The 0 g.s. 1 ˜2 1 1 transition in 38 Ca and isospin symmetry in A 5 38 nuclei P. D. Cottle, 1 M. Fauerbach, 1 T. Glasmacher, 2,3 R. W. Ibbotson, 3 K. W. Kemper, 1 B. Pritychenko, 2,3 H. Scheit, 2,3, * and M. Steiner 2,3 1 Department of Physics, Florida State University, Tallahassee, Florida 32306-4350 2 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 3 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824 ~Received 6 May 1999; published 4 August 1999! The B ( E 2;0 g.s. 1 ˜2 1 1 ) value for 38 Ca has been measured via the technique of intermediate energy Coulomb excitation using a beam of radioactive 38 Ca nuclei. The present result is used to test isospin purity in the mass 38 system by comparing the isoscalar multipole matrix element M 0 extracted from the 0 g.s. 1 ˜2 1 1 transitions in 38 Ca and 38 Ar to the corresponding matrix element obtained from the T 51 states of the T z 50 nucleus 38 K. A discrepancy between the two values of M 0 is found, suggesting that isospin symmetry is broken in A 538 nuclei. Similar discrepancies occur for A 534 and 42. Experiments for addressing these discrepancies are proposed. @S0556-2813~99!50109-X# PACS number~s!: 23.20.Js, 27.30.1t, 21.10.Hw The advent of methods for producing radioactive beams and the development of experimental techniques for exploit- ing these beams have provided new avenues for detailed studies of the isospin symmetry in nuclei. While isospin symmetry is broken by the Coulomb force, the approximate conservation of isospin has been assumed in many nuclear structure calculations, such as the shell model calculations of Brown, Chung, and Wildenthal @1,2#. In the present work, we report on a measurement of B ( E 2;0 g.s. 1 ˜2 1 1 ) in the short-lived ( T 1/2 50.44 s) nucleus 38 Ca using the method of intermediate energy Coulomb excitation of radioactive beams ~a review of this technique is given in @3#!. This mea- surement enables us to examine the isospin purity of the mass 38 system. As pointed out in @4#, we can test isospin purity by extracting the isoscalar multipole matrix element M 0 from the present result on 38 Ca and the previously mea- sured B ( E 2;0 g.s. 1 ˜2 1 1 ) value in the mirror nucleus 38 Ar and comparing it to the isoscalar matrix element obtained from the corresponding transition between T 51 states in the N 5Z nucleus 38 K. Our data suggest that these two values of M 0 are not equal and that isospin symmetry is broken to a surprisingly large degree in the mass 38 system. We demon- strate here that an examination of previous measurements on the mass 34 and 42 systems also reveals similar effects. Fi- nally, we discuss experiments which would provide further information on this apparent breakdown in isospin symme- try. To produce the 38 Ca beam, a 80 MeV/nucleon 40 Ca beam from the K1200 cyclotron at the National Superconducting Cyclotron Laboratory irradiated a 202 mg/cm 2 target of 9 Be located at the midacceptance target position of the A1200 fragment separator @5#. The energy spread of the resulting 38 Ca fragments was limited to 61% with an aperture. Iso- tope separation was obtained by placing a thin, achromatic wedge ( 27 Al, 64 mg/cm 2 ) at the second dispersive image of the A1200. A ‘‘cocktail’’ beam containing several fragment species was used to perform the experiment in order to study other nuclei in the vicinity simultaneously. This could be done because the counting rate was not a limiting factor and the fragment identification, which is described below, was unambiguous. After passing through the secondary target ~ 197 Au, 184.1 mg/cm 2 ), the secondary beams were stopped in a cylindrical fast-slow plastic phoswich detector ~called the ‘‘zero-degree detector,’’ or ZDD! which allowed charge identification of the secondary beam particles. The time of flight between a thin plastic scintillator located after the A1200 focal plane and the ZDD was recorded for each sec- ondary beam particle and provided positive mass identifica- tion. About 20% of the mixed beam was 38 Ca ( ’12 000 38 Ca particles/s!. The average energy of the incoming 38 Ca particles was 56.1 MeV/nucleon. The ZDD had an opening angle of u lab 54.0° with respect to the secondary target; Cou- lomb excitation is the dominant excitation process in this range of scattering angles. The secondary target was sur- rounded by an array of 38 position sensitive NaI~Tl! g-ray detectors arranged in three concentric rings around the target and shielded from background photons by 16.5 cm thick lead walls. A more detailed description of the experimental and analysis procedures can be found in Ref. @6#, which also illustrates the Doppler-shift correction technique used for analysis of the g-ray spectra. The Doppler-corrected g-ray energy spectrum for 38 Ca ~recorded under the condition that a 38 Ca fragment was de- tected in the zero-degree detector! is shown in Fig. 1~a!. The spectrum includes a strong peak at 2.206~10! MeV, a weaker peak at 3.685~21! MeV, and a weak peak at 1.448~25! MeV. The 2.206 MeV peak corresponds to the 2 1 1 ˜0 g.s. 1 transition in 38 Ca. There are two nearly degenerate states in 38 Ca near 3.685 MeV, one having J p 52 1 and the other J p 53 2 @7#. The possibility that the observed 3.685 MeV g-ray peak could correspond to transitions from either or both of these states to the ground state must be considered. Finally, it has been demonstrated previously that the 2 1 state at 3.685 MeV deexcites to the 2.206 MeV state via a 1.479 MeV g ray @7#. We identify the weak peak at 1.448~25! MeV as this con- necting transition. *Present address: Max-Planck-Institut fu¨r Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany. RAPID COMMUNICATIONS PHYSICAL REVIEW C, VOLUME 60, 031301 0556-2813/99/60~3!/031301~4!/$15.00 ©1999 The American Physical Society 60 031301-1