The annihilating photons produced by the positrons on the Be target have a large advantage in
counting rate relative to the tagged photon beam method. For the 5 muA positron beam intensity,
the gain is about 85, considering that an intensity of 108 is the upper limit for the tagged beam
intensity. The annihilating photon energy could be accurately reconstructed using a correlation of
the photon energy and emission angle, which is obtained from the transverse coordinate of the event
vertex at the target.