides with a large contribution to the decay heat and to the neutron emission in irradiated fuels proposed for the transmutation of nuclear waste and fast critical reactors. The available experimental data for both isotopes are very scarce. We measured the neutron capture cross section with isotopically enriched samples of $^{244}$Cm and $^{246}$Cm provided by JAEA. The measurement covers the range from 1 eV to 250 eV in the n_TOF Experimental Area 2 (EAR-2). In addition, a normalization measurement with the $^{244}$Cm sample was performed at Experimental Area 1 (EAR-1) with the Total Absorption Calorimeter (TAC)

Ermittlung von Dekompositionsraten mittels des Tea Bag Index in einem subtropischen Waldökosystem in China

Dekompositionsprozesse sind maßgeblich an den Kohlenstoff- und Nährstoffumsätzen in ter-restrischen Ökosystemen beteiligt. Während die Primärproduktion von Pflanzen CO2 aus der At-mosphäre festsetzt, wird CO2 durch die Dekomposition im Boden freigesetzt. Die Geschwindig-keit der Dekomposition hängt von verschiedenen abiotischen und biotischen Einflussfaktoren ab und variiert dadurch in verschiedenen Ökosystemen. In einem subtropischen Waldökosystem in China wurden im Rahmen des BEF-China-Forschungs-projekts Dekompositionsraten mittels des Tea Bag Index nach der Methode von KEUSKAMP et al. (2013) ermittelt. Dafür wurden zwei Teesorten – Grüntee und Rooibostee –als Substrate ver-wendet, welche die Laubstreu repräsentieren. Es stellte sich heraus, dass die Streuqualität von entscheidender Bedeutung in Bezug auf die Umsetzgeschwindigkeit und die prozentuale Zerset-zung des Materials ist. Grüntee wurde nach einer Inkubation von 90 Tagen im Boden durch-schnittlich zu 76 % zersetzt, während Rooibostee durchschnittlich zu 22 % zersetzt wurde. Zusätzlich wurden die Zersetzungsraten des Tees in Zusammenhang mit potentiellen Einflusspa-rametern im Untersuchungsgebiet gebracht und analysiert. Zur Verfügung standen plotbasierte Daten zu pH-Wert, TOC (Total Organic Carbon), N (Stickstoff) sowie zur Hangneigung, Höhen-lage, Exposition und weitere topographische Parameter. Die Auswertung mittels Klassifikationsbäumen (Classification And Regression Trees, CART und Classification And Regression Training, CART) ergab, dass Stickstoff im umgebenden Boden der einflussreichste Parameter ist. Dies wurde durch die Auswertung der Rangkorrelationskoeffi-zienten ρ nach Spearman bestätigt. Hohe Stichstoffgehalte im Boden sind für eine hohe A-bundanz und Aktivität der Bodenorganismen verantwortlich und ermöglichen dadurch eine schnellere Dekomposition. Hohe Temperaturen und eine hohe relative Luftfeuchtigkeit beein-flussen die Zersetzung ebenfalls positiv. Des Weiteren wurden verschiedene Standorte inner-halb der Plots verglichen. Totholz oder lebende Bäume in unmittelbarer Umgebung beeinflussen die Dekomposition der Streu demnach nicht signifikant. Die Ermittlung von Dekompositionsraten mittels des Tea Bag Index soll eine globale Vergleich-barkeit schaffen. Die Methode ist jedoch kritisch zu bewerten, da sie in ihrer Aussagekraft stark limitiert ist. Schwankungen in den Umweltbedingungen, v. a. Temperaturänderungen während der Inkubationszeit, können Verzerrungen der Dekompositionsrate k hervorrufen. Dadurch ist der Zeitpunkt der Durchführung des Experimentes entscheidend und eine globale Vergleichbar-keit aufgrund der Saisonalität nicht möglich. Es wird vorgeschlagen Dekompositionsraten wei-terhin mit Litterbags mit Laubstreu aus dem jeweiligen Ökosystem zu ermitteln. Dadurch können aussagekräftigere Daten zur Dekomposition im Ökosystem gewonnen werden.Decomposition processes control carbon and nutrient turnover in terrestrial ecosystems and thus play an important role in terrestrial ecosystems. While primary production of plants accu-mulates CO2 from the atmosphere, CO2 is released by decomposition in soils. The speed of de-composition depends on different abiotic and biotic influencing factors and thus varies in differ-ent ecosystems. Decomposition rates were calculated in a subtropical forest ecosystem in China using a method by KEUSKAMP et al. (2013), so-called Tea Bag Index. The experiment is part of the BEF-China re-search project. Two types of tea – green tea and rooibos tea – were used as substrates, which represent the leaf litter. It turned out that the litter quality is crucial in terms of the decomposi-tion rate. Green tea was decomposed by an average of 76 % after an incubation of 90 days in the soil, while rooibos tea was decomposed by an average of 22%. In addition, the decomposition rates of the tea were investigated and analyzed regarding poten-tial influencing parameters. Plot-based data were available for pH, TOC (total organic carbon), N (nitrogen), slope, altitude and exposure, as well as other topographical parameters. The evaluation by classification and regression trees (CART) and classification and regression training (CARET) revealed that nitrogen in surrounding soils is the most influential parameter. This was confirmed by rank correlation coefficient ρ (Spearman). High concentrations of N in soils are responsible for high abundance and activity of soil organisms, and thus enable a faster decomposition. Additionally, high temperatures and high relative humidity influence the decom-position positively. Furthermore, different locations within the plots were compared. Deadwood or living trees in immediate vicinity do not significantly influence the decomposition of litter. Determination of decomposition rates using the Tea Bag Index is intended to create global com-parability. The method, however, is to be assessed critically since it is strongly limited in its power of expression and comparability. Variations in environmental conditions, in particular temperature changes during the incubation period, can cause distortions of the decomposition rate k. Therefore, the date of the experiment is decisive and a global comparability is not possi-ble due to seasonality. It is suggested to determine decomposition rates with litter bags with foliage from the ecosystem of interest. In this way, more accurate data for decomposition in a specific ecosystem can be obtained

Spallation-based neutron target for direct studies of neutron-induced reactions in inverse kinematics

We discuss the possibility to build a neutron target for nuclear reaction studies in inverse kinematics utilizing a storage ring and radioactive ion beams. The proposed neutron target is a specially designed spallation target surrounded by a large moderator of heavy water (D2O). We present the resulting neutron spectra and their properties as a target. We discuss possible realizations at different experimental facilities

Reactor neutrons in nuclear astrophysics

The huge neutron fluxes offer the possibility to use research reactors to produce isotopes of interest, which can be investigated afterwards. An example is the half-lives of long-lived isotopes like 129I. A direct usage of reactor neutrons in the astrophysical energy regime is only possible, if the corresponding ions are not at rest in the laboratory frame. The combination of an ion storage ring with a reactor and a neutron guide could open the path to direct measurements of neutron-induced cross sections on short-lived radioactive isotopes in the astrophysically interesting energy regime

Reactor neutrons in nuclear astrophysics

The huge neutron fluxes offer the possibility to use research reactors to produce isotopes of interest, which can be investigated afterwards. An example is the half-lives of long-lived isotopes like 129I. A direct usage of reactor neutrons in the astrophysical energy regime is only possible, if the corresponding ions are not at rest in the laboratory frame. The combination of an ion storage ring with a reactor and a neutron guide could open the path to direct measurements of neutron-induced cross sections on short-lived radioactive isotopes in the astrophysically interesting energy regime