Proton scattering from the unstable neutron-rich nucleus 43 Ar F. Mare ´ chal, 1, * T. Suomija ¨ rvi, 1 Y. Blumenfeld, 1 A. Azhari, 2,3,² D. Bazin, 2 J. A. Brown, 2,‡ P. D. Cottle, 4 M. Fauerbach, 2,3, * T. Glasmacher, 2,3 S. E. Hirzebruch, 1 J. K. Jewell, 4,§ J. H. Kelley, 1,i K. W. Kemper, 4 P. F. Mantica, 2,5 D. J. Morrissey, 2,5 L. A. Riley, 4,¶ J. A. Scarpaci, 1 H. Scheit, 2,3, ** and M. Steiner 2 1 Institut de Physique Nucle ´aire, IN 2 P 3 -CNRS, 91406 Orsay, France 2 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824 3 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 4 Department of Physics, Florida State University, Tallahassee, Florida 32306 5 Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 ~Received 2 June 1999; published 22 November 1999! The neutron-rich argon isotope 43 Ar has been studied by quasielastic and inelastic proton scattering per- formed in inverse kinematics. The measured inelastic angular distribution for the second excited state is in good agreement with an L 52 transition. Assuming this transition to be E 2, yields a b 2 value for this state of 0.2560.03 when compared with distorted-wave Born approximation calculations. This value is comparable to the one reported for the stable isotope 40 Ar. Moreover it is similar to those measured by Coulomb excitation for the neighboring even-even isotopes 42 Ar and 44 Ar indicating that the structure of the argon isotopes is stable as a function of neutron number. @S0556-2813~99!03012-5# PACS number~s!: 21.10.Re, 25.40.Cm, 25.40.Ep, 27.40.1z I. INTRODUCTION The availability of radioactive beams with sizeable inten- sities and good optical qualities makes possible the study of direct reactions induced by unstable nuclei. The study of nuclear matter distributions, deformation, and the modifica- tion of shell structure far from stability can be addressed through inverse kinematics reactions on light targets. Con- siderable interest is currently being focused on neutron-rich nuclei near the N 528 magic number for which shell closure is expected to vanish, yielding a new region of deformation @1,2#. The 0 gs 1 →2 1 1 transition in even-even neutron rich sul- fur and argon isotopes was recently studied by intermediate energy Coulomb excitation @3,4#. The measurement of the excitation energies and the B ( E 2) reduced transition prob- abilities showed a weakening of the N 528 shell closure that was more pronounced for 44 S than for 46 Ar. Additional in- formation on the structure of nuclei in this mass region can be obtained from proton scattering experiments. Elastic scat- tering will give insight into nuclear densities and interaction potentials, while the comparison of Coulomb excitation with proton inelastic scattering should allow neutron and proton deformations to be separated. We have undertaken a study of the neutron-rich sulfur isotopes through elastic and inelastic scattering of protons in inverse kinematics. We performed experiments on 38 S and 40 S by using secondary fragmentation beams of 38,40 S deliv- ered by the National Superconducting Cyclotron Laboratory at Michigan State University @5,6#. During the second ex- periment, data were also collected for the neutron-rich iso- tope 43 Ar which was present as a byproduct in the secondary beam. Few of the properties of 43 Ar are known, even though several excited states were identified in a previous exotic transfer reaction study @7#. For instance, no spin assignments exist for either the ground or excited states. Here, we present the results of quasielastic and inelastic proton scattering on the unstable nucleus 43 Ar performed in inverse kinematics. The low-lying level structure of 43 Ar is discussed. The in- elastic scattering data are shown to be best described by an L 52 transition when compared with distorted-wave Born approximation ~DWBA! calculations. The b 2 value extracted from these data, assuming an E 2 inelastic transition, is com- pared to the values obtained for the nearby argon isotopes. II. EXPERIMENT The secondary 43 Ar beam was produced by fragmentation of a primary 48 Ca beam at 60 MeV/nucleon, provided by the K1200 cyclotron at the National Superconducting Cyclotron Laboratory, on a 285 mg/cm 2 Be production target. The fragments were analyzed using the A1200 fragment separa- tor @8# and the resulting beam was purified by using a 70 mg/cm 2 aluminum wedge. While the beam optics and A1200 parameters were both optimized for the production of 40 S at 30 MeV/nucleon, a final intensity of about 16 000 particles per second ~pps! for 43 Ar at 33 MeV/nucleon was obtained. The 40 S intensity was only 2000 pps. The incident beam nuclei were identified event by event by the combination of a *Present address: Department of Physics, Florida State Univer- sity, Tallahassee, FL 32306. ² Present address: Cyclotron Institute, Texas A&M University, College Station, TX 77843. ‡ Present address: Department of Physics, Millikin University, De- catur, IL 62522. § Present address: Idaho National Engineering Laboratory, Idaho Falls, ID 83415. i Present address: TUNL, Duke University, Durham, NC 27708. ¶ Present address: Department of Physics and Astronomy, Earlham College, Richmond, IN 47374. **Present address: Max Plank Institut fu ¨r Kernphysik, 69029 Heidelberg, Germany. PHYSICAL REVIEW C, VOLUME 60, 064623 0556-2813/99/60~6!/064623~4!/$15.00 ©1999 The American Physical Society 60 064623-1