Algora, A.

Aumann, T.

Csatlos, M.

Gulyas, J.

Kalantar-Nayestanaki, N.

Krasznahorkay, A.

Stuhl, L.

Timar, J.

GSI Repository

A New Method for Measuring Neutron-skin Thickness by Exciting theAnti-analog Giant Dipole Resonance ∗A. Krasznahorkay1, T. Aumann2,3, A. Algora1, M. Csatlo´s1, J. Gulya´s1, N. Kalantar-Nayestanaki4,L. Stuhl1, and J. Tı´ma´r for the R3B and EXL collaborations11Inst. for Nucl. Res., Hung. Acad. of Sci. (MTA Atomki), Debrecen, Hungary; 2Technische Universita¨t Darmstadt,Germany; 3GSI, Darmstadt, Germany; 4KVI, Univ. of Groningen, The NetherlandsA new method, based on the excitation of the anti-analoggiant dipole resonance (AGDR) in (p,n) reaction, for mea-suring the neutron skin thickness has been tested. The γ-decay of the AGDR to the isobaric analog state (IAS) hasbeen measured. The difference in excitation energy of theAGDR and IAS was calculated. By comparing the the-oretical results with the measured one, the ΔRpn valuefor 124Sn was deduced to be 0.21±0.07 fm. The presentmethod provides a new possibility for measuring neutronskin thickness of very exotic nuclei.The experiments, aiming at studying the neutron-skinthickness of 124Sn, were performed at GSI using 600MeV/nucleon 124Sn relativistic heavy-ion beams on CH2and C targets. This allowed us to subtract the contribu-tion of the C to the yield measured from the CH2 targetduring the analysis. The ejected neutrons were detectedby a low-energy neutron-array (LENA) ToF spectrometer,which was developed in Debrecen [1].The energy of de-exciting γ-transitions was measuredin coincidence with slow neutrons by six large cylindrical(Φ = 3.5′′, L = 8′′) state-of-the-art LaBr3 γ-detectors.The Doppler shift was taken into account in the analysis.The precise energy and efficiency calibrations of the detec-tors were performed after the experiments by using differ-ent radioactive sources and (p,γ) reactions on different tar-gets. The γ-ray energy spectrum measured in coincidencewith the low-energy neutrons is shown in Fig. 1.The direct γ-branching ratio of the AGDR to the IAS isexpected to be similar to that of the GDR to the g.s. inthe parent nucleus, which can be calculated from the pa-rameters of the GDR [2]. In contrast, in the investigationof the electromagnetic decay properties of the spin dipoleresonance (SDR) by Rodin and Dieperink [3] the γ-decaybranching ratio was in the range of 10−4. Therefore, thecoincidence measurements deliver a precise energy for theAGDR.The theoretical analysis is performed using the fully self-consistent relativistic proton-neutron quasiparticle random-phase approximation based on the relativistic Hartree-Bogoliubov model (RHB) [4] as described previously inRef. [5].By comparing the experimental result for E(AGDR) -E(IAS) to the theoretical energy differences, we deducethe value of the neutron-skin thickness in 124Sn: ΔRnp =∗Work supported by EU, ENSAR, No. 262010, and HIC for FAIR.70080090010001100120013001400150016006 8 10 12 14 16Eγ (MeV)Counts(CH 2 -C)Eγ= 11.09(22) MeV124SnFigure 1: The γ-ray energy spectrum measured in coinci-dence with the low-energy neutrons that fulfill the condi-tions of 1.0 ≤ En ≤ 3.5 MeV and 67◦ ≤ ΘnLAB ≤ 70◦,which corresponds to the exciation of the AGDR in inversekinematics. The calibrated energy scale was corrected al-ready for the Doppler effect. The solid line shows the resultof the fit using Gaussian line shape and a third order poli-nomial background.0.21 ± 0.07 fm (including theoretical uncertainties). Thevery good agreement with previously determined values [5]reinforces the expected reliability of the proposed method.References[1] C. Langer et al., Nucl. Instr. Meth. Phys. Res. A659 (2011)411.[2] A. Krasznahorkay et al., Nucl. Phys. A567 (1994) 521.[3] V. A. Rodin and A. E. L. Dieperink, Phys. Lett. B 541 (2002)7.[4] D. Vretenar, A. V. Afanasjev, G. A. Lalazissis, and P. Ring,Phys. Rep. 409 (2005) 101.[5] A. Krasznahorkay et al., AIP Conf. Proc. 1491 (2012) 190;in print in Phys. Lett. B, Acta Phys. Pol. B, Phys. Scripta.GSI SCIENTIFIC REPORT 2012 PHN-ENNA-EXP-21151

A New Method for Measuring Neutron-skin Thickness by Exciting the Anti-analog Giant Dipole Resonance

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A New Method for Measuring Neutron-skin Thickness by Exciting the Anti-analog Giant Dipole Resonance

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