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EPJ Web of Conferences 146, 03005 (2017) DOI: 10.1051/epjconf/201714603005 ND2016

Isotope production and target preparation for nuclear data

Dorothea Schumann, Rugard Dressler, and Emilio Andrea Maugeri, and Stephan Heinitz

Laboratory of Radiochemistry, Nuclear Energy and Safety Research Department, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

Abstract. Targets are in many cases an indispensable ingredient for successful experiments aimed to produce nuclear data. With the recently observed shift to study nuclear reactions on radioactive targets, this task can become extremely challenging. Concerted actions of a certain number of laboratories able to produce and manufacture radioactive targets are urgently needed. We present here some examples of successful and target production at PSI, in particular the production of 60Fe samples used for half-life measurements and capture cross section experiments, the chemical processing and fabrication of lanthanide targets for capture cross section experiments at n TOF (European Organization for Nuclear Research (CERN), Switzerland) as well as the recently performed manufacturing of highly-radioactive 7Be targets for the measurement of the 7Be(n,α)4He cross section in the energy range of interest for the Big-Bang contributing to the solving of the cosmological Li-problem. The two future projects: “Determination of the half-life and experiments on cross sections of 53Mn” and “32Si – a new chronometer for nuclear dating” are briefly described. Moreover, we propose to work on the establishment of a dedicated network on isotope and target producing laboratories.

1. Introduction 2. Requirements for radioactive Over the past 20 years, we observe an increasing target production demand on precise, accurate and reliable experimentally Availability. determined nuclear data in a certain number of scientific research fields like nuclear astrophysics, basic nuclear The wanted sample material shall be available in sufficient , nuclear medicine, geoscience, super heavy amount. element research and many others. This demand results Radioactive isotopes can be produced using nuclear likewise in an increasing number of requests for reactors, neutron sources or particle accelera- high-quality sample material in sufficient amounts and tors. In some special cases, radionuclides can be purchased well-characterized isotopic composition as well as the from industrial companies. However, the majority of manufacturing of targets in a shape suitable for the special radioactive samples have to be dedicatedly produced. envisaged application. The desired samples often represent These methods require access to large-scale facilities and or contain rare radioactive isotopes, which are very exotic are extremely cost-intensive. and available in limited amounts only. In dependence on Purity. their activity and/or dose rate, the handling of such samples can be extremely challenging. Thus, only a handful of The sample shall not contain unwanted by-products, laboratories world-wide are allowed and capable to work elements and/or isotopes. with this kind of material. Correspondingly high is the Even if a sample can be purchased from a commercial waiting list for sample and target preparation, even if company, the chemical composition and the delivered the isotope itself is available in sufficient amounts. To form do in many cases not fit to the envisaged effectively exploit the very limited resources, a new experiment. Additional purification procedures and/or level of collaboration between the target laboratories transformations are necessary. For isotopes, which were in Europe and also world-wide is urgently needed. A produced dedicated to an experiment, as a first step the dedicated workshop on target preparation was performed extraction from the matrix material is necessary, followed in November 2015 at Paul-Scherrer-Institute (PSI) [1], by several special purification procedures. The applied addressing this important issue. procedures shall not involve chemicals which later on can In the present contribution, we want to illuminate this disturb the measurement. Due to the high dose rates of essential problem, but also show some highlights from the the radioactive samples, radioprotection concerns often PSI isotope and target production activities, beginning with require special measures, for instance hotcells and remote- a brief summary of successful experiments and mentioning controlled handling. In a certain number of cases, also at the end some future projects. stable isotopes of the same element are hampering a

c The Authors, published by EDP Sciences. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 146, 03005 (2017) DOI: 10.1051/epjconf/201714603005 ND2016 successful experiment. These isotopes cannot be separated initiative (Exotic Radionuclides from Accelerator Waste by chemical means, but mass separation methods have to for Science and Technology) [3], aimed to extract be applied. urgently needed radioisotopes from suitable matrices and make them available for a broad community of Suitability. researchers. ERAWAST is also attempting to improve the communication with the users in order to manufacture The target shall be in a shape and form suitable for the tailored samples for the planned experiments. Two experiment. workshops have been performed at PSI in 2006 and 2011, After preparing the sample material, the target shape respectively, where experimenters had the opportunity and form should also meet the requirements of the planned to get in close contact with target producers to discuss experiment. A number of issues have to be addressed, their problems and requirements, look for new, innovative for example: i) Dimensions fitting to the installations of solutions and also propose new experiments, thus the facility ii) Is a target on a backing required? Which improving the quality of the samples and their experiment thickness shall the backing have? Which backing material outcome in general. is required? iii) Shall the sample be covered, or must it be In the following, we give an overview on the isotope an open source? Methods used to meet these requirements resources at PSI, the methods for sample preparation as are for instance rolling or punching [2], electrodeposition well as some of the highlights in scientific applications. We and molecular plating, evaporation, sputtering, and others. also mention two examples of prospective projects, which It has to be noticed, that – in dependence on the nuclear are currently ongoing or envisaged in the near future. properties of the radioisotope – for these procedures huge efforts concerning radioprotection can be necessary. 3.1. Isotope sources Characterization. PSI operates the Spallation Neutron Source SINQ, which The target composition shall be known. is driven by the most powerful accelerator world- For correct interpretation of the experimental data, wide (590 MeV, 2.4 mA), and is therefore best-suited as the knowledge of the exact composition of the target is a producer of rare exotic radionuclides. In the frame of mandatory. This includes the isotopic composition of the the ERAWAST initiative isotope separation from different primary material used for target production as well as a matrices has been established at PSI within the past detailed characterization of the deposited target layer and decade. In particular, the following sources for isotope the backing. Applied methods are for instance α-, β- and extraction are available: γ -spectrometry as well as Inductively-Coupled Plasma • Beam dumps, targets, shielding components (copper, Mass Spectrometry (ICP-MS) or Inductively-Coupled , graphite and others) Plasma Optical Emission Spectrometry (ICP-OES) for • Samples from the SINQ Target Irradiation program measuring the atomic composition. Layer morphology STIP (stainless steels and other metals) can be investigated using Scanning Electron Microscopy • SINQ cooling water (SEM) and Atomic-Force Microscopy (AFM). A overview • Special positions in the SINQ target for dedicated on the methods can be found in [2]. Measurements of the irradiations thicknesses of both the sample material and the backing as well as the evaluation of the homogeneity are also For further descriptions see [4,5]. mandatory. Also here, the radioprotection issue often complicates the procedures. Although ICP-MS and ICP- 3.2. Selected examples OES are nowadays well-established common methods, 3.2.1. 60Fe – world-wide unique production at PSI only a few laboratories, equipped with such instruments, are able to handle radioactive samples. Three Nature respective Science papers on the importance of this isotope concerning studies of supernovae phenom- 3. Isotope and target production ena were published this year [6–8]. This recently observed 60 possibilities at PSI increasing attention on the radioactive isotope Fe in nuclear astrophysics research implicates the necessity of High-energetic and secondary particles induce in reliable knowledge on its nuclear properties, in particular matter the production of a big variety of radionuclides, the decay scheme including the half-life and the branching some of them being very rare, exotic, and, in several ratios, as well as cross sections of key nuclear reactions cases, difficult to obtain by complementary reactions. leading to its production and/or destruction. Since 60Fe Several of these isotopes are of high importance in nuclear is two mass units heavier than the heaviest stable iron astrophysics, and sufficient sample material for scientific isotope 58Fe, its production possibilities are limited. experiments is urgently needed. Double neutron capture on 58Fe is one production path, Highly-activated components stemming from the but this cross section (still not experimentally determined!) surroundings or parts of a high-power particle accelerator is expected to be comparable low. For experiments are a unique possibility to gain such valuable isotopes. requiring samples without a macro amount of stable The advantage of “mining” isotopes from waste materials carrier, production via spallation reactions is the only consists in their principal availability, not requiring “extra” possibility. beam time. The challenge is their radiochemical isolation At PSI, a considerable amount of 60Fe was found from the matrix. during systematic analytical examinations in a proton For the exploitation of this valuable resource, irradiated copper beam dump [9]. The isotope-enriched we initiated a working program called ERAWAST part of the beam dump was drilled out and radiochemical

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Figure 2. Picture of a 7Be-target on a C backing. Figure 1. Photo of the final target as used in the (n,g) experiment at stellar energies (measures in cm; figure taken from [14]). a picture of deposited Be on a thin graphite backing is shown as an example. separation techniques were applied to extract about Two collection runs at SINQ were carried out in 2015 1016 60Fe atoms. A number of half-life measurements and 2016, respectively, gaining each several hundred GBq were performed with this material by several research of 7Be. From the 2015 batch, two targets were produced for groups, beginning in 2009 by Rugel et al. [10], who the investigation of the 7Be(n,a)4He reaction at n TOF, one found a considerable disagreement in comparison to the by molecular plating on an aluminum foil, the other one by formerly accepted value of Kutschera et al. [11], followed evaporating droplets on a thin PE foil. The experiment has by Wallner et al. [12] and Ostiek [13], who confirmed been performed with extraordinary success [23]. the “Rugel”-value of 2.62 · 106 years. A complementary From the batch produced in 2016, an aliquot has been measurement at PSI is currently ongoing. transferred to CERN, where the Isotope mass Separator Furthermore, we produced two 60Fe targets [14], which On-Line facility ISOLDE was used for implanting mass- have been used for the first measurements of neutron separated 7Be into suitable backing materials. Two capture cross sections at stellar [15] and thermal [16] targets were manufactured by this method, one for energies. The picture (Fig. 1)showsthe60Fe target which studies of the 7Be(n,p)7Li cross section at n TOF and was used for the determination of the neutron capture cross one for determination of the cross section for the section at stellar energies [15] as an example: reaction 7Be(n,a) 4He at thermal energies at the Institut Standard material for Accelerator Mass Spectrometry Laue-Langevin (ILL) Grenoble. A detailed description on measurements has been manufactured and is already in the manufacturing of 7Be targets is given in [24]. use at University Munich and the Australian National University (ANU) Canberra [17]. 3.2.3. 171Tm and 147Pm – branching points in element synthesis 3.2.2. 7Be – the clue to the cosmological The origin of the elements heavier than Li is determined Li-problem? by nuclear processes occurring during stellar . One of the most controversial and still pending problems One of these processes is the so-called s−process, where in nuclear astrophysics is the so-called “Cosmological elements heavier than Fe are produced by a series of problem” [18,19]. While the standard Big-Bang neutron capture and β-decay close to the stability line. nucleosynthesis (BBN) model predicts the abundance of In case of isotopes, where neutron capture and β-decay 2H, 3He and 4He correctly, there is a discrepancy between competes due to a comparable long half-life, a branching the calculated 7Li abundance and the one deduced from towards a higher or lower proton number becomes observations. In fact, the abundance of 7Li observed in the possible. These isotopes are called branching points. A list atmosphere of low-metallicity in the Galactic halo of 21 branching point isotopes, where a measurement of [20,21] is a factor of 2–3 less than the value predicted by the neutron capture cross section is of great significance in the BBN. Furthermore, new data of 3He(α,γ )7Be reaction the near future, has been published recently [25]. rate and more precise values of cosmic baryon density led Since all branching point isotopes are radioactive, their to a higher value of the estimated amount for the 7Li/H production, handling and measurement are challenging. ratio increasing the discrepancy to a factor between 2.4 and For the manufacturing of targets consisting of 171Tm 4.3 [22]. and 147Pm, respectively, enriched 170Er and 146Nd samples Up to now, the performance of experiments involving were neutron irradiated in the high-flux reactor at ILL 7Be as target material was hindered by the limited Grenoble. After the β−decay of the primary neutron availability of the isotope and the extremely difficult activation products 171Er and 147Nd, the wanted isotopes handling due to the high specific activity. However, at were chemically separated using ion exchange on the PSI a method for filtering considerable amounts of 7Be strongly acidic cation exchanger AMINEX and following from the SINQ cooling water has been developed and purification with commercially available lanthanide- successfully applied [5]. Moreover, systematic studies on specific columns. Finally, 140 GBq of 171Tm and 3 GBq target preparation using molecular plating were carried of 147Pm have been deposited on two 5 µm thick Al out, investigating the dependence on the most important foils, which are positioned face to face between two parameters (backing material, acidity of the solution, 6 µm thick Mylar foils (Fig. 3). Details of the target current, voltage and duration of deposition). In Fig. 2, fabrication and characterization are described in [26]. The

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EAF-2010 JEFF-3.2 TENDL-2013 exp. data Millard 1k UCN ICON exp. data Wölfle BOA 1 cross section [ b ] NEUTRA

n_TOF FRANZ 1m Figure 3. Picture of the 171Tm-target. Figure taken from [26]. 1µ 1m 1 1k 1M neutron energy [ eV ] two experiments on the neutron capture cross sections Figure 4. Comparison of known neutron capture cross-sections were successfully performed both at the Experimental of 53Mn with model calculations. Areas (EAR I and EAR II) at n TOF (CERN) and at the Soreq Applied Research Accelerator Facility (SARAF). A few measurements have been performed to determine the half-life of this isotope. The obtained 3.2.4. 53Mn – potential candidate for early solar values varied from 2 Ma to 11 Ma. Finally, the works system investigations and geochronology of Honda et al. [32], Wolfle¬ et al. [33], and Heimann et al. [34] give a consistent value of 3.8 Ma with Short-lived cosmogenic radio- with half-lives of 10 26 an uncertainty of about 10%. These experiments were less than hundred million years (100 Ma) (e.g. Be, Al, . × 11 . × 36 53 60 59 carried out with samples containing 2 5 10 to 1 3 Cl, Mn, Fe, Ni) are produced via neutron capture in 1013 atoms of 53Mn limiting the precision of the explosive states of the development or via spallogenic measurements. However, the above discussed data are processes induced by high energetic protons or . contradictory with other observations. Nyquist et al. Physically relevant parameters in stellar models such as showed in [35] that the isotopic ratio of 53Mn to 55Mn neutron density, temperature, and pressure can be tested of different meteoritic objects is in disagreement with the and constrained using reliable production rates in neutron deduced age using the 206Pb – 207Pb chronometer. The capture reactions. More accurate data will be essential obtained data suggest that the half-life of 53Mn should be for the understanding of the galactic stellar evolution of in the range of about 4.8 Ma, i.e. significant higher than the successive stellar generations. so far accepted value of 3.8 Ma. Contrary to extra-terrestrial production, the terrestrial Due to this unsatisfying situation, the performance production of cosmogenic nuclides is dominated by of new, more sophisticated experiments with sufficient neutron spallation reactions; requiring knowledge of amounts of samples material is urgently needed. spatial and temporary variations of the neutron flux and Since considerable amounts of 53Mn were detected in the energy spectrum as well as reaction cross sections for accelerator waste in the order of 1018 atoms per g matrix neutron spallation. material, we performed an extended chemical processing Another important parameter, both for the interpreta- of proton irradiated stainless steel samples, ending up tion of the galactic processes and the dating of terrestrial with a total amount of 1019 atoms of 53Mn [36]. The samples, is the half-life. manufacturing of standard material for Accelerator Mass Due to the limited availability, up to now nuclear data Spectrometry (AMS) is currently ongoing [37]. on decay properties and cross sections are scarce. In 1960 We intend to start now a series of experiments J.P. Shedlovsky [27] revealed iron meteorites containing determining the neutron capture cross section of 53Mn 53 · 11 Mn in an amount of about 5 10 atoms per g of using thermal-, cold-, and ultracold neutron beams iron. Using manganese extracted from such meteorites available at several experimental installations at PSI as first measurements of the neutron capture cross-section at well as neutrons with a quasi-stellar energy spectrum soon thermal neutron energy were carried out by H.T. Millard σ ≈ available at FRANZ (SGC). A re-measurement of the half- [28] in 1965 giving th 170 b and by R. Wolfle¬ et al. life is also planned. [29] in 1972 obtaining σth ≈ 66 ± 7 b. So far no other measurements of the neutron capture cross-section of 53Mn were performed. calculations (available 3.2.5. 32Si – a new chronometer for nuclear dating in nuclear data libraries as EAF-2007 [30], and TENDL- 2008 [31]) use the value of [29] for further predictions. The predicted climate changes on Earth will have a high In Fig. 4, known neutron capture cross-sections of 53Mn impact on the environment and the human society. The are displayed together with the mean neutron energies climate pattern observed within the past centuries at the UltraCold Neutrons (UCN), Beamline for neutron to a better understanding of the drivers for such changes. and other Approaches (BOA), Imaging with Cold They are the main basis for extrapolating and modelling Neutrons (ICON), NEUtron Transmission RAdiography future climate changes. More studies using new, innovative (NEUTRA) installations at PSI, the n TOF setups at techniques are necessary to broaden the knowledge on CERN, and the Frankfurt Neutron Source (FRANZ) the involved processes and accomplish a reliable scientific facilities at Stern-Gerlach Center (SGC). basis for decision makers.

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The radioactive isotope 32Si (Z = 14, N = 18) with a ground-breaking step in investigating processes in the a half-life of approximately 153 years (calculated average recent past on Earth and in stars aside the commonly value taken from [38]) is a cosmogenic that accepted doctrines. is produced in the upper atmosphere by bombardment of cosmic rays on argon and then quickly transferred to the Earth surface by precipitation. It has the 4. The need for a target preparation potential to serve as a powerful tool to understand environmental processes such as glacier dynamics, ocean network and atmospheric circulation, sedimentation in lakes At present, several large-scale facilities run own, dedicated and oceans or groundwater flow in the recent past target laboratories. These laboratories work mainly (300–1000 years). After the discovery of naturally existing isolated or on a very limited network level between each 32 cosmogenic Si, Lal et al. [39] proposed this nuclide as other, even in the frame of pure nuclear energy research a candidate isotope for tracing and dating applications programs. Links to other research areas are missing or in Earth sciences. Subsequent attempts, often with heroic only little developed. Sharing of resources for isotope efforts including processing large samples, have been production and target preparation does practically not take made to employ this nuclide for dating of old ice, place; coordination of requests is often left to the users. e.g. by Dansgaard et al. [40]. However, for precise Moreover, the collaboration with the users in order to dating the accurate knowledge of the radionuclide‘s half- manufacture tailored samples or targets for their specific life is a precondition. So far, the inconsistent database application needs urgent improvement. Owed to this 32 on the nuclear properties of Si but also its extreme situation, the development of the corresponding research rareness, made its application difficult if not impossible. infrastructures networks according to the special demands To overcome this, the following qualifications have to be of the user communities as well as global coordination fulfilled: of the capabilities of the target laboratories in Europe in order to optimize the use of the very limited resources 32 1) the Si half-life needs to be known with uncertainty is mandatory for future ground-breaking experiments in < 5%; the field of nuclear sciences and nuclear astrophysics in 2) reliable and reproducible routine measurement particular. techniques including certified reference material are The global aim is to establish an overarching research needed; infrastructure service for target production and develop a 3) sufficient sample material for both basic and applied tight cooperation between the target laboratories in Europe investigations and the routine measurements are in order to improve the production technique of well- necessary. characterized samples/targets. Moreover, the interaction We propose to solve these challenges in the following way: of the target laboratories with the research teams using these targets shall be triggered and/or intensified to deliver a) Around 200 MBq of 32Si were produced at PSI targets best-suited for the envisaged experiments. by bombarding metallic vanadium with 590 MeV The following objectives are in the focus: protons. A radiochemical separation system will be developed and applied for isolating and • Identification and promotion of synergies with other 32 purifying Si from the matrix material as well target producers 44 as from other rare exotic isotopes (e.g., Ti, • Establishing of a network between the presently 41 39/42 26 40 Ca, Ar, Al, K), which are urgently leading European infrastructures for isotope and target needed for applications in nuclear medicine, nuclear production astrophysics, environmental research, basic nuclear • Development of an open access data base on isotope physics and others. production facilities and target manufacturers b) Measurement techniques as Liquid Scintillation • Coordination of targets requests, sharing of capacities, Counting (LSC), Cerenkovˇ measurement (CM), distribution of demands Plastic Scintillation (PS) counting, Ionization Cham- • Transnational visits of scientists and especially students ber (IC) measurements, AMS and ICP-MS will and postdoctoral fellows for knowledge exchange and be utilized and refined. The techniques developed training in this course will subsequently meet the special • Development of new, innovative equipment for specific 32 requirements for the half-life determination of Si applications, accessible for the entire community 32 as well as future routine measurements of Si. • Basic research on target preparation methods and 32 Using a fraction of the isolated Si sample, characterization techniques; development of new, complementary half-life measurements via different improved techniques techniques will be performed. • Acquisition of new partners and, also outside Europe, c) Standard material for all applicable measurement to broaden the network techniques shall be produced and made available for • Networking with the user communities. routine measurements world-wide. Establishing 32Si as a new chronometer would fill the gap Such a network could enable the target producers 210 between the relatively short-lived Pb (T1/2 ≈ 22 a) and to early identify actual needs and requirements for 14 32 longer-lived C(T1/2 ≈ 5730 a). The availability of Si new developments, thus triggering foresight studies for will foster its routine application in environmental and new instrumentations, methods and concepts. Dedicated astrophysical applications. This has to be considered as projects can then be launched for concerted actions.

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5. Conclusions and outlook [11] W. Kutschera et al., NIMB 5, 430 (1984) [12] A. Wallner et al., PRL 114, 041101 (2015) Isotope production and radioactive target preparation are [13] K Ostieck et al., NIMB 361, 638–642 (2015) challenging tasks, requiring a tight collaboration between [14] D. Schumann et al., NIMA 613, 347 (2010) users and producers to obtain suitable samples, which [15] E. Uberseder et al., PRL 102, 151101 (2009) meet the boundary conditions of the envisaged experiment. [16] T. Heftrich et al., PRC 92, 015806 (2015) We presented here some results of such successful target [17] A. Wallner, personal communication production at PSI. However, the independent and isolated [18] R.H. Cyburt et al., PRD 69, 123519 (2004) acting of target-producing groups is not always the best [19] B.D. Fields, Ann. Rev. of Nuclear and Particle solution. A new, innovative target producer network, aimed Science 61, 47 (2011) to effectively explore the limited resources by sharing [20] F. Spite et al., & Astrophysics 115, 357 equipment and exchange knowledge, would improve (1982) the situation essentially. The bi-annual conferences of [21] A. Martin et al., The Astrophysical Journal 644, 229 the International Nuclear Target Development Society (2006) (INTDS) [41] are a good basis for interactions of target [22] R.H. Cyburt et al., JCAP 11, 012 (2008) producers. A first attempt to intensify the collaboration [23] M. Barbagallo et al., PRL 117, 152701 (2016) was started with the creation of the ANITA network [24] E.A. Maugeri et al., JINST 12, PO2016 (2016) (Advanced Network for I sotope and TArget Laboratories), [25] F. Kaeppeler et al., Reviews of 83, including 6 target producing laboratories (PSI Villigen, 157 (2011) European Commission Joint Research Centre Directorate [26] S. Heinitz et al. RCA 2017, in press G in Geel, Belgium, Johannes Gutenberg University [27] J.P. Shedlovsky, Geochim. Cosmochim. Acta 21, 156 Mainz & Helmholtzzentrum fur¨ Schwerionenforschung (1960) Darmstadt, Germany, Grand Accel´ erateur´ National d’Ions [28] H.T. Millard Jr, Science 147, 503 (1965) Lourds Caen, France and University of Warsaw, Poland). [29] R. Wolfle,¨ et al., Radiochim. Acta 18, 207 (1972) Possibilities for funding are currently explored. [30] R.A. Forrest, et al.: UKAEA FUS 535 (2007) [31] A.J. Koning, D. Rochman, JEFF-DOC 1262 (2008) References [32] M. Honda, M. Imamura, Phys. Rev. C 4, 1182 (1971) [33] R. Wolfle,¨ et al., Radiochim. Acta 18, 207 (1972) [1] http://indico.psi.ch/conferenceDisplay. [34] M. Heimann, et al., Geochim. Cosmochim. Acta 38, py?confId= 3671 217 (1974) [2] A. Stolarz, J. Radioanal. Nucl. Chem. 299, 913–931 [35] L.E. Nyquist, et al., Geochim. Cosmochim. Acta 73, (2014) 5115 (2009) [3] D. Schumann et al., RCA 101, 501–508 (2013) [36] R. Dressler et al., Nucl. Phys.G 39, 105201 (2012) [4] D. Schumann et al., J. Phys. G 35, 014046 (2008) [37] B.-A. Dittmann et al., Annual Report LCH-PSI 45 [5] D. Schumann et al., RCA, 509–514 (2013) (2016) [6] W.R. Binns et al., Science 352, 677 (2016) [38] Ch. Oellet, et al., Nuclear Data Sheets for A=32, [7] D. Breitschwerdt et al., Nature 532, 73 (2016) Nuclear Data Sheets 112, 2199 (2011) [8] A. Wallner et al., Nature 532, 69 (2016) [39] D. Lal, et al., Science 131, 332 (1960) [9] D. Schumann et al., RCA 97, 123–131 (2009) [40] W. Dansgaard et al., Tellus 18, 187 (1966) [10] G. Rugel et al., PRL 103, 072502 (2009) [41] http://www.intds.org/

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