FN Tandem Accelerator Research Articles

Production of 43Sc and 44gSc from natural CaF2 material using an FN Tandem Accelerator

The radioisotopes of 43Sc and 44Sc are promising in the field of theranostics for their role as β+ emitters in theranostic pairs with 47Sc. Production of these isotopes through various nuclear reactions using either cyclotrons or linear accelerators is of particular interest and previous studies have provided results using accelerated beams of protons, deuterons, and alpha particles. A novel production technique, using an ion source cathode packed with natural calcium fluoride material and irradiated with a 3He beam, was tested at the Nuclear Science Laboratory at the University of Notre Dame in order to initially study the production of 41Ca. Gamma-ray spectrometry revealed presence of 43Sc and 44Sc in the target, and allowed for the first measurement of their yield due to reactions of 3He on natural calcium. The calculated thick target yields of 43Sc and 44gSc from the reactions natCa(3He,x)43Sc and natCa(3He,x)44gSc are compared to theoretical results using TALYS cross section models. Overall, results agree well with models at lower beam energies but tend to diverge at higher energies.

Observation of d(t,n)α Neutrons Following d(d,p)t Reactions in a Deuterium Gas Cell: An Attempt to Repeat Ruhlig’s 1938 Observation of Secondary Reactions

In order to benchmark methods used to calculate reaction-in-flight fusion reactions in inertial confinement fusion and address issues related to the first claimed observation of d(t,n)α reactions in 1938, secondary d(t,n)α reactions have been observed following d(d,p)t reactions in deuterium gas. A pulsed 200-nA, 2.2-MeV deuterium beam from the Triangle Universities Nuclear Laboratory FN tandem accelerator was injected into a cylindrical multiatmosphere deuterium gas target. The incident beam traversed along the target cylinder’s 3-cm symmetry axis after its passage through a Havar entrance foil. Two different Havar foil thicknesses were used to obtain 1.5- and 0.6-MeV deuteron beams entering the deuterium cell. The cylinder’s radius was 2 cm to allow for d(d,p)t tritons emitted perpendicular to the beam to range out in the deuterium gas. The neutron emission from the cell was observed via its time of flight to a liquid scintillator placed at various angles to the beam direction, at a distance of 243 cm. Pulse-shape-discrimination techniques were used to separate neutron and gamma-ray signals seen in the liquid scintillator. The observed probability of ~2 × 10–4 for inducing secondary d(t,n)α fusion in the gas cell per d(d,p)t reaction is consistent with theoretical expectations.

Determination of 170,172Yb(α,n)173,175Hf reaction cross sections in a stacked-target experiment

Background: For understanding the synthesis of elements in the universe, precise knowledge of reaction rates and cross sections is paramount. This is especially true for the $p$ process because its study requires large network calculations including thousands of nuclei and nuclear reactions with little room for simplification. Therefore, robust theoretical methods for predicting cross sections are needed which are usually based on Hauser-Feshbach calculations. These calculations use physics input in the form of $\ensuremath{\gamma}$-ray strength functions, nuclear level densities, and particle $+$ nucleus optical-model potentials. These have to be constrained using experimental results.Purpose: To constrain the $\ensuremath{\alpha}$ optical-model potential $\ensuremath{\alpha}$-induced reactions are well suited. The ytterbium isotopic chain not only offers multiple stable isotopes on which cross sections can be measured and insights into the evolution of the $\ensuremath{\alpha}$ optical-model potential with the neutron-to-proton ratio can be gained but also includes the $p$ nucleus $^{168}\mathrm{Yb}$. Its abundance is significantly impacted by the $^{164,166}\mathrm{Yb}(\ensuremath{\alpha},\ensuremath{\gamma})$ reactions and is, therefore, also affected by the constraints resulting from the experiment presented in this paper.Method: To study the $^{170,172}\mathrm{Yb}(\ensuremath{\alpha},n)^{173,175}\mathrm{Hf}$ reaction cross sections the activation method was used. During irradiation the targets were arranged in stacks of four to reduce the required irradiation time. Aluminum degrader foils served as backings. The average interaction energy inside each ytterbium layer was determined via geant4 simulations. A third layer, consisting of manganese, was used to verify the simulations by comparing the measured $^{55}\mathrm{Mn}(\ensuremath{\alpha},(2)n)^{57,58}\mathrm{Co}$ reaction cross sections to previous results. For irradiation the 10 MV FN tandem accelerator located at the University of Cologne was used and the activation measurement was performed utilizing the Cologne Clover Counting setup consisting of two clover-type high-purity germanium detectors in a face-to-face geometry.Results: For the $^{170}\mathrm{Yb}(\ensuremath{\alpha},n)$ reaction seven cross sections at center-of-mass energies between 12.7 and 16.5 MeV were measured. For the $^{172}\mathrm{Yb}(\ensuremath{\alpha},n)$ reaction six cross sections for center-of-mass energies of 13.1 to 16.5 MeV could be determined with an additional upper limit at ${E}_{\mathrm{c}.\mathrm{m}.}=12.3$ MeV.Conclusion: Comparisons to theoretical models show that state-of-the-art $\ensuremath{\alpha}$-optical model potentials reproduce the measured cross sections very well. The ratios of $(\ensuremath{\alpha},n)$ reaction cross sections in the ytterbium isotopic chain can be accurately reproduced as well.

Results of total cross-section measurements of the 87Rb(p,γ)88Sr reaction

The existence of a set of stable proton-rich nuclei - the p nuclei - cannot be explained via neutron-capture reactions. Therefore, another mechanism has to exist in order to explain their origin, the most probable, especially at high masses, being photodisintegration reactions. This gives rise to the γ process. Since most photodisintegration reactions involved in the process are not experimentally accessible, reliable statistical model calculations are needed to predict cross sections and reaction rates. To improve these calculations nuclear input parameters need to be constrained and a large experimental database is needed. Via comparison of experimental data to theoretical predictions different models can be tested and constrained. In order to study the 87Rb(p,γ)88Sr reaction, for which previously no experimental data have been available, an in-beam experiment at at the University of Cologne’s high-efficiency HPGe γ-ray spectrometer HORUS was performed. Proton beams with energies between Ep = 2000 to 5000 keV reaching deep into the Gamow window of the reaction were provided by the 10 MV FN Tandem accelerator. Cross-section values at six proton beam energies were determined. The experimental results are in good agreement with statistical model calculations. The obtained results are the first experimental cross-section values for the87Rb(p,γ)88Sr reaction and help to constrain the nuclear physics input for statistical model calculations.

Investigation of γ softness: Lifetime measurements in Ru104,106

Lifetimes of the ${2}_{1}^{+}, {4}_{1}^{+}, {6}_{1}^{+}, {2}_{\ensuremath{\gamma}}^{+}$, and ${3}_{\ensuremath{\gamma}}^{+}$ states in $^{104,106}\mathrm{Ru}$ were measured by using the recoil-distance Doppler-shift technique and the Cologne Plunger device. Low-lying excited states in both nuclei were populated in a $^{104}\mathrm{Ru}(^{18}\mathrm{O},^{18}\mathrm{O})^{104}\mathrm{Ru}$* inelastic scattering and in a $^{104}\mathrm{Ru}(^{18}\mathrm{O},^{16}\mathrm{O})^{106}\mathrm{Ru}$ two-neutron transfer reaction using the Cologne FN Tandem accelerator. The experimental energy levels and deduced electromagnetic transition probabilities are compared in the context of $\ensuremath{\gamma}$ softness and the mapped interacting boson model with input from the microscopic self-consistent mean-field calculation using a Gogny interaction. The newly obtained results for the $\ensuremath{\gamma}$ band give a more detailed insight about the triaxial behavior of $^{104,106}\mathrm{Ru}$. The results will be discussed in the context of $\ensuremath{\gamma}$ soft and rigid triaxial behavior which is present in the neutron-rich Ru isotopes.

Development of the multi-purpose Cologne Compact Differential Plunger (CoCoDiff) for the measurement of nuclear level lifetimes with the Recoil Distance Doppler-shift method

A new 3-foil plunger device, the Cologne Compact Differential (CoCoDiff) plunger has been built and commissioned. Due to its compact size, it can be used together with many different spectrometers and auxiliary detectors. As a commissioning experiment, level lifetimes of the 21+ and the 41+ excited states of 50Cr have been measured, using the Differential Decay Curve method (DDCM). A derivation is given on how this method can be applied to a differential plunger measurement, in order to measure distances for lifetimes from two different regions of sensitivity at the same time. The commissioning experiment took place at the Cologne FN tandem accelerator, using the reaction 24Mg(32S,4p2n)50Cr. Lifetimes, deduced from this measurement, agree well with literature values from earlier measurements.

Lifetime measurements to investigate γ softness and shape coexistence in Mo102

Lifetimes of low-spin excited states in $^{102}\mathrm{Mo}$ populated in a $^{100}\mathrm{Mo}(^{18}\mathrm{O},\phantom{\rule{0.16em}{0ex}}^{16}\mathrm{O})^{102}\mathrm{Mo}$ two-neutron transfer reaction were measured using the recoil-distance Doppler-shift technique at the Cologne FN Tandem accelerator. Lifetimes of the ${2}_{1}^{+}$, ${4}_{1}^{+}$, ${6}_{1}^{+}$, ${0}_{2}^{+}$, ${2}_{\ensuremath{\gamma}}^{+}$, ${3}_{\ensuremath{\gamma}}^{+}$ states and one upper limit for the lifetime of the ${4}_{\ensuremath{\gamma}}^{+}$ state were obtained. The energy levels and deduced electromagnetic transition probabilities are compared with those obtained within the mapped interacting boson model framework with microscopic input from Gogny mean-field calculations. With the newly obtained signatures a more detailed insight in the $\ensuremath{\gamma}$ softness and shape coexistence in $^{102}\mathrm{Mo}$ is possible and discussed in the context of the $Z\ensuremath{\approx}40$ and $N\ensuremath{\approx}60$ region. The nucleus of $^{102}\mathrm{Mo}$ follows the $\ensuremath{\gamma}$ soft trend of the Mo isotopes. The properties of the ${0}_{2}^{+}$ state indicate, in contrast with the microscopic predictions, shape coexistence which also occurs in other $N=60$ isotones.

Triaxiality in the mid-shell nucleus Pd112

Lifetimes of low-spin excited states in $^{112}\mathrm{Pd}$ were measured using the recoil-distance Doppler-shift technique. The nucleus of interest was populated in a $^{110}\mathrm{Pd}(^{18}\mathrm{O},^{16}\mathrm{O})^{112}\mathrm{Pd}$ reaction using the Cologne FN Tandem accelerator. Three lifetimes of ground-state band members and one lifetime of the $\ensuremath{\gamma}$ band were measured. From these lifetimes reduced transition probabilities were extracted and compared to interacting boson model, $\ensuremath{\gamma}$-soft calculations, and Davydov calculations. The lifetime of the ${2}_{\ensuremath{\gamma}}^{+}$ gives some insights on the nuclear shape and structure of the $\ensuremath{\gamma}$ band. The deduced transition rates show an indicator for a rigid triaxial nucleus as well as more indicators for a $\ensuremath{\gamma}$-soft nucleus.

Lifetime measurements in ^{182}hbox {Pt} using gamma \u2013gamma fast-timing

The level lifetimes of the 2_1^+ and 4_1^+ states in ^{182}hbox {Pt} have been re-measured employing the gamma –gamma fast-timing technique using fast hbox {LaBr}_3(Ce) scintillators. Excited states in the nucleus of interest were populated by the fusion-evaporation reaction ^{170}hbox {Yb}(^{16}hbox {O},hbox {4n})^{182}hbox {Pt} at a beam energy of 87 MeV provided by the FN Tandem accelerator of the University of Cologne. The lifetime of the 2_1^+ state was re-measured with high accuracy to be tau = 563(12)~ps and resolves inconsistencies from previous measurements. Experimental results are compared to theoretical calculations in the framework of the sd-IBM with and without configuration mixing.

Deducing primary γ -ray intensities and the dipole strength function in Mo94 via radiative proton capture

Background: The nucleosynthesis of heavy nuclei is affected by the reaction rates of radiative capture reactions. Many of the astrophysical relevant rates cannot be obtained from experiments but are obtained from theoretical models. The $\ensuremath{\gamma}$-decay widths that are derived from radiative strength functions are one of the key nuclear physics input parameters in those calculations. The explicit study of $\ensuremath{\gamma}$-ray strength functions has been thoroughly addressed in the last decade and various methods have been established to extract the dipole strength in atomic nuclei.Purpose: The investigation of primary $\ensuremath{\gamma}$-ray transitions from the $^{93}\mathrm{Nb}(p,\ensuremath{\gamma})^{94}\mathrm{Mo}$ reaction allows deducing the $\ensuremath{\gamma}$-ray strength function in $^{94}\mathrm{Mo}$. The results are compared to results obtained using other techniques and the impact of level densities, excitation mechanism, and excitation energy will be thoroughly investigated.Method: The proton beam was delivered by the 10 MV FN-Tandem accelerator located at the Institute for Nuclear Physics at the University of Cologne, Germany. By means of in-beam $\ensuremath{\gamma}$-ray spectroscopy the intensity of primary $\ensuremath{\gamma}$ rays is determined. Additionally, the first generation $\ensuremath{\gamma}$-ray intensity is deduced from the secondary $\ensuremath{\gamma}$ rays using their branching and the coincidence detection efficiency. Absolute $\ensuremath{\gamma}$-ray strength function values as well as relative values using the ratio method will be extracted.Results: Numerous states in $^{94}\mathrm{Mo}$ at excitation energies above 3 MeV have been identified for the first time and their decay behavior has been studied. We disentangled the effects of $M1$ and $E1$ radiation for the $\ensuremath{\gamma}$-ray emission channel in $^{94}\mathrm{Mo}$ and extracted strength function curves for both radiation types. Our results are in good agreement with experimental results using the Oslo method and photoinduced experiments as well as with recent theoretical quasiparticle random-phase approximation (QRPA) calculations. An enhancement of the $\ensuremath{\gamma}$-ray strength below the neutron separation energy was found that is most likely of $E1$ character. In addition, a significant increase of the dipole strength at low $\ensuremath{\gamma}$-ray energies was found which is most likely due to $M1$ strength.Conclusion: Radiative proton capture reactions are a well-suited tool to study the $\ensuremath{\gamma}$-ray strength function in atomic nuclei. From the intensity of primary $\ensuremath{\gamma}$-ray transitions as well as from secondary $\ensuremath{\gamma}$-rays, information about the radiative dipole strength can be extracted. The $\ensuremath{\gamma}$-ray emission seems to be independent of the excitation energy in the studied mass and energy range. Detailed knowledge about the intrinsic properties of the nuclear states is very important and their uncertainties affect the uncertainty of the extracted $\ensuremath{\gamma}$-ray strength functions heavily.

Tests of collectivity in Zr98 by absolute transition rates

Lifetimes of low-spin excited states in $^{98}$Zr were measured using the recoil-distance Doppler-shift technique and the Doppler-shift attenuation method. The nucleus of interest was populated in a $^{96}$Zr($^{18}$O,$^{16}$O)$^{98}$Zr two-neutron transfer reaction at the Cologne FN Tandem accelerator. Lifetimes of six low-spin excited states, of which four are unknown, were measured. The deduced $B(E2)$ values were compared with Monte Carlo shell model and interacting boson model with configuration mixing calculations. Both approaches reproduce well most of the data but leave challenging questions regarding the structure of some low lying states.

Lifetime measurement of excited states in Ce144 : Enhanced E1 strengths in a candidate for octupole deformation

A lifetime measurement of excited states in $^{144}\mathrm{Ce}$ using the $^{142}\mathrm{Ce}(^{18}\mathrm{O},^{16}\mathrm{O})$ reaction with a beam energy of 67 MeV and the recoil distance Doppler-shift method was performed at the Cologne FN Tandem accelerator. Lifetimes of the three lowest yrast states in $^{144}\mathrm{Ce}$ have been measured as well as for the ${3}_{1}^{\ensuremath{-}}$ state and an effective lifetime of the ${4}_{2}^{+}$ state. Reduced $E2$ transition strengths determined using these results have been compared to predictions from recent shell-model calculations. From the interband transitions reduced $E1$ strengths could be determined, which are strongly enhanced.

Experimental techniques to study the [formula omitted] process for nuclear astrophysics at the Cologne accelerator laboratory

The nuclear astrophysics setup at the Institute for Nuclear Physics, University of Cologne, Germany is dedicated to measurements of total and partial cross sections of charged-particle induced reactions at astrophysically relevant energies. These observables are key ingredients for reaction network calculations of various stellar scenarios, and crucial for the understanding of the nucleosynthesis of elements. The experiments utilize the high-efficiency γ-ray spectrometer HORUS, and the 10 MV FN-Tandem accelerator. An updated target chamber as well as further experimental methods established in the last years will be presented which allow to measure cross sections down to the nb region. The reliability of the measured cross sections is proven by a 89Y(p, γ)90Zr commissioning experiment. Additionally, an application for nuclear astrophysics will be presented. The results of a 93Nb(p, γ)94Mo experiment will be discussed as well as their deviations compared to formerly reported results.

Comparing fission-product yields from photon-induced fission of 240Pu and neutron-induced fission of 239Pu as a test of the Bohr hypothesis in nuclear fission

Fission product yields (FPYs) are a uniquely sensitive probe of the fission process, with well established dependence on the species of nucleus undergoing fission, its excitation energy and spin. Thus FPYs are well suited for testing Bohr’s hypothesis in the context of nuclear fission, which states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using FPYs, we have performed a new highprecision test of the combined effects of the entrance channel, spin and parity on the fission process from two of the most commonly used particles to induce fission neutrons and photons. The 239 Pu(n,f) reaction at En = 4.6 MeV and the 240 Pu(γ,f) reaction at Eγ = 11.2 MeV were used to produce a 240 Pu∗ compound nucleus with the same excitation energy. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL’s FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ-ray Source (HIγS) facility. The FPYs from these two reactions are compared quantitatively for the first time.

Validating the Bohr hypothesis: Comparing fission-product yields from photon-induced fission of 240Pu and neutron-induced fission of Pu

The Bohr hypothesis, one of the most fundamental assumptions in nuclear fission theory, states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using fission product yields (FPYs) as a sensitive probe, we have performed new high precision test of the combined effects of the entrance channel, spin and parity on the fission process. Two different reactions were used in a self-consistent manner to produce a compound 240Pu nucleus with the same excitation energy: neutron induced fission of 239Pu at En = 4.6 MeV and photon-induced fission of 240Pu at Eγ = 11.2 MeV. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL's FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ-ray Source (HlγS) facility. The first results comparing the FPYs from these two reactions will be presented. Implications for validating the Bohr hypothesis will be discussed.

Method developments for accelerator mass spectrometry at CologneAMS, 53Mn/3He burial dating and ultra-small 14CO2 samples

Abstract Accelerator mass spectrometry (AMS) has a great potential for various geological applications. The precision and sensitivity is directly linked to technical developments. Therefore CologneAMS, the center for accelerator mass spectrometry of the University of Cologne ( Dewald et al., 2013a , Dewald et al., 2013b ), does extensive work on method developments for isotopic ratio measurement, for new applications and to improve the precision. We present two important technical developments: (1) dating of sediments with 53Mn and (2) ultra-small 14CO2 samples. (1) The age determination of discontinuously deposited sediments by measurement of cosmogenic 53Mn and 3He concentration in iron‑titaniumoxides (hematite, magnetite, titanomagnetite, ilmenite), where the measurement of the medium mass isotope 53Mn is difficult because of the interfering isobar 53Cr. It can be measured at only 2 laboratories worldwide. The new AMS system at the Cologne 10 MV FN tandem accelerator is therefore designed especially for the measurement of medium mass isotopes, e.g. 53Mn. It provides the opportunity to use several detector systems in different combinations: degrader foils with an electrostatic analyzer, a 4 m time-of-flight system, a new 135∘ gas-filled magnet, different gas ionization detectors and a projectile x-ray detector. With the implementation of the new 135∘ gas-filled magnet to the 10 MV FN-Tandem accelerator AMS beamline the first tests have shown an increase in transmission and reduction of blank value in comparrison to the measurement with the degrader foil technique (Schiffer et al. 2018). The background level was reduced by a factor of 3.3 and the error normalized sensitivity (Schiffer et al. 2018) was increased by a factor of 1.6. (2) For the measurement of extremely low carbon from cosmogenic in-situ produced 14C or from 14C content of bulk sediments, with 0.01% carbon, technical developments are required to measure low concentrations in terrestrial rocks of 100,000 atoms g−1 in quartz and to trace organic compounds with 10–50 μg of 14CO2 samples. It turned out, that the measurement of 14CO2 micro-samples with the gas-ion source of the Cologne 6 MV AMS system in combination with a gas-handling system is an alternative to the development of a micro-scale graphitization. Modern 1 μg CO2 samples, (14C/12C) = 1.18×10−12, can be measured with this system with a precision of 2.8%. The overall efficiency is 4%–6% and the reproducibility of standard samples is 0.9% with a blank level of 3–6×10−15.

First AMS measurements of 60Fe/Fe isotopic ratios at the Cologne 10 MV tandem accelerator

The measurement capabilities of the new accelerator mass spectrometry (AMS) system at the 10 MV FN tandem accelerator of CologneAMS has been extended with AMS measurements of 60Fe/Fe in the range of 10-8, 10-10 and 10-12. The measurements were conducted combining a 135° gas-filled magnet and a subsequent energy loss measurement in a 5 anode gas ionization chamber. This technique allows a good separation of the stable isobar 60Ni resulting in a suppression by a factor of 107 for the high energy mass spectrometer. With this a limit for the background level of 7·10-14 was achieved. The calculation of the error normalized sensitivity allowed a first comparison to other AMS laboratories.

59 POLYGENIC RISK SCORES ANALYSES IN ANTIPSYCHOTIC-INDUCED WEIGHT GAIN

The nuclear astrophysics setup at the Institute for Nuclear Physics, University of Cologne, Germany is dedicated to measurements of total and partial cross sections of charged-particle induced reactions at astrophysically relevant energies. These observables are key ingredients for reaction network calculations of various stellar scenarios, and crucial for the understanding of the nucleosynthesis of elements. The experiments utilize the high-efficiency γ-ray spectrometer HORUS, and the 10 MV FN-Tandem accelerator. An updated target chamber as well as further experimental methods established in the last years will be presented which allow to measure cross sections down to the nb region. The reliability of the measured cross sections is proven by a 89Y(p, γ)90Zr commissioning experiment. Additionally, an application for nuclear astrophysics will be presented. The results of a 93Nb(p, γ)94Mo experiment will be discussed as well as their deviations compared to formerly reported results.

Isomer spectroscopy in Ba133 and high-spin structure of Ba134

The transitional nuclei $^{134}$Ba and $^{133}$Ba are investigated after multinucleon transfer employing the high-resolution Advanced GAmma Tracking Array coupled to the magnetic spectrometer PRISMA at the Laboratori Nazionali di Legnaro, Italy, and after fusion-evaporation reaction at the FN tandem accelerator of the University of Cologne, Germany. The Jπ=19/2+ state at 1942 keV in $^{133}$Ba is identified as an isomer with a half-life of 66.6(20)ns corresponding to a B(E1) value of 7.7(4)×10−6e2fm2 for the Jπ=19/2+ to Jπ=19/2− transition. The level scheme of $^{134}$Ba above the Jπ=10+ isomer is extended to approximately 6 MeV. A pronounced backbending is observed at ℏω=0.38 MeV along the positive-parity yrast band. The results are compared to the high-spin systematics of the Z=56 isotopes. Large-scale shell-model calculations employing the GCN50:82, SN100PN, SNV, PQM130, Realistic SM, and EPQQM interactions reproduce the experimental findings and elucidate the structure of the high-spin states. The shell-model calculations employing the GCN50:82 and PQM130 interactions reproduce alignment properties and provide detailed insight into the microscopic origin of this phenomenon in transitional $^{134}$Ba.

Lifetime measurement of excited states in 46Ti

The level lifetimes of the yrast 21+, 41+ and 61+ states and an upper limit of the lifetime of the 81+ state in 46Ti have been measured with high accuracy exploiting the recoil distance Doppler-shift method (RDDS) and using $\gamma\gamma$ coincidences. The nuclei were populated by the fusion evaporation reaction 40Ca(9Be, 2p1n)46Ti at a beam energy of $E=33$ MeV at the FN tandem accelerator of the University of Cologne, Germany. Lifetimes were extracted using the established differential decay curve method (DDCM).