An optical readout TPC (O-TPC) for studies in nuclear astrophysics with gamma-ray beams at HIγS¹

We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO₂(80%) + N₂(20%) gas mixture at 100 and 150 Torr. It was designed to measure the cross sections of several key nuclear reactions involved in stellar evolution. In particular, a study of the rate of formation of oxygen and carbon during the process of helium burning will be performed by exposing the chamber gas to intense nearly mono-energetic gamma-ray beams at the High Intensity Gamma Source (HIγS) facility. The O-TPC has a sensitive target-drift volume of 30x30x21 cm³. Ionization electrons drift towards a double parallel-grid avalanche multiplier, yielding charge multiplication and light emission. Avalanche-induced photons from N₂ emission are collected, intensified and recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional track images. The event's time projection (third coordinate) and the deposited energy are recorded by photomultipliers and by the TPC charge-signal, respectively. A dedicated VME-based data acquisition system and associated data analysis tools were developed to record and analyze these data.

The O-TPC has been tested and calibrated with 3.183 MeV alpha-particles emitted by a ¹⁴⁸Gd source placed within its volume with a measured energy resolution of 3.0%. Tracks of alpha and ¹²C particles from the dissociation of ¹⁶O and of three alpha-particles from the dissociation of ¹²C have been measured during initial in-beam test experiments performed at the HIγS facility at Duke University. The full detection system and its performance are described and the results of the preliminary in-beam test experiments are reported.