Activations with neutrons in the keV energy range were routinely performed at the Karlsruhe Institute of Technology (KIT) in Germany in order to simulate stellar conditions for neutron-capture cross ...sections. A quasi-Maxwell-Boltzmann neutron spectrum of
kT
= 25 keV, being of interest for the astrophysical s-process, was produced by the
7
Li(p,n) reaction utilizing a 1912 keV proton beam at the Karlsruhe Van de Graaff accelerator. Activated samples resulting in long-lived nuclear reaction products with half-lives in the order of yr 100 Myr were analyzed by Accelerator Mass Spectrometry (AMS). Comparison of the obtained reaction cross sections to literature data from previous Time-of-Flight (ToF) measurements showed that the selected AMS data are systematically lower than the ToF data. To investigate this discrepancy,
54
Fe(n,γ)
55
Fe and
35
Cl(n,γ)
36
Cl reaction cross sections were newly measured at the Frankfurt Neutron Source (FRANZ) in Germany. To complement the existing data, an additional neutron activation of
54
Fe and
35
Cl at a proton energy of 2 MeV was performed. The results will give implications for the stellar environment at
kT
= 90 keV, reaching the not yet experimentally explored high-energy s-process range. AMS measurements of the activated samples are scheduled.
The
Pu/
Pu atom ratio is a very effective tool for the identification of the origin of plutonium (Pu) in the soil environment. We examine a dataset of
Pu/
Pu atom ratios determined from surface and ...core soils at 240 sites across China. The data were compiled from 18 separate literature sources from the last 20 years. For the first time the spatial distribution (3 latitude bands and 7 natural regions) of the weighted average
Pu/
Pu atom ratios in Chinese soils is investigated. An area to the West of Xining City, shows a weighted average
Pu/
Pu atom ratio of 0.167 ± 0.002, lower than that of average global fallout, which likely arises from the addition of local fallout radionuclides from the Chinese nuclear weapon tests at Lop Nor between 1964 and 1980. The Yumen and Jiuquan areas of Northwest China in particular show evidence of very low ratio material from the Chinese nuclear weapon tests. Excluding the impacted area around the test site the weighted average
Pu/
Pu ratio of 0.182 ± 0.002 suggests that global fallout is the main source of Pu in most Chinese soils.
Recent advances in AMS of 36Cl with a 3-MV-tandem Martschini, Martin; Forstner, Oliver; Golser, Robin ...
Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms,
12/2011, Letnik:
269, Številka:
24
Journal Article
Recenzirano
Accelerator mass spectrometry (AMS) of
36Cl (
t
1/2
=
0.30
Ma) at natural isotopic concentrations requires high particle energies for the separation from the stable isobar
36S and was so far the ...exclusive domain of tandem accelerators with at least 5
MV terminal voltage. Using terminal foil stripping and a detection setup consisting of a split-anode ionization chamber and an additional energy signal from a silicon strip detector, a
36S suppression of >10
4 at 3
MV terminal voltage was achieved. To further increase the
36S suppression energy loss straggling in various counter gases (C
4H
10, Ar–CH
4 and C
4H
10–Ar) and the effect of “energy focusing” below the maximum of the Bragg curve was investigated. The comparison of experimental data with simulations and published data yielded interesting insights into the physics underlying the detectors. Energy loss, energy straggling and angular scattering determine the
36S suppression. In addition, we improved ion source conditions, target backing materials and the cathode design with respect to sulfur output and cross contamination. These changes allow higher currents during measurement (
35Cl
− current
≈
5
μA) and also increased the reproducibility. An injector to detector efficiency for
36Cl ions of 8% (16% stripping yield for the 7+ charge state in the accelerator, 50%
36Cl detection efficiency) was achieved, which can favorably be compared to other facilities. The memory effect in our ion source was also thoroughly investigated. Currently our measured blank value is
36Cl/Cl
≈
3
×
10
−15 when samples with a ratio of 10
−11 are used in the same sample wheel and
36Cl/Cl
≈
5
×
10
−16 if measured together with samples with a ratio of 10
−12 or below. This is in good agreement with the lowest so far published isotope ratios around 5
×
10
−16 and demonstrates that 3
MV tandems can achieve the same sensitivity for
36Cl as larger machines.
Since 1990 the International VLBI Service for Geodesy and Astrometry (IVS) has been performing geodetic Very Long Baseline Interferometry (VLBI) observations within the European geodetic VLBI ...network. In this work, 114 European VLBI sessions from January 1990 to September 2011 are analysed using the Vienna VLBI Software (VieVS). A total of 58 baselines with lengths ranging from 59 m to 4581 km are investigated and the lengths of most of them indicate repeatabilities at the sub-centimetre level. The horizontal station motions which describe the motion of the Eurasian plate are compared to the NUVEL-1A and MORVEL tectonic plate models. Intraplate crustal motions are investigated by estimating the station velocities with respect to Wettzell (Germany), a station on the geodynamically stable part of Eurasia. The northern part of Europe is dominated by the postglacial isostatic rebound, confirmed by four VLBI sites in this region with an uplift from 2.89±0.71 mm/yr (Svetloe, Russia) to 7.23±1.00 mm/yr (Ny-Ålesund, Norway) with respect to the central part of the European plate. Besides the vertical uplift, these radio telescopes evidence a horizontal motion from the centre of the former ice sheet towards its border. In the southern part of Europe the motion of the VLBI sites is caused by the collision of the African plate with the Eurasian plate, while the stations on the stable part of Europe do not present any significant relative motions. Our results are compared against those by Haas et al. (J. Geodyn. 35:391–414,
2003
) and with velocities of the current reference frame of the International Global Navigation Satellite Systems Service.
AMS of 36Cl with the VERA 3MV tandem accelerator Martschini, Martin; Andersson, Pontus; Forstner, Oliver ...
Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms,
January 2013, 2013-01-00, 20130101, Letnik:
294
Journal Article
Recenzirano
Recent progress with compact ionization chambers has opened new possibilities for isobar suppression in accelerator mass spectrometry (AMS). Separation of 36Cl (t1/2=0.30Ma) at natural isotopic ...levels from its stable isobar 36S became feasible at particle energies of 24MeV, which are also accessible for medium-sized tandem accelerators with 3MV terminal voltage like VERA (Vienna Environmental Research Accelerator). Investigations with an ionization chamber revealed how physics favors isobar separation even at energies below the maximum of the Bragg curve. The strong energy focusing effect at high energy losses reduces energy straggling significantly and isobar separation steadily increases up to almost full energy loss. With an optimized detection setup, sulfur suppression factors of 2×104 have been achieved.
Refraining from the additional use of degrader foils has the benefit of high transmission to the detector (∼16%), but requires a low sulfur output from the ion source. Therefore several backing materials have been screened for sulfur content. The dependence of the sulfur output on the AgCl sample size has been investigated as well.
Precision and accuracy have been thoroughly assessed over the last two years. Since drifts in the spectra are efficiently corrected by monitoring the position of the 36S peak, the reproducibility for high ratio samples (36Cl/Cl>10−12) is better than 2%. Our blank value of 36Cl/Cl≈(5±5)×10−16 is competitive to other labs. 36Cl has become a routine AMS-isotope at VERA.
Recently we also explored novel techniques for additional sulfur suppression already in the ion source. While results with a small gas reaction cell in front of the sputter target were discouraging, a decrease in the sulfur/chlorine ratio by one order of magnitude was achieved by directing 300mW continuous wave laser beam at 445nm towards the cathode in the ion source.
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