Performance of the UConn-TUNL O-TPC with the Upgraded Optical Readout System*

An Optical-Readout Time Projection Chamber (O-TPC) is being used at the High-Intensity $\gamma$amma Source (HI$\gamma$S) at TUNL to study oxygen formation during stellar helium burning by measuring the time-reversed $^{16}O$($\gamma$,$\alpha$)$^{12}$C reaction. The photodissociation of $^{16}$O generates ionization tracks in the 30cm x 30cm x 20.5 cm drift chamber. The electrons drift toward the multiplication region composed of three grids separated by 5 mm. The avalanche electrons induce scintillation light (primarily $\lambda$ = 338 nm) in N$_2$ gas, and a CCD camera records an image of the two-dimensional projection of the tracks; hence the in-plane angle ($\alpha$). Photomultiplier tubes measure the time-projection; hence the out-of-plane angle ($\beta$). The azimuthal ($\phi$) and scattering ($\theta$) angles are reconstructed from $\alpha$ and $\beta$. The total energy deposited in the detector is measured by the charge signal from the last grid, as well as from the photomultiplier tubes. Recent upgrades of the optical-readout system have improved the event identification. The larger design lens (142mm diameter) was installed leading to a factor of 15 increase in the light collection. The operation of the upgraded O-TPC system and measured in-beam events will be discussed. *Work supported by the USDOE grant No. DE-FG02-94ER40870 and DE-FG02-97ER41033.