arXiv subjects

P. Mohr

Publications and source records attributed to P. Mohr.

At least 19 recordsLinked to original sources

arXiv2026

Competing decay modes and stability analysis of superheavy nuclei with Z = 120 using relativistic mean-field approach

We systematically study the competition between {\alpha}-decay and spontaneous fission in even-even superheavy nuclei with (Z=120) and 256 \leq A \leq 304 within the preformed cluster-decay model using microscopic inputs from relativistic mean-field calculations with the NL3 parameter set. The {\alpha}-decay half-lives are obtained from WKB barrier penetration with empirically determined preformation factors, self-consistent Q_{\alpha} values from RMF, and nuclear interaction potentials constructed using both M3Y and relativistic R3Y nucleon-nucleon forces, and are benchmarked against standard semi-empirical formulas. Our results predict reduced spontaneous fission probabilities and extended {\alpha}-decay chains toward the fermium region for isotopes with 296 \leq A \leq 304, with enhanced stability reflected in maxima of log_{10} T_{1/2} around neutron numbers N \approx 166-182. In particular, the nuclei 296,298,300,302,304_{120} are identified as the most favorable candidates for survival against fission, demonstrating the crucial role of shell effects, deformation, and pairing correlations and providing quantitative guidance for future experimental searches of Z=120 nuclei.

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arXiv2026

Direct Measurement of the $^{59}$Cu$(p,\alpha)^{56}$Ni Excitation Function to Constrain the Ni--Cu Cycle Strength and Its Impact on Explosive Nucleosynthesis

A new direct measurement of the 59Cu(p,a)56Ni excitation function from 2.43-5.88 MeV in the center-of-mass was performed in inverse kinematics using the high-efficiency MUSIC active-target detector at FRIB. This reaction plays a critical role in constraining the strength of the NiCu cycle in different explosive astrophysical scenarios such as Type I X-ray bursts and the nu-p process in neutrino-driven winds after a core-collapse supernova. The newly derived stellar rate is systematically lower than previous estimates, suppressing NiCu cycle recycling in X-ray bursts to below 3% and enhancing nu-p process efficiency throughout the relevant temperature range, potentially extending its effective operation to temperatures as high as T9 = 3.94 (+0.99/-0.85).

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arXiv2026

Detailed Study of the $^{59}$Cu(p,$\alpha)^{56}$Ni Reaction and Constraints on Its Astrophysical Reaction Rate

The $^{59}$Cu$(p,\alpha)^{56}$Ni reaction plays an important role in explosive astrophysical scenarios such as Type I X-ray bursts and the $\nu p$-process in neutrino-driven winds following a core-collapse supernova, where it regulates the flow of nucleosynthesis through the NiCu cycle and the synthesis of heavier nuclei. We present a direct measurement of the $^{59}\mathrm{Cu}(p,\alpha)^{56}\mathrm{Ni}$ excitation function from 2.43--5.88~MeV in the center-of-mass frame, performed in inverse kinematics with the high-efficiency MUSIC active-target detector at FRIB. The angle- and energy-integrated cross sections extend direct measurements to lower energies than previously reported and remove the angular-integration model dependence of earlier work. To extrapolate the rate to astrophysical energies, we constrain the statistical-model description through a systematic optimization of the DEM-3 $\alpha$-optical model potential geometry, and quantify the model-selection uncertainty with a Bayesian model averaging analysis over 96 TALYS combinations. The resulting stellar rate carries a temperature-dependent uncertainty factor of 1.26--1.63 over $T_9 = 0.2$--10 and is systematically lower than the REACLIB evaluation, remaining below the competing $(p,\gamma)$ rate for $T_9 \lesssim 3.94$. These results substantially weaken the inferred NiCu cycle strength and establish the $^{59}$Cu$(p,\gamma)^{60}$Zn rate as the dominant remaining uncertainty.

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arXiv2025

First measurement of 87Rb({\alpha}, xn) cross sections at weak r-process energies in supernova {\nu}-driven ejecta to investigate elemental abundances in low-metallicity stars

Observed abundances of Z ~ 40 elements in metal-poor stars vary from star to star, indicating that the rapid and slow neutron capture processes may not contribute alone to the synthesis of elements beyond iron. The weak r-process was proposed to produce Z ~ 40 elements in a subset of old stars. Thought to occur in the {\nu}-driven ejecta of a core-collapse supernova, ({\alpha}, xn) reactions would drive the nuclear flow toward heavier masses at T = 2-5 GK. However, current comparisons between modelled and observed yields do not bring satisfactory insights into the stellar environment, mainly due to the uncertainties of the nuclear physics inputs where the dispersion in a given reaction rate often exceeds one order of magnitude. Involved rates are calculated with the statistical model where the choice of an {\alpha}-optical-model potential ({\alpha}OMP) leads to such a poor precision. The first experiment on 87Rb({\alpha}, xn) reactions at weak r-process energies is reported here. Total inclusive cross sections were assessed at Ec.m. = 8.1 - 13 MeV (3.7 - 7.6 GK) with the active target MUlti-Sampling Ionization Chamber (MUSIC). With a N = 50 seed nucleus, the measured values agree with statistical model estimates using the {\alpha}OMP Atomki-V2. A re-evaluated reaction rate was incorporated into new nucleosynthesis calculations, focusing on {\nu}-driven ejecta conditions known to be sensitive to this specific rate. These conditions were found to fail to reproduce the lighter-heavy element abundances in metal-poor stars.

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arXiv2024

Exploring the effect of positive Q-value neutron transfer in coupled-channels calculations using microscopic nuclear potentials

We investigated the effect of the degree of freedom of neutron transfer on the cross section of heavy-ion fusion reactions, using the relativistic mean-field formalism within the coupled channel approach (CCFULL). We obtain the microscopic nuclear interaction potential in terms of the density distributions for the targets and projectiles with the NL3$^*$ parameter set and corresponding R3Y nucleon-nucleon potential. The present analysis includes the $^{18}$O-induced reactions, for which experimental fusion cross-section is available around the Coulomb barrier. It is evident from the results that including vibrational and/or rotational degrees of freedom enhances the fusion cross-section at energies below the barrier. However, fusion hindrance persists in this energy region. To address this, we incorporated the two-neutron $(2n)$ transfer channels in the Coupled Channel calculation. A comparison with the Woods-Saxon potential (WS) shows that the R3Y nucleon-nucleon (NN) potential, with intrinsic degrees of freedom, is superior to it, especially at energies below the barrier. This superiority can be attributed to the observed higher barrier heights and lower cross-section of the WS potential compared to the relativistic R3Y NN potential for the considered reaction systems. Consequently, we employed the relativistic mean-field formalism to estimate fusion characteristics for the unknown $^{18}$O-induced reactions, namely $^{18}$O + $^{62}${Ni}, $^{18}$O + $^{70,72,76}${Ge}, $^{18}$O + $^{144,150}${Nd}, and $^{18}$O + $^{144,148,152,154}${Sm}. Our analysis highlights the significant role of positive $Q$-value neutron transfer in enhancing the sub-barrier fusion cross-section for the $^{18}$O + $^{148}${Nd} reaction with the R3Y NN potential. However, the effect of this transfer channel for the other considered reactions is comparatively less pronounced.

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arXiv2024

Low energy alpha-nucleus optical potential studied via (a,n) cross section measurements on Te isotopes

In several processes of stellar nucleosynthesis, like the astrophysical gamma-process, nuclear reactions involving alpha particles play an important role. The description of these reactions necessitates the knowledge of the alpha-nucleus optical model potential (AOMP) which is highly ambiguous at low, astrophysical energies. This ambiguity introduces a substantial uncertainty in the stellar models for predicting elemental and isotopic abundances. The experimental study of the AOMP is thus necessary which can be implemented by measuring the cross section of alpha-induced nuclear reactions. At low energies, (a,n) reactions are suitable for such a purpose. Therefore, in the present work, the (a,n) cross sections of four Te isotopes have been measured, mostly for the first time, and compared with theoretical predictions. The (a,n) cross sections of 120,122,124,130Te have been measured in the energy range between about 10 and 17 MeV using the activation method. The detection of the gamma radiation following the decay of the radioactive reaction products were used to determine the cross sections. The measured cross sections are compared with statistical model calculations obtained from the widely used TALYS nuclear reaction simulation code. Predictions using various available AOMPs are investigated. It is found that the recently developed Atomki-V2 AOMP provides the best description for all studied reactions and this potential also reproduces well the total reaction cross sections from elastic scattering experiments, when they are available in literature. We recommend therefore to use the astrophysical reaction rates based on this potential for nucleosynthesis models of heavy elements.

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arXiv2024

Direct cross-section measurement of the weak r-process 88Sr(α,n)91Zr reaction in ν-driven winds of core collapse supernovae

About half of the heavy elements beyond iron are known to be produced by the rapid neutron capture process, known as r-process. However, the astrophysical site producing the r-process is still uncertain. Chemical abundances observed in several cosmic sites indicate that different mechanisms should be at play. For instance, the abundances around silver measured in a subset of metal-poor stars indicate the presence of a weak r-process. This process may be active in neutrino-driven winds of core collapse supernovae where ($α$,n) reactions dominate the synthesis of Z ~ 40 elements in the expelled materials. Scarcely measured, the rates of ($α$,n) reactions are determined from statistical Hauser-Feshbach calculations with $α$-optical-model potentials, which are still poorly constrained. The uncertainties of the ($α$,n) reaction rates therefore make a significant contribution to the uncertainties of the abundances determined from stellar modeling. In this work, the $^{88}$Sr($α$,n)$^{91}$Zr reaction which impacts the weak r-process abundances has been probed at astrophysics energy for the first time; directly measuring the total cross sections at astrophysical energies of 8.37 - 13.09 MeV in the center of mass (3.8 - 7.5 GK). Two measurements were performed at ATLAS with the electrically-segmented ionization chamber MUSIC, in inverse kinematics, while following the active target technique. The cross sections of this $α$-induced reaction on $^{88}$Sr, located at the shell closure N = 50, have been found to be lower than expected, by a factor of 3, despite recent statistical calculations validated by measurements on neighboring nuclei. This result encourages more experimental investigations of ($α$,n) reactions, at N = 50 and towards the neutron-rich side, to further test the predictive power and reliability of such calculations.

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arXiv2023

The $γ$-process nucleosynthesis in core-collapse supernovae. I. A novel analysis of $γ$-process yields in massive stars

The $γ$-process nucleosynthesis in core-collapse supernovae is generally accepted as a feasible process for the synthesis of neutron-deficient isotopes beyond iron. However, crucial discrepancies between theory and observations still exist: the average production of $γ$-process yields from massive stars are too low to reproduce the solar distribution in galactic chemical evolution calculations, and the yields of the Mo and Ru isotopes are by a further factor of 10 lower than the yields of the other $γ$-process nuclei. We investigate the $γ$-process in 5 sets of core-collapse supernova models published in literature with initial masses 15, 20, and 25 M$_{\odot}$ at solar metallicity. We compared the $γ$-process overproduction factors from the different models. To highlight the possible effect of nuclear physics input, we also considered 23 ratios of two isotopes close to each other in mass, relative to their solar values. Further, we investigated the contribution of C-O shell mergers in the supernova progenitors as an additional site of the $γ$-process. Our analysis shows that a large scatter among the different models exists for both the $γ$-process integrated yields and the isotopic ratios. We found only 10 ratios that agree with their solar values, all the others differ by at least a factor of 3 from the solar values in all the considered sets of models. The $γ$-process within C-O shell mergers mostly influence the isotopic ratios that involve intermediate and heavy proton-rich isotopes with $\rm A>100$.

astro-ph.SR↗

arXiv2023

Cross Section Measurement of the $^{82}$Kr(p,$γ$)$^{83}$Rb Reaction in Inverse Kinematics

The total cross section of the $^{82}$Kr(p,$γ$)$^{83}$Rb reaction was measured for the first time at effective center-of-mass energies between 2.4 and 3.0 MeV, within the relevant Gamow window for the astrophysical $γ$ process. The experiment took place at the National Superconducting Cyclotron Laboratory at Michigan State University using the ReA facility. A $^{82}$Kr beam was directed onto a hydrogen gas cell located at the center of the Summing NaI(Tl) (SuN) detector. The obtained spectra were analyzed using the $γ$-summing technique and the extracted cross section was compared to standard statistical model calculations using the \textsc{non-smoker} and \textsc{talys} codes. The comparison indicates that standard statistical model calculations tend to overproduce the cross section of the $^{82}$Kr(p,$γ$)$^{83}$Rb reaction relative to the experimentally measured values. Furthermore, the experimental data was used to provide additional constraints on the nuclear level density and $γ$-ray strength function used in the statistical model calculations.

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arXiv2023

Cross section measurement of the 144Sm(alpha,n)147Gd reaction for studying the alpha-nucleus optical potential at astrophysical energies

Nuclear reactions involving alpha particles play an important role in various astrophysical processes such as the gamma-process of heavy element nucleosynthesis. The poorly known low-energy alpha-nucleus optical (AOMP) potential is a key parameter to estimate the rates of these reactions. The AOMP can be tested by measuring the cross section of alpha-scattering as well as alpha-induced reactions. Low energy elastic alpha-scattering on 144Sm has recently been measured with high precision. The aim of the present work was to complement that work by measuring the (a,n) cross sections on 144Sm at low energies. The experimental data shall be used to constrain the AOMP. From this potential the 144Sm(a,g)148Gd reaction rate can be derived with reduced uncertainties. The 144Sm(a,n)147Gd reaction was studied by bombarding Sm targets with alpha-beams provided by the cyclotron accelerator of Atomki. The cross section was determined using the activation method. The gamma-radiation following the decay of the 147Gd reaction product was measured with a HPGe detector. The experimental data are analyzed within the statistical model. The cross section was measured in the alpha-energy range between 13 and 20 MeV in 1 MeV steps, i.e., from close above the (a,n) threshold. The results were compared with statistical model calculations using various approaches and parametrizations for the AOMP, and excellent agreement was obtained for two recent potentials. However, these potentials cannot reproduce literature data for the 144Sm(a,g)148Gd reaction with the same accuracy. Constraints for the AOMP were derived from an analysis of the new 144Sm(a,n)147Gd data and literature data for 144Sm(a,g)148Gd. These constraints enable a determination of the reaction rate of the 144Sm(a,g)148Gd reaction with significantly reduced uncertainties of less than a factor of two.

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arXiv2022

Study of the $^{22}$Mg waiting point relevant for x-ray burst nucleosynthesis via the $^{22}$Mg($α$,$p$)$^{25}$Al reaction

The $^{22}$Mg($α$,$p$)$^{25}$Al reaction rate has been identified as a major source of uncertainty for understanding the nucleosynthesis flow in Type-I x-ray bursts (XRBs). We report a direct measurement of the energy- and angle-integrated cross sections of this reaction in a 3.3-6.9 MeV center-of-mass energy range using the MUlti-Sampling Ionization Chamber (MUSIC). The new $^{22}$Mg($α$,$p$)$^{25}$Al reaction rate is a factor of $\sim$4 higher than the previous direct measurement of this reaction within temperatures relevant for XRBs, resulting in the $^{22}$Mg waiting point of x-ray burst nucleosynthesis flow to be significantly bypassed via the ($α,p$) reaction

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arXiv2022

First direct measurement of the $^{13}$N($α$,$p$)$^{16}$O reaction relevant for core-collapse supernovae nucleosynthesis

Understanding the explosion mechanism of a core-collapse supernova (CCSN) is important to accurately model CCSN scenarios for different progenitor stars using model-observation comparisons. The uncertainties of various nuclear reaction rates relevant for CCSN scenarios strongly affect the accuracy of these stellar models. Out of these reactions, the $^{13}$N($α$,$p$)$^{16}$O reaction has been found to affect various stages of a CCSN at varying temperatures. This work presents the first direct measurement of the $^{13}$N($α$,$p$)$^{16}$O reaction performed using a 34.6 MeV beam of radioactive $^{13}$N ions and the active-target detector MUSIC (MUlti-Sampling Ionization Chamber) at Argonne National Laboratory. The resulting total $^{13}$N($α$,$p$)$^{16}$O reaction cross sections from this measurement in the center-of-mass energy range of 3.26 - 6.02 MeV are presented and compared with calculations using the Hauser-Feshbach formalism. Uncertainties in the reaction rate have been dramatically reduced at CCSN temperatures.

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arXiv2022

Constraining nucleosynthesis in neutrino-driven winds: observations, simulations and nuclear physics

A promising astrophysical site to produce the lighter heavy elements of the first $r$-process peak ($Z = 38-47$) is the moderately neutron rich ($0.4 < Y_e < 0.5$) neutrino-driven ejecta of explosive environments, such as core-collapse supernovae and neutron star mergers, where the weak $r$-process operates. This nucleosynthesis exhibits uncertainties from the absence of experimental data from $(α,xn)$ reactions on neutron-rich nuclei, which are currently based on statistical model estimates. In this work, we report on a new study of the nuclear reaction impact using a Monte Carlo approach and improved $(α,xn)$ rates based on the Atomki-V2 $α$ Optical Model Potential ($α$OMP). We compare our results with observations from an up-to-date list of metal-poor stars with [Fe/H] $<$ -1.5 to find conditions of the neutrino-driven wind where the lighter heavy elements can be synthesized. We identified a list of $(α,xn)$ reaction rates that affect key elemental ratios in different astrophysical conditions. Our study aims on motivating more nuclear physics experiments on $(α, xn)$ reactions using current and the new generation of radioactive beam facilities and also more observational studies of metal-poor stars.

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arXiv2021

The $^{59}$Fe(n, γ)$^{60}$Fe Cross Section from the Surrogate Ratio Method and Its Effect on the $^{60}$Fe Nucleosynthesis

The long-lived $^{60}$Fe (with a half-life of 2.62 Myr) is a crucial diagnostic of active nucleosynthesis in the Milky Way galaxy and in supernovae near the solar system. The neutron-capture reaction $^{59}$Fe(n,$γ$)$^{60}$Fe on $^{59}$Fe (half-life = 44.5 days) is the key reaction for the production of $^{60}$Fe in massive stars. This reaction cross section has been previously constrained by the Coulomb dissociation experiment, which offered partial constraint on the $E$1 $γ$-ray strength function but a negligible constraint on the $M$1 and $E$2 components. In this work, for the first time, we use the surrogate ratio method to experimentally determine the $^{59}$Fe(n,$γ$)$^{60}$Fe cross sections in which all the components are included. We derived a Maxwellian-averaged cross section of 27.5 $\pm$ 3.5 mb at $kT$= 30 keV and 13.4 $\pm$ 1.7 mb at $kT$= 90 keV, roughly 10 - 20% higher than previous estimates. We analyzed the impact of our new reaction rates in nucleosynthesis models of massive stars and found that uncertainties in the production of $^{60}$Fe from the $^{59}$Fe(n,$γ$)$^{60}$Fe rate are at most of 25%. We conclude that stellar physics uncertainties now play a major role in the accurate evaluation of the stellar production of $^{60}$Fe.

astro-ph.GA↗

arXiv2021

First direct measurement of $^{59}$Cu(p,$α$)$^{56}$Ni: A step towards constraining the Ni-Cu cycle in the Cosmos

Reactions on the proton-rich nuclides drive the nucleosynthesis in Core-Collapse Supernovae (CCSNe) and in X-ray bursts (XRBs). CCSNe eject the nucleosynthesis products to the interstellar medium and hence are a potential inventory of p-nuclei, whereas in XRBs nucleosynthesis powers the light curves. In both astrophysical sites the Ni-Cu cycle, which features a competition between $^{59}$Cu(p,$α$)$^{56}$Ni and $^{59}$Cu(p,$γ$)$^{60}$Zn, could potentially halt the production of heavier elements. Here, we report the first direct measurement of $^{59}$Cu(p,$α$)$^{56}$Ni using a re-accelerated $^{59}$Cu beam and cryogenic solid hydrogen target. Our results show that the reaction proceeds predominantly to the ground state of $^{56}$Ni and the experimental rate has been found to be lower than Hauser-Feshbach-based statistical predictions. New results hint that the $νp$-process could operate at higher temperatures than previously inferred and therefore remains a viable site for synthesizing the heavier elements.

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arXiv2021

Low Energy measurement of the $^{96}\mathrm{Zr}(α,n)^{99}\mathrm{Mo}$ reaction cross section and its impact on weak r-process nucleosynthesis

Lighter heavy elements beyond iron and up to around silver can form in neutrino-driven ejecta in core-collapse supernovae and neutron star mergers. Slightly neutron-rich conditions favour a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion time scales, and ($α$,n) reactions become critical to move matter towards heavier nuclei. The rates of these reactions are calculated with the statistical model and their main uncertainty, at energies relevant for the weak r-process, is the $α$+nucleus optical potential. There are several sets of parameters to calculate the $α$+nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude. Recently the $^{96}$Zr($α$,n)$^{99}$Mo reaction has been identified as a key reaction that impacts the production of elements from Ru to Cd. Here, we present the first cross section measurement of this reaction at energies (6.22 MeV $\leq$ E$_\mathrm{c.m.}$ $\leq$ 12.47 MeV) relevant for the weak r-process. The new data provide a stringent test of various model predictions which is necessary to improve the precision of the weak r-process network calculations. The strongly reduced reaction rate uncertainty leads to very well-constrained nucleosynthesis yields for $Z = 44 - 48$ isotopes under different neutrino-driven wind conditions.

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arXiv2020

Successful prediction of total $α$-induced reaction cross sections at astrophysically relevant sub-Coulomb energies using a novel approach

The prediction of stellar ($γ$,$α$) reaction rates for heavy nuclei is based on the calculation of ($α$,$γ$) cross sections at sub-Coulomb energies. These rates are essential for modeling the nucleosynthesis of so-called $p$-nuclei. The standard calculations in the statistical model show a dramatic sensitivity to the chosen $α$-nucleus potential. The present study explains the reason for this dramatic sensitivity which results from the tail of the imaginary $α$-nucleus potential in the underlying optical model calculation of the total reaction cross section. As an alternative to the optical model, a simple barrier transmission model is suggested. It is shown that this simple model in combination with a well-chosen $α$-nucleus potential is able to predict total $α$-induced reaction cross sections for a wide range of heavy target nuclei above $A \gtrsim 150$ with uncertainties below a factor of two. The new predictions from the simple model do not require any adjustment of parameters to experimental reaction cross sections whereas in previous statistical model calculations all predictions remained very uncertain because the parameters of the $α$-nucleus potential had to be adjusted to experimental data. The new model allows to predict the reaction rate of the astrophysically important $^{176}$W($α$,$γ$)$^{180}$Os reaction with reduced uncertainties, leading to a significantly lower reaction rate at low temperatures. The new approach could also be validated for a broad range of target nuclei from $A \approx 60$ up to $A \gtrsim 200$.

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arXiv2019

Cross section of $α$-induced reactions on $^{197}$Au at sub-Coulomb energies

Statistical model calculations have to be used for the determination of reaction rates in large-scale reaction networks for heavy-element nucleosynthesis. A basic ingredient of such a calculation is the a-nucleus optical model potential. Several different parameter sets are available in literature, but their predictions of a-induced reaction rates vary widely, sometimes even exceeding one order of magnitude. This paper presents the result of a-induced reaction cross-section measurements on gold which could be carried out for the first time very close to the astrophysically relevant energy region. The new experimental data are used to test statistical model predictions and to constrain the a-nucleus optical model potential. For the measurements the activation technique was used. The cross section of the (a,n) and (a,2n) reactions was determined from g-ray counting, while that of the radiative capture was determined via X-ray counting. The cross section of the reactions was measured below E$_a=20.0$~MeV. In the case of the $^{197}$Au(a,2n)$^{199}$Tl reaction down to 17.5~MeV with 0.5-MeV steps, reaching closer to the reaction threshold than ever before. The cross section of $^{197}$Au(a,n)$^{200}$Tl and $^{197}$Au(a,g)$^{201}$Tl was measured down to E$_a=13.6$ and 14.0~MeV, respectively, with 0.5-MeV steps above the (a,2n) reaction threshold and with 1.0-MeV steps below that. The new dataset is in agreement with the available values from the literature, but is more precise and extends towards lower energies. Two orders of magnitude lower cross sections were successfully measured than in previous experiments which used g-ray counting only, thus providing experimental data at lower energies than ever before. The new precision dataset allows us to find the best-fit a-nucleus optical model potential and to predict cross sections in the Gamow window with smaller uncertainties.

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