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

Abstract

We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO 2 (80%) + N 2 (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) JINST 5 P12004facility. The O-TPC has a sensitive target-drift volume of 30x30x21 cm 3 . Ionization electrons drift towards a double parallel-grid avalanche multiplier, yielding charge multiplication and light emission. Avalanche-induced photons from N 2 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 148 Gd source placed within its volume with a measured energy resolution of 3.0%. Tracks of alpha and 12 C particles from the dissociation of 16 O and of three alpha-particles from the dissociation of 12 C 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.

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

“…This will result in better than 10% energy resolution. In comparison, the bubble chamber experiment of Reference [3] provides no kinematic information, while the O-TPC experiment of Reference [4] provides only energy information but with a resolution several times better than the present proposed technique. However, in the present case, both angle and energy information will be recorded.…”

“…The latter produces an intense monochromatic gamma-ray beam with small angular divergence (<2 × 10 −4 rad) [4,5]. Methylal ((OCH 3 ) 2 CH 2 ) and hexane (C 6 H 14 ), at a pressure of a few Torr, serve as the filling gases for operation of the MWPC.…”

“…One is based on the bubble chamber technique and a bremsstrahlung photon beam [3]; the other employs an Optical Time Projection Chamber (O-TPC) [4] and a Laser Compton Backscattered (LCB) γ-ray beam [5][6][7].…”

“…Thus, at an equivalent center of mass energy of~0.9 MeV, the direct (α, γ) cross-section is~60 pb, while the inverse (γ, α) cross-section at 8 MeV is 3 nb. The photodisintegration of 16 O at 8.0 MeV produces 4 He and 12 C with energies 675 and 225 keV, respectively, flying back-to-back, which aids the positive identification of this channel. We present a new active target for measuring the cross-section of the 16 O(γ, α) 12 C reaction, using an LCB γ-ray beam of energy E γ = 8-10 MeV.…”

“…It is a position-and time-sensitive detector system based on low-pressure Multi Wire Proportional Chamber (MWPC) technology. This enables the trajectories and velocities of 4 He and 12 C fragments of a few hundred keV energy to be determined. The experimental method is described in Section 2, while Section 3 summarizes the experimental setup and the results of test measurements of timing and position resolution using α-particles from a 239 Pu source.…”