In a high resolution 48Ca(d,3He) experiment at 80 MeV evidence has been found for a 7/2 state at 1.97 MeV in 47K with a spectroscopic factor of C2S = 0.08 corresponding to a N 4 % admixture of ~f2(s,d) -2 configurations in the ground state of 48Ca. We report on first direct observation of a deviation from proton shell closure in the ground state of 48Ca. This observation bears among others on calculations of charge distributions, quenching of MI giant resonances and Coulomb displacement energies. So far most calculations assumed a perfect proton shell closure largely based upon the results of a previous 48Ca(d,3He) experiment at 52 MeV [i] in which no evidence for ~(f,p)2(s,d)-2 configurations in the ground state of 48Ca was found, in contrast to the situation in 4~ [1,2]. The recent interest in MI and Gamow-Teller giant resonances made a scrutiny of this assumption necessary, all the more since in the meantime evidence for a deviation from neutron shell closure of 48Ca had been reported [3]. In order to discover weak two particle-two hole components expected to be predominantly ~(f,p)2(s,d)-2 configurations in nature we utilized the 48Ca(d,3He) reaction at 80 MeV which yields good momentum matching for 1 = 1 and 1 = 3 transfer. A beam of vector polarized deuterons from the Indiana University Cyclotron Facility was directed onto a 330 Ug/cm 2 thick 48Ca target. Crucial for the success of this experiment was the low level of carbon and oxygen impurities in the target achieved by transferring the target to the scattering chamber in vacuum. In consequence, the relative size of impurity peaks in the spectra was significantly less than in the most recent study [l] of this reaction.
*Present address: PhysikalischesThe emitted 3He particles were momentum analysed in a QDDM spectrograph, and detected in a position sensitive focal plane detector with an overall energy resolution of 45 keV. Spectra were taken at lab angles between 7 ~ and 34 ~ in 3 ~ steps. A sample 3He spectrum, corresponding to low excitation in the residual nucleus 47K ( fig.l) shows the population of four states of 47K, as well as other peaks which arise from the presence of carbon and oxygen impurities. 400 z 200 &8 ~ 3 ) 1,7 K CO (d, He Spin Up 8 L = 16 ~ t'TK 0.36 MeV ~K 3.42 MeV 15N 6.32 MeV ~IB 2.12 MeV 47 . .
2004~ 600 800 I0~
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Fig.1.A sample 3He spectrum with spin up incident deuterons.Of the four 47K states identified, three are known from the previous work [i]. These are the ground state, which is populated via 2Sl/2 proton pickup, the 0.36 MeV state (id3/2 proton pickup), and the 3.42 MeV state, which is part of a very fragmented id5/2 hole strength. The fourth state, at an excitation of 1.97 MeV, is weakly populated, and is the state of interest here. Spectroscopic factors were determined from local, zero-range DWBA calculations, using DWUCK4 and optical model parameters determined in part by fitting differential cross section and vector analyzing powers for elastic deuteron scattering which we have also measured [4]. Fig.2 sh...
