APH N.S., Heavy Ion Physics 13 (2001) 203-205

A New Method to Measure the Neutron Skin

HEAVY IONPHYSICS@Akademiai Kiad6

M. Csatlos,' A. Krasznahorkay,' A.M. van den Berg,"M.N. Harakeh," S.Y. van der Werf,2 M. Hagemann," M. de HUU,2H. Akimune," H. Fujimura," M. Fujiwara," K. Hara5 andT. Ishikawa"

1 MTA, ATOMKI Debrecen, Hungary2 Kernfysisch Versneller Instituut, 9747AA Groningen, The Netherlands3 Laboratorium voor Kernfysica Universiteit Gent, B-9000 Gent, Belgium4 Japan Atomic Energy Research Institute, Advanced Science Research Center

319-1195 Tokai, Japan5 Research Center for Nuclear Physics, 567-0047 Osaka, Japan

Received 20 May 2000

Abstract. Dipole resonances have been excited in 112-124Sn isotopes using(SHe,t) charge exchange reaction at E3He = 177 MeV and the cross sectionswere related to the neutron-skin thicknesses. The well understood correlationmay be used to determine the neutron-skin thicknesses in the future.

Keywords: spin-dipole resonance, charge exchange reaction, deduced neutron­skin thicknessesPACS: 24.30.Cz, 21.10.Gv, 25.55.Kr, 27.60.j

1. Introduction

The advent of radioactive beams has made it possible to study the structure of nucleifar from the stability. Recently a new possible tool was developed for studying theneutron-skin thickness. It is based on the excitation of the spin dipole resonance(SDR). This resonance can be strongly excited in the charge-exchange reactions. Inan experiment the SDR in the Sb isotopes was excited by the eHe,t) reaction atE3He = 450 MeV [1]. It was demonstrated that there exists a correlation betweenthe SDR cross section and the neutron-skin of nuclei.

The aim of a new experiment, which was performed at the KVI in Groningen,at E3He = 177 MeV, was to check the method at lower bombarding energy, whenthe spin-independent part of the nucleon-nucleon interaction is stronger.

1219-7580/01/ $ 5.00@2001 Akademiai Kiad6, Budapest

204 M. Csatl6s et ai.

2. Experiment

The experiment was carried out at the KVI, Groningen. The 3He++ beam used forthe experiment was accelerated with the super-conducting cyclotron AGOR to anenergy of 177 MeV. The targets were 112,114,116,118,120,122,124Sn, with a thickness of4.75-12.7 mg/cm'", their isotopic enrichment was 75.0% - 96.6%, respectively.

The tritons from the (3He,t) reaction were detected in the focal plane of the"Big Bite" magnetic spectrometer. The focal plane detection system used in thisexperiment was two pairs of Vertical Drift Chambers of the Focal Plane Polarimeterdetection system constructed within the European Supernova Collaboration.

3. Results

Measuring the horizontal and the vertical angles of the outgoing tritons, the scatter­ing angle can be determined. The triton spectra for angles 8Lab = 0° and 8Lab = 2°are shown in Fig. 1a) and 1b), respectively.

16 931100 lll:

~~aj e=2° lll:

~a 60 ~b)

0.3~ c)...

.:. IIIc:>

~ 0.25~u *

110 S'" 0.2... /\

~ 8l/ 0.015

S'·g 6

... /\ 0.1..U v *0.05

1011'80

o110112114116118120122U4126

0 0 A150 160 170 180 150 160 170 180

Triton Energy (MeV) Triton Energy (MeV)

Fig. 1. a), b) Triton energy spectra from (3He,t) reaction. The isobaric analogstate (lAS), the Gamow-Teller resonance and the SDR are indicated. c) Thedifference of the neutron and proton rms radii as a function of the mass numberof the Sn isotopes. The full squares with error bars show the present results. Theresults from the (p,p') reaction [2] and the GDR excitation [3] are shown as opencircles and open squares, respectively. The results from the eHe,t) reaction atE3He = 450 MeV [1] are presented as open triangles. The open and full stars showthe theoretical results of Angeli et al. [4] and Decharge et al. [5], respectively.

A New Method to Measure the Neutron Skin 205

The broad peak corresponding to the dipole strength is clearly visible at eLab =2°. For a preliminary analysis the difference of the 2° and 0° spectra have beeninvestigated for the SnR energy region and the neutron-skin thickness have beencalculated in a similar way as in Ref. [1]. In Fig. lc) our result is compared to theprevious ones and to the theoretical predictions.

4. Conclusion

In conclusion, excitation of the dipole strength in charge exchange reactions providesa new method for studying the neutron-skin thickness and it can be used for a widerange of neutron-rich unstable nuclei.

Acknowledgement

This work has been supported by the Hungarian OTKA Foundation N26675 andthe Nederlandse Organisatie voor Wetenschapelijk Onderzoek (NWO).

References

1. A. Krasznahorkayet al., Phys. Rev. Lett. 82 (1999) 3216.2. C. Batty et al., Adv. Nucl.Phys. 66 (1991) 1.3. A. Krasznahorkayet al., Phys.Rev.Lett. 66 (1991) 1287; Nucl. Phys. A567

(1994) 521.4. I. Angeli et al., J. Phys. G6 (1980) 303.5. J. Deeharge and D. Gogny, Phys. Rev. C21 (1980) 1568.