Observation of pion double charge exchange on 16O and 18O

The zero degrees cross section for the reaction 18O (pi+, pi-) 18Ne was measured at three incident pion energies between 95 and 139 MeV, and 16O(pi+,pi-) 16Ne at an incident energy of 145 MeV. In addition, the previously unpublished mass of 16Ne was determined. The experiment was done at the Los Alamos Meson Physics Facility on the Low Energy Pion Channel (LEP) by a collaboration including the University of South Carolina, Los Alamos Scientific Laboratory and Argonne National Laboratory. The LEP channel was used as both a pion channel and an energy loss spectrometer (fixed at zero degrees) by placing the double charge exchange targets at the channel midpoint. In this configuration, the second half of the channel has an acceptance solid angle of 7.2 msr as determined by a Monte Carlo technique. At an average primary proton beam current of one hundred microamperes and a one percent momentum bite, the pion flux at the target was several times ten to the seventh positive pions per second. The targets were approximately 3 gm/cm2 gelatin discs. A resolution of about 5 MeV FWHM was achieved in all cases allowing the identification of transitions to the ground states of the final nuclei. A detector array at the end of the spectrometer was used to identify the emerging particles and determine their momenta. A large background from e+→ gamma →e- processes in the target was discriminated against using lucite Cerenkov counters. A time-of-flight system and a delta-E scintillator telescope also aided in the identification of pions. The cross sections for the 18O target are approximately constant at about 2 μb/sr with absolute errors of about twenty percent. The 16O cross section is just under 1 μb/sr with an approximately twenty-five percent absolute error