Fusion cross section measurements of astrophysical interest for light heavy ions systems within the STELLA project

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1 EPJ Web of Conferences, 8 (7) DOI:.5/ejconf/78 Fusion cross section measurements of astrohysical interest for light heavy ions systems within the STELLA roject Guillaume Fruet,, Sandrine Courtin,, David G. Jenkins, Marcel Heine, Daniele Montanari,, Luke G. Morris, Phili Adsley,, Christian Beck, Serge Della Negra, Florent Haas, Fairouz Hammache, Oliver S. Kirsebom 5, Anne Meyer, Patrick H. Regan,7, Matthias Rudiger, Nicolas de Séréville, and Christelle Stodel 8 Université de Strasbourg, CNRS, IPHC UMR 778, F-7 Strasbourg, France USIAS, Université de Strasbourg, F-7 Strasbourg, France Deartment of Physics, University of York, YO 5DD York, United Kingdom Institut de Physique Nucléaire d Orsay, UMR88, CNRS/INP, Université Paris-Sud, F-9 Orsay, France 5 Deartment of Physics and Astronomy, Aarhus University, DK-8 Århus C, Denmark Deartment of Physics, University of Surrey, GU 7XH Guildford, United Kingdom 7 AIR Division, National Physical Laboratory, TW LW Teddington, United Kingdom 8 Grand Accélérateur National d Ions Lourds, CNRS/INP, F-7 Caen, France Abstract. This contribution is focused on the STELLA roject (STELlar LAboratory), which aims at the measurement of fusion cross sections between light heavy ions like C+ C, C+ O or O+ O at dee subbarrier energies. The gamma-article coincidence technique is used in order to reduce background contributions that become dominant for measurements in the nanobarn regime. The exerimental setu comosed of an ultra high vacuum reaction chamber, a set of silicon stri detectors, u to LaBr (Ce) scintillators from the UK FATIMA collaboration, and a fast rotating target system will be described. The C+ C fusion reaction has been studied from = to 5. MeV using STELLA at the Andromède facility in Orsay, France. commissioning results are resented in this article. Introduction The measurement of fusion cross sections between light heavy ions like C, O or Ne is at the center of interest to both nuclear hysics and astrohysics. In articular, the C+ C fusion is a key reaction in the nucleosynthesis rocess during the C burning hase in the core of massive stars, in tye Ia suernovae, and in suerbursts from accreting neutron stars []. For reactions involving light ions, structural effects may seriously affect the fusion rocess. For instance, the C- C system exhibits strong resonances from a few MeV/A down to the sub-barrier regime, suggesting ossible cluster formation in the comound nucleus [], while an alternative exlanation involves relatively large sacings and narrow widths of Mg comound nucleus energy levels []. The fusion reaction mechanism has been widely studied in the ast [ ], and desite considerable efforts in both exerimental and theoretical studies questions concerning the behaviour of such reactions at very low colliding energies remain oen. Indeed, it can be seen in figure that data show discreancies among the various ex- guillaume.fruet@ihc.cnrs.fr eriments with large uncertainties at the lowest measured energies. Moreover, extraolations towards the astrohysical region differ by orders of magnitude [7]. Figure. S-factor as a function of center of mass energy for the C+ C reaction. Theoretical calculations differ by orders of magnitude in the Gamow window E G =.5±. MeV at T = 5 8 K for this system. The measurement technique will be resented in Sect.. The dedicated STELLA setu as well as technical develoments will then be resented in Sect.. Fi- The Authors, ublished by EDP Sciences. This is an oen access article distributed under the terms of the Creative Commons Attribution License. (htt://creativecommons.org/licenses/by/./).

2 EPJ Web of Conferences, 8 (7) DOI:.5/ejconf/78 nally, reliminary results from the commissioning hase of STELLA will be discussed in Sect.. Exerimental aroach The C+ C fusion reaction has been studied at several beam energies around and below the Coulomb barrier (E C. MeV). In this regime, the three main exit channels are: C + C Ne + α (Q =. MeV), () C + C Na + (Q =. MeV), () C + C Mg + n (Q =. MeV). () At stellar temerature T = 5 8 K, the corresonding Gamow energy for C+ CisE G =.5 ±. MeV, below the neutron emission threshold. Thus, the exit channel () has not been studied within this work. The detection of either light charged articles or γ s from the deexcitation of both residual nuclei Ne and Na has been widely used in the ast to identify fusion events (see [,, 8]). Recently, it has been shown in Ref. [9] that the detection of the light charged articles in coincidence with the γ-ray from the heavy artner allows a very reliable selection of fusion events, due to a drastical rejection of background coming essentially from target contaminants ( C, H, H,...) or room background radiation. For the resent measurement, C targets of, and 5 µg/cm are used. It is of great imortance to avoid carbon build-u on them, a rocess that will affect recise determination of the reaction energy and robability by increasing the target thickness []. The differential cross section is then given by: ( ) dσ dω lab = N I ɛ γ Ω t N t, () where N is the number of coincidence events, I the beam intensity, t the acquisition time, N t the number of atoms in the target, Ω the solid angle covered by the article detector, and ɛ γ the γ efficiency. To clarify the evolution of the S-factor towards the Gamow window, fusion cross sections below the nanobarn range should be measured. One ossibility to obtain sufficient statistics is to increase the beam intensity, tyically at values µa. Under such conditions, the usual thin fixed targets have a relatively short lifetime of few hours and an imroved target system would be useful. Technical develoments. Reaction chamber and article detectors A reaction chamber has been designed and machined at IPHC-CNRS Strasbourg, France. The bottom art consists of a stainless steel cylinder used to accomodate the article detector suort, feedthrough and the uming system. The uer art is a.5 mm thick aluminium dome of cm diameter around which are laced LaBr (Ce) scintillators for gamma detection. Light charged articles like alhas and rotons are detected using annular double-sided silicon stri detectors (DSSD), S and S design by Micron Semiconductor. These detectors are divided into and rings, resectively. A PCB to suort the silicon chis has been develoed and built using RC material, which is known for low outgazing to revent carbon build-u in targets. Along with this develoment, a new in connection system has been conceived. It allows to read signals of detectors from below the target lane, thus reducing material budget inside the reaction chamber. In the arrangment configuration seen in figure, the total solid angle coverage is % of π. Figure. Mechanical drawing of the reaction chamber and the arrangment of annular Si-stri detectors around the target which is at the center of the chamber. The beam is going from right to left. The resolution of these detectors is about 5- kev (FWHM) for article energies around 5.5 MeV giving sufficient resolution to searate the different energies associated to rotons and alhas coming from the fusion reaction. Aluminium foils of.8 µm thickness rotect detectors at backward angles from low energy electrons emitted from the target, while a µm foil revents intense elastically scattered C beam to reach the forward detector. To obtain an absolute normalisation for the fusion cross section two Si surface barrier detectors are used to measure scattered beam. They are laced symetrically at an angle θ lab = 5 with resect to the beam axis, where the Mott cross section has a rather flat and energy-indeendent local maximum decreasing systematic uncertainties associated with the recise location of the monitors. A Faraday cu after the chamber was also used to control the beam intensity. The signals from the Si detectors are rocessed using a data acquisition system based on the µtca technology. It is a digital time stam acquisition system suorting 9

3 EPJ Web of Conferences, 8 (7) DOI:.5/ejconf/78 channels, and a dedicated card distributes an external 5 MHz frequency signal used to synchronise the clocks of article and γ cards to reconstruct coincident events offline.. Gamma detectors At bombarding energies below the Coulomb barrier, the states that can be oulated in Ne and Na exhibit a deexcitation attern which cascades essentially through their first excited state [, ]. The energies of the corresonding γ transitions are E γ = kev and E γ = kev for exit channel () and (), resectively. Novel generation scintillators are widely used in nuclear hysics as they rovide sufficient energy resolution combined with a high detection efficiency. In this roject, LaBr (Ce) detectors from the UK FATIMA collaboration have been combined with our system. Details concerning these detectors can be found in Ref. []. Comrehensive Geant simulations have been carried out to retrieve the highest hotoeak efficiency for the detection of γ lines of interest. A cylindrical setu where all the detectors are facing the beam line has been constructed. A mechanical drawing of the suorting structure is shown figure and a detailed descrition of the efficiency studies can be found in Ref. []. Figure. Cylindrical suorting structure of the LaBr (Ce) crystals. The detectors face the beam line and are arranged in the most comact configuration. In the otimal configuration where LaBr (Ce) are used the hotoeak efficiency is ɛ % for E γ = kev, and ɛ 8% for E γ = kev. The detectors self activity, which originates from the decay of the 8 La isotoe as well as the Ac contaminant, roduces a constant background in γ sectra that is irrelevant in coincidence analysis. This can be used to erform an online calibration of the sectra allowing to correct for the gain drift of the PMTs that may occur with change of room temerature. A reliminary framework for this rocedure is also resented in Ref. [].. Target develoments In order to avoid carbon build-u on targets, several efforts have been made to guarantee an ultra high vacuum in the reaction chamber. Thus, only comatible materials have been used inside STELLA, like suited high vacuum connectors and PCB for DSSDs. The uming system is comosed of a rimary dry um and a cryoum cooled down to a temerature of about 5 K. The latter has a sufficient diameter to cover the entire volume of the cylindrical art of the chamber, and a ressure of 8 mbar is obtained close to the target osition. A serious limitation while using thin carbon targets is their lifetime when beam intensity is higher than µa. One ossibility to overcome this difficulty is to use a fast rotating target system where the beam sot location is distributed along a ath on the target surface. This enables better heat dissiation which is one of the cause of the target breaking rocess. In collaboration with GANIL, a dedicated rotating target system has then been develoed for the STELLA roject and tested under beam at the end of the commissioning hase. Details of the aaratus will be given in a forthcoming technical aer.. The Andromède facility For the first camaign, the STELLA station has been installed to the Andromède facility [5] in Orsay, France. Andromède is a MV Pelletron accelerator which can rovide various beams from light ions such as He, C, O, u to heavy Au-cluster or CH molecules. STELLA was laced at the dedicated 9 line where the resence of a magnetic diole ensures the delivery of a high urity C beam. The accelerator ran in very stable energy and intensity conditions during the whole data taking. During the exeriment, we used C + and C + beams from = to 5. MeV iminging onto fix targets of, and 5 µg/cm with beam intensity varying from na to 5 na. results are resented in the next section. results A tyical matrix of the article energy as a function of ring number of a DSSD located at backward angles (8 θ lab 8 ) is shown in figure. This sectrum was obtained at a beam energy of MeV and the solid lines corresond to kinematics calculations for the different rotons and alhas ( i and α i ) associated to excited states in the heavy reaction ejectile. No contaminant contribution is resent in the sectrum at this energy, where the total fusion cross section is mb. The corresonding single γ sectrum is shown in figure 5. This is the sum over the 8 detectors used during

4 EPJ Web of Conferences, 8 (7) DOI:.5/ejconf/78 SB - Without coincidences Time difference between articule and γ module Ring # 8 8 7,8,9 α,5 alha s roton s = MeV 8 θ 8 lab α = MeV 9 coincidences E (kev) 5 t art. - t γ (ns) Figure. Angular distribution of articles obtained with a DSSD located at backward angles. The solid lines are kinematics calculations at = MeV for the roton (red) and alha (black) exit channels. Figure. Time difference between γ and article detection. A background free coincident eak is clearly visible. together. The and α exit channels are associated to the first hase of the exeriment. As mentioned in Sect.., a continuous background is visible in the single γ sectrum mainly due to LaBr self activity and gamma transitions originating from the first excited states of Ne and Na are masked by this contribution. 5 γ sectrum - All LaBr No coincidence = MeV Without coincidence 8,9 7 Particle sectrum,5 α kev 5 kev self activity LaBr 8 LaBr kev 8 8 E γ (kev) Figure 5. Gamma sectrum summed over 8 LaBr obtained at = MeV. The self activity as well as the two imortant γ lines at and kev from the C+C fusion are indicated by arrows. Nevertheless, this background in the single γ sectrum can be suressed when analysing coincident events. A tyical sectrum of the detection time difference between γ s and articles is shown in figure where a well defined eak around 5 ns with few random coincidences sread all over the sectrum is visible. This allows a reliable selection of coincident events. As an examle, the effect of requiring a γ coincidence in the article sectrum from a DSSD located at backward angles when gating around E γ = kev is deicted in figure 7, where single and coincident sectra are shown Coincidence with E γ = kev E (kev) Figure 7. Single article sectrum (uer art) and the corresonding one obtained by gating around E γ = kev (lower art) for a DSSD located at backward angles. The and α exit channels, which have no true coincidence with γ s, are effectively suressed in the coincident sectrum. the oulation of the ground state of the heavy artner Na and Ne, resectively. No coincident gammas are exected and their contributions are effectively suressed in the coincident sectrum. In figure 7, the rotons associated to the different excited states in Na are searated enough to obtain artial fusion cross sections after correcting for the decay branching ratios tabulated in Ref. []. From the sectra dislayed in figure 7, a first exerimental value can be obtained for the detection efficiency of E γ = kev by taking the ratio of integrated coincident and single eaks. This gives a result of about %, which is in agreement with Geant simulation of the 8 detectors setu used during the exeriment. Further checks

5 EPJ Web of Conferences, 8 (7) DOI:.5/ejconf/78 will be done by comaring simulation results to γ source runs where a 5 Eu source has been utilized. 5 Summary A new dedicated exerimental station (STELLA) has been develoed and built in IPHC-CNRS, Strasbourg, to measure fusion cross sections of light heavy ions. It makes use of the γ-article coincidence technique to ensure reliable selection of fusion events at dee sub-barrier energies. During the first camaign, STELLA has been couled to the LaBr detectors from the UK FATIMA collaboration and installed at the Andromède facility in Orsay, France, to study the C+ C fusion reaction from = to 5. MeV. Identification of the various exit channels of the reaction has been roven ossible using both single and coincident article sectra. This allows to take into account in the total fusion cross section the feeding of the ground and excited states in the Na and Ne exit channels. In collaboration with GANIL, a fast rotating target system has been develoed and tested under beam. With this aaratus, a beam intensity of 5µA may be used in the future. Then, considering the measured resonance of Sillane et al. at E cm =. MeV [] a statistical uncertainty of % can be obtained in about days, whereas several weeks are needed following Gasques et al. extraolation [7]. Acknowledgement We wish to thank G. Frémont (GANIL) and M. Loriggiola (LNL, Legnaro) for rearation of C targets for this commissioning run. References [] L.R. Gasques et al., PRC 7, 58 (7) [] R.G. Stokstad et al., PRL 7, (97) [] C.L. Jiang et al., PRL, 77 () [] L.J. Patterson et al., The Astr. J. 57, 77 (99) [5] B. Cujec et al., NPA, 9 (97) [] H.W. Becker et al., ZPA, 5 (98) [7] C.L. Jiang et al., PRC 75, 58 (7) [8] K.U. Kettner et al., PRL 8, 7 (977) [9] C.L. Jiang et al., NIMA 8, 5 () [] E.F. Aguilera et al., PRC 7, () [] D.R. Tilley et al., NPA, 9 (998) [] R.B. Firestone, Nuclear Data Sheets 8, (7) [] O.J. Roberts et al., NIMA 78, 995 () [] M. Heine et al., JoP: Conf. Ser. 7, 5 () [5] M.J. Eller et al., NIMB 5, 77 (5) [] T. Sillane et al., PRL 98, 5 (7) [7] L.R. Gasques et al., PRC 7, 58 (5) 5