AMS measurement of the reaction 35Cl(n,gamma)36Cl and its relevance to astrophysics and nuclear technology

Das langlebige Radionuklid 36Cl (t1/2 = 301000 a) wird hauptsächlich durch die Neutroneneinfangreaktion 35Cl(n,gamma)36Cl produziert. 35Cl, das stabile Saatnuklid für diese Reaktion fungiert in späteren Brennphasen von Sternen als sogenanntes Neutronengift. Dies macht die obige Reaktion für astrophysikalische Netzwerkberechnungen, deren Ziel die Bestimmung der Element- und Isotopenhäufigkeiten ist, relevant. Wegen der langen Halbwertszeit von 36Cl ist der Wirkungsquerschnitt von 35Cl(n,gamma)36Cl auch für die Kerntechnologie interessant. Diese Arbeit verfolgt folgende Ziele: Erstens die Produktion von unabhängigen 36Cl/35Cl-Referenzmaterial für AMS Messungen (accelerator mass spectrometry) und zweitens die Bestimmung des MACS (Maxwellian averaged cross-section) der Reaktion 35Cl(n,gamma)36Cl für ein Maxwell-Boltzmann Neutronenenergiespektrum von 25 keV. Für die Produktion des unabhängigen 36Cl/35Cl-Referenzmaterials wurden NaCl-Tabletten an zwei Forschungsreaktoren mit Neutronen bestrahlt. Die zur Berechnung des 36Cl/35Cl-Verhältnisses verwendete Neutronenfluenz wurde durch Goldmonitore ermittelt. Die AMS-Messung, der bestrahlten Proben wurde mit VERA (Vienna Environmental Research Accelerator) durchgeführt. Zur Bestimmung des 25 keV-MACS von 35Cl(n,g)36Cl wurden AMS- Messungen an Proben aus zwei NaCl Tabletten durchgeführt, die zuvor mit einem Neutronenspektrum, welches fast einer Maxwell-Boltzmann-Verteilung von 25 keV entsprach, bestrahlt wurden. Durch die Kombination der mit AMS gemessenen Isotopenverhältnisse und der Neutronenfluenz bei der Bestrahlung wurde der MACS errechnet. Für die beiden bestrahlten Tabletten KIT1 und KIT2 erhielten wir folgende Resultate: MACS(KIT1)=(10.5+/-0.6) mbarn; MACS(KIT2) =(8.8+/-0.5) mbarn. Eine mögliche Ursache dieser Abweichung, die in Zukunft untersucht werden wird, ist die Geometrie bei der Neutronenaktivierung.36Cl is a long-lived radionuclide (t1/2 = 301000 a), which is dominantly produced via the reaction 35Cl(n,gamma)36Cl. The seed nuclei of this reaction, the stable 35Cl, acts as neutron poison in nucleosynthesis processes in stars. This makes the reaction important for astrophysical network calculations. Due to the long half-life of 36Cl, the cross-section of 35Cl(n,gamma)36Cl is also important for nuclear technology. The two main goals of the present work are: First the production of an independent 36Cl/35Cl reference material for accelerator mass spectrometry (AMS) measurements and second the determination of the Maxwellian averaged cross-section (MACS) of 35Cl(n,gamma)36Cl at 25 keV using AMS. Approaching the first goal, NaCl-pellets were irradiated with neutrons in two research reactors. The neutron fluence used for the calculation of the 36Cl/35Cl-ratio of the irradiated samples was determined via gold fluence monitors. The AMS measurements on these samples were performed at VERA (Vienna Environmental Research Accelerator). To determine the 25 keV-MACS of 35Cl at stellar energies, AMS measurements were performed on two samples, which were irradiated with neutrons of a Maxwell-Boltzmann energy distribution of 25 keV. Combining the AMS- and the neutron-fluence data, the MACS for 35Cl at 25 keV was calculated. From the first irradiated sample we obtained a MACS of (10.5+/-0.6) mbarn. The second sample resulted in a MACS-value of (8.8+/-0.5) mbarn. A possible origin of this difference might be the geometry of the neutron activation, which will be investigated in the future

Calculation of the rockwall recession rate of a limestone cliff, affected by rockfalls, using cosmogenic chlorine-36 : case study of the Montsec Range (Eastern Pyrenees, Spain)

Cliff erosion may be a major problem in settled areas affecting populations and producing economic and ecological losses. In this paper we present a procedure to calculate the long-term retreat rate of a cliff affected by rockfalls in the Montsec Range, Eastern Pyrenees (Spain). It is composed of low, densely fractured limestones; and the rockwall is affected by rockfalls of different sizes. The rockfall scars are clearly distinguishable by their regular boundaries and by their orange colour, which contrast with the greyish old reference surface (S0) of the cliff face. We have dated different stepped surfaces of the rockwall, including S0, using cosmogenic 36Cl. The total amount of material released by rockfall activity was calculated using a high definition point cloud of the slope face obtained with a terrestrial laser scanner (TLS). The present rockwall surface has been subtracted from the reconstructed old cliff surface. This has allowed the calculation of the total volume released by rockfalls and of the retreat rate. The latter ranges from 0.31 to 0.37 mm·a−1 . This value is of the same order of magnitude as that obtained by other researchers in neighbouring regions in Spain, having similar geology and affected by rockfalls

Record of ³H and ³⁶Cl from the Fukushima nuclear accident recovered from soil water in the unsaturated zone at Koriyama

The opportunity to measure the concentrations of ³H and ³⁶Cl released by the Fukushima nuclear accident in 2011 directly in rain was lost in the early stage of the accident. We have, however, been able to reconstruct the deposition record of atmospheric ³H and ³⁶Cl following the accident using a bore hole that was drilled in 2014 at Koriyama at a distance of 60 km from the accident. The contributions of ³H and ³⁶Cl from the accident are 1.4 × 10¹³ and 2.0 × 10¹² atoms m⁻² respectively at this site. Very high concentrations of both ³H (46 Bq L⁻¹) and ³⁶Cl (3.36 × 10¹¹ atoms L⁻¹) were found in the unsaturated soil at depths between 300 and 350 cm. From these, conservative estimates for the ³H and ³⁶Cl concentrations in the precipitation in the ～ 6 weeks following the accident were 607 Bq L⁻¹ and 4.74 × 10¹⁰ atoms L⁻¹, respectively. A second hole drilled in 2016 showed that ³H concentrations in the unsaturated soil and shallow groundwater had returned to close to natural levels, although the ³⁶Cl concentrations were still significantly elevated above natural levels

A multi-environmental tracer study to determine groundwater residence times and recharge in a structurally complex multi-aquifer system

Despite being the main drinking water resource for over 5 million people, the water balance of the Eastern Mountain Aquifer system on the western side of the Dead Sea is poorly understood. The regional aquifer consists of fractured and karstified limestone - aquifers of Cretaceous age, and it can be separated into a Cenomanian aquifer (upper aquifer) and Albian aquifer (lower aquifer). Both aquifers are exposed along the mountain ridge around Jerusalem, which is the main recharge area. From here, the recharged groundwater flows in a highly karstified aquifer system towards the east and discharges in springs in the lower Jordan Valley and Dead Sea region. We investigated the Eastern Mountain Aquifer system for groundwater flow, groundwater age and potential mixtures, and groundwater recharge. We combined 36Cl ∕ Cl, tritium, and the anthropogenic gases SF6, CFC-12 (chlorofluorocarbon) and CFC-11, while using CFC-113 as "dating" tracers to estimate the young water components inside the Eastern Mountain Aquifer system. By application of lumped parameter models, we verified young groundwater components from the last 10 to 30 years and an admixture of a groundwater component older than about 70 years. Concentrations of nitrate, simazine (pesticide), acesulfame K (ACE-K; artificial sweetener) and naproxen (NAP; drug) in the groundwater were further indications of infiltration during the last 30 years. The combination of multiple environmental tracers and lumped parameter modelling helped to understand the groundwater age distribution and to estimate recharge despite scarce data in this very complex hydrogeological setting. Our groundwater recharge rates support groundwater management of this politically difficult area and can be used to inform and calibrate ongoing groundwater flow models.This research has been supported by the Helmholtz Association (grant no. VH-VI527), the Federal Ministry of Education and Research (BMBF; grant no. 02WM0848) and the BMBF-MOST(grant no. YSEP111)

The old, unique C1 chondrite Flensburg - Insight into the first processes of aqueous alteration, brecciation, and the diversity of water-bearing parent bodies and lithologies

On September 12, 2019 at 12:49:48 (UT) a bolide was observed by hundreds of eye-witnesses from the Netherlands, Germany, Belgium, Denmark and the UK. One day later a small meteorite stone was found by accident in Flensburg. The presence of short-lived cosmogenic radionuclides with half-lives as short as 16 days proves the recent exposure of the found object to cosmic rays in space linking it clearly to the bolide event. An exceptionally short exposure time of ~5000 years was determined. The 24.5 g stone has a fresh black fusion crust, a low density of <2 g/cm^3, and a magnetic susceptibility of logX = 4.35 (X in 10^-9 m^3/kg). The rock consists of relict chondrules and clusters of sulfide and magnetite grains set in a fine-grained matrix. The most abundant phases are phyllosilicates. Carbonates (~3.9 vol.%) occur as calcites, dolomites, and a Na-rich phase. The relict chondrules (often surrounded by sulfide laths) are free of anhydrous silicates and contain abundant serpentine. Lithic clasts are also surrounded by similar sulfide laths partly intergrown with carbonates. 53^Mn-^53Cr ages of carbonates in Flensburg indicate that brecciation and contemporaneous formation of the pyrrhotite-carbonate intergrowths by hydrothermal activities occurred no later than 4564.6 +- 1.0 Ma (using the angrite D'Orbigny as the Mn-Cr age anchor). This corresponds to 2.6 +- 1.0 or 3.4 +- 1.0 Ma after formation of CAIs, depending on the exact absolute age of CAIs. This is the oldest dated evidence for brecciation and carbonate formation, which likely occurred during parent body growth and incipient heating due to decay of 26Al. In the three oxygen isotope diagram, Flensburg plots at the 16O-rich end of the CM chondrite field and in the transition field to CV-CK-CR chondrites. The mass-dependent Te isotopic composition of Flensburg is slightly different from mean CM chondrites and is most similar to those of the ungrouped C2 chondrite Tagish Lake. On the other hand, 50Ti and 54Cr isotope anomalies indicate that Flensburg is similar to CM chondrites, as do the ~10 wt.% H2O of the bulk material. Yet, the bulk Zn, Cu, and Pb concentrations are about 30% lower than those of mean CM chondrites. The He, Ne, and Ar isotopes of Flensburg show no solar wind contribution; its trapped noble gas signature is similar to that of CMs with a slightly lower concentration of 20Netr. Based on the bulk H, C, and N elemental abundances and isotopic compositions, Flensburg is unique among chondrites, because it has the lightest bulk H and N isotopic compositions of any type 1 or 2 chondrite investigated so far. Moreover, the number of soluble organic compounds in Flensburg is even lower than that of the brecciated CI chondrite Orgueil. The extraordinary significance of Flensburg is evident from the observation that it represents the oldest chondrite sample in which the contemporaneous episodes of aqueous alteration and brecciation have been preserved. The characterization of a large variety of carbonaceous chondrites with different alteration histories is important for interpreting returned samples from the OSIRIS-REx and Hayabusa 2 missions.This work is partly funded by the Deutsche Forschungs- gemeinschaft (DFG, German Research Foundation) – Project-ID 263649064 – TRR 170 (A.B., C.B., T.K.); this is TRR170 Publication No. 119. M.S. and C.M. thank the Swiss National Science Foundation (SNF) for support. The work of H.B. and M.S. has been in parts carried out within the framework of the NCCR PlanetS supported by SNF. D.H. thanks F. Langenhorst for support and access to the FIB-SEM and TEM facilities at FSU-IGW, which are funded by the DFG via grant LA830/14-1. D.F. (CIW) acknowledges the support of the NASA awards 80NSSC19K0559 and 80NSSC20K0344

93Zr developments at the Heavy Ion Accelerator Facility at ANU

The long-lived radionuclide 93Zr t1/2 = (1.61 +- 0.05) Ma plays an important role in nuclear astrophysics and nuclear technology. In stellar environments, it is mainly produced by neutron capture on the stable nuclide 92Zr. On Earth high amounts of radioactive 93Zr are produced in nuclear power plants directly from 235U fission, but also by neutron capture on 92Zr, as Zr-alloys are commonly used as cladding for nuclear fuel rods.This work was supported by the Australian Research Council DP140100136

Accelerator mass spectrometry measurement of the reaction 35Cl(n,γ)36Cl at keV energies

The nuclide 35Cl can act as a minor “neutron poison” in the stellar slow neutron capture process

The cosmic-ray exposure history of the Twannberg iron meteorite (IIG)

The Twannberg iron meteorite is one out of only six members of the group IIG. The combined noble gas and radionuclide data obtained in this new systematic study indicate that Twannberg with its ~570 recently recovered specimens was a large object with a preatmospheric radius in the range of ~2 m, which corresponds to ~250 × 103 kg. The cosmic-ray exposure age for Twannberg is 182 ± 45 Ma. The most surprising result is the long terrestrial age of Tterr = math formula ka, which is unexpected considering the humid conditions in Switzerland. However, this age is in accord with glaciation events, indicating that the less shielded samples from Mt. Sujet were found close to the position of the original strewn field, whereas the samples from Gruebmatt and Twannbach, which are from more shielded positions, were glacially transported to the east–northeast during the second last ice age (185–130 ka ago) from an original position west of Mt. Sujet.This work is supported bythe Swiss National Science Foundation (SNF)

The 240Pu/239Pu atom ratio in Chinese soils

The 240Pu/239Pu 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 240Pu/239Pu 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 240Pu/239Pu atom ratios in Chinese soils is investigated. An area to the West of Xining City, shows a weighted average 240Pu/239Pu 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 240Pu/239Pu ratio of 0.182 ± 0.002 suggests that global fallout is the main source of Pu in most Chinese soils.This work was supported by the National Key Research and Development of China (No.2016YFA0600502) and the National Natural Science Foundation of China (No. 41671466). In addition, Yanan Huang thanks the China Scholarship Council for offering a scholarship (201706190133) to support his PhD study at the Australian National University

The cosmic-ray exposure history of the Twannberg iron meteorite (IIG)

The Twannberg iron meteorite is one out of only six members of the group IIG. The combined noble gas and radionuclide data obtained in this new systematic study indicate that Twannberg with its ~570 recently recovered specimens was a large object with a preatmospheric radius in the range of ~2 m, which corresponds to ~250 9 10³ kg. The cosmic-ray exposure age for Twannberg is 182 ± 45 Ma. The most surprising result is the long terrestrial age of Tterr = 202±33 34 ka, which is unexpected considering the humid conditions in Switzerland. However, this age is in accord with glaciation events, indicating that the less shielded samples from Mt. Sujet were found close to the position of the original strewn field, whereas the samples from Gruebmatt and Twannbach, which are from more shielded positions, were glacially transported to the east–northeast during the second last ice age (185–130 ka ago) from an original position west of Mt. Sujet