THM determination of the 65 kev resonance strength intervening in the 17 O ( p,α) 14 N reaction rate

Transcription

1 THM determination of the 65 kev resonance strength intervening in the 17 O ( p,α) 14 N reaction rate M. L. Sergi, C. Spitaleri, S. V. Burjan, S. Cherubini, A. Coc, M. Gulino, F. Hammache, Z. Hons, B. Irgaziev, G. G. Kiss, V. Kroha, M. La Cognata, L. Lamia, A. Mukhamedzhanov, R. G. Pizzone, S. M. R. Puglia, G. G. Rapisarda, S. Romano, N. de Séréville, E. Somorjai, and A. Tumino Citation: AIP Conference Proceedings 1645, 392 (2015); doi: / View online: View Table of Contents: Published by the AIP Publishing Articles you may be interested in Impact of THM reaction rates for astrophysics AIP Conf. Proc. 1681, (2015); / Measurement of the 13 C (α,n) 16 O reaction with the Trojan horse method: Focus on the sub threshold resonance at 3 kev AIP Conf. Proc. 1594, 206 (2014); / Precision Measurements of the 278 kev 14 N ( p,γ) and the 151 kev 18 O ( p,α) Resonance Parameters AIP Conf. Proc. 1090, 450 (2009); / Tensor analyzing powers of 3 He(d,p) 4 He reactions around 430 kev resonance AIP Conf. Proc. 570, 887 (2001); / Stopping Cross Section of Low Atomic Number Materials for He+, kev J. Appl. Phys. 36, 391 (1965); /

2 THM determination of the 65 kev resonance strength intervening in the 17 O(p,α) 14 N reaction rate M.L. Sergi, C. Spitaleri, S.V.Burjan, S. Cherubini, A.Coc, M. Gulino,, F.Hammache, Z. Hons, B. Irgaziev, G.G. Kiss, V. Kroha, M. La Cognata, L. Lamia, A. Mukhamedzhanov, R.G. Pizzone, S.M.R. Puglia, G.G. Rapisarda, S.Romano, N. de Séréville, E. Somorjai and A. Tumino, INFN-Laboratori Nazionali del Sud, Catania, Italy Università di Catania, Catania, Italy and INFN-Laboratori Nazionali del Sud, Catania, Italy Nuclear Physics Institute of ASCR Rez near Prague, CzechRepublic CSNSM, UMR 8609, CNRS/IN2P3 and Universitè Paris Sud 11, Bâtiment 104, Orsay Campus, France Present address: Universitá Kore di Enna, Enna, Italy IPN, IN2P3-CNRS et Université de Paris-Sud Orsay Cedex, France GIK Institute of Engineering Sciences and Technology Topi District Swabi NWFP, Pakistan ATOMKI, Debrecen,Hungary Università di Catania, Catania, Italy Cyclotron Institute,Texas A&M University College Station, USA IPN, IN2P3-CNRS et Universitè de Paris-Sud 11, Orsay Cedex, France Abstract. The 17 O(p,α) 14 N reaction is of paramount importance for the nucleosynthesis in a number of stellar sites, including red giants (RG), asymptotic giant branch (AGB) stars, massive stars and classical novae. We report on the indirect study of the 17 O(p,α) 14 N reaction via the Trojan Horse Method by applying the approach recently developed for extracting the resonance strength of the narrow resonance at E R c.m.=65 kev (E X =5.673 MeV). The strength of the 65 kev resonance in the 17 O(p,α) 14 N reaction, measured by means of the THM, has been used to renormalize the corresponding resonance strength in the 17 O + p radiative capture channel. Keywords: Nuclear Astrophysics, Direct reactions, Nucleosynthesis in novae, supernovae, and other explosive environments. PACS: f, k, v, g INTRODUCTION The 17 O(p,α) 14 N reaction is of large importance for the 17 O nucleosynthesis in a number of stellar sites, including red giants (RG), asymptotic giant branch (AGB) stars, massive stars and classical novae [1, 2]. In particular it governs the destruction of 17 O and the formation of the short-live radio-isotope 18 F which is of special interest for gamma ray astronomy [1, 2]. In the stellar temperatures of primary importance for 17 O nucleosynthesis (T= GK), the cross section of the 17 O(p,α) 14 N reaction is dominated by two resonances, one at E R c.m.=65 kev above the 18 F proton threshold, corresponding to the MeV 18 F level and the other one at E R c.m.=183 kev (E X =5.786 MeV). While, in the last years, several measurements ([2] and references therein) of the E R c.m.=183 kev resonance have drastically reduced the uncertainties on 17 O(p,α) 14 N rate in the context of explosive H-burning, only one direct measurement for the E R c.m.=65 kev resonance was performed [3]. In fact, because of the presence of the Coulomb barrier, the direct measurements at very low energies are very difficult and large uncertainties are still present on the available direct data [2, 4]. In addition, some sub-threshold levels could contribute to the total reaction rate and then a further study of this reaction in the energy region relevant for astrophysics is necessary. In order to reduce the uncertainties affecting the direct measurements, in the last twenty years many indirect methods have been developed. In particular the Trojan Horse Method (THM) [5, 6] is a powerful tool which selects, under appropriate kinematical conditions, the quasi-free (QF) contribution of a suitable three-body reaction performed at energies well above the Coulomb barrier to extract a charged particle two-body cross section at astrophysical energies, free of Coulomb suppression and electron screening effects. Exotic Nuclei and Nuclear/Particle Astrophysics (V). From Nuclei to Stars AIP Conf. Proc. 1645, (2015); doi: / AIP Publishing LLC /$

3 FIGURE 1. Experimental set-up adopted for the study of the 2 H( 17 O,α 14 N)n reaction. The displacement of the detectors assures the covering of the QF angular region. In this paper, we report on the results of the indirect measurement of 17 O(p,α) 14 N reaction at energies below 300 kev, being the data analysis already described in [7]. As extensively discussed in [7], both the 65 and 183 kev resonances were observed and the 17 O(p,α) 14 N reaction rate was well established, for the first time, below T9=0.1[8]. To study the 17 O(p,α) 14 N reaction at such low-energies, we used the Trojan Horse Method (THM) [5, 6, 9, 10, 11, 12, 13]. This is an indirect technique that, by selecting the quasi-free (QF) contribution to a suitable three-body reaction A + a(x + s) c + C + s performed at energies well above the Coulomb barrier, leads to the extraction of the cross section of the binary reaction A+x c+c at astrophysical energies, unhindered by Coulomb suppression. In particular, the present study of the 17 O(p,α) 14 N reaction was performed in the energy window relevant for astrophysics by selecting the QF-contribution to the 2 H( 17 O, 14 N α)n reaction. The deuteron was used as the tro jan horse nucleus because of its p n structure; the proton is brought in the nuclear field of 17 O while the neutron acts like a spectator to the binary quasi-free reaction. THM has been used in studying several problems, ranging from light element burning reactions [12, 14, 15, 16, 17, 18, 19, 20, 22, 23, 24, 25] CNO reactions [7, 8, 13], and removing/producing neutron reactions [26, 27]. THE EXPERIMENT The study of the 17 O(p,α) 14 N via the THM application was performed at the Laboratori Nazionali del Sud (LNS) in Catania (Italy). The SMP Tandem Van de Graaff accelerator provided a 41 MeV 17 O beam, with a spot size on target of about 1.5 mm and intensities up to 2-3 na, impinging on a deuterated polyethylene target (CD 2 ) of about 150 µg/cm 2 placed at 90 with respect to the beam axis. The angles and the energies of the ejected α and 14 N were detected in coincidence by using an experimental setup, symmetric with respect to the beam axis, which consisted of six singlearea, resistive-readout position-sensitive silicon detectors (PSDs) with spatial resolution of 0.5 mm. The neutron was not detected in these experiments, and its energy and emission angle were reconstructed from the momenta of the detected particles. The PSD detectors covered the angular ranges in the laboratory reference system (PSD 1,4 ), (PSD 2,5 ) and (PSD 3,6 ). In Fig. 1, a schematic drawing of the detection setup is shown. In front of the two forward PSDs, two ionization chambers (IC) were further used as E detectors to discriminate the impinging 14 N reaction products from 14 C coming from the 2 H( 17 O,α 14 C)p reaction. The ionization chambers were filled with 60 mbar of isobuthane gas and were closed on both sides by 1.5 µm thick mylar foil windows. The gas pressure inside the ionization chambers was selected to yield an high enough signal-to-noise ratio. No energy threshold was introduced by the E detector on 14 N detection, since the 14 N particles were emitted with energies higher than 18 MeV, thus exceeding the energy threshold introduced by our telescope ( 11 MeV) for 14 N detection. An energy threshold of about 2.5 MeV was introduced on α-particle detection in the same telescopeó. On the other hand, no E detectors were put in front of detectors devoted to the α-particles detection, PSD 2, PSD 3, PSD 5 and PSD 6, to avoid to set detection thresholds on α particles spectra, since the energy range spanned by them extends down to zero energy. The displacement of the detection setup was chosen in order to cover the angular region at which a strong contribution of the QF reaction mechanism is expected on the total reaction yield and to span momentum values of the undetected neutron between 0 MeV/c and 100 MeV/c. This ensures that the bulk of the QF contribution for the breakup process of interest falls inside the investigated regions, because the momentum distribution for the n p system has its 393

4 FIGURE 2. Left panel: a typical two-dimensional spectrum of a Position Sensitive Detector before the calibration, where both energy and position signal are expressed in channel. The clear kinematical locus around the channels in x-axis and in y-axis corresponds to the events relative to elastic scattering 17 O+ 12 C. These points were used for angular and energy calibration. Right panel: the same two-dimensional spectrum of the left panel after position-energy calibration. Now, the energy of the x-axis represents the energy of the detected particles expressed in MeV, while the position on y-axis represents the angle at which the particles were emitted. A good agreement between the experimental kinematical locus and the calculated one makes us confident of a good calibration procedure. maximum at p n = 0 MeV/c. The angles corresponding to this condition are known as QF angles. Energy and position signals of the detected particles were processed by standard electronics, together with the coincidence relative time. Coincidences among either one of the two forward PSDs and one of the three placed on the opposite side with respect to the beam axis were recorded by the data acquisition system. At the initial stage of the measurement, masks with a number of equally spaced slits were placed in front of each PSD to perform position calibration. The angle of each slit with respect to the beam direction was measured by means of an optical system, making it possible to establish a correlation between position signal from the PSDs and detection angle of the impinging particles. Detectors were calibrated at low energies using a three peaks α source ( 239 Pu at 5.16 MeV, 241 Am at 5.48 MeV 244 Cm at 5.80 MeV). At higher energies, elastic scattering of 6 Li off a 197 Au target and a 12 C target was emplojed, using two 6 Li beam energies, 35.9 MeV and 14.8 MeV. Moreover, additional runs were performed to measure the 12 C particle from the elastic scattering 17 O+ 12 C at E beam = 41 MeV. In this way an accurate calibration of PSD 1, PSD 4 was obtained as shown in Fig. 2. The IC was calibrated by taking the differences of the residual energies measured by PSD 1 and PSD 4 when the IC was empty and filled with isobutane at the working pressure, respectively. The total kinetic energy of the detected particles was reconstructed off-line, taking into account the energy loss in the target and in the entrance and exit windows of the ICs and in other dead layers. THE RESULTS The data analysis has been already described step by step in [7, 28], here only the main result is reported together with a very short theoretical introduction. In fact, as extensively described in [13, 7], the experimental THM cross section for the 2 H( 17 O,α 14 N)n QF process is given by the following equation: d 2 de c.m. dω n = 3 i=1 N i exp[ 1 2 (E c.m. E Ri ) 2 ] σ + a 0 + a 1 E c.m., (1) where a first-order polynomial has been added to account for non-resonant contributions. In addition, in Eq. (1), E Ri represent the resonance energies, σ=20 kev is the experimental standard deviation of the Gaussian function 394

5 approximating the detector response function and the N i parameters represent the TH resonance strengths in the E c.m. =0-250 kev interval [13]. In the case of narrow resonance, the N i parameters bear a fundamental physical meaning. In fact, they are easily connected to the resonance strengths (ωγ) i for the 18 F levels [29], which are the key parameters to evaluate the reaction rate for astrophysical applications. Since in this work we did not measure the absolute value of the cross section, the absolute strength of the resonance at 65 kev was obtained from the ratio between the N 1 and N 2 peak values through the relation [7, 13] (ωγ) 1 = ω 1 Γ (p 17 O) 1 σ R2 (θ) N 1 (ωγ) 2 (2) ω 2 σ R1 (θ) Γ (p 17 O) 1 N 2 where the subscripts 1 and 2 refer to the 65 and 183 kev resonances respectively, ω i = (2J18 F i +1)/[(2J17 O +1)(2J p + 1)] (i = 1,2) is the statistical factor, σ Ri (θ) is the direct transfer reaction cross section for the binary reaction A+a F i +s populating the resonant state F i with resonance energy E Ri and Γ (p 17 O) i is the partial width for the p+ 17 O 18 F i channel, leading to the population of the i-th excited state in 18 F [7]. This normalization procedure introduces an sizable advantage because the resonance strength can be deduced without introducing a spectroscopic factor, thus greatly reducing the uncertainties affecting resonance strengths when indirectly established. The detailed procedure to determine the strength of the resonance at 65 kev was described in [7], here the By taking (ωγ) 2 =(1.66±0.10) 10 3 ev, namely the weighted average of the three values for the 183 kev resonance strength given in the literature [2, 30, 31], by means of eq. 4 one gets: (ωγ) 1 = ( ) 10 9 ev. (3) This obtained value differs by 20% from the direct data [2] and, even if the difference between the two measurements is within the quoted experimental uncertainties, a possible explanation of such discrepancy could be due to the electron screening effect that was not taken into account in the direct measurement [2]. The definition of the resonance strength [29] (ωγ) i = 2J18 F i + 1 Γ (p 17 O) i (E Ri )Γ (α 14 N) i (E Ri ) (2J17 O + 1)(2J, (4) p + 1) Γ i (E Ri ) makes us able to give a new value of the strength of the E R =65 kev resonance also in the 17 O(p,γ) 18 F channel. Since, the ωγ parameter of the E R =65 kev resonance in the 17 O(p,α) 14 N reaction is proportional to the proton partial width Γ p, the exit channel partial width essentially coinciding with the total width through the statistical factor, and since the strength of the 65 kev resonance in the 17 O(p,γ) 18 F channel is proportional to Γ p as well, by using the following formula (ωγ) T HM pγ = (ωγ) T HM pα Γ γ Γ α, (5) the 65 kev resonance strength in the (p,γ) channel can be evaluated. The Γ γ and Γ α values used in Eq. 5 are those adopted in [2]. The THM-scaled resonance strength of the lowest energy resonance is then (ωγ) T pγ HM =( ) ev, to be compared with (1.64 ± 0.28) ev as given in the literature and in the most recent reviews [2, 8, 32, 33, 34]. This 30% difference between the THM-scaled resonance strength and the value in the literature might determine significant consequences on astrophysics motivating an evaluation of its reaction rate as described in [8]. In order to increase both the statistics, a further experiment was performed at Nuclear Structure Lab of the University of Notre Dame (Indiana, USA) in November 2008 and the data analysis is still in progress. ACKNOWLEDGMENTS This work has been partially supported by the Italian Ministry of University MIUR under the grants RFBR (FIRB2008) and LNS-Astrofisica Nucleare (fondi premiali). It has been partially supported also by the grant LC of the Czech MŜMT, grant M of the Czech Academy of Science, grant LH1101 of the AMVIS project. A. M. M. acknowledges the support by U.S. Department of Energy under Grants No. DE-FG52-09NA29467, No. DE-FG02-93ER40773, and No. DE-SC and by NSF under Grant No. PHY

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