VOLUME 83, NUMBER 16 PHYSICAL REVIEW LETTERS 18 OCTOBER 1999 pn-Pair Coupling in the g , pn Reaction at 72 MeV L. Isaksson, J-O. Adler, B-E. Andersson,* K. I. Blomqvist, † A. Sandell, and B. Schröder Department of Physics, University of Lund, P.O. Box 118, S-221 00 Lund, Sweden P. Grabmayr, S. Klein, ‡ and G. Mauser § Physikalisches Institut der Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen, Germany J. R. M. Annand, G. I. Crawford, V. Holliday, J. C. McGeorge, and G. J. Miller** Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland (Received 27 May 1999) The 16 Og, pn reaction was measured at E g  72 MeV with resolution sufficient to distinguish the low-lying states of the residual 14 N nucleus. Cross sections averaged over the acceptance of the detector system were determined for each of the resolved states. The relative population of residual states indicates that proton-neutron pairs coupled to J p , T   0 1 ,1 play a minor role in the photon absorption process compared to 1 1 ,0 pairs and that both L  0 and L  2 pairs participate in the reaction. PACS numbers: 25.20.Dc, 21.10.Hw, 27.20. + n Photonuclear reactions provide an excellent tool for in- vestigating the structure of atomic nuclei due to the high degree of accuracy with which the underlying electromag- netic interaction is described by the theory of quantum electrodynamics. Furthermore, the relative weakness of the interaction causes a minimal perturbation of the ini- tial nuclear state, thus simplifying the description of the reaction process. However, in order to extract detailed nuclear structure information from photonuclear measure- ments the detailed reaction mechanism must be known, which is not the case for the energy region between the gi- ant dipole resonance and the D resonance. In this region, early experimental results [1,2] pointed toward a reaction mechanism involving absorption on proton-neutron pairs. Levinger coined the term “quasideuterons” for these pairs and introduced a corresponding model [3], further elabo- rated by Gottfried [4], in which the relative wave function for a pair was approximated by the wave function for a free deuteron. Implicit in Levinger’s model was the as- sumption that the pair had the same quantum numbers, 3 S 1 , as a free deuteron. The same assumption was used by Gottfried, who performed his calculation for higher photon energies (340 MeV). At these photon energies, the dominant reaction mechanism is expected to be ab- sorption on D resonance currents, for which absorption on pairs coupled to the singlet 1 S 0 state is suppressed by isospin and parity conservation. At lower photon ener- gies the reaction mechanism is expected to be dominated by absorption on meson exchange currents, for which it has been suggested that 1 S 0 pairs may contribute [5]. The present experiment aims primarily at determin- ing the quantum numbers of the proton-neutron pairs on which 72 MeV photons are absorbed. This was ac- complished by measuring the 16 Og, pn reaction with resolution sufficient to distinguish the low-lying states of the residual 14 N nucleus. The ground state of 16 O is characterized by quantum numbers J p , T   0 1 ,0, where J , p , and T are the angular momentum, parity, and isospin, respectively. Consequently, the quantum num- bers of a proton-neutron pair on which a photon is ab- sorbed are equal to those of the residual nucleus, assuming the latter has remained a spectator throughout the process. The quantum numbers for the ground state and second ex- cited 3.95 MeV state of 14 N are J p , T   1 1 ,0, while those for the first excited 2.31 MeV state are 0 1 ,1. Hence, the population of the ground state and second excited state relative to the first excited state give an indication of the relative importance of absorption on proton-neutron pairs coupled to 3 S 1 compared to 1 S 0 . The 1.5 MeV resolution in missing energy obtained in this measurement made it possible to distinguish these low- lying states. In the context of g, pn experiments this is a high resolution, the best achieved in any previous experi- ment being 7 MeV [6,7]. The measurement was performed at the tagged-photon facility at the MAX-lab accelerator laboratory in Lund, Sweden [8 – 10]. A pulsed electron beam from a 100 MeV microtron was stretched in a pulse stretcher ring to give 50% duty cycle, and the 30 nA extracted beam pro- duced bremsstrahlung in a 50 mm Al foil. Residual elec- trons corresponding to 67 – 76 MeV photons were detected by an array of 22 plastic scintillator detectors in the fo- cal plane of a magnetic tagging spectrometer [11]. A tagged-photon intensity of 10 7 s 21 and a photon en- ergy resolution of 400 keV were achieved. Because of collimation of the beam, the ratio of the number of tagged photons reaching the target to the number of residual elec- trons detected in the focal plane of the tagging spectro- meter (the tagging efficiency) was 33%, determined by a separate measurement at low intensity in which photons were detected by a 100% efficiency scintillation-glass de- tector inserted in the beam. 3146 0031-9007 99  83(16)  3146(4)$15.00 © 1999 The American Physical Society