Dipartimento di Fisica e Astronomia, Università degli Studi di Catania, Catania, Italy 3

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2 The 17 O(n,α) 14 C neutron induced reaction at the astrophysical energies studied via the Trojan Horse Method G.L. Guardo 1, C. Spitaleri 1,2, V. Burjan 3, S. Cherubini 1,2, S. Chesnevskaya 4, A. Cvetinovic 1, G. D Agata 1,2, M.Gulino 1,5, S. Hayakawa 6, I. Indelicato 1, M. La Cognata 1, L. Lamia 1, M. La Commara 7,8, C. Matei 4, M. Mazzocco 9, J. Mrazek 3, S. Palmerini 10, D. Pierroutsakou 8, R. G. Pizzone 1, O. Trippella 10, S.M.R. Puglia 1, G. G. Rapisarda 1, S. Romano 1,2, M.L. Sergi 1, R. Spartà 1, A. Tumino 1,5, Y. Xu 4, H. Yamaguchi 6 1 Laboratori Nazionali del Sud INFN, Catania, Italy 2 Dipartimento di Fisica e Astronomia, Università degli Studi di Catania, Catania, Italy 3 Nuclear Physics Institute of ASCR, Rez, Czech Republic 4 Extreme Light Infrastructure - Nuclear Physics, Magurele, Romania 5 Università di Enna "Kore", Enna, Italy 6 Center for Nuclear Study, The University of Tokyo, Tokyo, Japan 7 Dipartimento di Scienze Fisiche, Università di Napoli Federico II, Napoli, Italy 8 INFN Sezione di Napoli, Napoli, Italy 9 Dipartimento di Fisica e Astronomia, Università di Padova, and INFN Sezione di Padova, Padova, Italy 10 Dipartimento di Fisica, Università di Perugio and INFN Sezione di Perugia, Perugia, Italy 1. Astrophysical motivation and state of the art Recent studies have shown the role of the 17 O(n,α) 14 C reaction during the primordial nucleosynthesis in the framework of the Inhomogeneous Big Bang Nucleosynyhesis (IBBN). In [1] the authors suggest that it is possible to identify an enhanced baryon density zone against a less dense one. The baryon to photon ratio η is then fixed in average between the 2 zones to the value of 6.2 ± [2] deduced from WMAP measurements. In the high density region, the 14 C abundance increases from to ~ mass fraction [3]. Once produced, carbon could activate the network leading to 22 Ne: 14 C(α,γ) 18 O(n,γ) 19 O(β) 19 F(n,γ) 20 F(β) 20 Ne(n,γ) 21 Ne(n,γ) 22 Ne, (1) fundamental for the production of heavy elements. A major 14 C synthesis channel in neutron rich areas is the 17 O(n,α) 14 C reaction, in the energy range kev, corresponding to the temperatures that characterize the model: < T < K. In stellar structure of massive stars (with initial mass M > 8Mʘ), the 17 O(n,α) 14 C reaction may act as a neutron poison reducing the neutron flux available for the so-called weak s-process. In such stars, the main neutron source is provided by the 22 Ne(α,n) 25 Mg reaction, activated at the end of the convective core He-burning and in the successive convective shell C-burning [4]. However, the Hecore is enriched in oxygen because of the CNO cycle, thus an ignition of the 16 O(n,γ) 17 O reaction is also expected. The produced 17 O can experience both (α,n) or (n,α) reactions. The 17 O(α,n) 20 Ne reaction represents a recycle channel for the neutron flux while the 17 O(n,α) 14 C reaction is a neutron poison reaction. Therefore the knowledge of the ratio between the cross section of these reactions is important to determine the neutron flux available for the s-process. All the reasons given above justify the study of the 17 O(n,α) 14 C reaction in the energy window of interest for those contexts, namely from thermal up to 400 kev neutron energies. In this energy range, the intermediate compound 18 O nucleus exhibit four different states that could contribute in the final cross section evaluation [5]. The reaction has been largely studied in the past by different authors in direct experiments and by applying the detailed balance principle to the inverse reactions [6-9], but the total reaction rate calculation coming from different data sets show a difference evaluated in the work of Wagemans et al. [9] to be of about a factor in the astrophysically relevant energy region.

3 The previously mentioned direct measurements have been recently corroborated by the indirect 17 O(n,α) 14 C investigation performed via the Trojan Horse Method (THM) [10]. The THM study allowed us to cover the energy region of interest for astrophysics and to assess the contribution of the two already known resonant levels detected by Wagemans et al. [9]. In addition, the THM measurement allowed us to determine the influence of the 8121 kev resonant level that, because of its J π assignment, represents l=3 resonance for the 17 O+n system. Moreover, from such measurement, it is possible to excite the subthreshold level centered at -7 kev in the center-of-mass system corresponding to the MeV level of 18 O, which is important to determine the 17 O(n,α) 14 C reaction rate. Finally, the angular distribution of the resonance corresponding to the E = 8213 kev level is better reproduced by adopting l=2 instead of l=0, as usually assumed by considering the low value of the neutron energy availabe in the c.m. system. However, THM measurements performed so far experience significant discrepancy in the high energy region due to the different kinematical range covered that could negatively influence the normalization process. Moreover, the low statistic in the angular distribution led to a substandard assignement of the angular momentum that give and important contribution in the calculation of the total reaction rate. Thus, we need a new accurate experiment for a precise assessment of the reaction rate for the reaction under study. 2. Method The THM is an indirect technique to measure the relevant reaction a + x c + d at the Gamow peak, without the need of any extrapolation procedure. In particular, THM allows one to overcome Coulomb barrier suppression and electron screening effects in the measurement of the cross section. Conversely, neutron-induced direct reactions may suffer only of centrifugal barrier penetration effects if partial waves other than s-wave are present in the entrance channel. Recently, the method has been extended to the indirect study of neutron induced reactions, showing the possibility to bypass even the centrifugal barrier suppression effects and to pick out the contribution of the mere nuclear interaction. One can briefly describe the method as follows (more details can be found in refs. [11-12]). A projectile a strikes a target nucleus A, whose wave function has a large amplitude for a s-b cluster configuration. Under proper kinematical conditions, particle a interacts only with the part b (participant) of the target nucleus A, while the other part s behaves as a spectator to the process a+b(+s) c+d(+s) (quasi-free mechanism). To completely determine the kinematical properties of the spectator s, the other two particles have to be detected and identified and their energies and emission angles measured. Since the bombarding energy is chosen to overcome the Coulomb barrier in the entrance channel for the a+a c+d+s, the particle b can be brought into the nuclear field and induce the a+b c+d reaction. Moreover, assuming that the beam energy can be compensated for by the b+s binding energy, the two-body reaction can take place at very low a-b relative energy, e.g. in the region of astrophysical relevance. In the plane wave impulse approximation, the three body cross section can be expressed as: d /dec d c d d (KF) (ps) 2 d N /d (2) where KF is a kinematical factor containing the final state phase space factor, (ps) 2 is the momentum distribution of the spectator s inside A, and d N /d is the differential nuclear cross section for the two body reaction a+b c+d. The deduced two-body reaction cross-section N (E) represents the nuclear part alone, the Coulomb and centrifugal barriers being already overcome in the entrance channel. The absolute cross section is obtained by normalization to the direct data at higher energies. In order to compare the two sets of data it is necessary to correct the N (E) for the penetration function through the centrifugal barrier.

4 We would like to stress that the presence of the quasi-free mechanism is a necessary condition for the THM applicability, so that the experimental set-up will be chosen in order to optimise the corresponding kinematical properties. Moreover the comparison with direct data in the energy region above the Coulomb barrier represents a significant validity test for the method. In this proposal, we aim to apply the THM to study the 17 O(n,α) 14 C reaction by measuring the 2 H( 17 O, 14 C)p 3-body reaction cross section. 3. Experimental procedure We plan to perform an experiment aimed to investigate the S(E) factor of the 17 O(n,α) 14 C reaction in the energy range between ~40 kev and ~300 kev, where up to now no definite conclusions are drawn with direct measurements. To this purpose, we plan to apply the THM to the 2 H( 17 O, 14 C)p 3-body reaction, by using a 43,5 MeV 17 O beam impinging on a CD2 target, 150 μg/cm 2 thick. In this framework, the deuteron will act as TH-nucleus because of its obvious p n structure, its low binding energy (Bp-n~2.2 MeV), and a very well known l=0 p-n momentum distribution given in terms of the radial Hulthén wave function. The experimental setup is thought to detect the produced alpha particle in coincidence with 14 C. The former will be detected by means of a standard ΔE E telescope, by using a 50 mb isobutane filled ionization chamber (IC) as ΔE detector and a standard 1000 μm thick position sensitive detector (PSD) as E detector. The alpha particles will be detected by using two 500 μm thick PSD s. The ΔE E telescope and the other PSD will be placed at opposite sides with respect to the beam direction. Moreover a symmetric setup wil be arranged in order to increase the statistics. Figure 1 Left) schematical view of the proposed setup. Right) simulated angular range covered in the experiment: the black points correspond to a proton momentum pp <30 MeV/c while the red points show the contribution of pp <5 MeV/c. The kinematical conditions for the 2 H( 17 O, 14 C)p reaction channel will allow the selection of the locus corresponding to very small proton momentum values, i.e. pp <30 MeV/c. This constraint strongly underlines the basic feature of the THM, that is the selection of the quasi-free reaction mechanism on the 3-body coincidence yield. To cover the kinematical region where a strong (even dominant) presence of the QF mechanism is expected on the 2 H( 17 O, 14 C)p reaction yield, we plan to use the CAMERA2000 at LNS, to fix the ΔE E telescope at a distance of about 50 cm from the target, thus allowing us to span the angular region of 7.5 ±3 for the 14 C detection. The other PSD s, devoted to α detection, will be placed at about 40 cm from the target on the opposite side with respect to the beam line direction and covering the angular range 18 ±3 and 27 ±3. The angular position of the detectors is chosen in order to increase the kinematical region with respect the previous experiments allowing us to avoid the problem arising previously in the high energy region and to improve the angular resolution in order to magnify the angular distribution of interest. In fig.1 it is reported a schematic view of the expected setup (right panel) with a simulated plot showing the

5 expected angular region covered in the experiment (left panel). The black points represent the event corresponding to a proton momentum pp <30 MeV/c, while the red points refer to the contribution of a proton momentum pp <5 MeV/c. 4. Beam time request For the requested beam time evaluation, we refer to a typical value of cross section for deuteron break-up processes of about 100 mbarn/sr 2 MeV and to a conservative estimation of the QF mechanism contribution (about 10%, assumed from similar reactions studied in the past). Thus, considering a 17 O beam intensity of 0.5 pna (~ pps), a CD2 target of about 150 μg/cm 2 (~10 19 atoms/cm 2 ) we should expect about 0.01 cps. Thus, aiming at covering the energy range between 0-1 MeV (including that one for normalization to direct data) with a bin of 10 kev, events are required for having a statistical error of 10%. This corresponds to about 15 days (45 BTU) of beamtime for the THM measurement. Moreover, we do need 3 days (9 BTU) of beamtime for energy and position calibration of the detectors. 5. References [1] T. Kajino et al., Nuclear Physics A588 (339c) [2] D. J. Fixen & J. C. Mather, Astrophysical Journal 581 (817) [3] J. H. Applegate et al., Astrophysical Journal 329 (572) [4] M. Pignatari et al., Astrophysical Journal 710 (1557) [5] F. Ajzenberg-Selove, Nuclear Physics A475 (1) [6] R. M. Sanders, Physical Review 104 (1434) [7] P. E. Koehler & S. M. Graff, Physical Review C44 (2788) [8] H. Schatz et al., Astrophysical Journal 413 (750) [9] J. Wagemans et al., Physical Review C65 (34614) [10] M. Gulino et al., Physics Review C87 (012801) [11] Spitaleri et al., Phys. At. Nucl. 74, 1725 (2011) [12] Tribble et al. Rep. Progr. Phys. 77, (2014)