A universal function for heavy-ion fusion cross sections, Y = root pi X erfc(-X ) + exp(-X2) is proposed. By scaling both the cross section sigma (E) and the energy E, heavy-ion fusion cross section data are found to follow closely a universal function over the whole energy range. The scaling is developed from either a simple, empirical single-Gaussian barrier distribution model for the representation d2(sigma E)/dE2, or the modified SiwekWilczynski model. The cross section expressions of these models are analytical, and can be easily used for all heavy-ion fusion excitation functions. Thus a bench marking of heavy-ion fusion excitation functions has been achieved. A general discussion regarding the universal function is given.

Negative components are observed in the representation d2(sigma E)/dE2 in heavy-ion fusions related to recent studies of noticeable oscillations in the high-energy region of the fusion excitation functions. A modified multiGaussian model has been developed, which includes contributions for both positive and negative components in the d2(sigma E)/dE2 representation. The model reproduces the heavy-ion fusion excitation functions very well, including oscillations. The property of these negative components in the d2(sigma E)/dE2 spectrum and the reaction mechanism of fusion at energies above the Coulomb barrier are investigated. The model also reproduces well light heavy-ion fusion reactions between 12C and 16O at high energy; results imply that the compound nucleus channel effect may have an important contribution at the high energy region for all fusion systems.

Understanding the role of optical absorbers is critical for linking the properties of the dayside and terminator atmospheres of hot Jupiters. This study aims to identify the signatures of optical absorbers in the atmosphere of the hot Jupiter HAT-P-41b. We conducted five transit observations of this planet to obtain its optical transmission spectra using the Gran Telescopio Canarias (GTC). We performed atmospheric retrievals assuming free abundances of 12 chemical species. Our Bayesian model comparisons revealed strong evidence for TiO absorption (∆ ln 𝒵 = 21.02), modest evidence for CrH (∆ ln 𝒵 = 3.73), and weak evidence for MgH (∆ ln 𝒵 = 2.32). When we combined the GTC transmission spectrum with previously published Hubble Space Telescope and Spitzer data, the retrieval results and model inferences remained consistent. In conclusion, HAT-P-41b has a metal-rich atmosphere with no high-altitude clouds or hazes. Further observations of its dayside atmosphere should be made to confirm the hints of a thermal inversion in the upper atmosphere suggested by our results.

An unexpected overlapping behavior has been observed for fusion excitation functions sigma(E) from different entrance channels that fuse to the same compound nucleus in energies above the barrier. The overlap appears when the center of mass energy of each excitation function is multiplied by a constant scaling factor. The scaling factors are determined by adjusting and are nearby the ratios of corresponding Coulomb barriers of each entrance channel. Moreover, the corresponding d(sigma E)/dE spectra also overlap well in this energy range, including their fine structures. The reasoning behind these behaviors are unknown. One possibility is the compound-channel effect, since the overlap behavior does not occur between collision systems leading to different compound nuclei.

Exoplanet atmospheres are the key to understanding the nature of exoplanets. To this end, transit spectrophotometry provides us opportunities to investigate the physical properties and chemical compositions of exoplanet atmospheres. We aim to detect potential atmospheric signatures in 12 gaseous giant exoplanets using transit spectrophotometry and we try to constrain their atmospheric properties. The targets of interest were observed using transit spectrophotometry with the GTC OSIRIS instrument. We estimated the transit parameters and obtained the optical transmission spectra of the target planets using a Bayesian framework. We analyzed the spectral features in the transmission spectra based on atmospheric retrievals. Most of the observed transmission spectra were found to be featureless, with only the spectrum of CoRoT-1b showing strong evidence for atmospheric features. However, in combination with the previously published near-infrared transmission spectrum, we found multiple interpretations for the atmosphere of CoRoT-1b due to the lack of decisive evidence for alkali metals or optical absorbers. Featureless spectra are not necessarily indicative of cloudy atmospheres if they poorly constrain the altitudes of cloud decks. Precise constraints on the models of hazes and clouds strongly depend on the significance of the observed spectral features. Further investigations on these exoplanets, especially CoRoT-1b, are required to confirm the properties of their atmospheres.

In the present work the fusion cross section of the 12 C+ 24 Mg system has been measured down to energies far below the coulomb barrier around 4μ b . This system is slightly heavier than those of astrophysical interest, like 12 C+ 12 C and 16 O+ 16 O. The data points highlight the presence of hindrance in 12 C+ 24 Mg because the excitation function is over-estimated by standard Coupled-Channels calculations, and a clear maximum of the S factor has been observed. The cross section at hindrance threshold is found to be remarkably large (σ ≈0.75mb). The S-factor maximum is nicely fitted using both an empirical interpolation in the spirit of the adiabatic model, and the hindrance parametrisation. The data far below the barrier may suggest that the coupling strengths gradually decrease and vanish, so that the excitation function seems to be well reproduced by a simple one-dimensional tunnelling through the potential barrier in that energy range. On the other hand, the equally good fit obtained with the hindrance model, indicates that discriminating between the two approaches would require further precise measurements at slightly lower energies.

Context. Ground-based transit observations are affected by both telluric absorption and instrumental systematics, which can affect the final retrieved transmission spectrum of an exoplanet. To account for these effects, a better understanding of the impact of different data analyses is needed to improve the accuracy of the retrieved transmission spectra. Aims. We propose validating ground-based low-resolution transmission spectroscopy using transiting white dwarfs. These targets are selected to have transit parameters comparable with typical transiting hot Jupiters but nondetectable transmission signals due to their extremely high surface gravities. The advantage here is that we know beforehand what the final transmission spectrum should be: a featureless flat spectrum. Methods. We analyzed two transiting white dwarfs analogous to hot Jupiters, KIC 10657664B and KIC 9164561B. We used various noise models to account for the systematic noise in their spectroscopic light curves following common procedures of transmission spectroscopy analyses. We compared the derived transmission spectra with the broadband transit depth to determine whether there are any artificial offsets or spectral features arising from light-curve fitting. Results. The results show a strong model dependence, and the transmission spectra exhibit considerable discrepancies when they are computed with different noise models, different reference stars, and different common-mode removal methods. Nonetheless, we can still derive relatively accurate transmission spectra based on a Bayesian model comparison. Conclusions. With current ground-based instrumentation, the systematics in transit light curves can easily contaminate a transmission spectrum, introducing a general offset or some spurious spectral features and thus leading to a biased interpretation on the planetary atmosphere. Therefore, we suggest that any wiggle within the measurement errors in a transmission spectrum should be interpreted with caution. It is necessary to determine the dependence of results on the adopted noise model through model comparison. The model inferences should be examined through multiple observations and different instruments.

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 13N(??, p) 16O reaction has been found to affect various stages of a CCSN at varying temperatures. This work presents the first direct measurement of the 13N(??, p) 16O reaction performed using a 34.6 MeV beam of radioactive 13N ions and the active-target detector MUSIC (MUlti-Sampling Ionization Chamber) at Argonne National Laboratory. The resulting total 13N(??, p) 16O 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.

Methods for obtaining fusion cross section formulas are discussed, especially for those that take on an empirical form. A new expression starting with sigma (E) = 1/E integral(E) (E0) (integral (E ') (E0) B(E '')dE '')dE ' has been explored. Here, B(E) is a reasonable, assumed function of the barrier height distribution, d(2) (sigma E)/dE(2). The resulting analytic cross section formula reproduces very well the excitation functions for many light and heavy fusion systems across wide energy ranges, when B(E) is assumed to be a multi-Gaussian function. This study offers an improved determination of the fusion barrier height distribution over other numerical techniques.

By assuming one parameter of the Wong formula, the curvature of the potential barrier, as an energy-dependent one, namely $$ \hbar \omega \rightarrow \hbar \omega \exp \bigl [ \lambda \frac{E - V}{V} \bigr ]$$ , a modified-Wong formula has been developed. Here, $$\lambda $$ is an additional adjustable parameter and $$\hbar \omega $$ becomes the curvature at the top of the barrier, $$E = V$$ . This modified-Wong formula, with four parameters, reproduces fusion excitation functions very well for many light to heavy systems across the whole energy range, including fusion hindrance phenomenon.

As the scope of Accelerator Mass Spectrometry (AMS) expands, there is an increased need to extend the capability of isobaric separation to the medium-heavy mass region. Existing AMS facilities are limited in their ability to separate radioactive nuclei in the A = 100-200 range of interest from their neighboring stable isobars, as such measurements require higher energies than available in most facilities. ATLAS is one of the highest energy system used for AMS based experiments and has enabled isobaric discrimination for medium to heavy nuclides, notably via the Gas-Filled Magnet technique. A preparatory experiment performed in November, 2019, successfully demonstrated isobaric separation of 92Zr-92Mo using the Argonne Gas-Filled Analyzer (AGFA) with high magnetic rigidity. Since that time, MONICA, an eight-anode ionization chamber that measures both energy loss and position with two sets of split anodes, has been developed to aid in AMS experiments at AGFA and has undergone four commissioning runs at the Nuclear Science Laboratory at the University of Notre Dame utilizing Si, Fe/Ni, and Mn beams. This report presents the AGFA AMS run (November 2019) and the subsequent commissioning runs of the MONICA detector, including preliminary measurements on the long-lived isotopes 39Ar (268 y) and for the first time on 42Ar (33 y).

The study of fusion reactions at extreme sub-barrier energies has seen an increased interest in recent years, although difficult to measure due to their very small cross sections. Such reactions are extremely important for our understanding of the production of heavy elements in various environments. In this article, the status of the field is reviewed covering the experimental techniques, the available data, and the theoretical approaches used to describe such reactions. The fusion hindrance effect, first discovered in medium-mass systems, has been found to be relevant also for lighter systems. In some light systems, resonance structures are found to be important, while for heavy systems, the fission process plays an important role. In the near barrier region, couplings to collective excitations in the fusion participants and transfer reactions have been found to give a good description of the measured fusion cross sections and it results in a distribution of fusion barrier heights. New physics ingredients, related to the overlap process of the two projectiles, have to be introduced to describe the hindrance behavior. In addition, it has recently been found that the fusion cross section in both near-barrier and sub-barrier regions can be described very well in many cases using simple, analytical forms of the barrier-height distributions or a modified version of the classic Wong formula.

Context. Transmission spectroscopy characterizes the wavelength dependence of transit depth, revealing atmospheric absorption features in planetary terminator regions. In this context, different optical transmission spectra of HAT-P-12b reported in previous studies exhibited discrepant atmospheric features (e.g., Rayleigh scattering and alkali absorption). Aims. We aim to understand the atmosphere of HAT-P-12b using two transit spectroscopic observations by the Gran Telescopio Canarias (GTC) and to search for evidence of stellar activity contaminating the transmission spectra, which might be the reason behind the discrepancies. Methods. We used Gaussian processes to account for systematic noise in the transit light curves and used nested sampling for Bayesian inferences. We performed joint atmospheric retrievals using the two transmission spectra obtained by GTC OSIRIS, as well as previously published results, coupled with stellar contamination corrections for different observations. Results. The retrieved atmospheric model exhibits no alkali absorption signatures, but shows tentative molecular absorption features including H 2 O, CH 4 , and NH 3 . The joint retrieval of the combined additional public data analysis retrieves similar results, but with a higher metallicity. Conclusions. Based on Bayesian model comparison, the discrepancies of the transmission spectra of HAT-P-12b can be explained by the effect of different levels of unocculted stellar spots and faculae. In addition, we did not find strong evidence for a cloudy or hazy atmosphere from the joint analysis, which is inconsistent with prior studies based on the observations of the Hubble Space Telescope.

Background: Nova explosions synthesize elements up to A similar or equal to 40, and discrepancies exist between calculated and observed abundances of Ar and Ca created in the explosion. The K-38(p, gamma) Ca-39 reaction rate has been shown to be influential on these isotopic abundances at the endpoint of nova nucleosynthesis. The energies of the three most important resonances, corresponding to J(pi) = 5/2(+) excited states in the Ca-39 nucleus above the proton separation threshold, are uncertain and one has been measured with conflicting values [E-r = 679(2) versus E-r = 701(2) keV] in previous experiments. Purpose: Reducing the uncertainties on the resonance energies would allow for a better understanding of the reaction rate. To improve these uncertainties, we searched for gamma rays from the depopulation of the corresponding excited states in Ca-39. Methods: We report a new measurement of these resonance energies via the observation of previously unobserved gamma-ray transitions. These transitions were observed by studying the Ca-40(He-3, alpha gamma) Ca-39 reaction with Gammasphere ORRUBA Dual Detectors for Experimental Structure Studies (GODDESS). The updated resonance energies were then used to calculate the K-38(p, gamma)Ca-39 reaction rate and assess its uncertainties. Results: In total, 23 new transitions were found, including three gamma -ray transitions corresponding to the three J(pi)= 5/2(+) states of astrophysical interest at energies of 6156.2(16), 6268.8(22), and 6470.8(19) keV. These correspond to resonance energies in the K-38(p, gamma)Ca-39 reaction of 386(2), 498(2), and 701(2) keV. Conclusions: Updated K-38(p, gamma) Ca-39 reaction rate calculations show a reduced upper limit at nova temperatures. However, the lower-than-previously-measured energy of the 498-keV resonance and uncertainty in its resonance strength increases the upper limit of the rate close to previous estimates at 0.4 GK.

Background: The phenomenon of fusion hindrance may have important consequences on the nuclear processes occurring in astrophysical scenarios, if it is a general behavior of heavy-ion fusion at extreme subbarrier energies, including reactions involving lighter systems, e.g., reactions in the carbon and oxygen burning stages of heavy stars. The hindrance is generally identified by the observation of a maximum of the S-factor vs energy. Whether there is an S-factor maximum at very low energies for systems with a positive fusion Q value is an experimentally challenging question. Purpose: Our aim has been to search for evidence of fusion hindrance in C-12 + (24)g which is a medium-light m system with positive Q value for fusion, besides the heavier cases where hindrance is recognized to be a general phenomenon. C-12 + (24)mg is very close to the O-16 + O-16 and C-12 + C-12 systems that are important for the late evolution of heavy stars. Methods: The experiment has been performed in inverse kinematics using the Mg-24 beam from the XTU Tandem accelerator of LNL in the energy range 26-52 MeV with an intensity of 4-8 pnA. The targets were C-12 evaporations 50 mu g/cm(2) thick, isotopically enriched to 99.9%. The fusion-evaporation residues were detected at small angles by a E-Delta E-ToF detector telescope following an electrostatic beam deflector. Results: Previous measurements of fusion cross section for C-1(2) + (24)g were limited to above-barrier energies. m In the present experiment the excitation function has been extended down to similar or equal to 15 mu b and it appears that the S factor develops a clear maximum vs energy, indicating the presence of hindrance. This is the first convincing evidence of an S factor maximum in a medium-light system with a positive fusion Q value. These results have been fitted following a recently suggested method and a detailed analysis within the coupled-channels model that has been performed using a Woods-Saxon potential and including the ground state rotational band of 24 Mg. The coupled-channels calculations give a good account of the data near and above the barrier but overpredict the cross sections at very low energies. Conclusions: The hindrance phenomenon is clearly observed in C-12 + Mg-24, and its energy threshold is in reasonable agreement with the systematics observed for several medium-light systems. The fusion cross sections at the hindrance threshold show that the highest value (sigma(s) = 1.6 mb) is indeed found for this system. Therefore it may even be possible to extend the measurements further down in energy to better establish the position of the S-factor maximum.

The phenomenon of fusion hindrance may have important consequences on the nuclear processes occurring in astrophysical scenarios, if it is a general behaviour of heavy-ion fusion at extreme sub-barrier energies, including reactions involving lighter systems, e.g. reactions in the carbon and oxygen burning stages of heavy stars. The hindrance is generally identified by the observation of a maximum of the S-factor vs. energy. Whether there is an S-factor maximum at very low energies for systems with a positive fusion Q-value is an experimentally challenging question. Our aim has been to search evidence for fusion hindrance in 12C + 24Mg which is a medium-light systems with positive Q-value for fusion, besides the heavier cases where hindrance is recognised to be a general phenomenon. The experiment has been performed at the XTU Tandem accelerator of LNL by directly detecting the fusion evaporation residues at very forward angles. The excitation function has been extended down to ≃10μb, i.e. 4 orders of magnitude lower than previous measurements and we observe that the S-factor develops a clear maximum vs. energy. Coupled-Channels calculations using a Woods-Saxon potential give a good account of the data near and above the barrier but over predict the cross sections at very low energies. Therefore the hindrance phenomenon is clearly recognised in 12 C + 24 Mg with an energy threshold that nicely fits the systematics in several medium-light systems. The fusion cross sections at the hindrance threshold show that the highest value (as=1.6mb) is indeed found for this system. It may be possible to extend the measurements further down in energy.

Background: 19Ne is an important isotope in nuclear astrophysics due to its role in both the F-18(p, alpha)O-15 and O-15(alpha, gamma )19Ne reactions in novae and Type I x-ray bursts, respectively. The energy levels of 19Ne near the alpha and proton thresholds (S alpha = 3529 keV, Sp = 6410 keV) correspond to resonances in both of these reactions. Previous measurements to study the structure of Ne-19 have focused on both regions in an effort to constrain these reaction rates. Purpose: Discrepancies in the energies, spins, and parities for levels in Ne-19 from previous measurements contribute to the reaction-rate uncertainties. Gamma rays from the depopulation of excited states in Ne-19 were measured to reduce the level-energy uncertainties and inconsistencies in previous spin-parity assignments. Methods: The F-19(He-3, t)Ne-19 reaction was used to elucidate the structure of Ne-19 levels up to Ex = 6.9 MeV. The reaction products were measured using Gammasphere ORRUBA: Dual Detectors for Experimental Structure Studies-a coupling of the Oak Ridge Rutgers University Barrel Array and Gammasphere at Argonne National Laboratory. Tritons produced in the reaction were measured in coincidence with gamma rays from the deexcitation of Ne-19 energy levels. Results: Previously unobserved transitions allowed for discrepancies in the resonance properties relevant to these two reactions to be resolved. In total, 41 transitions from 21 energy levels were measured in Ne-19, with 21 of those transitions being previously unobserved. Of particular importance, transitions from two 3/2+ states with energies of 6423(3) and 6441(3) keV, crucial for accurate estimations of the F-18(p, alpha)15O reaction rate, were found. Conclusions: Energies and spin-parities of important energy levels near the proton and alpha thresholds were measured and some of the discrepancies in previous measurements were resolved. Measurement of the two near-threshold 3/2+ states reduced the calculated upper limit of the F-18(p, alpha)O-15 reaction rate by factors of 1.5-17 in the nova temperature range.

Background: The phenomenon of fusion hindrance may have important consequences on the nuclear processes occurring in astrophysical scenarios, if it is a general behavior of heavy-ion fusion at extreme subbarrier energies, including reactions involving lighter systems, e.g., reactions in the carbon and oxygen burning stages of heavy stars. The hindrance is generally identified by the observation of a maximum of the S factor vs energy. Whether there is an S -factor maximum at very low energies for systems with a positive fusion Q value is an experimentally challenging question. Purpose: Our aim has been to search for evidence of fusion hindrance in 12 C + 24 Mg which is a medium-light system with positive Q value for fusion, besides the heavier cases where hindrance is recognized to be a general phenomenon. 12 C + 24 Mg is very close to the 16 O + 16 O and 12 C + 12 C systems that are important for the late evolution of heavy stars. Methods: The experiment has been performed in inverse kinematics using the 24 Mg beam from the XTU Tandem accelerator of LNL in the energy range 26–52 MeV with an intensity of 4–8 pnA. The targets were 12 C evaporations 50 μ g / cm 2 thick, isotopically enriched to 99 . 9%. The fusion-evaporation residues were detected at small angles by a E - (cid:2) E -ToF detector telescope following an electrostatic beam deflector. Results: Previous measurements of fusion cross section for 12 C + 24 Mg were limited to above-barrier energies. In the present experiment the excitation function has been extended down to (cid:2) 15 μ b and it appears that the S factor develops a clear maximum vs energy, indicating the presence of hindrance. This is the first convincing evidence of an S factor maximum in a medium-light system with a positive fusion Q value. These results have been fitted following a recently suggested method and a detailed analysis within the coupled-channels model that has been performed using a Woods-Saxon potential and including the ground state rotational band of 24 Mg. The coupled-channels calculations give a good account of the data near and above the barrier but overpredict the cross sections at very low energies. Conclusions: The hindrance phenomenon is clearly observed in 12 C + 24 Mg, and its energy threshold is in reasonable agreement with the systematics observed for several medium-light systems. The fusion cross sections at the hindrance threshold show that the highest value ( σ s = 1 . 6 mb) is indeed found for this system. Therefore it may even be possible to extend the measurements further down in energy to better establish the position of the S -factor maximum.