Central Tracking in SASY

The present focus of our experimental program is the measurement of polarized asymmetries and cross sections with the aim of constraining pi-production amplitudes, particularly from the neutron for which there is little available data. Thus, at present, target development work is concentrated on increasing D polarization.

A series of polarized D experiments will begin in late 2003, with measurements of the D(γ, pi⁰n) channel using the neutron barrel. Charged pion production from the neutron requires a separation of the D(γ, pi^-p) channel from the D (γ, pi⁺n) channel. The energy of the recoil proton can distinguish the two reactions, but this only works if the proton has sufficient energy to emerge from the target cryostat. Below 280 MeV gamma energy, this is not the case over more than half of the angular distribution. Therefore, the charged pion final state on the neutron will be isolated by tracking the pions in a magnetic field to measure the pion charge. For this purpose a time projection chamber (TPC) is being constructed, which will surround the target, to track, identify, and determine the momentum of protons and charged pions over the full energy range available to LEGS and for polar angles from 20 to 150 degrees.

The TPC will operate in a 1.8 Tesla superconducting solenoid mounted inside the XTAL Box. The active volume of this device is 0.38 m diameter by 0.5 m long, with a single drift region. Electron multiplication at the readout end of the TPC will be provided by two stages of gaseous electron multiplier (GEM). Following multiplication by the GEM stages, the charge and time of arrival will be determined at each of 9200 pads by 290 custom designed 32 channel ASICs. Expected event multiplicities are no higher than 3, although the segmentation will easily allow events with multiplicities several times higher to be processed. This design will accommodate event rates up to 1 kHz at 10% deadtime. Operation is planned for early 2005.