The 19F(7Li,7Be)19O charge-exchange reaction at 52 MeV incident energy has been performed at INFN-LNS in Catania using the MAGNEX spectrometer. The use of an algebraic ray-reconstruction technique has allowed to extract the 19O excitation energy spectrum and the experimental angular distributions obtained with a single angular setting of the spectrometer

Borello-Lewin, T

Cappuzzello, F

Carbone, D

Cavallaro, M

Cunsolo, A

Foti, A

Orrigo, S E A

Petrascu, H

Rodrigues, M R D

Schillaci, M

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181  Preliminary study of the 19F(7Li,7Be)19O reaction at 52 MeV with MAGNEX M. Cavallaroa,b, A. Cunsoloa,b, F. Cappuzzelloa,b, D. Carbonea,b, A. Fotic,  S.E.A.Orrigoa,b, M.R.D. Rodriguesb, M. Schillacia,b, T.Borello-Lewine, H.Petrascuf aDipartimento di Fisica e Astronomia, Università di Catania, Italy bINFN-Laboratori Nazionali del Sud, Catania, Italy cINFN-Sezione di Catania, Italy eNIPNE, Bucarest, Romania Abstract The 19F(7Li,7Be)19O charge-exchange reaction at 52 MeV incident energy has been performed at INFN-LNS in Catania using the MAGNEX spectrometer. The use of an algebraic ray-reconstruction technique has allowed to extract the 19O excitation energy spectrum and the experimental angular distributions obtained with a single angular setting of the spectrometer.  1 Introduction Over the last years, a general interest has concerned the study of nuclear structure and reaction dynamics far off the stability valley, arising from the possibility to explore new and exciting phenomena characteristic of those systems. The present study of the 19O neutron-rich nucleus via the (7Li,7Be) charge-exchange reaction is well inserted into this general context.  The investigation of this nucleus belongs to a research line that aims to a systematic exploration of both structural properties of a particular category of light neutron-rich nuclei, for which an inner core of an integer number of α particles is coupled to three external neutrons (Nα + 3 neutrons). The (7Li, 7Be) reaction at about 8 MeV/A incident energy has shown to be a suitable tool to explore such systems, as demonstrated by previous studies on other nuclei belonging to this category: 11Be and 15C [1], [2]. In fact, this process in such energy range proceeds with a considerable predominance of the direct one-step mechanism, thus being an useful probe for spectroscopic studies. In addition it has turned out to be suitable to populate configurations more complex than the single-particle ones, providing, thanks also to the good resolution achievable, complementary information on nuclear structure. 2 Experimental Setup The 7Li+++ beam at 52.2 MeV was accelerated by the Tandem facility of INFN-LNS. The 19F target was a 80 μg/cm2 thick AlF3 foil evaporated on a gold backing of 250 μg/cm2 produced at the chemical laboratory of LNS. A 27Al target (116 μg/cm2) was also used in order to estimate the aluminium presence in the target compound and subtract it in the final spectra. Supplementary runs were done also on a WO3 target (150 μg/cm2 on 20 μg/cm2 carbon backing) and on a carbon target (76 μg/cm2) for the subtraction of the contribution in the final spectra due to the oxygen and carbon impurities in the AlF3 target. The reaction ejectiles were momentum analyzed by the MAGNEX large acceptance magnetic spectrometer A picture of MAGNEX is shown in Fig.1. The MAGNEX quadrupole and dipole fields and the α-surface coil, together with the position of the Focal Plane Detector (FPD), were set in order to focus the 7Be ejectiles relative to the 19Og.s. in the focal plane position corresponding to a momentum deviation δ = 0.08 with respect to the central one.  The FPD [3],[4] was filled with 99.95% pure isobutane gas at 7 mbar pressure. The cathode was supplied at –950 V while the Frish-grid was grounded. The high voltage in the proportional wires was +750 V, and the lateral shaping wires between the Frish-grid and the proportional wires were maintained at increasing voltage by a separate power supply at +400 V. The silicon detectors were powered with +60 V voltage in a full depletion mode.  182  In the data analyzed up to now the spectrometer was located at a central angle of 25.12lab with respect to the beam incidence direction. Due to the large angular acceptance of MAGNEX (horizontally –0.090 rad, +0.110 rad, vertically ±0.125 rad in the spectrometer reference frame), this angular setting corresponds to a covered angular range 7.1° < θlab < 19.8° in the laboratory reference frame. Measurements at 0lab  and 25.6lab were also performed and the data analysis is in progress.  Fig.1: General view of the MAGNEX spectrometer. From the left to the right the scattering chamber, the quadrupole, the dipole and the Focal Plane Detector are visible. 3 19O excitation energy spectra The identification of the 7Be ejectiles was done by the MAGNEX Focal Plane Detector through a ΔE-E technique combined with the measurement of the magnetic rigidity vs. the kinetic energy of each detected particle [5].  An algebraic technique of ray-reconstruction of the detected ions [6] has been used to relate the final parameters measured at the focal plane to the initial phase space parameters. Such ray-reconstruction technique is based on the formalism of Differential Algebra [7]. It is a perturbative technique to solve the differential equations describing the motion of ions through the spectrometer and to obtain the Taylor coefficients of the flow linking the initial phase space with the final one. In this mathematic environment, the integration of the differential equations results a simpler algebraic task and very high order of the perturbation series can be treated.  With this technique, if the appropriate positions and directions of the detected ions are determined by measurement at the focal plane, one can reconstruct the full trajectories back to the reaction target and consequently obtain the scattering angles and the initial momenta of the reaction products, by the application of the inverse map of the measured phase space parameters.  In Fig.2 a reconstructed spectrum of the 19O excitation energy is shown. The spectrum obtained from the data with the AlF3 target is shown with superimposed that related to the aluminium target. The contribution of the Al-derived reaction is the only one up to about 6 MeV 19O excitation energy.  Several excited states are observed and identified in the low excitation energy region. The well isolated peaks are labelled with the relative excitation energy in MeV. Peaks marked with an asterisk refer to the transitions in which 7Be ejectiles are in the first excited state at 0.43 MeV. Most of the 19O states have been observed in the past by one and two neutron transfer reactions [8], thus confirming the capability of the (7Li,7Be) reactions to populate such states. For most of them the shell structure configuration is quite well 183  known as for example for the 1.47 MeV excited state (1/2+) which is mainly a single-particle state with the configuration of one neutron in the 2s1/2 orbital on a 18O(0+) core.  The estimated energy resolution is about 80 KeV (FWHM). In a previous test run, realized with the same experimental setting in order to check the feasibility and study the working conditions, a better resolution was achieved (about 50 KeV) [9]. There are two main reasons causing the lack of resolution in the present experiment. First, the gold backing of the AlF3 target was thicker (250 μg/cm2) in the present experiment compared to 120 μg/cm2 in the test run, thus producing less straggling in the target. In addition, in the test run the magnetic fields were set to focus the 7Be relative to the 19Og.s. in a different focal plane position, closer to the spectrometer optical axis (Xfoc = 0), where a minor contribution of aberrations is expected.   Fig.2 19O excitation energy spectrum obtained using the AlF3 target and, superimposed, the normalized spectrum obtained from the aluminium target. The known states of 19O are indicated with their energy. The presence of 27Mg and 16N impurity states are also shown. Peaks marked with an asterisk refer to the transitions in which 7Be ejectiles are in the first excited state at 0.43 MeV. 4 Angular Distributions One of the advantage of working with a large acceptance spectrometer is that, in a single setting of the instrument, several scattering angles are covered. As a consequence, a consistent part of a cross section angular distribution can be measured in a single run in the same experimental conditions, resulting in a reduction of the uncertainty due to the normalization of runs at different angles. In the present case, a single set of measurements at central angle 25.12lab allows to obtain an angular distribution for scattering angles about 10° < CM < 21° in the centre of mass reference frame. In Fig. 3 the measured angular distributions for the transition to the not resolved doublet of 19O ground and first excited state at 0.096 MeV is shown. In Fig. 4 the angular distribution for the transition to the 19O(1.47MeV, ½+) excited state is shown. A theoretical analysis of the 19F(7Li,7Be)19O reaction in the framework of the Charge-Exchange Quasiparticle Random Phase Approximation (CEX-QRPA) [10] is on the way. This approach is very powerful since can be used to describe the 19O nuclear structure and also to calculate the transition densities connecting the 19F ground to the 1p-1h states of 19O, thus allowing a direct connection to reaction cross section calculations by a suitable Distorted Wave Born Approximation (DWBA). The use of a realistic semi-184  microscopic interaction [11], that includes the tensor contribution, in both structure and reaction calculations, has allowed in the past to describe very well reactions similar to the present one as the 11B(7Li,7Be)11Be and 15N(7Li,7Be)15C in the same energy region [1], [2].  Fig.3 Measured angular distribution for the 7Li + 19F → 7Be + 19O(g.s. + 0.096MeV) transition.  Fig.4 Measured angular distribution for the 7Li + 19F → 7Be + 19O(1.47MeV) transition. References [1] F. Cappuzzello et al., Nucl. Phys. A 739 (2004) 30. [2] S.E.A. Orrigo et al., Phys. Lett. B 633 (2006) 469. [3] M. Cavallaro, “First Application of the MAGNEX Spectrometer: Investigation of the 19F(7Li,7Be)19O reaction at 52 MeV”, Ph.D. Thesis, University of Catania, 2009. [4] C. Boiano at al., IEEE Transactions On Nuclear Science, 55 (2008) 3563. [5] F. Cappuzzello et al., Nucl. Instr. Meth. A (submitted). [6] A.Lazzaro et al., Proceedings of the “7th International Computational Accelerator Physics Conference”, East Lansing, Michigan, 2002. IOP series 175 (2002) 171. dσ/dΩ (μb/sr) θCM (deg) 7Li + 19F → 7Be + 19Og.s.+0.096  7Li + 19F → 7Be + 19O1.47 θCM (deg) dσ/dΩ (μb/sr) 185  [7] M. Berz, Nucl. Instr. Meth. A 298 (1990) 426. [8] J.L.Wiza et al., Phys. Rev. 143 (1966) 676. [9] M. Cavallaro et al., Proceedings of the Conference: “Nuclear Physics and Astrophysics: From Stable Beams to Exotic Nuclei, 2008, Cappadocia, Turkey. AIP Conf. Proc. 1072 (2008) 249. [10] H. Lenske, Nucl.Phys. A482 (1988) 343. [11] H. Hoffmann et al., Phys. Rev.C 57 (1998) 2281.                                        

Preliminary study of the 19F(7Li,7Be)19O reaction at 52 MeV with MAGNEX

Preliminary study of the 19F(7Li,7Be)19O reaction at 52 MeV with MAGNEX

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