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P. Mohr

Publications and source records attributed to P. Mohr.

At least 73 records · Page 4Linked to original sources
arXiv2003

Low-Energy Direct Capture in the 8Li(n,gamma)9Li and 8B(p,gamma)9C Reactions

The cross sections of the 8Li(n,gamma)9Li and 8B(p,gamma)9C capture reactions have been analyzed using the direct capture model. At low energies which is the astrophysically relevant region the capture process is dominated by E1 transitions from incoming s-waves to bound p-states. The cross sections of both mirror reactions can be described simultaneously with consistent potential parameters, whereas previous calculations have overestimated the capture cross sections significantly. However, the parameters of the potential have to be chosen very carefully because the calculated cross section of the 8Li(n,gamma)9Li reaction depends sensitively on the potential strength.

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arXiv2002

The $^{15}$N($\bmα$,$\bmγ$)$^{19}$F reaction and nucleosynthesis of $^{19}$F

Several resonances in the $^{15}$N($α$,$γ$)$^{19}$F reaction have been investigated in the energy range between 0.6 MeV and 2.7 MeV. Resonance strengths and branching ratios have been determined. High sensitivity could be obtained by the combination of the {\sc{dynamitron}} high current accelerator, the windowless gas target system {\sc{rhinoceros}}, and actively shielded germanium detectors. Two levels of $^{19}$F could be observed for the first time in the ($α$,$γ$) channel, and several weak branchings below the detection limits of previous experiments were measured. Two observed resonances correspond to $α$-cluster states in $^{19}$F which have been assigned unambiguously. The astrophysical reaction rate is derived from this set of resonance strengths.

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arXiv2002

The s-process branching at 185W

The neutron capture cross section of the unstable nucleus 185W has been derived from experimental photoactivation data of the inverse reaction 186W(gamma,n)185W. The new result of sigma = (687 +- 110) mbarn confirms the theoretically predicted neutron capture cross section of 185W of sigma = 700 mbarn at kT = 30 keV. A neutron density in the classical s-process of n_n = (3.8 +0.9 -0.8} * 1e8 cm-3 is derived from the new data for the 185W branching. In a stellar s-process model one finds a significant overproduction of the residual s-only nucleus 186Os.

astro-ph↗
arXiv2002

Alpha-decay Rates of Yb and Gd in Solar Neutrino Detectors

The $α$-decay rates for the nuclides $^{168,170,171,172,173,174,176}$Yb and $^{148,150,152,154}$Gd have been estimated from transmission probabilities in a systematic $α$-nucleus potential and from an improved fit to $α$-decay rates in the rare-earth mass region. Whereas $α$-decay of $^{152}$Gd in natural gadolinium is a severe obstacle for the use of gadolinium as a low-energy solar-neutrino detector, we show that $α$-decay does not contribute significantly to the background in a ytterbium detector. An extremely long $α$-decay lifetime of $^{168}$Yb is obtained from calculation, which may be close to the sensitivity limit in a low-background solar neutrino detector.

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arXiv2001

$^{92}$Mo($α,α$)$^{92}$Mo scattering,the $^{92}$Mo--$α$ optical potential, and the $^{96}$Ru($γ,α$)$^{92}$Mo reaction rate at astrophysically relevant energies

The elastic scattering cross section of$^{92}$Mo($α$,$α$)$^{92}$Mo has been measured at energies of $E_{\rm{c.m.}} \approx$ 13, 16, and 19 MeV in a wide angular range. The real and imaginary parts of the optical potential for the system $^{92}$Mo - $α$ have been derived at energies around and below the Coulomb barrier. The result fits into the systematic behavior of $α$-nucleus folding potentials. The astrophysically relevant $^{96}$Ru($γ$,$α$)$^{92}$Mo reaction rates at $T_9=2.0$ and $T_9=3.0$ could be determined to an accuracy of about 16 % and are compared to previously published theoretical rates.

astro-ph↗
arXiv2001

Measurement of the (gamma,n) reaction rates of the nuclides 190Pt, 192Pt, and 198Pt in the astrophysical gamma-process

The nucleosynthesis of heavy neutron-deficient nuclei in a stellar photon bath at the temperatures relevant for the astrophysical gamma process was investigated. In order to derive (gamma,n) cross sections and reaction rates, the stellar photon bath was simulated by the superposition of several bremsstrahlung spectra with different endpoint energies. As a first test for this method, the (gamma,n) reaction rates of the platinum isotopes 190Pt, 192Pt, and 198Pt were derived. The results are compared to other experimental data and theoretical calculations.

astro-ph↗
arXiv1999

Measurement of neutron capture on 50Ti at thermonuclear energies

At the Karlsruhe and Tuebingen 3.75 MV Van de Graaff accelerators the thermonuclear 50Ti(n,gamma)51Ti(5.8 min) cross section was measured by the fast cyclic activation technique via the 320.852 and 928.65 keV gamma-ray lines of the 51Ti-decay. Metallic Ti samples of natural isotopic composition and samples of TiO2 enriched in 50Ti by 67.53 % were irradiated between two gold foils which served as capture standards. The capture cross-section was measured at the neutron energies 25, 30, 52, and 145 keV, respectively. The direct capture cross section was determined to be 0.387 +/- 0.011 mbarn at 30 keV. We found evidence for a bound state s-wave resonance with an estimated radiative width of 0.34 eV which destructively interfers with direct capture. The strength of a suggested s-wave resonance at 146.8 keV was determined. The present data served to calculate, in addition to the directly measured Maxwellian averaged capture cross sections at 25 and 52 keV, an improved stellar 50Ti(n,gamma)51Ti rate in the thermonuclear energy region from 1 to 250 keV. The new stellar rate leads at low temperatures to much higher values than the previously recommended rate, e.g., at kT=8 keV the increase amounts to about 50 %. The new reaction rate therefore reduces the abundance of 50Ti due to s-processing in AGB stars.

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arXiv1998

Absolute measurement of the 5/2+ resonance of 36Ar(p,gamma)37K at E_p = 918 keV

The strength of the 5/2+ resonance of the reaction 36Ar(p,gamma)37K at E_p = 918 keV was measured absolutely: omega gamma = (258 \pm 24) meV. The new result is consistent with previous results. This resonance was used in several experiments as a calibration standard. Therefore we discuss our experimental uncertainties in detail which is important for the reliability of previous experiments and our new result.

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arXiv1998

Neutron capture of 26Mg at thermonuclear energies

The neutron capture cross section of 26Mg was measured relative to the known gold cross section at thermonuclear energies using the fast cyclic activation technique. The experiment was performed at the 3.75 MV Van-de-Graaff accelerator, Forschungszentrum Karlsruhe. The experimental capture cross section is the sum of resonant and direct contributions. For the resonance at E(n,lab) = 220 keV our new results are in disagreement with the data from Weigmann et al. An improved Maxwellian averaged capture cross section is derived from the new experimental data taking into account s- and p-wave capture and resonant contributions. The properties of so-called potential resonances which influence the p-wave neutron capture of 26}Mg are discussed in detail.

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arXiv1997

Measurement and analysis of neutron capture reaction rates of light neutron-rich nuclei

Several neutron capture cross sections of light neutron-rich nuclei were measured in the astrophysically relevant energy region of 5 to 200 keV. The experimental data are compared to calculations using the direct capture model. The results are used for the calculation of neutron capture cross sections of unstable isotopes. Furthermore, neutron sources with energies below E_n \approx 10 keV are discussed.

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arXiv1997

Direct neutron capture of 48Ca at kT = 52 keV

The neutron capture cross section of 48Ca was measured relative to the known gold cross section at kT = 52 keV using the fast cyclic activation technique. The experiment was performed at the Van-de-Graaff accelerator, Universitaet Tuebingen. The new experimental result is in good agreement with a calculation using the direct capture model. The 1/v behaviour of the capture cross section at thermonuclear energies is confirmed, and the adopted reaction rate which is based on several previous experimental investigations remains unchanged.

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arXiv1997

Astrophysical neutron capture rates in s- and r-process nucleosynthesis

Astrophysical neutron capture rates of light and heavy nuclei have been measured and calculated. For the measurements the activation technique was applied at the 3.75 MV Karlsruhe Van de Graaff accelerator, and at the Geel electron linear accelerator (GELINA) the time-of-flight (TOF) method was used. The calculations were performed using direct and compound nuclear capture models.

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arXiv1997

Comparison of low--energy resonances in 15N(alpha,gamma)19F and 15O(alpha,gamma)19Ne and related uncertainties

A disagreement between two determinations of Gamma_alpha of the astro- physically relevant level at E_x=4.378 MeV in 19F has been stated in two recent papers by Wilmes et al. and de Oliveira et al. In this work the uncertainties of both papers are discussed in detail, and we adopt the value Gamma_alpha=(1.5^{+1.5}_{-0.8})10^-9eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei Gamma_gamma(19F) approx Gamma_gamma(19Ne), theta^2_alpha(19F) approx theta^2_alpha(19Ne) are discussed, together with the resulting uncertainties on the resonance strengths in 19Ne and on the 15O(alpha,gamma)19Ne rate.

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arXiv1996

Spectroscopic factors for bound s-wave states derived from neutron scattering lengths

A simple and model-independent method is described to derive neutron single-particle spectroscopic factors of bound s-wave states in $^{A+1}Z = ^{A}Z \otimes n$ nuclei from neutron scattering lengths. Spectroscopic factors for the nuclei ^{13}C, ^{14}C, ^{16}N, ^{17}O, ^{19}O, ^{23}Ne, ^{37}Ar, and ^{41}Ar are compared to results derived from transfer experiments using the well-known DWBA analysis and to shell model calculations. The scattering length of ^{14}C is calculated from the ^{15}C_{g.s.} spectroscopic factor.

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arXiv1996

The ^{144}Sm-αoptical potential at astrophysically relevant energies derived from ^{144}Sm(α,α)^{144}Sm elastic scattering

For the determination of the $^{144}Sm-α$ optical potential we measured the angular distribution of $^{144}Sm(α,α)^{144}Sm$ scattering at the energy $E_{lab} = 20 MeV$ with high accuracy. Using the known systematics of $α$-nucleus optical potentials we are able to derive the $^{144}Sm-α$ optical potential at the astrophysically relevant energy $E_{c.m.} = 9.5 MeV$ with very limited uncertainties.

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arXiv1996

Astrophysically Relevant Neutron Capture near the Border of Stability

The neutron capture cross section on $^{26}$Mg was measured in the astrophysically relevant energy region from 25 keV to 220 keV. The experimental results agree well with a calculation using the Direct Capture (DC) model together with systematic folding potentials. This experiment confirms for the $^{26}$Mg(n,$γ$)$^{27}$Mg reaction that the DC process is at least comparable to the Compound--Nucleus (CN) process. The reliability of DC calculations is discussed, and we present some ideas for future experiments which could reduce the theoretical uncertainties of DC calculations.

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