Proton Capture Reactions Research Articles

Discrepancies between spectroscopy and HST photometry in tagging multiple stellar populations in 22 globular clusters

Multiple populations (MPs) in globular clusters (GCs) are stars that are distinct for their abundances of light elements. The MPs can be directly separated by measuring abundances of C, N, O, Na, Al, and Mg with spectroscopy or indirectly from photometric sequences created by the impact of different chemistry on band passes of particular filters, such as the HST pseudo-colours in the ultraviolet. An attempt to link HST pseudo-colour maps (PCMs) and spectroscopy was made by Marino et al. (2019, MNRAS, 487, 3815), using abundances mostly from our FLAMES survey. However, we found that an incomplete census of stars in common was used in their population tagging. We corrected the situation by building our own PCMs and matching them with our abundances in 20 GCs, plus two GCs from other sources, doubling the sample with spectroscopic abundances available. We found that the pseudo-colour (magF275W − 2 × magF336W + magF438) does not have a monotonic trend with Na abundances, enhanced by proton-capture reactions in MPs. Moreover, on average about 16% of stars with spectroscopic Na abundances show a discrepant tagging of MPs with respect to the HST photometry. Stars with second generation (SG) chemistry are mistaken for first generation (FG) objects according to HST photometry and vice versa. In general, photometric indices tend to overestimate the fraction of FG stars, in particular in low mass GCs. We offer a simple explanation for these findings. Finally, we publish all our PCMs (with more than 31 800 stars in 22 GCs) with star ID and coordinates; this is done to ensure easy verification and reproduction, as should be the case in scientific papers.

Astrophysical S-factor and reaction rate for 15N(p,γ)16O within the modified potential cluster model* *Supported by the Ministry of Science and Higher Education of the Republic of Kazakhstan (AP09259174)

We study radiative capture on the ground state of 16O at stellar energies within the framework of a modified potential cluster model (MPCM) with forbidden states, including low-lying resonances. The investigation of the 15N( )16O reaction includes the consideration of resonances due to transitions and the contribution of the scattering wave in the p + 15N channel due to the transition. We calculated the astrophysical low-energy S-factor, and the extrapolated turned out to be within 34.7−40.4 keV·b. The important role of the asymptotic constant (AC) for the 15N( )16O process with interfering (312) and (962) resonances is elucidated. A comparison of our calculation for the S-factor with existing experimental and theoretical data is addressed, and a reasonable agreement is found. The reaction rate is calculated and compared with the existing rates. It has negligible dependence on the variation of AC but shows a strong impact of the interference of (312) and (962) resonances in reference to the CNO Gamow windows, especially at low temperatures. We estimate the contribution of cascade transitions to the reaction rate based on the exclusive experimental data from Phys. Rev. C. 85, 065810 (2012). The reaction rate enhancement due to the cascade transitions is observed from and reaches the maximum factor ~ 1.3 at . We present the Gamow energy window and a comparison of rates for radiative proton capture reactions 12N( )13O, 13N( ) 14O, 14N( )15O, and 15N( )16O obtained in the framework of the MPCM and provide the temperature windows, prevalence, and significance of each process.

Isotopic abundance estimation of $${}^{\bf{22}}$$Ne and $${}^{\bf{24}}$$Mg using proton capture reaction network in the extended Ne–Al chain

Isotopic abundance estimation of $${}^{\bf{22}}$$Ne and $${}^{\bf{24}}$$Mg using proton capture reaction network in the extended Ne–Al chain

Photon strength functions from (p,γ) reactions

The 50Cr(p,γ)51Mn proton capture reaction has been used to study the photon strength functions by utilizing primary gamma ray transitions from the entry states to discrete states of known spins and parities. The reaction was conducted with the 3 MV Tandetron accelerator at iThemba LABS which delivered proton beams of 2.5 to 2.740, 2.760 to 3.0 MeV and 3.675 to 4.498 MeV in intervals of 20-25 keV with beam currents of up to 5 μA. In this work the proton capture reaction was employed together with the Average Resonance Capture method to extract the shape of the PSF of 51Mn.

Contribution of proton capture reactions to the abundances of phosphorus and sulfur in FGK stars

Contribution of proton capture reactions to the abundances of phosphorus and sulfur in FGK stars

Astrophysical S factor and reaction rate of 92,94Mo(p, γ) relevant to the p-process

Abstract. Low energy proton capture cross sections on heavy isotopes are necessary for a better understanding of the astrophysical p-process. There are around 35 proton-rich stable isotopes between 74S e and 196Hg which are bypassed by the s- and r- processes. These are commonly referred as p-nuclei whose origin is still not completely understood. In the present study, proton capture reactions are studied on Mo isotopes at astrophysically relevant energies using nuclear modular code TALYS. Astrophysical S factor and reaction rates are also calculated inside a core-collapse supernova. The obtained results are compared with the literature data taken from EXFOR data library. In addition, the effect of different combinations of the nuclear input parameters entering the stellar reaction rate have been investigated.

A new light particle is being born

A few years ago we observed anomalous electron-positron angular correlations for the 18.15 MeV M1 transition of 8Be. This was interpreted as the creation and decay of an intermediate bosonic particle with a mass of m 0 c 2=16.70±0.35(stat )±0.5(sys) MeV, which is now called X17. The possible relation of the X17 boson to the dark matter problem triggered an enormous interest in the wider physics community. We also observed a similar anomaly in 4He, which could be described also by the creation and subsequent decay of the same X17 particle. Very recently, the 11B proton capture reaction was used for exciting the 17.2 MeV broad (Γ=1.15 MeV) resonance in 12C and studying their internal pair creation decay. Anomalies were observed in the angular correlation of the electron-positron pairs, which together with the 8Be and 4He data provides kinematic evidence for the X17 particle and supports their vector boson and fifth force explanation.

Stable nickel production in type Ia supernovae: A smoking gun for the progenitor mass?

Context. At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit (MCh ≈ 1.4 M⊙) contribute a significant fraction of SN Ia progenitors. The relative fraction of stable iron-group elements synthesized in the explosion has been suggested as a possible discriminant between MCh and sub-MCh events. In particular, it is thought that the higher-density ejecta of MCh WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Ni II] lines in late-time spectra (≳150 d past explosion). Aims. We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from MCh and sub-MCh progenitors. We explore the potential for lines of [Ni II] in the optical an near-infrared (at 7378 Å and 1.94 μm) in late-time spectra to serve as a diagnostic of the exploding WD mass. Methods. We reviewed stable Ni yields across a large variety of published SN Ia models. Using 1D MCh delayed-detonation and sub-MCh detonation models, we studied the synthesis of stable Ni isotopes (in particular, 58Ni) and investigated the formation of [Ni II] lines using non-local thermodynamic equilibrium radiative-transfer simulations with the CMFGEN code. Results. We confirm that stable Ni production is generally more efficient in MCh explosions at solar metallicity (typically 0.02–0.08 M⊙ for the 58Ni isotope), but we note that the 58Ni yield in sub-MCh events systematically exceeds 0.01 M⊙ for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction 57Co(p, γ)58Ni is the dominant production mode for 58Ni in both MCh and sub-MCh models, while the α-capture reaction on 54Fe has a negligible impact on the final 58Ni yield. More importantly, we demonstrate that the lack of [Ni II] lines in late-time spectra of sub-MCh events is not always due to an under-abundance of stable Ni; rather, it results from the higher ionization of Ni in the inner ejecta. Conversely, the strong [Ni II] lines predicted in our 1D MCh models are completely suppressed when 56Ni is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements. Conclusions. [Ni II] lines in late-time SN Ia spectra have a complex dependency on the abundance of stable Ni, which limits their use in distinguishing among MCh and sub-MCh progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Ni II] lines would most likely result from a Chandrasekhar-mass progenitor.

Radiative Capture on Nuclear Isomers: Direct Measurement of the ^{26m}Al(p,γ)^{27}Si Reaction.

We present the first direct measurement of an astrophysical reaction using a radioactive beam of isomeric nuclei. In particular, we have measured the strength of the key 447-keV resonance in the ^{26m}Al(p,γ)^{27}Si reaction to be 432_{-226}^{+146} meV and find that this resonance dominates the thermally averaged reaction rate for temperatures between 0.3 and 2.5GK. This work represents a critical development in resolving one of the longest standing issues in nuclear astrophysics research, relating to the measurement of proton capture reactions on excited quantum levels, and offers unique insight into the destruction of isomeric ^{26}Al in astrophysical plasmas.

Radiative proton–capture reactions with 112,114Cd at astrophysically important energies

The reaction network in the neutron-deficient part of the nuclear chart around A ≈ 100 contains several nuclei of importance to astrophysical processes, such as the p–process. This work reports on the results from recent experimental studies of the radiative proton–capture reactions 112,114Cd(p, γ)113,115 In. Experimental cross sections for the reactions have been measured for proton beam energies residing inside the respective Gamow windows for each reaction, using isotopically enriched 112Cd and 114Cd targets. Two different techniques, the in–beam γ–ray spectroscopy and the activation method have been implemented, where the latter is considered mandatory to account for the presence of low–lying isomers in 113In (E ≈ 392 keV, t1/2 ≈ 100 min), and 115In (E ≈ 336 keV, t1/2 ≈ 4.5 h). Following the measurement of the cross sections, the astrophysical S factors have been subsequently

Oxygen and aluminum-magnesium isotopic systematics of presolar nanospinel grains from CI chondrite Orgueil

Presolar oxide grains have been divided into several groups (Group 1 to 4) based on their O isotopic compositions, which can be tied to several stellar sources. Much of the available data was acquired on large grains, which may not be fully representative of the presolar grain population present in meteorites. We present here new O isotopic data for 74 small presolar oxide grains (∼200 nm in diameter on average) from Orgueil and Al-Mg isotopic systematics for 25 of the grains. Based on data-model comparisons, we show that (i) Group 1 and Group 2 grains more likely originated in low-mass first-ascent (red giant branch; RGB) and/or second-ascent (asymptotic giant branch; AGB) red giant stars and (ii) Group 1 grains with (26Al/27Al)0 ⪆ 5 × 10−3 and Group 2 grains with (26Al/27Al)0 ⪅ 1 × 10−2 all likely experienced extra circulation processes in their parent low-mass stars but under different conditions, resulting in proton-capture reactions occurring at enhanced temperatures. We do not find any large 25Mg excess in Group 1 oxide grains with large 17O enrichments, which provides evidence that 25Mg is not abundantly produced in low-mass stars. We also find that our samples contain a larger proportion of Group 4 grains than so far suggested in the literature for larger presolar oxide grains (≥400 nm). We discuss this observation in the light of stellar dust production mechanisms.

Cross section measurements of proton capture reactions on Sr isotopes for astrophysics applications

Background: Abundance calculations of a certain class of proton-rich isotopes, known as $p$ nuclei, require knowledge of the cross sections of thousands of nuclear reactions entering a reaction network. As a result, the solution of the latter relies on the predictions of the Hauser-Feshbach (HF) theory and, hence, on the reliability of the models describing the nuclear parameters entering the HF calculations, notably the optical model potential (OMP), the nuclear level density (NLD) and the $\ensuremath{\gamma}$-ray strength function ($\ensuremath{\gamma}\mathrm{SF}$).Purpose: The present work reports on a systematic study of proton capture reactions on Sr isotopes at energies relevant to the $p$ process which is responsible for the production of the $p$ nuclei at explosive stellar sites. The purpose of the work reported here is to perform a validity test of the different OMP, NLD, and $\ensuremath{\gamma}\mathrm{SF}$ models through extensive and detailed comparisons between HF calculations and experimental cross section data. This test is necessary to understand the origin of discrepancies between the $p$-nuclei abundances observed in the solar system and those predicted by the different astrophysical models, known as $p$-process models, aiming at describing the nucleosynthesis of the $p$ isotopes.Method: Cross sections were determined from $\ensuremath{\gamma}$-angular distribution measurements and from angle-integrated $\ensuremath{\gamma}$ spectra taken with the $4\ensuremath{\pi}\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$-summing technique. Cross-section data and the resulting astrophysical $S$ factors were compared with Hauser-Feshbach calculations obtained with the latest version 1.95 of the nuclear reaction code talys using combinations of global semi-microscopic and phenomenological models of optical potentials (OMPs), nuclear level densities (NLDs), and $\ensuremath{\gamma}$-ray strength functions ($\ensuremath{\gamma}\mathrm{SFs}$).Results: Total cross sections as well as cross sections to the ground and metastable states were determined for the reactions $^{86}\mathrm{Sr}(p,\ensuremath{\gamma})^{87}\mathrm{Y}$, $^{87}\mathrm{Sr}(p,\ensuremath{\gamma})^{88}\mathrm{Y}$, and $^{88}\mathrm{Sr}(p,\ensuremath{\gamma})^{89}\mathrm{Y}$ at incident proton-beam energies from 2.5 to 3.6, 2 to 5, and 1.5 to 5 MeV, respectively.Conclusions: The experimental data reported in the present work are in very good agreement with the talys 1.95 calculations obtained with the default combination of OMP, NLD, and $\ensuremath{\gamma}\mathrm{SF}$ models. This combination is based on purely phenomenological models. A semimicroscopic proton-nucleus optical model potential was optimized at low energies leading to an equally good agreement between experimental data and theoretical calculations based solely on combinations of fully semimicroscopic models of OMP, NLD, and $\ensuremath{\gamma}\mathrm{SF}$. Our results highlight the need for a continued effort on the systematic study of proton-capture reactions to reduce the range of uncertainties arising from global nuclear models for as wide a range of relevant nuclei as possible. In this regard, new ($p,\ensuremath{\gamma}$) data at the lowest possible energies below the opening of the neutron channel are of key importance to improve global proton-nucleus optical model potentials.

An investigation of radiative proton–capture reactions in the Cd–In mass region

The reaction network in the neutron–deficient part of the nuclear chart around A∼100 contains several nuclei of importance to astrophysical processes, such as the p–process. This work reports on the results from recent experimental studies of the radiative proton-capture reactions 112,114Cd(p,γ)113,115In. Experimental cross sections for the reactions have been measured for proton beam energies residing inside the respective Gamow windows for each reaction, using isotopically enriched 112Cd and 114Cd targets. Two different techniques, the in–beam γ–ray spectroscopy and the activation method have been employed, with the latter considered necessary to account for the presence of low–lying isomers in 113In (Eγ≈392 keV, t1/2≈100 min), and 115In (Eγ≈336 keV, t1/2≈4.5 h). Following the measurement of the total reaction cross sections, the astrophysical S factors have been additionally deduced. The experimental results are compared with Hauser–Feshbach theoretical calculations carried out with the most recent version of TALYS. The results are discussed in terms of their significance to the various parameters entering the models.

Direct measurement of the resonance strengths and branching ratios of low-energy (p, γ) reactions on Mg isotopes * *Supported by the National Natural Science Foundation of China (11490563, 11961141003) and the Continuous Basic Scientific Research Project (WDJC-2019-13)

Proton capture reactions on Mg isotopes are significant in the Mg-Al cycle in stellar H-burning. In particular, the resonance strengths and branching ratios of low-energy resonances in 25Mg( )26Al reactions determine the production of 26Al, which is one of the most important long-lived radioactive nuclei in nuclear astrophysics. In this article, we report our first experiment using the intense proton beam of approximately 2 mA provided by the JUNA accelerator ground laboratory and a new technique that can minimize the composition change of targets under intense beam irradiation. The resonance strengths and branching ratios of E = 214, 304, and 326 keV resonances in the reactions of 24Mg( )25Al, 25Mg( )26Al, and 26Mg( )27Al, respectively, were measured with high accuracy. The success of this experiment provides a good calibration for the nuclear astrophysical experiment at the Jinping underground laboratory.

Evaluation the nonlinear response function of a HPGe detector for 59 keV to 10.7 MeV gamma-rays using a Monte Carlo simulation and comparison with experimental data

Modeling of High Purity Germanium (HPGe) detector on a wide energy range is important in gamma-ray spectroscopy. The precisely modeled detector can be used for proton-induced prompt gamma-ray spectroscopy. In this work, we used both the gamma-rays of calibration sources and prompt gamma-rays emitted in proton capture reactions to model a coaxial p-type HPGe detector using Geant4 Monte Carlo simulation for the gamma-ray energy range of 59–10764 keV. The calibration sources were 137Cs, 241Am, 60Co, 152Eu, and 133Ba, while the prompt gamma-rays were due to the gamma-ray cascades following the 27Al(p,γ)28Si reaction capture at the resonant energies of 992, 1317 and 2483 keV, as well as the 23Na(p,γ)24Mg reaction capture at the resonant energies of 1417 and 1395 keV. According to the results obtained, the presented simulated and experimental spectra show a good agreement. In addition, the mean and maximum relative deviations between experimental and simulated efficiencies corresponding to 27 major peaks are smaller than 5% and 15%, respectively.

Investigating the Ag109(p,γ)Cd110 reaction and its underlying nuclear physics

Background: The nucleosynthesis of neutron-deficient $p$ nuclei remains an unsolved puzzle in nuclear astrophysics. Most likely, huge networks containing hundreds of nuclear reactions are responsible for the creation of this group of nuclei. In reality, many of the relevant reaction rates cannot be studied experimentally but need to be estimated using global and robust theoretical approaches. The underlying nuclear physics that enters these calculations is often still not constrained well enough, especially for nuclei further off the valley of stability.Purpose: Here, we complete the systematic measurement of radiative proton-capture reactions on stable Ag isotopes. The results will be used to test existing theoretical models and are crucial to constrain underlying nuclear physics properties. For this purpose, total cross sections of the $^{109}\mathrm{Ag}(p,\ensuremath{\gamma})^{110}\mathrm{Cd}$ reaction have been measured at proton energies between 2.5 and 5.0 MeV.Method: The cross-section measurements have been carried out by means of in-beam $\ensuremath{\gamma}$-ray spectroscopy and the observation of all ground-state transitions in $^{110}\mathrm{Cd}$. In general, the total cross sections depend strongly on the $\ensuremath{\gamma}$-ray strength function ($\ensuremath{\gamma}$-SF) and the nuclear level density (NLD). While the former one is taken from the systematics in other even-even Cd isotopes, the NLD has been constrained by renormalizing microscopic, tabulated values onto the low-lying cumulative number of levels.Results: For the first time, $^{109}\mathrm{Ag}(p,\ensuremath{\gamma})^{110}\mathrm{Cd}$ reaction cross sections have been reported over a wide range of beam energies. The total cross-section results are in good agreement with a rather limited, formerly published dataset. A convincing agreement with recent Hauser-Feshbach calculations is received when using existing descriptions for the $\ensuremath{\gamma}$-SF and a renormalized, microscopic NLD model.Conclusion: The measured cross sections confirmed the reliability of existing theoretical models used in Hauser-Feshbach statistical model calculations. Coherent results are obtained for the statistical nuclear physics properties in the even-even Cd isotopes as well as for the derived experimental cross sections.

Deducing primary γ -ray intensities and the dipole strength function in Mo94 via radiative proton capture

Background: The nucleosynthesis of heavy nuclei is affected by the reaction rates of radiative capture reactions. Many of the astrophysical relevant rates cannot be obtained from experiments but are obtained from theoretical models. The $\ensuremath{\gamma}$-decay widths that are derived from radiative strength functions are one of the key nuclear physics input parameters in those calculations. The explicit study of $\ensuremath{\gamma}$-ray strength functions has been thoroughly addressed in the last decade and various methods have been established to extract the dipole strength in atomic nuclei.Purpose: The investigation of primary $\ensuremath{\gamma}$-ray transitions from the $^{93}\mathrm{Nb}(p,\ensuremath{\gamma})^{94}\mathrm{Mo}$ reaction allows deducing the $\ensuremath{\gamma}$-ray strength function in $^{94}\mathrm{Mo}$. The results are compared to results obtained using other techniques and the impact of level densities, excitation mechanism, and excitation energy will be thoroughly investigated.Method: The proton beam was delivered by the 10 MV FN-Tandem accelerator located at the Institute for Nuclear Physics at the University of Cologne, Germany. By means of in-beam $\ensuremath{\gamma}$-ray spectroscopy the intensity of primary $\ensuremath{\gamma}$ rays is determined. Additionally, the first generation $\ensuremath{\gamma}$-ray intensity is deduced from the secondary $\ensuremath{\gamma}$ rays using their branching and the coincidence detection efficiency. Absolute $\ensuremath{\gamma}$-ray strength function values as well as relative values using the ratio method will be extracted.Results: Numerous states in $^{94}\mathrm{Mo}$ at excitation energies above 3 MeV have been identified for the first time and their decay behavior has been studied. We disentangled the effects of $M1$ and $E1$ radiation for the $\ensuremath{\gamma}$-ray emission channel in $^{94}\mathrm{Mo}$ and extracted strength function curves for both radiation types. Our results are in good agreement with experimental results using the Oslo method and photoinduced experiments as well as with recent theoretical quasiparticle random-phase approximation (QRPA) calculations. An enhancement of the $\ensuremath{\gamma}$-ray strength below the neutron separation energy was found that is most likely of $E1$ character. In addition, a significant increase of the dipole strength at low $\ensuremath{\gamma}$-ray energies was found which is most likely due to $M1$ strength.Conclusion: Radiative proton capture reactions are a well-suited tool to study the $\ensuremath{\gamma}$-ray strength function in atomic nuclei. From the intensity of primary $\ensuremath{\gamma}$-ray transitions as well as from secondary $\ensuremath{\gamma}$-rays, information about the radiative dipole strength can be extracted. The $\ensuremath{\gamma}$-ray emission seems to be independent of the excitation energy in the studied mass and energy range. Detailed knowledge about the intrinsic properties of the nuclear states is very important and their uncertainties affect the uncertainty of the extracted $\ensuremath{\gamma}$-ray strength functions heavily.

Excess of Ca (and Sc) produced in globular cluster multiple populations: a first census in 77 Galactic globular clusters

Multiple stellar populations in globular clusters (GCs) are distinct by their different abundances of light elements. The abundance anti-correlations point towards a nucleosynthesis origin due to high-temperature H burning, but it remains to be assessed which type of stars altered primordial abundances in GCs. In particular, the regime at very high temperature that shapes the variations in potassium as well as calcium and scandium, which has been detected in a few notable cases such as NGC 2419 and NGC 2808, is still poorly explored. We started a systematic search for excess of Ca (and Sc) in GC stars with respect to the level of unmodified field stars. This method has recently been proven to be highly efficient in revealing the outcome of the proton-capture reactions at very high temperatures. Statistically robust evidence of such excess was found in a small number of GCs (NGC 4833, NGC 6715, NGC 6402, NGC 5296, NGC 5824, and NGC 5139/ωCentauri) that join the previously known two clusters. For the first time we show that NGC 4833 is likely to host anti-correlated K and Mg abundances. All these GCs are among the most massive ones in the Galaxy. We found that the fraction of stars with Ca enhancement at 3σabove the field star distribution is a multivariate function of the GC mass and metallicity, as in other manifestations of the multiple population phenomenon in GCs. We argue that these alterations in only a few GCs can be reproduced by two different channels: either a class of ordinary stars, that is common to all GCs, acts only in particular environments, or an on-off mechanism is generated by the occurrence of a peculiar type of stars (or lack of such stars). Hot bottom-burning in asymptotic giant branch stars in the low-metallicity regime is a good candidate for the first class. Alternatively, a metallicity dependence is also expected for supermassive stars, which are predicted to preferentially form in massive GCs.

Primary γ -ray intensities and γ -strength functions from discrete two-step γ -ray cascades in radiative proton-capture experiments

Background: Reaction rates of radiative capture reactions can play a crucial role in the nucleosynthesis of heavy nuclei in explosive stellar environments. These reaction rates depend strongly on $\ensuremath{\gamma}$-ray decay widths in the reaction products, which are, for nonresonant capture reactions at high excitation energies, derived from the $\ensuremath{\gamma}$-ray strength function and the nuclear level density. Recently, the ratio method was applied to primary $\ensuremath{\gamma}$ rays observed from ($d,p$) reactions and nuclear resonance fluorescence measurements to extract the dipole strength in atomic nuclei and to test the generalized Brink-Axel hypothesis.Purpose: The purpose of this work is to apply the ratio method to primary $\ensuremath{\gamma}$-ray intensities of the $^{63,65}\mathrm{Cu}(p,\ensuremath{\gamma})$ reactions to extract $\ensuremath{\gamma}$-ray strength information on the nuclei $^{64,66}\mathrm{Zn}$. The impact of spin distribution, total $\ensuremath{\gamma}$-ray decay widths, level densities, and width fluctuations on the application of the ratio method will be discussed. Additionally, by comparing the relative $\ensuremath{\gamma}$-ray strength at different excitation energies, conclusions on the validity of the generalized Brink-Axel hypothesis can be made.Method: The radiative proton capture reaction measurements have been performed at the HORUS $\ensuremath{\gamma}$-ray spectrometer of the University of Cologne at one excitation energy for each reaction. Primary $\ensuremath{\gamma}$-ray intensities have been determined by normalizing secondary $\ensuremath{\gamma}$-ray transitions in two-step cascades using their absolute branching ratio. The ratio method was applied to the measured primary $\ensuremath{\gamma}$-ray intensities as well as to previous measurements by Erlandsson $et$ $al.$ at different excitation energies.Results: The relative strength function curve for $^{64}\mathrm{Zn}$ from our measurement shows no significant deviation from the previous measurement at a different excitation energy. The same is true for $^{66}\mathrm{Zn}$ where both measurements were at almost the same excitation energy. Absolute $\ensuremath{\gamma}$-strength function values have been obtained by normalizing the relative curves to quasiparticle random phase approximation calculations because of the absence of experimental data in the respective energy region.Conclusion: The generalized Brink-Axel hypothesis, i.e., the independence of the strength function on the excitation energy, seems to hold in the studied energy region and nuclei. The method to obtain primary $\ensuremath{\gamma}$-ray intensities from two-step cascade spectra was shown to be a valuable and sensitive tool although its uncertainties are connected to the knowledge of the low-energy level scheme of the investigated nucleus. The scaling in the ratio method should be taken with care, because the relative strength is not a simple sum of ${f}_{E1}$ and ${f}_{M1}$ but a somewhat complex linear combination dependent on the excitation energy of the nucleus.

Proton-capture Reaction Cross Sections on the Sr Isotopes Relevant to the p-process Nucleosynthesis

Proton-capture reaction cross sections on the 86,87,88Sr isotopes have been determined at energies from 1.4 to 5 MeV by measuring γ-angular distributions at the 4 MV single-ended Dynamitron accelerator of the University of Stuttgart as well as at the 5 MV VdG Tandem accelerator of NCSR "Demokritos", Athens. In the former case an array of 4 HPGe detectors with relative efficiency εr≈100%, each shielded with BGO crystals, were used. In the case of the measurements carried out at "Demokritos" we used only one HPGe detector (εr≈80%) with no BGO shield. Cross sections ranging from 0.5 μb to 5 mb as well as the relevant S factors were obtained. The data were compared with statistical model calculations using the code MOST. In the calculations, various combinations of microscopic and phe- nomenological models of the nucleon-Nucleus Optical Model Potentials (OMP) and Nuclear Level Densities (NLD) were used and a good agreement between the data and theoretical predictions was found.