arXiv subjects

A. Junghans

Publications and source records attributed to A. Junghans.

At least 19 recordsLinked to original sources

arXiv2026

Improved $^{94}$Mo neutron resonance parameters from neutron capture and transmission measurements at n_TOF and GELINA

We report high-resolution measurements of the $^{94}\mathrm{Mo}(\mathrm{n},\gamma)^{95}\mathrm{Mo}$ cross section in the neutron energy range from a few eV up to about 250 keV, performed at the n_TOF facility (CERN), and of the $^{94}\mathrm{Mo}(\mathrm{n},\mathrm{tot})$ cross section up to 32 keV, measured at GELINA (JRC Geel). A combined R-matrix analysis of capture and transmission data yields significantly improved neutron-resonance parameters for $^{94}$Mo. A total of 186 resonances were observed in this analysis of which 127 reported here for the first time. The resulting Maxwellian-averaged cross section at stellar temperatures relevant to the slow neutron-capture process (s-process) is approximately 25% lower than previous evaluations and literature values. To assess the astrophysical impact of the revised cross section, we performed s-process nucleosynthesis calculations for low-mass asymptotic giant branch stars. Despite the substantial reduction in the $^{94}\mathrm{Mo}(\mathrm{n},\gamma)^{95}\mathrm{Mo}$ rate, the final yields vary by only 3-4%. This demonstrates that the isotopic budget of $^{94}$Mo in AGB stars is primarily governed by the branching flow at $^{94}\mathrm{Nb}$ rather than by the neutron-capture destruction on $^{94}$Mo itself. The new cross section thus helps disentangle nuclear cross-section uncertainties from branching-flow effects in the s-process production of $^{94}$Mo.

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arXiv2026

Energy-differential measurement of the $^{\mathrm{nat}}$C(n,p) and $^{\mathrm{nat}}$C(n,d) reactions at the n_TOF facility at CERN

Energy-differential cross section of the $^{\mathrm{nat}}$C(n,p) and $^{\mathrm{nat}}$C(n,d) reactions was measured at the neutron time of flight facility n_TOF at CERN. The measurement was performed in the first experimental area (EAR1; flight path of 182.5 m). Two position-sensitive $\Delta E$-$E$ silicon telescopes were used. Two naturally occurring carbon isotopes, $^{12}$C and $^{13}$C, contribute to the reactions on natural carbon, with the (n,p) reaction threshold at 13.7 MeV and the (n,d) threshold at 14.9 MeV (determined by the $^{12}$C isotope for both reactions). This paper provides the details of the analysis leading to the final results published previously as a Letter. The cross section results are reported up to 25 MeV. During the data analysis the population of the excited states in the daughter nuclei $^{11}$B, $^{12}$B, $^{13}$B had to be considered, requiring the adoption of the branching ratios and angular distributions of the emitted particles from an external source of information. TALYS-2.0 calculations were used as the main source and an in-depth analysis of the model-related uncertainties was performed. The n_TOF results are largely inconsistent with the major evaluation libraries. On the other hand, an unexpected agreement is found with TALYS-2.0 calculations. Specifically, the obtained cross section for the (n,p) reaction is significantly higher than in the available evaluations, fully supporting the earlier finding from an integral measurement at n_TOF.

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arXiv2026

First $^{94}$Nb($n,\gamma$) Measurement: Constraining the Nucleosynthetic Origin of $^{94}$Mo in Presolar Grains

Isotopic measurements of presolar silicon carbide grains from dying stars have revealed a puzzling overabundance of $^{94}$Mo that stellar nucleosynthesis models have failed to reproduce for two decades. This discrepancy challenged our understanding of the slow neutron-capture process ($s$-process) that forges approximately half of the elements heavier than iron. The key uncertainty lies at $^{94}$Nb, a radiactive branching point where competition between neutron capture and beta decay governs the $^{94}$Mo production, yet the neutron-capture cross section had never been measured. Here we report the first experimental determination of the $^{94}$Nb(n,$\gamma$)$^{95}$Nb cross section important for Mo isotopic abundances. The measurement was enabled by a coordinated effort involving high-purity target preparation at Institute of Solid State and Materials Research (IFW) Dresden, radioactive sample production at the Institut Laue-Langevin (ILL) Grenoble, radiochemical characterization at Paul Scherrer Institute (PSI) Villigen, and the Time-of-Flight CERN n$\_$TOF facility using for the first time segmented total-energy detectors. Incorporation of the resulting Maxwellian-averaged cross section into fully coupled nucleosynthesis models of low-mass asymptotic giant branch (AGB) stars brings them into agreement with the presolar grain data. These results remove a major nuclear-physics input uncertainty at the $^{94}$Nb branching point and provide a firmer foundation for understanding the origin of $^{94}$Mo in the solar system.

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arXiv2025

Measurement of the $^\text{nat}$C(n,p) and $^\text{nat}$C(n,d) reactions from n_TOF at CERN

The energy dependence of the cross section of the (n,p) and (n,d) reactions on $^\text{nat}$C has been studied for the first time at the n_TOF facility at CERN, from the particle detection threshold up to 25 MeV. The measurement was performed with two telescopes made of position-sensitive silicon $\Delta E$-$E$ detectors, covering the angular range from 20{\deg} to 140{\deg}. A detector efficiency has been determined by means of Monte Carlo simulations of the experimental setup. Various assumptions on the angular distributions and branching ratios of the excited levels of the residual $^{11}$B, $^{12}$B, $^{13}$B nuclei were considered. In particular, theoretical calculations based on the TALYS-2.0 code were used and the systematic uncertainties in the analysis results were determined from the variations in these distributions. The n_TOF data on the (n,p) and (n,d) reaction on carbon are characterized by a higher accuracy and wider energy range than currently available in literature. A comparison with current evaluations from different libraries reveals a rather significant disagreement with the n_TOF results, while a remarkable agreement is observed with the prediction of TALYS-2.0 for this light element.

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arXiv2025

Neutron capture measurements for s-process nucleosynthesis; A review about CERN n_TOF developments and contributions

This article presents a review about the main CERN n\_TOF contributions to the field of neutron-capture experiments of interest for $s$-process nucleosynthesis studies over the last 25 years, with special focus on the measurement of radioactive isotopes. A few recent capture experiments on stable isotopes of astrophysical interest are also discussed. Results on $s$-process branching nuclei are appropriate to illustrate how advances in detection systems and upgrades in the facility have enabled increasingly challenging experiments and, as a consequence, have led to a better understanding and modeling of the $s$-process mechanism of nucleosynthesis. New endeavors combining radioactive-ion beams from ISOLDE for the production of radioisotopically pure samples for activation experiments at the new NEAR facility at n\_TOF are briefly discussed. On the basis of these new exciting results, also current limitations of state-of-the-art TOF and activation techniques will be depicted, thereby showing the pressing need for further upgrades and enhancements on both facilities and detection systems. A brief account of the potential technique based on inverse kinematics for direct neutron-capture measurements is also presented.

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arXiv2024

Towards a new generation of solid total-energy detectors for neutron-capture time-of-flight experiments with intense neutron beams

Challenging neutron-capture cross-section measurements of small cross sections and samples with a very limited number of atoms require high-flux time-of-flight facilities. In turn, such facilities need innovative detection setups that are fast, have low sensitivity to neutrons, can quickly recover from the so-called $\gamma$-flash, and offer the highest possible detection sensitivity. In this paper, we present several steps toward such advanced systems. Specifically, we describe the performance of a high-sensitivity experimental setup at CERN n\_TOF EAR2. It consists of nine sTED detector modules in a compact cylindrical configuration, two conventional used large-volume C$_{6}$D$_{6}$ detectors, and one LaCl$_{3}$(Ce) detector. The performance of these detection systems is compared using $^{93}$Nb($n$,$\gamma$) data. We also developed a detailed \textsc{Geant4} Monte Carlo model of the experimental EAR2 setup, which allows for a better understanding of the detector features, including their efficiency determination. This Monte Carlo model has been used for further optimization, thus leading to a new conceptual design of a $\gamma$ detector array, STAR, based on a deuterated-stilbene crystal array. Finally, the suitability of deuterated-stilbene crystals for the future STAR array is investigaged experimentally utilizing a small stilbene-d12 prototype. The results suggest a similar or superior performance of STAR with respect to other setups based on liquid-scintillators, and allow for additional features such as neutron-gamma discrimination and a higher level of customization capability.

physics.ins-det↗

arXiv2023

Pushing the high count rate limits of scintillation detectors for challenging neutron-capture experiments

One of the critical aspects for the accurate determination of neutron capture cross sections when combining time-of-flight and total energy detector techniques is the characterization and control of systematic uncertainties associated to the measuring devices. In this work we explore the most conspicuous effects associated to harsh count rate conditions: dead-time and pile-up effects. Both effects, when not properly treated, can lead to large systematic uncertainties and bias in the determination of neutron cross sections. In the majority of neutron capture measurements carried out at the CERN n\_TOF facility, the detectors of choice are the C$_{6}$D$_{6}$ liquid-based either in form of large-volume cells or recently commissioned sTED detector array, consisting of much smaller-volume modules. To account for the aforementioned effects, we introduce a Monte Carlo model for these detectors mimicking harsh count rate conditions similar to those happening at the CERN n\_TOF 20~m fligth path vertical measuring station. The model parameters are extracted by comparison with the experimental data taken at the same facility during 2022 experimental campaign. We propose a novel methodology to consider both, dead-time and pile-up effects simultaneously for these fast detectors and check the applicability to experimental data from $^{197}$Au($n$,$γ$), including the saturated 4.9~eV resonance which is an important component of normalization for neutron cross section measurements.

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arXiv2023

First measurement of the $^{94}$Nb($n$,$γ$) cross section at the CERN n\_TOF facility

One of the crucial ingredients for the improvement of stellar models is the accurate knowledge of neutron capture cross-sections for the different isotopes involved in the $s$-,$r$- and $i$- processes. These measurements can shed light on existing discrepancies between observed and predicted isotopic abundances and help to constrain the physical conditions where these reactions take place along different stages of stellar evolution.In the particular case of the radioactive $^{94}$Nb, the $^{94}$Nb($n$,$γ$) cross-section could play a role in the determination of the $s$-process production of $^{94}$Mo in AGB stars, which presently cannot be reproduced by state-of-the-art stellar models. There are no previous $^{94}$Nb($n$,$γ$) experimental data for the resolved and unresolved resonance regions mainly due to the difficulties in producing high-quality samples and also due to limitations in conventional detection systems commonly used in time-of-flight experiments.Motivated by this situation, a first measurement of the $^{94}$Nb($n$,$γ$) reaction was carried out at CERN n\_TOF, thereby exploiting the high luminosity of the EAR2 area in combination with a new detection system of small-volume C6D6-detectors and a high quality $^{94}$Nb-sample. The latter was based on hyper-pure $^{93}$Nb material activated at the high-flux reactor of ILL-Grenoble. An innovative ring-configuration detection system in close geometry around the capture sample allowed us to significantly enhance the signal-to-background ratio. This set-up was supplemented with two conventional C$_{6}$D$_{6}$ detectors and a high-resolution LaCl$_{3}$(Ce)-detector, which will be employed for addressing reliably systematic effects and uncertainties.At the current status of the data analysis, 18 resonance in $^{94}$Nb+$n$ have been observed for the first time in the neutron energy range from thermal up to 10 keV.

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arXiv2022

The CERN n TOF NEAR station for astrophysics- and application-related neutron activation measurements

A new experimental area, the NEAR station, has recently been built at the CERN n TOF facility, at a short distance from the spallation target (1.5 m). The new area, characterized by a neutron beam of very high flux, has been designed with the purpose of performing activation measurements of interest for astrophysics and various applications. The beam is transported from the spallation target to the NEAR station through a hole in the shielding wall of the target, inside which a collimator is inserted. The new area is complemented with a γ-ray spectroscopy laboratory, the GEAR station, equipped with a high efficiency HPGe detector, for the measurement of the activity resulting from irradiation of a sample in the NEAR station. The use of a moderator/filter assembly is envisaged, in order to produce a neutron beam of Maxwellian shape at different thermal energies, necessary for the measurement of Maxwellian Averaged Cross Sections of astrophysical interest. A new fast-cycling activation technique is also being investigated, for measurements of reactions leading to isotopes of very short half life.

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arXiv2022

Advances and new ideas for neutron-capture astrophysics experiments at CERN n_TOF

This article presents a few selected developments and future ideas related to the measurement of $(n,γ)$ data of astrophysical interest at CERN n_TOF. The MC-aided analysis methodology for the use of low-efficiency radiation detectors in time-of-flight neutron-capture measurements is discussed, with particular emphasis on the systematic accuracy. Several recent instrumental advances are also presented, such as the development of total-energy detectors with $γ$-ray imaging capability for background suppression, and the development of an array of small-volume organic scintillators aimed at exploiting the high instantaneous neutron-flux of EAR2. Finally, astrophysics prospects related to the intermediate $i$ neutron-capture process of nucleosynthesis are discussed in the context of the new NEAR activation area.

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arXiv2022

New limits on double-beta decay of $^{190}$Pt and $^{198}$Pt

A search for double-beta decay of $^{190}$Pt and $^{198}$Pt with emission of $γ$-ray quanta was realized at the HADES underground laboratory with a 148 g platinum sample measured by two ultralow-background HPGe detectors over 8946 h. The isotopic composition of the platinum sample has been measured with high precision using inductively coupled plasma mass spectrometry. New lower limits for the half-lives of $^{190}$Pt relative to different channels and modes of the decays were set on the level of $\lim T_{1/2}\sim 10^{14}-10^{16}$ yr. A possible exact resonant $0νKN$ transition to the 1,2 1326.9 keV level of $^{190}$Os is limited for the first time as $T_{1/2} \geq 2.5 \times10^{16}$ yr. A new lower limit on the double-beta decay of $^{198}$Pt to the first excited level of $^{198}$Hg was set as $T_{1/2} \geq 3.2\times10^{19}$ yr, one order of magnitude higher than the limit obtained in the previous experiment.

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arXiv2021

Vacuum laser acceleration of super-ponderomotive electrons using relativistic transparency injection

Intense lasers can accelerate electrons to very high energy over a short distance. Such compact accelerators have several potential applications including fast ignition, high energy physics, and radiography. Among the various schemes of laser-based electron acceleration, vacuum laser acceleration has the merits of super-high acceleration gradient and great simplicity. Yet its realization has been difficult because injecting free electrons into the fast-oscillating laser field is not trivial. Here we demonstrate free-electron injection and subsequent vacuum laser acceleration of electrons up to 20 MeV using the relativistic transparency effect. When a high-contrast intense laser drives a thin solid foil, electrons from the dense opaque plasma are first accelerated to near-light speed by the standing laser wave in front of the solid foil and subsequently injected into the transmitted laser field as the opaque plasma becomes relativistically transparent. It is possible to further optimize the electron injection/acceleration by manipulating the laser polarization, incident angle, and temporal pulse shaping. Our result also sheds new light on the fundamental relativistic transparency process, crucial for producing secondary particle and light sources.

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arXiv2021

High-yield and high-angular-flux neutron generation from deuterons accelerated by laser-driven collisionless shock

A bright collimated neutron source is an essential tool for global security missions and fundamental scientific research. In this paper, we study a compact high-yield and high-angular-flux neutron source utilizing the break-up reaction of laser-driven deuterons in a $^9\text{Be}$ converter. The neutron generation scaling from such a reaction is used to guide the choice and optimization of the acceleration process for the bulk ions in a low density $\text{CD}_2$ foam. In particular, the collisionless shock acceleration mechanism is exploited with proper choice in the laser and target parameter space to accelerate these ions towards energies above the temperature of the distribution. Particle-In-Cell and Monte Carlo simulations are coupled to investigate this concept and possible adverse effects, as well as the contribution from the surface ions accelerated and the optimal converter design. The simulation results indicated that our design can be a practical approach to increase both the neutron yield and forward flux of laser-driven neutron sources, reaching peak angular neutron flux $>10^{11}$ neutron/sr and yield $>10^{11}$ neutron/pulse with present-day kJ-class high-power lasers. Such developments will advance fundamental neutron science, high precision radiography and other global security applications with the laser-driven sources.

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arXiv2020

Imaging neutron capture cross sections: i-TED proof-of-concept and future prospects based on Machine-Learning techniques

i-TED is an innovative detection system which exploits Compton imaging techniques to achieve a superior signal-to-background ratio in ($n,\gamma$) cross-section measurements using time-of-flight technique. This work presents the first experimental validation of the i-TED apparatus for high-resolution time-of-flight experiments and demonstrates for the first time the concept proposed for background rejection. To this aim both $^{197}$Au($n,\gamma$) and $^{56}$Fe($n, \gamma$) reactions were measured at CERN n\_TOF using an i-TED demonstrator based on only three position-sensitive detectors. Two \cds detectors were also used to benchmark the performance of i-TED. The i-TED prototype built for this study shows a factor of $\sim$3 higher detection sensitivity than state-of-the-art \cds detectors in the $\sim$10~keV neutron energy range of astrophysical interest. This paper explores also the perspectives of further enhancement in performance attainable with the final i-TED array consisting of twenty position-sensitive detectors and new analysis methodologies based on Machine-Learning techniques.

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arXiv2019

Review and new concepts for neutron-capture measurements of astrophysical interest

The idea of slow-neutron capture nucleosynthesis formulated in 1957 triggered a tremendous experimental effort in different laboratories worldwide to measure the relevant nuclear physics input quantities, namely ($n,γ$) cross sections over the stellar temperature range (from few eV up to several hundred keV) for most of the isotopes involved from Fe up to Bi. A brief historical review focused on total energy detectors will be presented to illustrate how, advances in instrumentation have led, over the years, to the assessment and discovery of many new aspects of $s$-process nucleosynthesis and to the progressive refinement of theoretical models of stellar evolution. A summary will be presented on current efforts to develop new detection concepts, such as the Total-Energy Detector with $γ$-ray imaging capability (i-TED). The latter is based on the simultaneous combination of Compton imaging with neutron time-of-flight (TOF) techniques, in order to achieve a superior level of sensitivity and selectivity in the measurement of stellar neutron capture rates.

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arXiv2017

Astrophysical S-factor of the $^{14}\textrm{N(p,}γ\textrm{)}^{15}\textrm{O}$ reaction at 0.4 -- 1.3\,MeV

The $^{14}\textrm{N(p,}γ\textrm{)}^{15}\textrm{O}$ reaction is the slowest reaction of the carbon-nitrogen cycle of hydrogen burning and thus determines its rate. The precise knowledge of its rate is required to correctly model hydrogen burning in asymptotic giant branch stars. In addition, it is a necessary ingredient for a possible solution of the solar abundance problem by using the solar $^{13}$N and $^{15}$O neutrino fluxes as probes of the carbon and nitrogen abundances in the solar core. After the downward revision of its cross section due to a much lower contribution by one particular transition, capture to the ground state in $^{15}$O, the evaluated total uncertainty is still 8\%, in part due to an unsatisfactory knowledge of the excitation function over a wide energy range. The present work reports precise S-factor data at twelve energies between 0.357-1.292~MeV for the strongest transition, capture to the 6.79~MeV excited state in $^{15}$O, and at ten energies between 0.479-1.202~MeV for the second strongest transition, capture to the ground state in $^{15}$O. An R-matrix fit is performed to estimate the impact of the new data on astrophysical energies. The recently suggested slight enhancement of the 6.79~MeV transition at low energy could not be confirmed. The present extrapolated zero-energy S-factors are $S_{6.79}(0)$~=~1.24$\pm$0.11~keV~barn and $S_{\rm GS}(0)$~=~0.19$\pm$0.05~keV~barn.

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arXiv2012

Coulomb excitation of exotic nuclei at the R3B-LAND setup

Exotic Ni isotopes have been measured at the R3B-LAND setup at GSI in Darmstadt, using Coulomb excitation in inverse kinematics at beam energies around 500 MeV/u. As the experimental setup allows kinematically complete measurements, the excitation energy was reconstructed using the invariant mass method. The GDR and additional low-lying strength have been observed in 68Ni, the latter exhausting 4.1(1.9)% of the E1 energy-weighted sum rule. Also, the branching ratio for the non-statistical decay of the excited 68Ni nuclei was measured and amounts to 24(4)%.

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arXiv2010

Solar fusion cross sections II: the pp chain and CNO cycles

We summarize and critically evaluate the available data on nuclear fusion cross sections important to energy generation in the Sun and other hydrogen-burning stars and to solar neutrino production. Recommended values and uncertainties are provided for key cross sections, and a recommended spectrum is given for 8B solar neutrinos. We also discuss opportunities for further increasing the precision of key rates, including new facilities, new experimental techniques, and improvements in theory. This review, which summarizes the conclusions of a workshop held at the Institute for Nuclear Theory, Seattle, in January 2009, is intended as a 10-year update and supplement to Reviews of Modern Physics 70 (1998) 1265.

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