Volume 150B, number 6 PHYSICS LETTERS 24 January 1985 ELECTROEXCITATION OF THE FIRST EXCITED STATE IN 39K C.W. DE JAGER, P.H.M. KEIZER, E.A.J.M. OFFERMANN, H. DE VRIES NIKHEF-K, PO Box 4395, 1009 AJ Amsterdam, The Netherlands and M.V. HYNES 1, S. KOWALSKI, C.N. RAD 2 and C.F. WILLIAMSON Bates Linear A ccelerator and Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 2 November 1984 Electroexcitation of the first excited state in 39K has been studied in the momentum transfer region of 0.8-2.5 fm- 1. Separation of the longitudinal and transverse form-factor components has been obtained. The longitudinal form factor has been analyzed model-independently. A B (M1) value for this/-forbidden transition was also obtained. Presently available theoretical predictions are unable to reproduce the B (M1) value or the transverse form-factor data. The ground state to first excited state transition in 39K is considered to be of quite pure 1 d3/2 -1 to 2Sl/2 -I proton single-particle character. The M 1 con- tribution of this mixed C2/E2/MI transition is/-for- bidden. As a consequence core-polarization contribu- tions to the B(M 1) value vanish in zeroeth and first order. Existing electron-scattering data, which are lim- ited to low momentum transfer or are of poor resolu- tion, only yielded information on the longitudinal form factor. In the present experiment the longitudi- nal and the transverse form factors were determined separately up to a momentum transfer of 2.5 fm -1. The longitudinal form factor, which is of pure C2 character, allows a sensitive study of the radial shape of the core polarization, since the ld3/2 -1 -~ 2Sl/2 -1 transition charge density has a node in the nuclear in- terior. The experiment was performed at the MIT-Bates and the NIKHEF-K facilities. At Bates the high-reso- lution spectrometer [1 ] was used in conjunction with the 180 ° scattering set-up [2] in some cases. At 1 Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 2 Present address: Fermi National Accelerator Laboratory, Batavia, IL 60510, USA. NIKHEF likewise the high-resolution spectrometer [3] was used. The targets consisted of metallic potassium of natu- ral isotopic composition. The material was rolled in an inert argon atmosphere to a thickness of approximate- ly 40 mg/cm 2 or 100 mg/cm 2 for use at the highest energies. The targets were mounted in the scattering chamber target ladder while minimizing exposure to the atmosphere. Notwithstanding this precaution, small contamination peaks of carbon and oxygen were visible in all spectra, but these could easily be sepa- rated from the level under study. The low beam densi- ty, due to the dispersed beam technique used at both facilities, allowed beam currents of up to 10/IA to be used without any indication of evaporation. In total 33 spectra were accumulated. At Bates 13 spectra at 90 °, covering energies from 100 to 310 MeV, 7 spectra at 160 ° from 130 to 240 MeV and 5 spectra at 180 ° from 90 to 120 MeV; at NIKHEF 8 spectra at 154 ° from 70 to 150 MeV. In the 90 ° measurements the targets were placed in transmission geometry, while at the backward angles reflection geometry was used. This resulted in a resolution (FWHM) of about 30 keV at forward angles and 100 keV at backward angles. At the highest q-values the thick target was used with a resolution of 250 keV. In the latter cases additional 0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 421