15O+α resonant elastic scattering to study cluster states in19Ne

1 Content from this work may be used under the terms of theCreativeCommonsAttribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1234567890 11th International Conference on Clustering Aspects of Nuclear Structure and Dynamics IOP Publishing IOP Conf. Series: Journal of Physics: Conf. Series 863 (2017) 012026 doi :10.1088/1742-6596/863/1/012026 15O+αresonant elastic scattering to study cluster states in 19Ne D Torresi1, C Wheldon1, Tz Kokalova1, S Bailey1, A Boiano2, C Boiano3, M Cavallaro4, S Cherubini4, A Di Pietro4, J P Fernandez Garcia4, M Fisichella4, T R Glodariu5, J Grebosz6, M La Cognata4, M La Commara2,7, M Lattuada4,8, M Mazzocco9,10, D Mengoni9,10, C Parascandolo2,7, D Pierroutsakou4, G Pizzone4,8, C Signorini10, C Stefanini9, L Stroe5, C Spitaleri4,8, E Strano9,10 and M Zadro12 1School of Physics, University of Birmingham, Birmingham, B15 2TT, UK 2INFN-Sezione di Napoli, Via Cinthia, I-80126 Napoli, Italy 3INFN-Sezione di Milano, Via Celoria 16, I-20133 Milano, Italy 4INFN-Laboratori Nazionali del Sud, Via S. Sofia 62 Catania, Italy 5TNIPNE, 407 Atomistilor Street, 077125 Magurele, Romania 6Department of Nuclear Reactions, Institute for Nuclear Studies, ul. Hoza 69, 00-681 Warsaw, Poland 7Dipartimento di Fisica, Universit´a di Napoli, Via Cinthia, I-80126 Napoli, Italy 8Dipartimento di Fisica, Universit´a di Catania, Via A. Doria 64, I-95123 Catania, Italy 9INFN-Sezione di Padova, Via F. Marzolo 8, I-35131 Padova, Italy 10INFN-Laboratori Nazionali di Legnaro, Viale dellUniversit´a 2, I-35020 Legnaro, Italy E-mail: [email protected] Abstract. Clustering phenomena are well known in nuclear physics for stable nuclei, both αconjugate (N=Z, A=2N), like 8Be, 16O, 20Ne, and non-α-conjugate, like 6Li and 7Li. In general, it is expected that light exotic nuclei may also exhibit cluster behavior. Moving out of the valley of stability configurations can be found where at least one of the clusters is unbound or weakly bound, thus not satisfying the strong internal correlation requirement of classical clusters. This is so-called exotic clustering. The study of such systems presents many difficulties, due, mainly, to the low intensities typical of radioactive ion beams. Therefore, few significant experimental studies have been performed so far. In this work we searched for α-cluster states in 19Ne above its α-decay threshold measuring, for the first time, the 15O(4He,4He) elastic scattering excitation function. Moreover, this study classified low-energy states in Ne in the astrophysically important region in this HCNO-break-out nucleus. 1. Introduction In recent years the isotope 19Ne has attracted much interest. This is mainly due to the astrophysical importance of the 15O(α,γ)19Ne reaction rate that was identified as a break out from the hot-CNO cycles and a trigger for the following rapid proton capture that populate masses higher then neon [1]. Therefore, the structure of the low-energy spectrum of 19Ne close to the 15O+αthreshold (Sα=3.528 MeV) is very important. Moreover, the states in 19Ne near the proton threshold (Sp=6.411 MeV), are again important, as the destruction of the key radionuclide 18F [2], occurs mainly via the 18F(p,α)15O reaction. 2 1234567890 11th International Conference on Clustering Aspects of Nuclear Structure and Dynamics IOP Publishing IOP Conf. Series: Journal of Physics: Conf. Series 863 (2017) 012026 doi :10.1088/1742-6596/863/1/012026 Neon isotopes exhibit strong α-clustering phenomena, even in the ground states. Neon-20 [3, 4, 5] and 21Ne [6, 7] are two of the best examples of nuclear clustering known to date. Therefore, it is important to extend this cluster study to the neutron poor side of the neon isotopes that is, at the moment, almost totally unknown concerning the clustering point of view. This is needed in order to understand if such clustering phenomena persist in 19Ne and, if so, to what extent. The present work reports the direct measurement of 15O+αelastic scattering using the technique of Thick Target Inverse Kinematics scattering [8] to investigate the structure of the low-lying-energy levels of 19Ne. The method as implemented here allows the elastic scattering excitation function to be measured and enables to measure the αreduced α-width directly across a large excitation-energy range using a single beam energy. The nature of the technique makes it ideally suited to use with low-intensity, large-energy-profile radioactive ion beams. 2. The Experimental Technique The TTIK method [9, 8, 10] with a gas target consists of delivering the beam into a reaction chamber filled with gas (typically 4He) at such a pressure as to fully stop the beam. At the same time the gas acts as both target and degrader of the beam energy. The interaction between projectile and target nuclei can occur at different depths in the chamber along the beam direction, and thus at different energies. Thanks to the different stopping power of the gas for heavy projectiles and light recoils, the light recoiling particles can reach the telescopes placed at the downstream end of the chamber with respect to the entrance window. From the measurement of the recoil energy one can deduce the interaction position along the beam direction and thus the energy of the scattering event. This relationship is unequivocal if the detected light recoils originate from elastic scattering. The α-particles are detected at small angles covering the 0◦ region in the laboratory system, corresponding to the region around 180◦in the center-of-mass. In this condition it is possible to extract the elastic scattering angular distribution in the region where the Coulomb scattering is minimized over the resonant elastic scattering. This technique is particularly suitable for measuring elastic scattering excitation functions with radioactive beams since it strongly reduces the beam time request with respect to the thin target technique. 2.1. The Experimental Apparatus The 15O radioactive ion beam was delivered by the EXOTIC facility [11, 12] at the Laboratori Nazionali di Legnaro (LNL, Italy) by means of the in-flight technique. A primary 80 MeV 15N5+ beam was delivered by the LNL-XTU tandem Van de Graaff accelerator and impinged on a H2cryogenic gas target at a pressure of 1 bar and a temperature of 24◦, corresponding to an equivalent target thickness of about 1.35 mg/cm2of H2. The 15O beam [13], produced by means of the two-body reaction p(15N,15O)n (Qval=-3.54 MeV), was selected and purified from the scattered primary 15N beam and other contaminant nuclei by mean of several optical elements: a 30◦-bending magnet, a Wien-filter, two quadrupole triplets and several collimators. Two parallel plate avalanche counters (PPACs) [14] were placed downstream prior to the beam entering the scattering chamber. The PPACs were utilized for on-line monitoring of the secondary 15O beam profile, to provide an event-by-event reconstruction of the trajectory of the beam and for timing purposes. Finally, 15O beam entered the the scattering chamber filled with 4He gas at a pressure of 467 mbar, that was separated from the high vacuum beam line by a 2.2 µm thick Havar window. The 15O beam intensity was in the range 1-2×104particles/s with ≈99% purity. The 4He gas pressure and the temperature were continuously monitored by two capacitance manometers with an accuracy of 0.3% and a thermocouple device with 0.1 K sensitivity. The detection apparatus, illustrated in Fig. 1, consisted of two Double-Sided Silicon Strip Detectors (DSSSDs) from the EXPADES detector array [14]. The detectors were placed, the first at 0◦ and the second off 0◦as shown in figure 1. 3 1234567890 11th International Conference on Clustering Aspects of Nuclear Structure and Dynamics IOP Publishing IOP Conf. Series: Journal of Physics: Conf. Series 863 (2017) 012026 doi :10.1088/1742-6596/863/1/012026 window 50 cm 52 cm 4He at 467 mbar DSSSD 14 cm PPAC B PPAC A triple collimator Figure 1. (color online) Schematic view of the experimental apparatus. The detection system consisted of two DSSSDs and two PPACs required for the time-of-flight measurement and beam tracking. 3. Data Analysis The goal of the analysis is the reconstruction of the elastic scattering excitation function from the energy of the detected α-particles and their impact point on the detectors. The data coming from the two detectors were divided into three angular ranges. The first one corresponding to the whole detector placed at 0◦while the dector off 0◦was divided in two angular ranges. The first step is the identification of α-particles produced in elastic scattering events. This was performed through measurements of the time-of-flight between the entering of the particle in the scattering chamber and the following recoiling particle on a DSSSD. The final results are summarized in Fig. 2 showing the three excitation functions obtained from the two detectors, all three reveal the presence of resonances. Unfortunately the elastic scattering excitation functions corresponding to the detector off 0◦have low statistics and very low resolution. This is mainly due to the fact that the effects of the energy and angular spread of the beam profile are larger for particles that are scattered at larger angles. Moreover due to the low statistics obtained for the detector off 0◦the angular range is relatively high and this can be a further cause of resolution worsening. Due to the low statistics and low resolution the data at larger angle were not used in the R-matrix calculation (see section 4) for the identification of the spins and parities of the resonances observed in the elastic scattering excitation function. 4. Results and Conclusions In order to determine the main properties of the observed resonances: widths, partial decay branches, energies and spins, an R-matrix calculation was performed. A comprehensive description of the R-matrix theory can be found in Ref. [15]. A full R-Matrix fit was performed using Azure2 [16] which uses the Brune transformation. This was done for 0◦(θcm = 180◦). The statistics at the other angles were too poor to provide any constraint to the fit. Instead, the spins and energies of states from the mirror nucleus, 19F [17], were used as a starting point for the fit; a powerful technique, with a good agreement between the energy levels in each nucleus. A two channel calculation were performed to fit the data: 15O(1 2 −) + α(0+) and 18F(1+) + p(1 2 +), with channel radii for the ith decay channel, ri=r0(A 1 3 recoil +A 1 3 i).and r0=1.4 fm. Levels in the 5.3 – 8.9 MeV excitation-energy region have been investigated, with a number of newly observed states in this work. Furthermore, partial widths have been extracted for the observed levels, for many of them it is the first time measure-width data has been available. Reliable cross sections have been measured across the full range of levels populated representing important input to reaction-rate models. 4 1234567890 11th International Conference on Clustering Aspects of Nuclear Structure and Dynamics IOP Publishing IOP Conf. Series: Journal of Physics: Conf. Series 863 (2017) 012026 doi :10.1088/1742-6596/863/1/012026 5 6 7 8 9 0 50 100 150 200 250 300 350 400 450 5 6 7 8 9 [mbarn/sr] Ω/dσd 0 20 40 60 80 100 120 140 [MeV] CM E 5 6 7 8 9 0 20 40 60 80 100 120 140 Figure 2. Elastic scattering excitation function for 15O+4He at the three different angular values: from top to down ∼180◦,∼160◦,∼155◦ Acknowledgments The present work has received funding from the European Unions Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 659744. The UK STFC is acknowledged for providing funding under grant ST/L005751/1. M.Z. was partially supported by the Croatian Science Foundation under Project No. 7194. References [1] Wiescher M et al 2007 Prog. Part. and Nucl. Phys. 59 51 [2] Bardayan D W et al 2015 Phys. Lett. B751 311 [3] von Oertzen W 2001 Eur. Phys. J. A11 403 [4] Freer M Rep. Prog. Phys. 70 2149 [5] von Oertzen W, Freer M and Kanada-En‘yo Y 2005 Phys. Rep. 432 43 [6] Thummerer S et al 2003 J. Phys. G29 509 [7] Wheldon C et al Eur. Phys. J. A26 321 [8] Artemov K P et al 1990 Sov. J. Nucl. Phys. 52 634 [9] Kallman K et al 1994 Nucl. Instr. and Meth. in Phys. Res. A338 413 [10] Zadro M et al 2007 Nucl. 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