Isobaric charge-exchange reactions induced by diﬀerent tin isotopes have been investigated at GSI. The high-resolving power of the FRS spectrometer made it possible to separate elastic and inelastic components in the missing-energy spectra of the ejectiles. The inelastic component was associated to the in-medium excitation of nucleon resonances such as the Delta and Roper resonances. These data are expected to contribute to better understand the in-medium properties of baryon resonances but also to investigate the abundance of protons and neutrons at the nuclear periphery

Alavarez-Pol,, H.

Aumann, T.

Benlliure, J.

Rodriguez-Sanchez, J. L.

Vargas, J.

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DOI 10.1393/ncc/i2016-16401-0Colloquia: IWM-EC 2016IL NUOVO CIMENTO 39 C (2016) 401Exciting baryon resonances in isobar charge-exchange reactionsJ. Benlliure(1), J. L. Rodriguez-Sanchez(1), J. Vargas(1), H. Alavarez-Pol(1),T. Aumann(2), J. Atkinson(2), Y. Ayyad(1), S. Beceiro(1), K. Boretzky(2),A. Chatillon(3), D. Cortina(1), P. Diaz(1), A. Estrade(2), H. Geissel(2),H. Lenske(4), Y. Litvinov(2), M. Mostazo(1), C. Paradela(1), S. Pietri(2),A. Prochazka(2), M. Takechi(2), I. Vidaña(5), H. Weick(2) and J. Winfield(2)(1) Universidade de Santiago de Compostela - 15782 Santiago de Compostela, Spain(2) GSI Helmholtzzentrum für Schwerionenforschung GmbH - D-64291 Darmstadt, Germany(3) CEA DAM - F-91297 Arpajon, France(4) Institut für Theoretische Physik der Universität - Giessen 35392, Germany(5) Centro de Fisica Computacional, Department of Physics, University of CoimbraPT-3004-516 Coimbra, Portugalreceived 10 January 2017Summary. — Isobaric charge-exchange reactions induced by different tin isotopeshave been investigated at GSI. The high-resolving power of the FRS spectrometermade it possible to separate elastic and inelastic components in the missing-energyspectra of the ejectiles. The inelastic component was associated to the in-mediumexcitation of nucleon resonances such as the Delta and Roper resonances. Thesedata are expected to contribute to better understand the in-medium properties ofbaryon resonances but also to investigate the abundance of protons and neutrons atthe nuclear periphery.1. – IntroductionCharge-exchange reactions are unique probes to investigate spin-isospin excitations innuclei. The broad range in energy that can be covered with these reactions makes it pos-sible to induce charge-exchange processes through nuclear (Gamow-Teller, spin-dipole,spin-quadrupole, ...) [1] or subnuclear (nucleon resonances) [2] excitations. During the80’s, subnuclear charge-exchange reactions were extensively investigated at Saturne(France) by measuring the ejectile missing energy spectra in relativistic heavy-ion charge-exchange reactions [3]. One of the main results of this research program was the obser-vation of a downward shift of the mass of the delta resonance attributed to in-mediummodifications of the baryon resonances.The in-medium properties of baryon resonances such as the Δ(1232) or the Roper(N∗(1440)) resonance are also of importance for the understanding of three-body forcesCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 J. BENLLIURE et al.[4,5] or the not fully solved problem of the missing Gamow-Teller strength [6]. Moreover,the excitation of baryon resonances in heavy-ion (p,n) and (n,p) reactions has also beenproposed to explore the radial distribution of protons and neutrons in nuclei [7].To investigate baryon resonances in asymmetric nuclear matter we used isobariccharge-exchange reactions induced by relativistic beams of stable and unstable projec-tile nuclei. These reaction channels guarantee that with a large probability the charge-exchange process is mediated by a quasi-free nucleon-nucleon collision. In the case ofinelastic nucleon-nucleon collisions the excited nucleon resonances, produced in the over-lapping region between projectile and target nuclei, decay by pion emission. To preservethe initial number of nucleons in the projectile remnant, pion decay should not induceany sizable excitation energy in the projectile, leaving then this nucleus without anyfurther interaction. Due to the strong absorption of the pion, we can then conclude thatthese reactions must be very peripheral.In this paper we will describe the experiment we performed at GSI using beams ofstable (112Sn and 124Sn) and unstable (120Sn and 110Sn) tin isotopes to induce isobariccharge exchange reactions. We will detail the experimental technique and set-up and wewill present preliminary results.Projectile residues were identified and momentum analysed using the zero-degreemagnetic spectrometer FRagment Separator (FRS) [8]. The momentum resolution ofthis spectrometer was proved to be sufficient to clearly identify the elastic (Gamow-Teller) and inelastic (nucleon excitations) components in the missing-energy spectra ofprojectile residues in isobaric charge-exchange reactions [9].2. – The experiment2.1. Experimental technique and set-up. – The experiment was performed at GSI(Germany) where the SIS18 synchrotron delivered beams of 112Sn and 124Sn with energiesbetween 400 and 1000 A MeV and intensities around 108 ions per second. High-resolutionmeasurements of the missing-energy spectra of the ejectiles produced in isobaric charge-exchange reactions induced by these primary beams were obtained by using thin targetsof carbon, polyethylene, copper and lead from the target station located at the entranceof the fragment separator. Secondary beams of 122Sn, 120Sn and 110Sn were producedfragmenting the primary beams of 112Sn and 124Sn in a beryllium target also locatedat the entrance of the spectrometer. The secondary beams were identified and separatedwith the first section of the fragment separator. Targets made of carbon and polyethylenewere placed at the intermediate image plane and the isobaric charge-exchange residuesidentified in the second section of the spectrometer. In this case and due to the lowerintensities of the secondary beams, around 103 ions per second, thicker targets were useto obtain a minimum statistics in the measurements.The fragment separator (FRS) is a zero-degree high-resolving power spectrometer witha typical resolving power of Bρ/ΔBρ ≈ 1500, a momentum acceptance of Δp/p ≈ 3%and and angular acceptance around the central trajectory around 15 mrad. The FRS is anachromatic spectrometer made of two symmetric stages with a dispersive intermediateimage plane [8]. The spectrometer can be used in a single and a two-step reactionmodes. In the first case the reaction target is placed at the entrance of the FRS andthe complete spectrometer is used to separate and identify the projectile residues (see,e.g., [10]). In the two-step reaction model the target used to produce the secondary beamsis placed at the entrance of the spectrometer and the secondary beams are identifiedand separated with the first section of the FRS (see, e.g., [11]). The reaction targetEXCITING BARYON RESONANCES IN ISOBAR CHARGE-EXCHANGE REACTIONS 3Fig. 1. – Identification matrices of the two isobar charge-exchange channels, (p,n) in the leftpanel and (n,p) in the right panel, which are produced in reactions induced by 112Sn projectilesimpinging a carbon target at 1000 A MeV.is thus located at the intermediate image plane and the reaction fragments identifiedwith the second section of the FRS. In both modes projectile residues are identified bymeasuring the Bρ, velocity and energy loss of the projectile residues using high-resolutiontime-projection chambers for tracking, scintillator detectors for time-of-flight and multi-sampling ionization chambers for energy loss. These measurements also allow for theaccurate determination of the longitudinal momentum of the reaction products.2.2. Observables. – With the measurements provided with the FRS we could definetwo important observables for the characterization of the isobar charge-exchange reaction,the cross section of the process and the ejectile missing energy spectra. Cross sectionswere accurately determined from the yields of the final reaction residues obtained in thecorresponding identification matrices, as the one shown in fig. 1, the number of scatteringcentres per surface unit in the target and the number of incoming projectiles determinedwith a secondary electron monitor placed at the entrance of the FRS. The measured yieldswere corrected by several factors accounting for the dead time of the acquisition system,fraction of atomic charge states in the final fragments, optical transmission through theFRS and secondary reaction in the target or any other layer of matter along the beamline. A detailed description of the method can be found in ref. [12].The second observable we used in this work is the missing energy spectra of therecoiling projectile residues. This observable was obtained from the measured longitu-dinal momentum of the residual nuclei transformed into the reference frame defined bythe velocity of the incoming projectiles in the middle of the reaction target. Kelic andcollaborators already determined this observable at the FRS for 208Bi residuals pro-duced in isobaric (n,p) reactions induced by 208Pb projectiles impinging hydrogen anddeuterium [9]. In these measurements the separation between the elastic and inelasticcomponents in the charge exchange reaction were shown.In the present experiment we improved the resolution using more precise positionmeasurements for tracking (≈ 200μm), optimizing the FRS optics and, the most im-portant, by reducing angular and energy straggling using thinner targets. In fig. 2 thered line in the left panel represents a typical spectrum of the recoiling energy of 124Sbresidual fragments produced in isobaric (n,p) reactions induced by 124Sn projectiles im-pinging into a carbon target. In this spectrum we observe two clear components, a peak4 J. BENLLIURE et al.Fig. 2. – Left panel: missing energy spectra of the 124Sb residual fragments produced in isobaric(n,p) reactions induced by 124Sn projectiles impinging in a carbon target.corresponding to residual nuclei having the same energy as the incoming projectiles, anda second peak for an energy loss around 300 MeV. We associate these two componentsto elastic spin-isospin nuclear excitations and to the excitation of the Delta resonance.We additionally improved the resolution of the measured missing energy spectra byunfolding the response of the experimental setup. This response function was definedas the missing energy spectra measured for the unreacted projectiles. Those nuclei weremeasured in its hydrogen-like charge state in the same magnetic tuning of the FRS usedfor the measurement of the (p,n) channel, as shown in the left panel of fig. 1. Thisresponse function is depicted by the blue line in the left panel of fig. 2. The unfoldingprocedure used in this work is based on the Richardson-Lucy method with a regulariza-tion technique to optimize the stability of the solution against statistical fluctuations [13].The result of the deconvolution is represented by the blue line in the right panel of fig. 2.As can be observed, the deconvolution clearly improves the separation between the elas-tic and inelastic charge-exchange components but also enhances some structures in theinelastic component that we will discuss in the next section.3. – ResultsUsing the setup described in the previous section we could determine the cross sec-tions and missing-energy spectra of projectile ejectiles issued in isobaric charge-exchangereactions ((p,n) and (n,p)) induced by several tin isotopes (110Sn, 112Sn, 120Sn and 124Sn)accelerated at 400, 700 and 1000 A MeV impinging on CH2, C, Cu and Pb targets. Thecombined measurements with CH2 and C targets made it possible to investigate thereactions induced by protons.In fig. 3 we report the unfolded missing energy spectra obtained in isobaric chargeexchange reactions, (p,n) (left column) and (n,p) (right column), induced by 112Sn pro-jectiles at 1000 A MeV on protons, carbon, copper and lead. In all spectra we observea clear separation between the elastic and inelastic components of the charge exchangereaction except for the (p,n) channel with protons. In this case, charge conservationforbids the elastic process. In reactions induced on protons the inelastic peak seems tobe dominated by a single structure with a mean missing energy around 300 MeV that weassociate to the excitation of the Delta resonance in these charge-exchange reactions.The missing-energy spectra measured in isobaric charge exchange reactions inducedon targets heavier than proton show clear substructures in the inelastic component. TheEXCITING BARYON RESONANCES IN ISOBAR CHARGE-EXCHANGE REACTIONS 5Fig. 3. – Missing energy spectra of the ejectiles measured in isobaric p,n (left panels) and n,p(right panels) induced by 112Sn projectiles on different targets.dominant structure at 300 MeV seems to correspond to the excitation of the Delta reso-nance. A second structure is clearly observed around 450 MeV, that we tentatively iden-tify as the Roper resonance. A third structure can also be seen in many cases, althoughit could not be unambiguously identified. The main structures of the inelastic componentin the missing-energy spectra were fitted to generalized Breit-Wigner functions just toguide the eye.The reduction of these structures in the inelastic component for reactions investigatedat lower projectile energies supports our identification of the different nucleon resonancesin this isobaric charge exchange reactions. Indeed we observe the disappearance of thestructure associated to the Roper resonance at projectile energies below 700 A MeV andthe one corresponding to the Delta resonance below 400 A MeV.The cross sections associated to the elastic and inelastic components of the isobariccharge-exchange process show a clear dependence with the target used to induce thereaction. As expected, the cross sections increase with the size of the target nucleus.We also observe that the cross section of the elastic channel increases for (p,n) reactionswith the mass of the target, but not really for the (n,p) ones. This evolution couldbe understood as a consequence of the relative abundance of protons and neutrons atthe target nuclei periphery. Such conclusion can be illustrated with the cross sectionsobtained with the lead target. In this case, the larger abundance of neutrons at theperiphery enhances elastic (n,p) reactions in the target, and therefore the complementary(p,n) reactions in the projectile.6 J. BENLLIURE et al.4. – ConclusionsIsobaric charge-exchange reactions induced by several stable and non-stable tin iso-topes on different targets, and at energies between 400 and 1000 A MeV have beeninvestigated at GSI. The experimental setup allowed us to measure the cross sections ofthese processes, but also to determine the missing-energy spectra of the correspondingejectiles. Due to the large kinetic energies of the projectiles and the high-resolving powerof the magnetic spectrometer FRS we could clearly identify in the missing-energy spec-tra the elastic and inelastic channels corresponding to nuclear spin-isospin and nucleonresonances excitations, respectively. Moreover, we implemented an unfolding method tofurther improve the resolution of the missing-energy spectra.The unfolded spectra not only showed a better separation between the elastic andinelastic components but also enhanced additional structures in the inelastic channel.These structures were associated to the excitation of different nucleon resonances. Thedominant structure located at a value of the missing energy around 300 MeV was assignedto the Delta resonance. A second structure corresponding to a missing-energy valuearound 400 was tentatively identified as the Roper resonance. An additional structurefor a missing energy around 550 MeV clearly appears in many of the investigated reactionsalthough its identification is still under investigation. Such an effect can be illustratedwith the lead target. Since lead has a larger abundance of neutrons at the periphery,(n,p) reactions in the target, and therefore (p,n) in the projectile, will be more probable.The preliminary results presented in this paper show the potentiality of these measure-ments. A forthcoming analysis of the missing-energy spectra will allow us to characterizethe in-medium excitation of nucleon resonances in terms of its mean mass and width.On going model calculations of these processes benchmarked with the measures crosssections will provide information on the relative abundance of protons and neutrons atthe nuclear periphery of the colliding nuclei. Moreover these kind of analysis could beextended with non stable projectiles giving access to the properties of nucleon resonancesin asymmetric nuclear matter and the evolution of the relative abundance of protons andneutrons along long isotopic chains.REFERENCES[1] Ichimura M., Sakai H. and Wakasa T., Prog. Part. Nucl. Phys., 56 (2006) 446.[2] Gaarde C., Annu. Rev. Part. Sci., 41 (1991) 187.[3] Bachelier C. et al., Phys. Lett. B, 172 (1986) 23.[4] Zuo W., Lejeune A., Lombardo U. and Mathiot J. F., Nucl. Phys. A, 706 (2002)418.[5] Li Z. H., Lombardo U., Schulze H.-J. and Zuo W., Phys Rev. C, 77 (2008) 034316.[6] Arima A., Prog. Part. Nucl. Phys., 46 (2001) 119.[7] Li B., Hussein M. S. and Bauer W., Nucl. Phys. A, 533 (1991) 749.[8] Geissel H. et al., Nucl. Instrum. Methods B, 70 (1992) 286.[9] Kelic A. et al., Phys. Rev. C, 70 (2004) 064608.[10] Armbruster P. et al., Phys. Rev. Lett., 93 (2004) 212701.[11] Cortina-Gil D. et al., Nucl. Phys. A, 720 (2003) 3.[12] Benlliure J. et al., Phys. Rev. C, 78 (2008) 054605.[13] Vargas J., Benlliure J. and Caamano M., Nucl. Instrum. Methods A, 707 (2013) 16.

Exciting baryon resonances in isobar charge-exchange reactions

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Exciting baryon resonances in isobar charge-exchange reactions

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