Radiative Pion Capture byC12

Abstract

ENERGY 0.5 (MeV) 1.0 100 200 300 400 CHANNEL FIG. 2. Spectrum of electrons coincident with photons of ~ 75 keV. The data bars correspond to N±^M 9The dashed curve shows the shape of the " singles" spectrum observed with the same source, which consisted of a few milligrams of TlNO s powder spread over an area of 3 cm 2 .is in distinct contrast to the results of Goldman et alJ for other nuclides in this region. In the high-geometry measurements of Howe and Lan-ger, 2 coincidence summing between conversion electrons and j3 rays would provide a contribution consistent with the nonstatistical shape reported for the ground-state transition.The neutron-photon angular-correlation and energy spectrum of neutrons from radiative pion capture by carbon are presented and provide new evidence for excitation of a J 1 * = 1" axial-vector collective mode. Asymmetry in the angular correlation increases with neutron energy and is accounted for by direct neutron emission.The spin-isospin structure of the nuclear giant dipole resonance can be studied by radiative pion capture. 1 The capture rate is dominated by the same axial-vector operator that appears in muon capture 2 and in transverse electromagnetic interactions. 3 Thus these processes complement each other with pion and muon capture exciting the AT e = -1 analog states.A recent measurement 4 of the high-energy photon spectrum from radiative pion capture at rest by 12 C has disclosed resonances in 12 B at 4.8 and 8.2 MeV with widths of 0.45 and 1.1 MeV, respectively. The 4.8-MeV peak can be associated with the 19.4-MeV complex observed by electroexcnation of 12 c. 5,6 For a momentum transfer appropriate to pion capture (-120 MeV/c), this complex is dominated by the J* -2" giant magnetic quadrupole excitation. The resonance at 8.2 MeV corresponds to an excitation energy of 23.3 MeV in 12 C and may be due to excitation of a J 71 " = 1" axial-vector collective mode. This interpretation is supported by a high-resolution electroexcitation experiment on carbon, 6 where evidence was found that this spin-isospin mode is at 22.7 MeV, rather than at 25 MeV as suggested by particlehole calculations. 3 The energy spectrum of neutrons from radiative pion capture will also reflect the nature of the capture process. 7 Furthermore, the n-y angular correlation can be observed. This correlation will be isotropic when a large number of highly excited states participate leading to exaporation neutrons, strongly peaked near 180° when the neutrons are emitted directly, and symmetric about 90° when isolated nuclear states predominate. In the latter case information on the spin of the excited states can be obtained.To explore these possibilities experimentally we have measured the n-y angular correlation following radiative capture of stopped pions by 12 C, together with the energy spectrum of neutrons. Neutrons between 1.7 and 28 MeV were 108