Neutron-Driven Nucleosynthesis in Stellar Plasma

1 Introduction

Neutron capture nucleosynthesis is the main responsible for the production of elements heavier than Fe. Such a process can be rapid (r-process) or slow (s-process), according with the comparison between the neutron capture reaction (n,γ) rates of the involved nuclei and their half-lives [1], but anyway they are a series of subsequent neutron captures, separated by β − decays. Therefore, the rapidity of the process is determined by the intensity flux of neutron available in the stellar environment in which occurs.

In recent years, nuclear astrophysicists focused their attention to the s-process relevant cross sections, also considering that r-process abundances can be obtained by the s-process ones [2]. The s-process nucleosynthesis follows a path that wanders along the valley of β-stability close to the strongly bound isotopes of a given atomic weight (or mass number) A. The main site of the slow neutron captures is the final Asymptotic Giant Branch (hereafter AGB) phase of low- and intermediate-mass stars. Stars with mass up to 3 M_⊙ are actually responsible for the production of the main component of the s-process (i.e., nuclei from Sr to Bi) and the main neutron source is known to be the ¹³C(α,n)¹⁶O reaction [3]. It operates at typical energies of 8 keV in radiative conditions and provides a neutron density of about 10⁶–10⁷ n/cm³ [4,5]. The ²²Ne(α,n)²⁵Mg reaction is the second neutron source in AGB stars and it is activated only marginally during the thermal pulses (periodic episodes of He-convective burning) in low mass stars. Only red giants more massive than 3 M_⊙ (the intermediate mass stars) burn ²²Ne efficiently, at maximum temperatures of 3–3.2 ⋅ 10⁸ K. Even stars with larger masses, terminating their evolution as core-collapse Supernovae, are sites of s-process nucleosynthesis, and in particular they are responsible of the so-called weak component, namely that devoted to the production of nuclei with A ≤ 85 [6].

The s-process is considered as being a rather well-known nucleosynthesis mechanism because 1) its main component occurs in common astrophysical objects and 2) neutron capture reactions are not hampered by the Coulomb barrier. This fact makes the most of (n,γ) reactions relatively easy to be studied experimentally. However, there are still theoretical problems to be understood ([7]), as like as the ¹⁹F abundance in AGB stars [8]. Thus, some nuclear reactions cross sections need to be measured with high precision in the energy range of AGB star nucleosynthesis, where objective experimental difficulties are present. Among them, we appoint:

1. Reactions delivering the neutrons for the s process, in particular ¹³C(α,n)¹⁶O (Section 3) and ²²Ne(α,n)²⁵Mg (Section 4), whose cross section determination is affected by the well-known problems in measuring the reactions between charged particles at low energy [9,10];

2. Neutron capture reactions on unstable nuclei, whose half-lives are characterised by time-scales close to those corresponding to neutron capture rates and thus might constitute branching points of the s-nucleosynthesis path;

3. Neutron capture reactions on the so-called poisons of the s-process, i.e. those relatively light nuclei that, having a very large neutron capture cross section, can absorb the neutrons delivered by the ¹³C(α,n)¹⁶O and the ²²Ne(α,n)²⁵Mg reactions, subtracting them to the s-nucleosynthesis. Among the poisons of the s-process we recall the ¹⁴N, the ¹⁶⁻¹⁷O and the ²⁵⁻²⁶Mg. For reactions like ¹⁴N (n,p)¹⁴C or ¹⁷O(n,α)¹⁴C (Section 5), a precision study of these reactions is very hard to perform, owing to the non-existence of a pure-neutron target and to the current unavailability of a neutron beam facility with sufficient energy precision.

4. β − decays of the nuclei involved in the s process in stellar plasma conditions are estimated only by theoretical calculations. A crucial contribution in this field will be provided by future experiments such as those planned at the PANDORA project [11,12], for which we refer to the specific paper in this volume.

Thanks to the indirect Trojan Horse Method (THM), described in the following section, one can extract the cross sections at the ultra-low energies of astrophysical interest for charged particle induced reactions, where this is made almost impossible by the Coulomb barrier presence. In more recent years, the method has been largely tested and applied with success to shed light on different problems, such as the different levels contributions to the cross sections [13,14], for neutron induced reactions [15–21] and for reactions induced by unstable beams [22,23], thus helping in solving the experimental difficulties of points 1 to 3.

2 The Trojan Horse Method

The idea at the base of THM is to obtain the binary reaction cross section of interest for astrophysics a + x → c + C, by measuring the reaction cross section of a given A+ a → c + C + s quasi-free (QF) break-up reaction.

This is possible if we use a suitable nucleus A (called TH nucleus) that has a high probability to be found in a x-s cluster configuration, so that A = x ⊕ s. The QF break-up of A occurs if, in the interaction between A and a, in the exit channel s keeps the same momentum distribution before its break-up. In this case s is the spectator to the virtual process a + x → c + C [24]. It has been experimentally demonstrated that changes in the Trojan Horse nucleus are not impacting the applicability of the THM, despite the change in the spectator cluster for example from a charged to a neutral particle [25,26]. Figure 1 represents a sketch of the TH process, where it is possible to apply the polar approximation [27] only for small spectator momenta p_s respecting the relation in [28].

FIGURE 1

FIGURE 1. Sketch of the polar diagram representing the TH process.

The possibility to reach ultra-low energies is given by the binding energy E_B of x inside A, which compensates for the beam energy [29–31]. One of the advantages of the THM is that, using only one beam energy, the variation of the spectator momentum values inside the limit in [28] makes it possible to scan almost the entire energy range of interest.

Once the QF break-up data have been selected from all other reaction mechanisms with the same final state, in the Plane Wave Impulse Approximation (PWIA) the three-body cross section can be factorized as:

$\frac{d^3\sigma }{d{\mathrm{\Omega }}_{c}d{\mathrm{\Omega }}_{C}d{E}_C}\propto KF|\varphi \left(p_s\right){|}^2{\left(\frac{d\sigma }{d\mathrm{\Omega }}\right)}_{Cc}^{\mathit{HOES}}(1)$

where the phase space is taken into account by the kinematical factor KF and |ϕ(p_s)|² is the squared Fourier transform of the radial wave function describing the intercluster motion inside the TH nucleus. The factor KF ⋅|ϕ(p_s)|² is calculated by means of a Montecarlo simulation where the momentum distribution has a width fixed to the value measured experimentally, to account for the distortion effects arising at low transferred momenta [32,33].

The extracted (dσ/dΩ)^HOES obtained inverting Eq. (1) is denoted half-off-energy-shell (HOES), being x virtual and then the entrance channel off-shell, while the exit channel in on-shell (OES). This cross section has to be corrected for the penetration factor, that in case of n-induced reaction is only relative to the centrifugal barrier, and then it has to be normalized to direct data at energies higher than the Coulomb barrier, where the HOES and OES cross sections are proportional.

Details and reviews of the THM can be found in [34,35].

3 The ¹³C(α,n)¹⁶O Neutron Source for the Main Component

The main s-process neutron source has been singled out in AGB stars where, following the injection of protons from the outer layers into the carbon- and helium-rich shell, they are captured by carbon nuclei, leading to the formation of a ¹³C pocket [36]. Then, at typical temperature range between 0.8 × 10⁸ K and 1 × 10⁸ K [3], ¹³C is burnt through the ¹³C(α, n)¹⁶O reaction producing neutrons. Such temperatures correspond to ¹³C − α center-of-mass energies of about E_13C−α ∼ 140−230 keV. A recent review can be found in Xu et al. [37] and besides, theoretical evaluations are available, using microscopic cluster approach, leading to results in good agreement with the experimental data in [38] and in [39]. It is worth noting that the crucial role of the $\frac{1}{2}+$ subthreshold state was already foreseen in [38] despite of the absence of very low energy data at the time.

Owing to its astrophysical importance, the ¹³C(α, n)¹⁶O reaction has been the subject of many direct and indirect studies. Indirect methods are mandatory since direct measurements could not span so far the energy window of astrophysical interest, which lies between 150 and 230 keV. For instance, the most recent direct measurement [40] could only reach about 230 keV in the center of mass system. Extrapolations to even lower energies are then necessary to establish the trend of the ¹³C(α, n)¹⁶O astrophysical factor inside the Gamow window. However, the understanding of the astrophysical region of the S-factor is complicated by the interplay between the rise due to the ¹⁷O 6.356 MeV 1/2⁺ level and the enhancement due to the electron screening effect (see e.g. La Cognata et al. [41] and references therein). The latter introduces a correction of about 20% at the lowest-energy data point of Drotleff et al. [42] at 279 keV, growing up exponentially approaching the energy region of astrophysical interest. Yet, larger uncertainties are introduced by the occurrence of the 6.356 MeV broad resonance in ¹⁷O. Indeed, until 2015 [43], it was commonly assumed that the aforementioned 1/2⁺¹⁷O state was lying −3 keV below the ¹⁷O →¹³C + α dissociation threshold [44]. Precision spectroscopy studies [43] have shown that the location of such near-threshold resonance should be 4.7 keV above the ¹⁷O →¹³C + α dissociation energy.

An additional challenge for direct measurements is the determination of the absolute value of the ¹³C(α, n)¹⁶O astrophysical factor, since neutron detectors are often used to measure the cross section. Already in Heil et al. [45], the scatter of the absolute normalizations of existing data was evident and as large as a factor two. This spread was realistically attributed to systematic errors affecting neutron detection efficiency as a function of its energy. This is the reason that lead Heil et al. [45] to measure the experimental detection efficiency using a setup identical to the one used for the study of the ¹³C(α, n)¹⁶O reaction and to cross check the results using a GEANT4 simulation. Then, the obtained astrophysical factor was used to scale existing ¹³C(α, n)¹⁶O data sets. Earlier THM studies of this reaction used the Heil et al. [45] astrophysical factor for normalization.

In the THM framework, the ¹³C(α, n)¹⁶O S-factor was extracted from the ¹³C(⁶Li, n¹⁶O)²H QF cross section, measured at 8 MeV beam energy. With this method, neither Coulomb nor centrifugal barriers quell the cross section and electron screening does not shield the ¹³C −⁴He interaction. However, astrophysical energies could be reached thanks to the energy spent to break ⁶Li into to the α − d system as well as to their relative motion. Details on the THM measurement can be found in La Cognata et al. [46,47]. A drawback of the THM is the need of normalization to existing direct data. In those works, the ${E_{13}}_{\mathrm{C}-\alpha }$ energy region between $\sim 0.6-1.2$ MeV was selected, namely a region where the contribution of the threshold state is negligible.

While a THM S-factor in good agreement with the direct ones scaled to match the Heil et al. [45] absolute value was found, a Coulomb modified Asymptotic Normalization Coefficient (ANC) [47] for the 1/2⁺¹⁷O threshold state equal to ${7.7\pm 0.3_{\mathit{stat}}}_{-1.5norm}^{+1.6}$ was determined, contradicting the existing independent assessments of the Coulomb modified ANCs, whose weighted average is 3.9 ± 0.5 fm⁻¹. This may be attributed to the systematic errors affecting direct data absolute normalization. This fact triggered further studies, which ended up in the work of Trippella and La Cognata [48]. Indeed, to unequivocally size the contribution of the ¹⁷O threshold state to the reaction rate, many coherent measurements of its ANC were carried out [49–52].

The ANC method is another powerful indirect method meant to extract the direct radiative part of the cross section for reactions of importance for astrophysics: with this technique, in principle, it is in fact possible to study (p,γ), (n,γ) and (α, γ) reactions of astrophysical importance using p-, n- or α-transfer. Such transfer, at low energies, must show a mostly peripheral character, and such a condition must be verified experimentally. The difference between ANC and THM lies in which kind of reaction can be studied–direct and resonant capture, respectively–and in the reaction products: the ANC method, in fact, can be applied just for reactions that have γ-rays in the exit channel (see also [53] for a recent review on the ANC and, particularly [54], for proton [55], for neutron and [56] for alpha captures at low energies from suitable transfer reactions, also using mirror nuclei [57]).

Therefore, a change in the paradigm usually adopted in the THM application was implemented and the THM S-factor was normalized to the ANC of 6.356 MeV state. This approach lead to a concordance scenario for the ¹³C (α, n)¹⁶O S-factor, where both direct (some data sets with a more appropriate normalization) and indirect data (both ANC and THM) accurately agree.

With the normalization of Trippella and La Cognata [48], the astrophysical factor in Figure 2 (shown as an orange band upper, where middle and lower orange lines are used for recommended + 1σ, recommended and recommended − 1σ THM S-factor) was deduced. An ANC of 3.6 ± 0.7 fm⁻¹ has been determined, in perfect agreement with the weighted average of the results in the literature. The corresponding THM S-factor at E_c.m. = 140 keV was found to be $1.80_{-0.17}^{+0.50}\times 10^6$ MeVb, to be compared with the astrophysical factor in Heil et al. [45] $S(140\mathrm{k}\mathrm{e}\mathrm{V})=2.2_{-0.8}^{+1.1}\times 10^6$ MeVb. More details can be found in Trippella and La Cognata [48]; a recent direct measurement reported in Ciani et al. [40] has confirmed such results, as it is clearly demonstrated in Figure 2, where other direct measurements with their original normalizations are shown.

FIGURE 2

FIGURE 2. Comparison of S(E)-factor obtained in Trippella and La Cognata [48] (orange band) with direct measurements: cyan solid circles are from Ciani et al. [40], black from Davids [58], open circles from Bair and Haas [59], red from Kellogg et al. [60], green from Drotleff et al. [42], blue from Harissopulos et al. [61], brown from Heil et al. [45].

4 The ²²Ne(α,n)²⁵Mg Neutron Source for the Weak Component

A key role in the s-process is played by the ²²Ne(α,n)²⁵Mg reaction, the major neutron producer for its weak component (60$<$A$<$0) in massive stars, influencing all the late evolution of stars, until their death. Despite of the importance of this reaction, in the energy region of astrophysical interest (E_cm = 600 ± 300 keV) three orders of magnitude discrepancies in the cross section of data available in literature are still present, making those data essentially not useful for astrophysical needs [62].

The trend of the excitation function measured so far shows a steep drop in the yield which makes it possible to provide only upper limits to the cross section already at center-of-mass energies of the order of 800 keV, that is at the edge of the region of interest. A clear picture of the status of the art can be found in [63]. Indeed, direct measurements at such low energies are very challenging due to the exponential Coulomb damping (Coulomb barrier is located at 3.5 MeV) of the cross section to values less than 1 μb, pushing the signal-to-noise ratio essentially to zero.

Indirect measurements, such as capture and transfer reactions [63], at low energy are thus needed to supply resonance parameters, such as spectroscopic factors, to be used in the calculation of the reaction rate. In this regard, a recent paper [10] recalculated the reaction rates of ²²Ne(α,n)²⁵Mg and its competitor ²²Ne(α,γ)²⁶Mg, updating energies and spin-parities of all the ²⁶Mg levels, particularly focusing on the results in [64], obtained with ²²Ne(⁶Li,d)²⁶Mg, and ²²Ne (⁷Li,t)²⁶Mg transfer reactions. The authors find that, excluding the results in [64], s-process nucleosynthesis will not substantially change, but including them leads to a better agreement of Ba and Zr isotopic ratios with the data from presolar SiC grains. However, the results reported in [10,64] suggest the need for further measurements, particularly focusing on the excitation function trend, to include also levels interference in the reaction rate evaluation.

To try to deal with this issue, a new measurement was performed at INFN-LNS of Catania (Italy), aimed at bypassing the challenges that prevented to obtain the result until now. To avoid problems related to the poor efficiency in neutron detection, the idea was to obtain the desired cross section applying the detailed balance principle to the inverse reaction: ²⁵Mg(n,α)²²Ne. Then, to avoid the use of neutron beam (and the consequent worsening of the energy resolution) the THM was used, exploiting the well-known p-n structure of the deuteron, used as neutron virtual source. The experiment to measure the 3-body ²H(²⁵Mg,αp)²²Ne cross section was performed in the Camera2000 scattering chamber, because of its 1 m radius that allowed to put detectors as far as it was needed to keep a good angular resolution to apply the THM. In Figure 3 is evident how the use of THM also helped to bypass complex setups. Moreover, THM allowed the use of thin CD₂ targets, to minimize the beam energy spread on the target. The first ²⁵Mg beam of LNS was accelerated with the Tandem accelerator for this experiment.

FIGURE 3

FIGURE 3. Setup of one of the experimental runs of the Magn-a experiment in Catania, at LNS, aimed to perform the ²H(²⁵Mg,αp)²²Ne reaction to extract the ²²Ne(α,n)²⁵Mg cross section with the THM and the detailed balance principle.

This measurement was performed with two different experimental approaches, and thus setups. At first, coincidences between α particles and the spectator proton, using Position Sensitive Detectors (PSD), were detected, reconstructing the ²²Ne kinematics offline, identifying the desired ions with ΔE-E technique using thin PSD (35 μm thickness) as ΔE stage. Dealing with the very low energy of the spectator proton below 1 MeV, forced to reduce the acquisition system threshold to its minimum. The second approach tried was to detect ²²Ne and α particles in coincidence, reconstructing the proton kinematics offline. This has implied to change beam energy to stay focused on the region where QF kinematics is most probable, as required by THM, from ⁽⁷⁺⁾²⁵Mg at 74 MeV to ⁽⁶⁺⁾²⁵Mg at 50 MeV. The ΔE stage PSDs were substituted with two Ionization Chambers (5 cm deep), filled with isobutane (p = 65 mb) to detect and separate ²²Ne. The analysis of these data is still ongoing and additional validity tests are mandatory in order to safely apply THM to this specific case.

It is worth noting that many other possibilities of indirect measurements can be considered. The possible extension of the THM to p-wave intercluster motion nuclei could open the doors to the use of ²³Ne as TH nucleus for its ²²Ne + p configuration, and another good candidate TH nucleus can be the ²⁶Mg as ²²Ne+α in s-wave. In any case, the possibility of new highly ⁴He-enriched solid targets obtained with NESTOR [65], the new ECR ion source at LNS, will be pivotal for these measurements.

5 Neutron Poisons Reactions

The presence of certain nuclei lighter than Fe produced in the previous stages of stellar evolution, causes the capture of a large fraction of the neutrons produced by the ¹³C(α,n)¹⁶O and ²²Ne(α,n)²⁵Mg reactions. These neutron poisons reactions significantly constrain the s-process efficiency [1]. As an example, due to the large abundances of ¹²C and ¹⁶O, the ¹²C(n,γ)¹³C and ¹⁶O(n,γ)¹⁷O reactions are serious neutrons consumer, removing a non-negligible fraction of the produced neutrons [5].

5.1 The ¹⁷O(n,α)¹⁴C

In massive stars (with initial mass M$>$M_⊙), due to the high abundance of oxygen from CNO cycle, one of the most important neutron poison reaction is the ¹⁶O(n,γ)¹⁷O [66]. The ¹⁷O produced by the ignition of this reaction can undergo the ¹⁷O(α,n)²⁰Ne reaction, that represents a recycling channel for neutrons, or the ¹⁷O(n,α)¹⁴C reaction that is another neutron-absorbing reaction [67]. Thus, in order to establish the total neutron flux available for the so-called weak s-process pathway, it is of fundamental importance to ascertain the ratio between the cross sections of the aformentioned reactions. For this reason, the ¹⁷O(n,α)¹⁴C reaction has triggered several measurements with direct experiments and by applying the detailed balance principle to the inverse reaction [68–72]. The population of two excited states at 8,213 keV and 8,282 keV and the contribution of a sub-threshold level at 8,038 keV are present in the available data, while no evidence for the 8,125 keV level is shown [73]. Indeed, the contribution of this resonance is suppressed by the centrifugal barrier, since this resonance is populated in f-wave. In addiction, in the astrophysically relevant temperature region, the reaction rate calculated from measurements available in literature, is affected by a difference of about a factor of 2.5–3, due to disagreement present in the different data sets [71].

For such a reason, two THM measurements were performed at different laboratories, namely at LNS-INFN in Catania (Italy) and at ISNAP (Institute for Structure and Nuclear Astrophysics) in Notre Dame (IN—United States). The experiments were devoted to the measurement of the cross section of the ²H (¹⁷O,α¹⁴C)p three-body reaction to cover the energy range from zero up to a few hundred keV in the binary reaction center-of-mass energy [74,75]. A ¹⁷O beam of 41 (43.5) MeV was delivered onto a CD₂ target in the LNS (ISNAP) experiment.

In order to probe the presence of the QF reaction process, the procedure described in [74,75] was followed, finally extracting the HOES cross section of the ¹⁷O (n,α)¹⁴C reaction.

The experimental data of the two data sets, weighted over the respective errors and summed in order to improve the statistical precision, were then normalized to the available direct measurements [71], integrated to the angular distribution and fitted following the modified R-matrix approach [76] in order to calculate the reduced γ-widths of the excited levels. Thus, the calculated reaction rate is shown in Figure 4 with a blue line, while the blue band highlights the region allowed by uncertainties (statistical and normalization). In comparison, reaction rates from [69] (black line) and [71] (red line) are reported.

FIGURE 4

FIGURE 4. THM reaction rate for the ¹⁷O (n,α)¹⁴C reaction (blue line) in comparison with the ones available in literature (see text for details).

In this measurement, it was possible to excite the sub-threshold level centered at −7 keV in the center-of-mass system corresponding to the 8.039 MeV level of ¹⁸O, which is important to determine the ¹⁷O(n,α)¹⁴C reaction rate. In addiction, it was found that the resonance corresponding to the 8.213 MeV level is better reproduced by adopting an angular momentum l = 2 instead of l = 0, as assumed in the past [68]. This result triggered the need of a new experiment with an improved detection setup and a wider angular coverage, whose analysis is still ongoing [77]. Finally, the use of deuteron as source of virtual neutrons allows us to populate the level centered at 75 keV in the ¹⁷O-n center-of-mass system, corresponding to the 8.121 MeV level of ¹⁸O. Due to its 5⁻ J^π assignment, the population of such level is suppressed in direct measurements because of its l = 3 angular momentum. The application of the modified R-matrix approach made it possible to measure the neutron and α partial widths that are in agreement with the ones available in the literature, while these partial widths were extracted for the first time in the case of the 8.125 MeV level. Therefore, extensive calculations are ongoing in order to understand the precise impact of the present results on astrophysics.

5.2 The ¹⁴N(n,p)¹⁴C

¹⁴N is very abundant in stars, due to its production during the stage prior to the s-process, namely the hydrogen-burning in the CNO cycle. Thus, with its relatively high cross section, this reaction represents a strong neutron poison, especially in low-mass AGB stars [78]—thus for the main component elements production. Moreover, ¹⁴N is of crucial importance in the nucleosynthetic origin of fluorine and the He-burning shell in AGB stars is thought to be the most likely site for this synthesis, mainly through the nuclear chain ¹⁴N(α,γ)¹⁸F(β⁺) ¹⁸O(p,α)¹⁵N(α, γ)¹⁹F. In this sense, the ¹⁴N(n,p)¹⁴C reaction plays a key role because of its double effect of removing neutrons and producing protons. In addition, the protons can trigger the ¹⁸O(p,α)¹⁵N or the ¹³C(p,γ)¹⁴N reactions, being the last one in competition with the ¹³C(α,n)¹⁶O reaction [5].

The first direct measurement of the stellar ¹⁴N (n,p)¹⁴C cross section was done by [79]. Their result for the reaction rate was about a factor three smaller than the rate used in most of the previous s-process calculations, but it was also 2–3 times smaller than rates estimated from the inverse reaction and extrapolations from the thermal cross section, for which an evaluated value of 1.83 b was adopted. Indeed, measurements performed from thermal neutron energy up to 35 keV [80,81] found clear evidence for a 1/v behavior of the ¹⁴N(n,p)¹⁴C reaction cross section up to approximately 30 keV and also their results for the stellar reaction rate at kT = 525 keV are about a factor of 3 higher than reported by [79]. Measurements with quasimonoenergetic neutrons at 25 keV from [82] are in fair agreement with the results from [80,81] and with the estimates from the inverse reaction, since again the same thermal value was used for the normalization. Another direct measurement of the ¹⁴N (n,p)¹⁴C stellar cross section at kT = 525 keV was done by [83]. The authors found a value approximately a factor of 2 higher than [79] and a rather good agreement with the other results. In 1999 a new measurement at neutron energies of 35.8 and 67.1 keV by [84] supports the previous measurements but have rather large (20 and 12%, respectively) uncertainties. In 2000 [85] determined at the high flux reactor of the ILL in Grenoble an accurate value of (1.93 ± 0.05) b for the ¹⁴N(n_thermal,p)¹⁴C cross section, that is in good agreement with some results present in literature but differs by 10% with the lower extreme value.

For this reason, a careful new evaluation is needed and the THM was applied to determine the cross section of the ¹⁴N (n,p)¹⁴C reaction by selecting the QF contribution to the ²H(¹⁴N,p ¹⁴C)p reaction. The experiment was performed at INFN-LNS where the SMP Tandem accelerator provided a 40 MeV ¹⁴N beam on deuterated polyethylene target (CD₂) of about 150 μg/cm² was placed at 90°with respect to the beam axis. The reaction channel selection has been performed via the prior selection of the Z = 6 locus in the ΔE-E telescope and the full kinematic reconstruction of the selected events with the hypothesis of an unitary mass for the third undetected particle.

The experimental Q-value spectrum has been reconstructed and the result is shown in Figure 5. The spectrum is centered at −1.61 MeV, showing a FWHM of about 130 keV, reflecting the energy loss of the incoming beam in the 150 μg/cm² CD₂ thick target. The experimental value nicely overlaps the expected one of −1.599 MeV, thus showing the correct selection of the reaction channel. Further studies on these data, presently under way and beyond the scope of this article, will be published in near future.

FIGURE 5

FIGURE 5. The experimental Q-value for the selected ²H(¹⁴N,p ¹⁴C)p events. The black arrow correspond to the theoretical Q value, Q_th = -1.6 MeV.

6 Conclusion

The use of THM is helping in achieving neutron producers and poisons reaction cross sections at energies relevant for the s-process in stars, where they are otherwise unreachable, because of the Coulomb barrier presence or the struggling with the neutron detection.

For ¹³C(α,n)¹⁶O the results obtained with THM has already moved forward our knowledge in this sense, as it summarized in Trippella and La Cognata [48]. On the contrary, much work remains to be done for ²²Ne(α,n)²⁵Mg, where the results of an ongoing analysis of a TH measurement can boost our knowledge on its reaction rate. This has not been found out yet, despite of recent indirect measurements clarifying the role of crucial levels in ²⁶Mg. The same holds for the neutron poisons reactions, which affect the neutron flux available for the s-process nucleosynthesis, mainly ¹⁷O(n,α)¹⁴C and ¹⁴N(n,p)¹⁴C. Also in these cases, ongoing analysis on TH measurements will help in completing the picture of our understanding of the neutron involving reactions in the stellar plasma.

Thus, the overall s-process nucleosynthesis understanding can be strongly constrained by the results of these reaction rates, jointly with the measurements of half-lives of radioactive nuclei at the s-process path branching points.

Author Contributions

RS, MLC, GLG, SP, and MLS wrote the paper. All the coauthors participated to the experiments described in the paper and revised the paper itself. Experimental data were analyzed by MLC, RS, GLG, and MLS for ¹³C(α,n)¹⁶O, ²²Ne(α,n)²⁵Mg, ¹⁷O(n,α)¹⁴C and ¹⁴N(n,p)¹⁴C, respectively.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

The authors acknowledge “Programma ricerca di ateneo UNICT2020-22 linea 2” and “Starting grant 2020” from the University of Catania.

References

1. Käppeler F, Gallino R, Bisterzo S, Aoki W. Thesprocess: Nuclear Physics, Stellar Models, and Observations. Rev Mod Phys (2011) 83:157–93. doi:10.1103/RevModPhys.83.157

CrossRef Full Text | Google Scholar

2. Pritychenko B (2020). Determination of Solar System R-Process Abundances Using endf/b-viii.0 and Tendl-2015 Libraries. doi:10.2172/1737434

CrossRef Full Text | Google Scholar

3. Busso M, Gallino R, Wasserburg GJ. Nucleosynthesis in Asymptotic Giant Branch Stars: Relevance for Galactic Enrichment and Solar System Formation. Annu Rev Astron Astrophys (1999) 37:239–309. doi:10.1146/annurev.astro.37.1.239

CrossRef Full Text | Google Scholar

4. Gallino R, Busso M, Picchio G, Raiteri CM, Renzini A. On the Role of Low-Mass Asymptotic Giant Branch Stars in Producing a Solar System Distribution of S-Process Isotopes. Astrophysical J (1988) 334:L45. doi:10.1086/185309

CrossRef Full Text | Google Scholar

5. Iliadis C, Longland R, Champagne AE, Coc A, Fitzgerald R. Charged-particle Thermonuclear Reaction Rates: Ii. Tables and Graphs of Reaction Rates and Probability Density Functions. Nucl Phys A (2010) 841:31–250. doi:10.1016/j.nuclphysa.2010.04.009

CrossRef Full Text | Google Scholar

6. The LS, El Eid MF, Meyer BS. s‐Process Nucleosynthesis in Advanced Burning Phases of Massive Stars. Astrophysical J (2007) 655:1058–78. doi:10.1086/509753

CrossRef Full Text | Google Scholar

7. Busso M, Vescovi D, Palmerini S, Cristallo S, Antonuccio-Delogu V. s-Processing in Agb Stars Revisited. Iii. Neutron Captures from Mhd Mixing at Different Metallicities and Observational Constraints. Astrophysical J (2021) 908:55. doi:10.3847/1538-4357/abca8e

CrossRef Full Text | Google Scholar

8. D’Agata G, Pizzone R, La Cognata M, Indelicato I, Spitaleri C, Palmerini S, et al. The 19F (α, P)22Ne Reaction at Energies of Astrophysical Relevance by Means of the Trojan Horse Method and its Implications in Agb Stars. Astrophysical J (2018) 860:61.

Google Scholar

9. Cristallo S, Cognata ML, Massimi C, Best A, Palmerini S, Straniero O, et al. The Importance of the 13C(α,n)16O Reaction in Asymptotic Giant Branch Stars. Astrophysical J (2018) 859:105. doi:10.3847/1538-4357/aac177

CrossRef Full Text | Google Scholar

10. Adsley P, Battino U, Best A, Caciolli A, Guglielmetti A, Imbriani G, et al. Reevaluation of the Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 Reaction Rates. Phys Rev C (2021) 103:015805. doi:10.1103/PhysRevC.103.015805

CrossRef Full Text | Google Scholar

11. Mascali D, Musumarra A, Leone F, Romano F, Galatà A, Gammino S, et al. Pandora, a New Facility for Interdisciplinary In-Plasma Physics. The Eur Phys J A (2017) 53:1–14. doi:10.1140/epja/i2017-12335-1

CrossRef Full Text | Google Scholar

12. Mascali D, Santonocito D, Amaducci S, Andò L, Antonuccio V, Biri S, et al. A Novel Approach to β-Decay: PANDORA, a New Experimental Setup for Future In-Plasma Measurements. Universe (2022) 8:80. doi:10.3390/universe8020080

CrossRef Full Text | Google Scholar

13. Cvetinović A, Spitaleri C, Spartá R, Rapisarda G, Puglia S, La Cognata M, et al. Trojan Horse Measurement of the ¹⁰B(p,α₀)⁷Be Cross Section in the Energy Range from 3 Kev to 2.2 Mev. Phys Rev C (2018) 97:065801.

Google Scholar

14. Rapisarda G, Spitaleri C, Cvetinović A, Spartà R, Cherubini S, Guardo G, et al. Study of the ¹⁰B(p,α₁)⁷Be Reaction by Means of the Trojan Horse Method. Eur Phys J A (2018) 54:1–12. doi:10.1140/epja/i2018-12622-3

CrossRef Full Text | Google Scholar

15. Spartà R, Lamia L, La Cognata M, Spitaleri C, Rapisarda G, Guardo G, et al. ¹⁰B(n,α₀)⁷Li and ¹⁰B(n,α₁)⁷Li Reactions Measured via Trojan Horse Method. Eur Phys J A (2021) 57:1–9.

Google Scholar

16. Hayakawa S, La Cognata M, Lamia L, Yamaguchi H, Kahl D, Abe K, et al. Constraining the Primordial Lithium Abundance: New Cross Section Measurement of the 7Be + N Reactions Updates the Total 7Be Destruction Rate. The Astrophysical JournalL (2021) 915:L13. doi:10.3847/2041-8213/ac061f

CrossRef Full Text | Google Scholar

17. Pizzone R, Spampinato C, Spartá R, Couder M, Tan W, Burjan V, et al. Indirect Measurement of the 3He(n, p)3H Reaction Cross Section at Big Bang Energies. Eur Phys J A (2020) 56:1–7. doi:10.1140/epja/s10050-020-00212-x

CrossRef Full Text | Google Scholar

18. Guardo G, Spitaleri C, Lamia L, Spartá R, Carlin N, Cherubini S, et al. The ¹⁰B(n,α)⁷Li Cross Sections at Ultra-low Energy through the Trojan Horse Method Applied to the ²H(¹⁰B,α⁷Li)H. Eur Phys J A (2019) 55:1–13. doi:10.1140/epja/i2019-12914-0

CrossRef Full Text | Google Scholar

19. Gulino M, Spitaleri C, Cherubini S, Crucillà V, Cognata ML, Lamia L, et al. Study of the6Li(n, α)3H Reaction via the2H Quasi-free Break-Up. J Phys G: Nucl Part Phys (2010) 37:125105. doi:10.1088/0954-3899/37/12/125105

CrossRef Full Text | Google Scholar

20. Lamia L, Spitaleri C, Carlin N, Cherubini S, Del Szanto M, Gulino M, et al. Indirect Study of (P, α) and (N, α) Reactions Induced on boron Isotopes. Il nuovo cimento C (2008) 31:423–31.

Google Scholar

21. Tumino A, Spitaleri C, Bonomo C, Cherubini S, Figuera P, Gulino M, et al. Quasi-free 6Li(n,α)3H Reaction at Low Energy from 2H Break-Up. Eur Phys J A (2005) 25:649–50. doi:10.1140/epjad/i2005-06-085-1

CrossRef Full Text | Google Scholar

22. Lamia L, Mazzocco M, Pizzone RG, Hayakawa S, La Cognata M, Spitaleri C, et al. Cross-section Measurement of the Cosmologically Relevant 7Be(n, α)4He Reaction over a Broad Energy Range in a Single Experiment. Astrophysical J (2019) 879:23. doi:10.3847/1538-4357/ab2234

CrossRef Full Text | Google Scholar

23. Gulino M, Cherubini S, Kubono S, Lamia L, Cognata ML, Pizzone RG, et al. Application of Trojan Horse Method to Radioactive Ion Beams Induced Reactions. J Phys Conf Ser (2020) 1610:012005. doi:10.1088/1742-6596/1610/1/012005

CrossRef Full Text | Google Scholar

24. Satchler G. Direct Nuclear Reactions, International Series of Monographs on Physics. Oxford, UK: Oxford University Press (1983).

Google Scholar

25. Pizzone R, Spitaleri C, Lamia L, Bertulani C, Mukhamedzhanov A, Blokhintsev L, et al. Trojan Horse Particle Invariance Studied with the ⁶Li(d,α)⁴He and ⁷Li(p,α)⁴He Reactions. Phys Rev C (2011) 83:045801.

Google Scholar

26. Pizzone R, Spitaleri C, Bertulani C, Mukhamedzhanov A, Blokhintsev L, La Cognata M, et al. Updated Evidence of the Trojan Horse Particle Invariance for the 2H(d, p)3H Reaction. Phys Rev C (2013) 87:025805.

CrossRef Full Text | Google Scholar

27. Baur G. Breakup Reactions as an Indirect Method to Investigate Low-Energy Charged-Particle Reactions Relevant for Nuclear Astrophysics. Phys Lett B (1986) 178:135–8. doi:10.1016/0370-2693(86)91483-8

CrossRef Full Text | Google Scholar

28. Shapiro IS, Kolybasov VM, Augst GR. Treiman-Yang Criterion for Direct Nuclear Reactions. Nucl Phys (1965) 61:353–67. doi:10.1016/0029-5582(65)90097-0

CrossRef Full Text | Google Scholar

29. Spitaleri C. Proceedings of the Fifth Hadronic Physics winter Seminar. Folgaria-TN, italy: World Scientific Singapore (1990).

Google Scholar

30. Spitaleri C, La Cognata M, Lamia L, Mukhamedzhanov A, Pizzone R. Nuclear Astrophysics and the Trojan Horse Method. Eur Phys J A (2016) 52:1–9. doi:10.1140/epja/i2016-16077-2

CrossRef Full Text | Google Scholar

31. Spitaleri C, La Cognata M, Lamia L, Pizzone R, Tumino A. Astrophysics Studies with the Trojan Horse Method. Eur Phys J A (2019) 55:1–29. doi:10.1140/epja/i2019-12833-0

CrossRef Full Text | Google Scholar

32. Pizzone R, Spitaleri C, Mukhamedzhanov A, Blokhintsev L, Bertulani C, Irgaziev B, et al. Effects of Distortion of the Intercluster Motion in ²H, ³He, ³H, ⁶Li, and ⁹Be on Trojan Horse Applications. Phys Rev C (2009) 80:025807.

Google Scholar

33. Pizzone R, Bertulani C, Lamia L, Cognata ML, Sergi M, Spartá R, et al. Clusters and Their Fundamental Role for Trojan Horse Method. Eur Phys J A (2020) 56:1–11. doi:10.1140/epja/s10050-020-00285-8

CrossRef Full Text | Google Scholar

34. Tribble R, Bertulani C, La Cognata M, Mukhamedzhanov A, Spitaleri C. Indirect Techniques in Nuclear Astrophysics: a Review. Rep Prog Phys (2014) 77:106901. doi:10.1088/0034-4885/77/10/106901

PubMed Abstract | CrossRef Full Text | Google Scholar

35. Tumino A, Bertulani CA, La Cognata M, Lamia L, Pizzone RG, Romano S, et al. The Trojan Horse Method: A Nuclear Physics Tool for Astrophysics. Annu Rev Nucl Part Sci (2021) 71:345–76. doi:10.1146/annurev-nucl-102419-033642

CrossRef Full Text | Google Scholar

36. Gallino R, Arlandini C, Busso M, Lugaro M, Travaglio C, Straniero O, et al. Evolution and Nucleosynthesis in Low‐Mass Asymptotic Giant Branch Stars. II. Neutron Capture and thes‐Process. Astrophysical J (1998) 497:388–403. doi:10.1086/305437

CrossRef Full Text | Google Scholar

37. Xu Y, Takahashi K, Goriely S, Arnould M, Ohta M, Utsunomiya H. NACRE II: an Update of the NACRE Compilation of Charged-Particle-Induced Thermonuclear Reaction Rates for Nuclei with Mass Number A&lt;16. Nucl Phys A (2013) 918:61–169. doi:10.1016/j.nuclphysa.2013.09.007

CrossRef Full Text | Google Scholar

38. Descouvemont P. Microscopic Analysis of the 13C(α, n) ¹⁶O and 13C(α,α)¹³C Reactions. Phys Rev C (1987) 36:2206–11. doi:10.1103/PhysRevC.36.2206¹³

CrossRef Full Text | Google Scholar

39. Dufour M, Descouvemont P. Multichannel Study of the 13C(α, n)16O and 16O(n,γ)17O Reactions. Phys Rev C (2005) 72:015801. doi:10.1103/PhysRevC.72.015801

CrossRef Full Text | Google Scholar

40. Ciani GF, Csedreki L, Rapagnani D, Aliotta M, Balibrea-Correa J, Barile F, et al. Direct Measurement of the ^{13}C(α,n)^{16}O Cross Section into the S-Process Gamow Peak. Phys Rev Lett (2021) 127:152701. doi:10.1103/PhysRevLett.127.152701

PubMed Abstract | CrossRef Full Text | Google Scholar

41. La Cognata M, Spitaleri C, Tumino A, Typel S, Cherubini S, Lamia L, et al. Bare-nucleus Astrophysical Factor of the 3He(d, p)4He Reaction via the “Trojan Horse” Method. Phys Rev C (2005) 72:065802. doi:10.1103/PhysRevC.72.065802

CrossRef Full Text | Google Scholar

42. Drotleff HW, Denker A, Knee H, Soine M, Wolf G, Hammer JW, et al. Reaction Rates of the S-Process Neutron Sources Ne-22(alpha, n)Mg-25 and C-13(alpha, n)O-16. Astrophysical J (1993) 414:735. doi:10.1086/173119

CrossRef Full Text | Google Scholar

43. Faestermann T, Mohr P, Hertenberger R, Wirth H-F. Broad Levels in O17 and Their Relevance for the Astrophysicalsprocess. Phys Rev C (2015) 92:052802. doi:10.1103/PhysRevC.92.052802

CrossRef Full Text | Google Scholar

44. Tilley DR, Weller HR, Cheves CM. Energy Levels of Light Nuclei A = 16-17. Nucl Phys A (1993) 564:1–183. doi:10.1016/0375-9474(93)90073-7

CrossRef Full Text | Google Scholar

45. Heil M, Detwiler R, Azuma RE, Couture A, Daly J, Görres J, et al. The 13C(α,n) reaction and its Role as a Neutron Source for The sprocess. Phys Rev C (2008) 78:025803. doi:10.1103/PhysRevC.78.025803

CrossRef Full Text | Google Scholar

46. La Cognata M, Spitaleri C, Trippella O, Kiss GG, Rogachev GV, Mukhamedzhanov AM, et al. Measurement of the -3 keV Resonance in the Reaction 13C(α,n)16O of Importance in the S-Process. Phys Rev Lett (2012) 109:232701. doi:10.1103/PhysRevLett.109.232701

PubMed Abstract | CrossRef Full Text | Google Scholar

47. La Cognata M, Spitaleri C, Trippella O, Kiss GG, Rogachev GV, Mukhamedzhanov AM, et al. On the Measurement of the 13C(α, n)16O S-Factor at Negative Energies and its Influence on the S-Process. Astrophysical J (2013) 777:143. doi:10.1088/0004-637X/777/2/143

CrossRef Full Text | Google Scholar

48. Trippella O, Cognata ML. Concurrent Application of ANC and THM to Assess the 13C(α,n)16O Absolute Cross Section at Astrophysical Energies and Possible Consequences for Neutron Production in Low-Mass AGB Stars. Astrophysical J (2017) 837:41. doi:10.3847/1538-4357/aa5eb5

CrossRef Full Text | Google Scholar

49. Pellegriti MG, Hammache F, Roussel P, Audouin L, Beaumel D, Descouvemont P, et al. Indirect Study of the 13C(α, n)16O Reaction via the 13C(7Li, t)17O Transfer Reaction. Phys Rev C (2008) 77:042801. doi:10.1103/PhysRevC.77.042801

CrossRef Full Text | Google Scholar

50. Guo B, Li ZH, Lugaro M, Buntain J, Pang DY, Li YJ, et al. New Determination of the 13C(a, n)16O Reaction Rate and its Influence on the S-Process Nucleosynthesis in AGB Stars. Astrophysical J (2012) 756:193. doi:10.1088/0004-637X/756/2/193

CrossRef Full Text | Google Scholar

51. Avila ML, Rogachev GV, Koshchiy E, Baby LT, Belarge J, Kemper KW, et al. New Measurement of Theαasymptotic Normalization Coefficient of The 1/2+state in O17 at 6.356 MeV that Dominates the C13(α,n)O16 reaction Rate at Temperatures Relevant for Thesprocess. Phys Rev C (2015) 91:048801. doi:10.1103/PhysRevC.91.048801

CrossRef Full Text | Google Scholar

52. Mezhevych SY, Rudchik AT, Rudchik AA, Ponkratenko OA, Keeley N, Kemper KW, et al. Cluster Structure of O17. Phys Rev C (2017) 95:034607. doi:10.1103/PhysRevC.95.034607

CrossRef Full Text | Google Scholar

53. Burjan V, Mrazek J, D’Agata G. Anc from Experimental Perspective. Front Astron Space Sci (2020) 2020:71. doi:10.3389/fspas.2020.562466

CrossRef Full Text | Google Scholar

54. Kiss GG, La Cognata M, Yarmukhamedov R, Tursunmakhatov KI, Wiedenhöver I, Baby LT, et al. Indirect Determination of the Astrophysical S Factor for the Li6 ( p,γ)Be7 Reaction Using the Asymptotic Normalization Coefficient Method. Phys Rev C (2021) 104:015807. doi:10.1103/PhysRevC.104.015807

CrossRef Full Text | Google Scholar

55. Burjan V, Hons Z, Kroha V, Mrázek J, Piskoř Š, Mukhamedzhanov AM, et al. The Determination of the Astrophysical S-Factor of the Direct ¹⁸O(p,γ)¹⁹F Capture by the Anc Method. Eur Phys J A (2019) 55:1–10. doi:10.1140/epja/i2019-12801-8

CrossRef Full Text | Google Scholar

56. Kiss GG, La Cognata M, Spitaleri C, Yarmukhamedov R, Wiedenhöver I, Baby LT, et al. Astrophysical S-Factor for the 3He(α,γ)7Be Reaction via the Asymptotic Normalization Coefficient (ANC) Method. Phys Lett B (2020) 807:135606. doi:10.1016/j.physletb.2020.135606

CrossRef Full Text | Google Scholar

57. D'Agata G, Kilic AI, Burjan V, Mrazek J, Glagolev V, Kroha V, et al. Si26(p,γ)P27 Direct Proton Capture by Means of the Asymptotic Normalization Coefficients Method for Mirror Nuclei. Phys Rev C (2021) 103:015806. doi:10.1103/PhysRevC.103.015806

CrossRef Full Text | Google Scholar

58. Davids CN. A Study of (α, N) Reactions on 9Be and 13C at Low Energies. Nucl Phys A (1968) 110:619–36. doi:10.1016/0375-9474(68)90377-1

CrossRef Full Text | Google Scholar

59. Bair JK, Haas FX. Total Neutron Yield from the Reactions 13C(α, n)16O and 17,18O(α, n)20,21Ne. Phys Rev C (1973) 7:1356–64. doi:10.1103/PhysRevC.7.1356

CrossRef Full Text | Google Scholar

60. Kellogg S, Vogelaar R, Kavanagh R. Bull Am Phys Soc (1989) 34:1192.

61. Harissopulos S, Becker HW, Hammer JW, Lagoyannis A, Rolfs C, Strieder F. Cross Section of the C13(α,n)O16 reaction: A Background for the Measurement of Geo-Neutrinos. Phys Rev C (2005) 72:062801. doi:10.1103/PhysRevC.72.062801

CrossRef Full Text | Google Scholar

62. Longland R, Iliadis C, Karakas AI. Reaction Rates for the S-Process Neutron Source ²²Ne+ α. Phys Rev C (2012) 85:065809. doi:10.1103/physrevc.85.065809

CrossRef Full Text | Google Scholar

63. Jaeger M, Kunz R, Mayer A, Hammer JW, Staudt G, Kratz KL, et al. N22e(α,n)M25g: The Key Neutron Source in Massive Stars. Phys Rev Lett (2001) 87:202501. doi:10.1103/physrevlett.87.202501

PubMed Abstract | CrossRef Full Text | Google Scholar

64. Jayatissa H, Rogachev GV, Goldberg VZ, Koshchiy E, Christian G, Hooker J, et al. Constraining the 22Ne(α,γ)26Mg and 22Ne(α,n)25Mg Reaction Rates Using Sub-coulomb α-transfer Reactions. Phys Lett B (2020) 802:135267. doi:10.1016/j.physletb.2020.135267

CrossRef Full Text | Google Scholar

65. Mazzaglia M, Amato AS, Battiato G, Calabrese G, Celona L, Dantoni L, et al. Design and First Operations of a ECR Based He⁻ Source at INFN-LNS. In: Proc. of ICIS’21 - International Conference on Ion Sources (IOP Publishing) (2022). accepted for publication.

Google Scholar

66. Mohr P, Heinz C, Pignatari M, Dillmann I, Mengoni A, Käppeler F. Re-evaluation of the 16O(N, γ)17O Cross Section at Astrophysical Energies and its Role as a Neutron Poison in the S-Process. Astrophysical J (2016) 827:29. doi:10.3847/0004-637x/827/1/29

CrossRef Full Text | Google Scholar

67. Pignatari M, Gallino R, Heil M, Wiescher M, Käppeler F, Herwig F, et al. The Weak S-Process in Massive Stars and its Dependence on the Neutron Capture Cross Sections. Astrophysical J (2010) 710:1557–77. doi:10.1088/0004-637x/710/2/1557

CrossRef Full Text | Google Scholar

68. Sanders RM. Study of the 14C(p, n)14N and 14C(α, n)17O Reactions. Phys Rev (1956) 104:1434–40. doi:10.1103/PhysRev.104.1434

CrossRef Full Text | Google Scholar

69. Koehler PE, Graff SM. O17(n,α)14C Cross Section from 25 meV to Approximately 1 MeV. Phys Rev C (1991) 44:2788–93. doi:10.1103/PhysRevC.44.2788

PubMed Abstract | CrossRef Full Text | Google Scholar

70. Schatz H, Kaeppeler F, Koehler PE, Wiescher M, Trautvetter H-P. O-17(n, alpha)C-14 - Closure of a Primordial CNO Bi-cycle? Astrophysical J (1993) 413:750. doi:10.1086/173042

CrossRef Full Text | Google Scholar

71. Wagemans J, Wagemans C, Goeminne G, Serot O, Loiselet M, Gaelens M. The 17O(n,α)14 Creaction from Subthermal up to Approximately 350 keV Neutron Energy. Phys Rev C (2002) 65:034614. doi:10.1103/PhysRevC.65.034614

CrossRef Full Text | Google Scholar

72. Avila ML, Rogachev GV, Goldberg VZ, Johnson ED, Kemper KW, Tchuvil'sky YM, et al. α-Cluster Structure of O18. Phys Rev C (2014) 90:024327. doi:10.1103/PhysRevC.90.024327

CrossRef Full Text | Google Scholar

73. Ajzenberg-Selove F. Energy Levels of Light Nuclei A = 18-20. Nucl Phys A (1987) 475:1–198. doi:10.1016/0375-9474(87)90205-3

CrossRef Full Text | Google Scholar

74. Gulino M, Spitaleri C, Tang XD, Guardo GL, Lamia L, Cherubini S, et al. Suppression of the Centrifugal Barrier Effects in the Off-Energy-Shell neutron+17Ointeraction. Phys Rev C (2013) 87:012801. doi:10.1103/PhysRevC.87.012801

CrossRef Full Text | Google Scholar

75. Guardo GL, Spitaleri C, Lamia L, Gulino M, La Cognata M, Tang X, et al. Assessing the Near Threshold Cross Section of the O17(n,α)C14 reaction by Means of the Trojan Horse Method. Phys Rev C (2017) 95:025807. doi:10.1103/PhysRevC.95.025807

CrossRef Full Text | Google Scholar

76. Cognata ML, Spitaleri C, Mukhamedzhanov A, Banu A, Cherubini S, Coc A, et al. A Novel Approach to Measure the Cross Section of the 18O(p, α)15N Resonant Reaction in the 0-200 keV Energy Range. Astrophysical J (2009) 708:796–811. doi:10.1088/0004-637x/708/1/796

CrossRef Full Text | Google Scholar

77. Oliva A, Guardo G, Lamia L, Cherubini S, Cvetinovic A, D’Agata G, et al. Study of the Neutron-Induced Reaction ¹⁷O(n,α)¹⁴C at Astrophysical Energies via the Trojan Horse Method. Nuovo Cim.C (2020) 43:111. doi:10.1393/ncc/i2020-20111-710.1051/epjconf/202022702007

CrossRef Full Text | Google Scholar

78. Lugaro M, Herwig F, Lattanzio JC, Gallino R, Straniero O. s‐Process Nucleosynthesis in Asymptotic Giant Branch Stars: A Test for Stellar Evolution. Astrophysical J (2003) 586:1305–19. doi:10.1086/367887

CrossRef Full Text | Google Scholar

79. Brehm K, Becker HW, Rolfs C, Trautvetter HP, Käppeler F, Ratynski W. The Cross Section of 14N(n, p)14C at Stellar Energies and its Role as a Neutron Poison for S-Process Nucleosynthesis. Z Physik A - At Nuclei (1988) 330:167–72. doi:10.1007/bf01293392

CrossRef Full Text | Google Scholar

80. Koehler PE, O’Brien HA. N14(n,p)14C Cross Section from 61 meV to 34.6 keV and its Astrophysical Implications. Phys Rev C (1989) 39:1655–7. doi:10.1103/physrevc.39.1655

PubMed Abstract | CrossRef Full Text | Google Scholar

81. Koehler PE. N14(n,p)14C Cross Section Near thermal Energy. Phys Rev C (1993) 48:439–40. doi:10.1103/PhysRevC.48.439

CrossRef Full Text | Google Scholar

82. Gledenov YM, Salatski VI, Sedyshev PV, Sedysheva MV, Pshenichnyj VA, Andrzejewski J. Cross Sections of the 14N(n, p)14C Reaction at 24.5, 53.5 and 144 keV. Z Physik A - Hadrons Nuclei (1994) 348:199–200. doi:10.1007/bf01291917

CrossRef Full Text | Google Scholar

83. Sanami T, Baba M, Matsuyama I, Matsuyama S, Kiyosumi T, Nauchi Y, et al. Measurement of 14N(n,p)14C Cross Section for kT = 25.3 keV Maxwellian Neutrons Using Gridded Ionization Chamber. Nucl Instr Methods Phys Res Section A: Acc Spectrometers, Detectors Associated Equipment (1997) 394:368–73. doi:10.1016/S0168-9002(97)00698-0

CrossRef Full Text | Google Scholar

84. Kii T, Shima T, Sato H, Baba T, Nagai Y. Cross Section of the 14N(n,p)14 Creaction from 10 to 100 keV Measured by a Gas Scintillation Drift Chamber. Phys Rev C (1999) 59:3397–401. doi:10.1103/PhysRevC.59.3397

CrossRef Full Text | Google Scholar

85. Wagemans J, Wagemans C, Goeminne G, Geltenbort P. Experimental Determination of the 14N(n,p)14 Creaction Cross Section for thermal Neutrons. Phys Rev C (2000) 61:064601. doi:10.1103/PhysRevC.61.064601

CrossRef Full Text | Google Scholar