arXiv subjects

Peter Mohr

Publications and source records attributed to Peter Mohr.

30 records · Page 2Linked to original sources

arXiv2015

Broad levels in $^{17}$O and their relevance for the astrophysical s-process

Levels in $^{17}$O affect the astrophysical s-process in two opposite ways. The neutron production is enhanced by resonances in the $^{13}$C($α$,$n$)$^{16}$O reaction at excitation energies around 7 MeV in $^{17}$O, and the number of available neutrons is reduced by low-lying resonances in the $^{16}$O($n$,$γ$)$^{17}$O reaction corresponding to levels in $^{17}$O with excitation energies of $4-5$ MeV. The present work uses the $^{19}$F($d$,$α$)$^{17}$O reaction to determine absolute widths of the relevant levels in $^{17}$O. The results improve the uncertainties of the previously adopted values and resolve a discrepancy between recent studies for the $1/2^+$ level close to the threshold of the $^{13}$C($α$,$n$)$^{16}$O reaction. In addition, improved excitation energies and widths are provided for several states in $^{17}$O up to excitation energies close to 8 MeV.

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arXiv2015

Addendum to "Determination of $γ$-ray widths in $^{15}$N using nuclear resonance fluorescence''

The determination of absolute widths of two observed levels above the proton threshold in $^{15}$N has been improved by a combined analysis of our recent $^{15}$N($γ$,$γ'$)$^{15}$N$^\ast$ photon scattering data, resonance strengths $ωγ$ of the $^{14}$C($p$,$γ$)$^{15}$N reaction, and $γ$-ray branchings $b_{γ,i}$ in $^{15}$N. The revised data are compared to the adopted values, and some inconsistencies in the adopted values are illustrated.

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arXiv2015

Cross sections of $α$-induced reactions for targets with masses $A \approx 20-50$ at low energies

A simple reduction scheme using so-called reduced energies $E_{\rm{red}}$ and reduced cross sections $σ_{\rm{red}}$ allows the comparison of heavy-ion induced reaction cross sections for a broad range of masses of projectile and target and over a wide energy range. A global behavior has been found for strongly bound projectiles whereas much larger reduced cross sections have been observed for weakly bound and halo projectiles. It has been shown that this simple reduction scheme works also well for $α$-particle induced reactions on heavy target nuclei, but very recently significant deviations have been seen for $α$+$^{33}$S and $α$+$^{23}$Na. Motivated by these unexpected discrepancies, the present study analyses $α$-induced reaction cross sections for targets with masses $A \approx 20-50$. The study shows that the experimental data for $α$-induced reactions on nuclei with $A \approx 20-50$ deviate slightly from the global behavior of reduced cross sections. However, in general the deviations evolve smoothly towards lower masses. The only significant outliers are the recent data for $^{33}$S and $^{23}$Na which are far above the general systematics, and some very old data may indicate that $^{36}$Ar and $^{40}$Ar are below the general trend. As expected, also the doubly-magic $^{40}$Ca nucleus lies slightly below the results for its neighboring nuclei. Overall, the experimental data are nicely reproduced by a statistical model calculation utilizing the simple $α$-nucleus potential by McFadden and Satchler. Simultaneously with the deviation of reduced cross sections $σ_{\rm{red}}$ from the general behavior, the outliers $^{23}$Na, $^{33}$S, $^{36}$Ar, and $^{40}$Ar also show significant disagreement between experiment and statistical model calculation.

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arXiv2014

Unexpected properties of the $^{33}$S($α$,p)$^{36}$Cl reaction cross section at low energies

New experimental data for the $^{33}$S($α$,p)$^{36}$Cl reaction show a very unusual energy dependence. Contrary to common findings for many other $α$-induced reactions, statistical model calculations underestimate the measured cross sections at very low energies. The relatively huge cross sections at these low energies require a significant amount of single-particle strength in the measured energy range which exceeds by far 100% as soon as the additional strength from the competing $^{33}$S($α$,n)$^{36}$Ar reaction is taken into account. In addition, the new data deviate from a general trend for the energy dependence of $α$-induced reaction cross sections.

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arXiv2013

Total reaction cross section $σ_{\rm{reac}}$ of $α$-induced reactions from elastic scattering: the example $^{140}$Ce($α$,$α$)$^{140}$Ce

Angular distributions of elastic $^{140}$Ce($α$,$α$)$^{140}$Ce scattering are analyzed in the framework of the optical model from low energies around the Coulomb barrier up to about 40\,MeV. From the local fits the total reaction cross section $σ_{\rm{reac}}$ is extracted. This procedure requires experimental scattering data with small uncertainties. The results for $σ_{\rm{reac}}$ are compared to predictions of different systematic global potentials. It is found that the total reaction cross section $σ_{\rm{reac}}$ is well predicted from all global potentials under study although the reproduction of the angular distributions is not perfect in all cases. In addition, the lower energy limit for the extraction of $σ_{\rm{reac}}$ from elastic scattering angular distributions is analyzed. Finally, the potentials under study are used to calculate the $^{143}$Nd(n,$α$)$^{140}$Ce cross section, and the predictions are compared to experimental data.

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arXiv2012

Direct capture in the $^{130}$Sn(n,$γ$)$^{131}$Sn and $^{132}$Sn(n,$γ$)$^{133}$Sn reactions under $r$-process conditions

The cross sections of the $^{130}$Sn(n,$γ$)$^{131}$Sn and $^{132}$Sn(n,$γ$)$^{133}$Sn reactions are calculated in the direct capture model at low energies below 1.5\,MeV. Using recent data from (d,p) transfer experiments on $^{130}$Sn and $^{132}$Sn, it is possible to avoid global input parameters with their inherent uncertainties and to determine all input to the direct capture model by local adjustments. The calculated direct capture cross sections of $^{130}$Sn and $^{132}$Sn are almost identical and have uncertainties of less than a factor of two. The stellar reaction rates $N_A < σv >$ show a slight increase with temperature. Finally an estimate for the influence of low-lying resonances to the stellar reaction rates is given.

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arXiv2011

Total reaction cross sections from $^{141}$141Pr($α$,$α$)$^{141}$Pr elastic scattering and $α$-induced reaction cross sections at low energies

Elastic scattering data for $^{141}$Pr($α$,$α$)$^{141}$Pr have been analyzed to derive a new energy-dependent local potential for the $^{141}$Pr-$α$ system. This potential is used successfully to predict the cross section of the $^{141}$Pr($α$,n)$^{144}$Pm reaction at low energies where new experimental data have become available very recently. Contrary to various global potentials, this new potential is able to reproduce simultaneously elastic scattering data around and above the Coulomb barrier and reaction data below the Coulomb barrier for the $^{141}$Pr-$α$ system. Reasons for the partial failure of the global potentials are explained by intrinsic properties of the scattering matrix and their variation with energy. The new local potential may become the basis for the construction of a new global $α$-nucleus potential.

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arXiv2008

alpha-cluster states in intermediate mass nuclei

Properties of intermediate mass nuclei have been investigated within the framework of the alpha-cluster model in combination with systematic double-folding potentials. Previously, this alpha-cluster model has been widely applied to light nuclei, in particular to 8Be = alpha \otimes alpha, 20Ne = 16O \otimes alpha, and 44Ti = 40Ca \otimes alpha, and to heavy nuclei, in particular to 212Po = 208Pb \otimes alpha. In the present work a wide range of nuclei is investigated with the magic neutron number N = 50 in the mass range around A \approx 80 - 100: (A+4,N=52) = (A,N=50) \otimes alpha. It is found that excitation energies, decay properties, and transition strengths can be described successfully within this model. The smooth and small variation of the underlying parameters of the alpha-nucleus potential may be used for extrapolations to predict experimentally unknown properties in the nuclei under study.

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arXiv2007

Super-allowed alpha-decay above doubly-magic 100Sn and properties of 104Te = 100Sn otimes alpha

Alpha-decay half-lives for 104,105,106Te and 108,109,110Xe close above the doubly-magic 100Sn are calculated from systematic double-folding potentials. The derived alpha preformation factors are compared to results for 212,213,214Po and 216,217,218Rn above the doubly-magic 208Pb. Alpha-decay energies of Q_alpha = 5.42 \pm 0.07 MeV and 4.65 \pm 0.15 MeV are predicted for 104Te and 108Xe; the corresponding half-lives are T_{1/2} approx 5 ns for 104Te and of the order of 60 μs for 108Xe. Additionally, the properties of rotational bands in 104Te are analyzed, and the first excited 2+ state in 104Te is predicted at E_x = 650 \pm 40 keV; it decays preferentially by gamma emission with a reduced transition strength of 10 Weisskopf units to the ground state of 104Te and with a minor branch by alpha emission to the ground state of 100Sn.

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arXiv2006

alpha-nucleus potentials, alpha-decay half-lives, and shell closures for superheavy nuclei

Systematic alpha-nucleus folding potentials are used to analyze alpha-decay half-lives of superheavy nuclei. Preformation factors of about several per cent are found for all nuclei under study. The systematic behavior of the preformation factors and the volume integrals of the potentials allows to predict alpha-decay energies and half-lives for unknown nuclei. Shell closures can be determined from measured alpha-decay energies using the discontinuity of the volume integral at shell closures. For the first time a double shell closure is predicted for Z_magic = 132, N_magic = 194, and A_magic = 326 from the systematics of folding potentials. The calculated alpha-decay half-lives remain far below one nanosecond for superheavy nuclei with double shell closure and masses above A > 300 independent of the precise knowledge of the magic proton and neutron numbers.

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arXiv2004

Photon-induced Reactions in Stars and in the Laboratory: A Critical Comparison

Photon-induced reactions during the astrophysical p- (or gamma-) process occur at typical temperatures of 1.8 < T9 < 3.3. Experimental data of (gamma,n), (gamma,p), or (gamma,alpha) reactions - if available in the relevant energy region - cannot be used directly to measure astrophysical (gamma,n), (gamma,p), or (gamma,alpha) reaction rates because of the thermal excitation of target nuclei at these high temperatures. Usually, statistical model calculations are used to predict photon-induced reaction rates. The relations between experimental reaction cross sections, theoretical predictions, and astrophysical reaction rates will be critically discussed.

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arXiv2000

alpha-nucleus potentials for the neutron-deficient p nuclei

alpha-nucleus potentials are one important ingredient for the understanding of the nucleosynthesis of heavy neutron-deficient p nuclei in the astrophysical gamma-process where these p nuclei are produced by a series of (gamma,n), (gamma,p), and (gamma,alpha) reactions. I present an improved alpha-nucleus potential at the astrophysically relevant sub-Coulomb energies which is derived from the analysis of alpha decay data and from a previously established systematic behavior of double-folding potentials.

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