Active Oxygen Target for Studies in Nuclear Astrophysics with Laser Compton Backscattered gamma-ray Beams

An active target is being developed to be used in low-energy nuclear astrophysics experiments. It is a position- and time-sensitive detector system based on the low-pressure Multi Wire Proportional Chamber (MWPC) technique. Methylal ((OCH3)(2)CH2), at a pressure of a few Torr, serves as the working gas for MWPC operation, and in addition, the oxygen atoms of the methylal molecules serve as an experimental target. The main advantage of this new target detector system is that it has high sensitivity to the low-energy, highly-ionizing particles produced after photodisintegration of O-16 and insensitivity to gamma-rays and minimum ionizing particles. This allows users to detect only the products of the nuclear reaction of interest. The threshold energies for detection of alpha particles and C-12 nuclei are about 50 keV and 100 keV, respectively. The main disadvantage of this detector is the small target thickness, which is around a few tens of mu g/cm(2). However, reasonable luminosity can be achieved by using a multimodule detector system and an intense, Laser Compton Backscattered (LCB) gamma-ray beam. This paper summarizes the architecture of the active target and reports test results of the prototype detector. The tests investigated the timing and position resolutions of 30 x 30 mm(2) low-pressure MWPC units using an alpha-particle source. The possibility of measuring the O-16(gamma, alpha)C-12 cross-section in the 8-10 MeV energy region by using a LCB gamma-ray beam is also discussed. A measurement of the O-16(gamma, alpha)C-12 cross-section will enable the reaction rate of C-12(alpha, gamma)O-16 to be determined with significantly improved precision compared to previous experiments.