G. G. Kiss

A Novel Approach to Measure the Cross Section of the 18O(p, α)15N Resonant Reaction in the 0-200 keV Energy Range

The 18O(p, ?)15N reaction is of primary importance to pin down the uncertainties, due to nuclear physics input, affecting present-day models of asymptotic giant branch stars. Its reaction rate can modify both fluorine nucleosynthesis inside such stars and oxygen and nitrogen isotopic ratios, which allow one to constrain the proposed astrophysical scenarios. Thus, an indirect measurement of the low-energy region of the 18O(p, ?)15N reaction has been performed to access, for the first time, the range of relevance for astrophysical application. In particular, a full, high-accuracy spectroscopic study of the 20 and 90 keV resonances has been performed and the strengths deduced to evaluate the reaction rate and the consequences for astrophysics.

New measurement of the 11B(p,α0)8Be bare-nucleus S(E) factor via the Trojan horse method

A new measurement of the 11B(p,?0)8Be has been performed applying the Trojan horse method (THM) to the 2H(11B,?80Be)n quasi-free reaction induced at a laboratory energy of 27 MeV. The astrophysical S(E) factor has been extracted from ?600 keV down to zero energy by means of an improved data analysis technique and it has been compared with direct data available in the literature. The range investigated here overlaps with the energy region of the light element LiBeB stellar burning and with that of future aneutronic fusion power plants using the 11B+p fuel cycle. The new investigation described here confirms the preliminary results obtained in the recent TH works. The origin of the discrepancy between the direct estimate of the 11B(p,?0)8Be S(E)-factor at zero energy and that from a previous THM investigation is quantitatively corroborated. The results obtained here support, within the experimental uncertainties, the low-energy S(E)-factor extrapolation and the value of the electron screening potential deduced from direct measurements.

Activation measurement of the he3(α,γ)be7 reaction cross section at high energies

Abstract The astrophysically important He 3 ( α , γ ) Be 7 reaction was studied at high energies where the available experimental data are in contradiction. A thin window 3He gas cell was used and the cross section was measured with the activation method. The obtained cross sections at energies between E c . m . = 1.5 and 2.5 MeV are compared with the available data and theoretical calculations. The present results support the validity of the high energy cross section energy dependence observed by recent experiments.

Determining reaction cross sections via characteristic X-ray detection: α-induced reactions on 169Tm for the astrophysical γ-process

Abstract The cross sections of the Tm 169 ( α , γ ) Lu 173 and Tm 169 ( α , n ) Lu 172 reactions have been measured first time using a new method, by detecting the characteristic X-ray radiation following the electron capture-decay of 172,173Lu. Despite the relatively long half-life of the reaction products ( T 1 / 2 = 500 and 6.7 days, respectively) it was possible to measure the cross section of the Tm 169 ( α , γ ) Lu 173 reaction close to the Gamow window ( T 9 = 3.5 GK ), between E c . m . = 13.16 and 17.08 MeV. The Tm 169 ( α , n ) Lu 172 reaction cross section was measured from E c . m . = 11.21 MeV up to E c . m . = 17.08 MeV . The experimental results have been compared to theoretical predictions.

Alpha-induced reaction cross section measurements on 151Eu for the astrophysical γ-process

In order to extend the experimental database relevant for the astrophysical -process towards the unexplored heavier mass region, the cross sections of the 151 Eu(�,) 155 Tb and 151 Eu(�,n) 154 Tb reactions have been measured at low energies between 12 and 17MeV using the activation technique. The results are compared with the predictions of statistical model calculations and it is found that the calculations overestimate the cross sections by about a factor of two. A sensitivity analysis shows that this discrepancy is caused by the inadequate description of the �+nucleus channel. A factor of two reduction of the reaction rate of 151 Eu(�,) 155 Tb in -process network calculations with respect to theoretical rates using the optical potential by McFadden and Satchler (1966) is recommended.

Elastic alpha scattering experiments and the alpha-nucleus optical potential at low energies

High precision angular distribution data of (α,α) elastic scattering are presented for the nuclei {sup 89}Y, {sup 92}Mo, {sup 106,110,116}Cd, {sup 112,124}Sn, and {sup 144}Sm at energies around the Coulomb barrier. Such data with small experimental uncertainties over the full angular range (20–170°) are the indispensable prerequisite for the extraction of local optical potentials and for the determination of the total reaction cross section σ{sub reac}. A systematic fitting procedure was applied to the experimental scattering data presented to obtain comprehensive local potential parameter sets that are composed of a real folding potential and an imaginary potential of Woods–Saxon surface type. The potential parameters obtained were used in turn to construct a new systematic α-nucleus potential with very few parameters. Although this new potential cannot reproduce the angular distributions with the same small deviations as the local potential, the new potential is able to predict the total reaction cross sections for all cases under study.

Investigation of α-induced reactions on the p nucleus Yb168

Abstract Cross sections for the Yb 168 ( α , γ ) Hf 172 and Yb 168 ( α , n ) Hf 171 reactions were measured by means of the activation method using α particles with energies between 12.9 MeV and 15.1 MeV. The spectroscopy of the γ rays emitted by the reaction products was performed using three different HPGe detector types, namely clover-type high-purity germanium detectors, a low-energy photon spectrometer detector, and a coaxial high-purity germanium detector. The results were compared to Hauser–Feshbach statistical model calculations. Within certain assumptions, astrophysical conclusions could be drawn concerning the production of the p nucleus Yb 168 . The data in this work can serve as a contribution to the current very fragmentary experimental data base for charged-particle induced reactions. In addition, the absolute intensity for nine γ-ray transitions following the electron capture decay of Hf 171 could be derived.

Total reaction cross sections from elastic α -nucleus scattering angular distributions around the Coulomb barrier

The total reaction cross section {sigma}{sub reac} is a valuable measure for the prediction of {alpha}-induced reaction cross sections within the statistical model and for the comparison of scattering of tightly bound projectiles to weakly bound and exotic projectiles. Here we provide the total reaction cross sections {sigma}{sub reac} derived from our previously published angular distributions of elastic {alpha}-nucleus scattering on {sup 89}Y, {sup 92}Mo, {sup 112,124}Sn, and {sup 144}Sm at energies around the Coulomb barrier.

The Trojan horse method in nuclear astrophysics: Recent results

Difficulties in cross-section measurements at very low energies, when charged particles are involved, led to the development of some indirect methods. The Trojan horse method (THM) allows us to bypass the Coulomb effects and has been successfully applied to several reactions of astrophysical interest. A brief review of the THM applications is reported together with some of the most recent results.

Measurement of (α,n) reaction cross sections of erbium isotopes for testing astrophysical rate predictions

The γ-process in core-collapse and/or type Ia supernova explosions is thought nto explain the origin of the majority of the so-called p nuclei (the 35 protonrich nisotopes between Se and Hg). Reaction rates for γ-process reaction network nstudies have to be predicted using Hauser–Feshbach statistical model ncalculations. Recent investigations have shown problems in the prediction of nα-widths at astrophysical energies which are an essential input for the statistical nmodel. It has an impact on the reliability of abundance predictions in the nupper mass range of the p nuclei. With the measurement of the 164,166Er(α,n) n167,169Yb reaction cross sections at energies close to the astrophysically relevant nenergy range we tested the recently suggested low energy modification of nthe α+nucleus optical potential in a mass region where γ-process calculations nexhibit an underproduction of the p nuclei. Using the same optical potential nfor the α-width which was derived from combined 162Er(α,n) and 162Er(α,γ) nmeasurement makes it plausible that a low-energy modification of the optical nα+nucleus potential is needed.