  We are developing an Optical Readout Time Projection Chamber (O-TPC) detector
for the study of the 12C(a,g)16O reaction that determines the ratio of carbon
to oxygen in helium burning. This ratio is crucial for understanding the final
fate of a progenitor star and the nucleosynthesis of elements prior to a Type
II supernova; an oxygen rich star is predicted to collapse to a black hole, and
a carbon rich star to a neutron star. Type Ia supernovae (SNeIa) are used as
standard candles for measuring cosmological distances with the use of an
empirical light curve-luminosity stretching factor. It is essential to
understand helium burning that yields the carbon/oxygen white dwarf and thus
the initial stage of SNeIa. The O-TPC is intended for use with high intensity
photon beams extracted from the HIgS/TUNL facility at Duke University to study
the 16O(g,a)12C reaction, and thus the direct reaction at energies as low as
0.7 MeV. We are conducting a systematical study of the best oxygen containing
gas with light emitting admixture(s) for use in such an O-TPC. Preliminary
results with CO_2 + TEA mixture were obtained
